Catalytic Hydroboration of Aldehydes, Ketones, and Alkenes Using Potassium Carbonate: A Small Key to Big Transformation.

An efficient transition-metal-free protocol for the hydroboration of aldehydes and ketones (reduction) was developed. The hydroboration of a wide range of aldehydes and ketones with pinacolborane (HBpin) under the K2CO3 catalyst has been studied. The reaction system is practical and reliable and proceeds under extremely mild and operationally simple conditions. No prior preparation of the complex metal catalyst was required, and hydroboration occurred stoichiometrically. Further, the chemoselective reduction of aldehydes over ketones was carried out. Moreover, we demonstrated the use of K2CO3 as an efficient catalyst for the hydroboration of alkenes. The operational simplicity, inexpensive and transition-metal-free catalyst, and the applicability to gram-scale synthesis strengthen its potential applications for hydroboration (reduction) at an industrial scale.

Introduction

One of the core principles of green chemistry is the development of atom-economic, low hazardous, and sustainable technologies that are easy to use.[1] Over recent decades, rapid advances in research and development have been realized in the pursuit of green processes.[2] Hydrofunctionalizations have found widespread applications in academia and industrial applications for the production of petrochemicals, automobiles, fine chemicals, pharmaceutical, perfumes, and consumer products.[3] Hydroboration constitutes one of the major hydrofunctionalizations owing to its broad applicability in the C–X bond formation (X = C, O, N, P) and functional-group manipulations.[4]

The operational simplicity and broad synthetic utility of functionalized boronates make catalytic hydroboration a highly practiced and economically favored reaction. These reasons have driven rapid growth in the applications of metal-mediated (including transition-metal, precious, main group, and lanthanide metal complexes) hydroborations of unsaturated hydrocarbons, carbonyl compounds, and imines.[5,6] Despite efficient conversions and good selectivities seen with the aforementioned catalysts/complexes, these systems suffer from the need for the use of complex ligands, prepared using tedious procedures.

The use of commercially available and simple hydroxides and hydrides for catalytic hydroboration has been demonstrated. In 2011, Query et al.[7] reported the alkoxide-mediated (NaOtBu) reduction of ketones with pinacolborane (HBpin) at ambient temperature. Recently, Wu[8] and co-workers developed a general method for catalytic hydroboration of aldehydes, ketones, alkynes, and alkenes with HBpin using powder NaOH under deuterated solvent and stated nucleophilic coordinations[9] with HBpin through natural bond orbital calculations. Lastly, green and sustainable approaches incorporating organoboron moieties into organic compounds have drawn attention of the scientists, starting from the pioneering work reported by Knochel[10] and ending with the conclusions provided by Bertrand and others.[11]

For instance, Stachowiak et al. have described the hydroboration of aldehydes under solvent- and catalyst-free conditions; however, this method is applicable only to aldehydes and requires higher temperatures for better conversions.[12] Recently, Zhu’s group reported protocols for the catalytic hydroboration of aldehydes, ketones, imines, and alkynes with n-BuLi.[13] More recently, the Leung group has reported catalyst-free and solvent-free hydroboration of ketones at an elevated temperature.[14] These results point toward strong possibility for the development of more efficient and eco-friendly conditions for the catalytic hydroboration of carbonyl compounds without the need for highly toxic metals and costly ligands, which is the objective of our present study. In addition, apart from the hydroboration of carbonyl compounds, catalytic hydroborations of unsaturated hydrocarbons (alkenes and alkynes) are scarcely reported with commercially available and simple salts. However, extensive study on these transformations using transition complexes (Scheme ) has been reported.

Scheme 1

Reported Catalyzed Hydroborations and Present Work

Results and Discussion

To address these challenges, the development of green protocols for sustained use, particularly in large-scale industrial applications, is highly necessary. To meet such demands, we attempted to find a convenient protocol for the catalytic hydroboration of carbonyl and unsaturated hydrocarbons, the results of which we report herein. Aldehydes and ketones were selectively reduced via catalytic hydroboration under extremely mild conditions (minimal solvent/low catalyst loading/room temperature), using 0.5–5 mol % of K2CO3 which delivered the hydroboration products in good to excellent yields (Scheme ).

Scheme 2

Potassium Carbonate (0.5–5 mol %)-Promoted Hydroboration of Aldehydes and Ketones

Initially, reaction parameters such as catalyst selection, loading, and reaction time were optimized for hydroboration of aldehydes using benzaldehyde as the model substrate.

For the catalyst selection, a variety of commercially available mild salts including Lewis and Brønsted acids (10 mol %) were screened (see Table S1 in the Supporting Information). To our delight, we found the simple and inexpensive potassium salts, K2CO3, KOAc, and K3PO4, to be the superior ones among the tested catalysts. Potassium salts were hence chosen for further optimization. The optimization results of the catalyst loading and screen are presented in Table .

Table 1

Optimization of Catalyst Loading for Hydroboration of the Aldehydea

entry	catalyst	loading (mol %)	conversion (%)b
1c	A	10	94
2c	A	1	17
3d	A	10	97
4d	B	10	97
5d	C	10	97
6d	A	1	96
7d	B	1	39
8d	C	1	11

A = K2CO3, B = KOAc, C = K3PO4.

Conversions were determined based on the consumption of aldehydes by GC using the internal standard (naphthalene).

Type of the catalyst was bead.

Type of the catalyst was powder.

At 10 mol % catalyst loading, these three potassium salts exhibited similar reactivity toward catalytic hydroboration of aldehydes (for a more detailed comparison, see Table S2 in the Supporting Information). However, a dramatic effect was observed when K2CO3 (1 mol %) was used as powder instead of bead because of a common rate enhancement phenomenon, observed in catalytic reactions because of increase in the surface area (Table ).

Next, we attempted optimization of the reaction time to obtain the maximum conversion. Interestingly, the results after modifying this parameter pointed to the possibilities for reduced catalyst loading. Accordingly, the use of 0.5 mol % K2CO3 was tested and provided a 97% conversion in 1 h (entry 2 in Table ). Even in short reaction time (30 min), K2CO3 was able to promote catalytic hydroboration at 0.5 mol % loading (97%, entry 3 in Table ). However, attempts to further reduce the loading to 0.1 mol % delivered only 36% conversion (entry 5 in Table ), while no hydroboration was observed in the absence of the catalyst under this protocol (entry 6 in Table ). Next, hydroboration was carried out in different solvent systems such as with diethyl ether, DCM, hexane, CHCl3, and toluene. However, tetrahydrofuran (THF) was found to be superior among the tested ones (entries 7–11). In addition, the neat condition facilitated moderate conversion to the product (entry 12).

Table 2

Optimization of Reaction Time for Catalytic Hydroboration of Aldehydes

entry	cat (mol %)	solvent	time (min)	conversion (%)a	yield of alcohol (%)b
1	1	THF	60	96	96
2	0.5	THF	60	97	97
3	0.5	THF	30	97	97
4	0.5	THF	10	50	50
5	0.1	THF	60	36	36
6c	-	THF	60	NR	NR
7	0.5	ether	30	81	81
8	0.5	MC	30	59	59
9	0.5	hexane	30	69	69
10	0.5	CHCl3	30	58	58
11	0.5	toluene	30	54	54
12	0.5	neat	30	69	69

Conversion was determined by GC based on the aldehyde consumption with the internal standard.

Yields of alcohols were determined by GC using standard samples.

Reaction was performed under no catalyst condition. NR: no reaction.

With the optimized parameters in hand, we tested the substrate scope of the developed protocol. The results in Table indicate that most of the aldehydes undergo smooth catalytic hydroboration and afforded the corresponding boronate esters in good to excellent yields with 0.5 mol % K2CO3. Substrates with electron-withdrawing substituents afforded hydroboration more quickly when compared to electron-donating ones. Among the electron-poor, chloro, bromo, and nitro substituents resulted in excellent conversion to products, whereas 4-bromo and 4-fluoro substituents resulted in hydroboration in reasonable yield. Further, heterocyclic and aliphatic aldehydes also furnished the corresponding boronate esters using the present protocol. Cinnamaldehyde, a conjugated aldehyde, yielded the corresponding 1,2 reduction product selectively (Table ).

Table 3

Substrate Scope for K2CO3-Catalyzed Hydroboration of Aldehydes

Conversions were determined by GC.

Yields were determined by 1H NMR spectroscopy using acetonitrile as an internal standard.

Having established the conditions for hydroboration of aldehydes, we attempted the catalytic hydroboration of ketones. However, lower conversion was obtained with ketone substrates under which the conditions were optimized for aldehyde hydroboration. Modifications to the optimized conditions were studied (Table ) and an increase in the catalyst loading to 5 mol % along with the increase in reaction time significantly improved the conversions and yield (96%) with 12 h being the optimal reaction time.

Table 4

Optimization of Catalyst Loading for Ketone Hydroboration

entry	HBpin (equiv)	K2CO3 (mol %)	conversoin (%) (alcohol)	yield of alcohol (%)a
1	1.5	0.5	22 (1)	15
2	1.5	0.5	69 (30)	66
3	1.5	0.5	95 (28)	95
4	1.5	5	96 (53)	96

Yields were determined by 1H NMR spectroscopy using acetonitrile as an internal standard.

With the optimized conditions available, the substrate scope was evaluated for the ketone functionality. As can be seen from Table , ketones afforded the corresponding alcohols after the base hydrolysis of the intermediate boronates. Acetophenone produced the boronate with 96% conversion. As seen with the aldehydes, ketones containing electron-withdrawing substituents underwent smooth hydroboration, while electron-rich substrates displayed poor reactivity in this system (Table ).

Table 5

Substrate Scope for K2CO3-Catalyzed Hydroboration of Ketones

Yields were determined by 1H NMR spectroscopy using acetonitrile as an internal standard.

Reacted with HBpin (3.0 equiv).

Next, the potential scalability of the present protocol was evaluated. Benzaldehyde (10 mmol) was treated with 0.5 mol % of K2CO3 and HBpin. Excellent conversion with a good isolated yield of benzyl alcohol was obtained under mild conditions (Scheme ).

Scheme 3

Scale-Up Reaction of Catalyzed Hydroboration using K2CO3 and HBpin

From the above results, we anticipated that the present system is suitable for chemoselective hydroboration. Accordingly, aldehydes were treated with HBpin and K2CO3 under the reaction conditions optimized for aldehydes, in the presence of ketones (Scheme ).

Scheme 4

Intermolecular Chemoselective Hydroboration of Aldehydes in the Presence of Ketones

Density functional theory calculations were performed to explore the reaction pathway for the K2CO3-catalyzed hydroboration of aldehydes. Scheme represents an energy profile for the reaction where the HBpin initially reacts with the catalyst to produce the zwitterionic intermediate A. The subsequent reaction of the intermediate A with aldehydes would generate the intermediate B which turns into the intermediate C through a hexagonal ring transition state TS1. Then, a ligand exchange reaction of the intermediate C with another HBpin yields the intermediate D which participates in a catalytic cycle. The subsequent reaction of the intermediate D with aldehydes leads to the intermediate E, followed by the generation of the intermediate F through TS2. The catalytic cycle is then accomplished when the intermediate F reproduces the intermediate D along with the boronate ester through a ligand exchange reaction with HBpin. A plausible reaction mechanism based on the energy profile in Scheme is proposed in Scheme .

Scheme 5

Energy Profile (in kcal/mol) for Potassium Carbonate-Catalyzed Hydroboration of Aldehydes

Scheme 6

Plausible Reaction Mechanism for Potassium Carbonate-Catalyzed Hydroboration of Aldehydes

We next evaluated the catalytic hydroboration of alkenes using K2CO3 and HBpin. The reaction conditions were optimized for the preparation of alkyl boronate esters. The hydroboration of styrene with HBpin (4.0 equiv) using 5 mol % catalyst loading at 60 °C furnished the corresponding alkyl boronic ester in 66% yield after 24 h (entry 1 in Table ).

Table 6

Optimization of Reaction Conditions for Catalyzed Hydroboration of Styrene

entry	K2CO3 (mol %)	HBpin (equiv)	time (h)	temp (°C)	yield (%)a,b
1	5	4	24	60	66 (4)
2	10	3	24	60	80 (0)
3	10	4	12	60	78 (0)
4	20	4	12	60	93 (4)
5	5	1.1	12	110	53 (0)
6	5	1.5	12	110	81 (2)
7	5	2	12	110	92 (6)
8	5	3	12	110	94 (5)
9	5	2	1	110	29 (0)
10	5	2	3	110	96 (2)
11	5	2	6	110	92 (6)
12		2	12	110	23 (2)

Yields were determined by 1H NMR.

Yields in parentheses report the branched products.

A small increase in product yield was obtained upon increasing the catalyst loading to 10 mol % and reducing the reaction time (entry 2 in Table ). An increase in the catalyst loading to 20 mol % afforded the desired product in 90% yield with the use of 4.0 equiv of HBpin after 12 h reaction time (entry 4 in Table ).

However, in order to reduce the catalyst loading and the equivalents of HBpin, the reaction temperature was increased to 110 °C. Hydroboration with 2.0 and 3.0 equiv of HBpin afforded the product in more than 90% of yield after 12 h (entries 7, 8 in Table ). The equivalents of the HBpin were then fixed to 2 and the reaction time was optimized. Using the optimized conditions, the product was synthesized in 96% yield after 3 h reaction time (entry 10 in Table ). To assess the role of the catalyst in conversion, we have carried out hydroboration using the optimized conditions in the absence of the catalyst which gave a much lower conversion to the boronate ester and a low yield (entry 12 in Table ). Given this, the conditions in entry 10, with 5 mol % of the catalyst, 2 equiv of HBpin, a reaction time of 3 h, and a 110 °C reaction temperature were chosen as the optimal conditions.

To assess the reactivity of the present catalytic system, a comparative study for hydroboration of alkene has been conducted with other metal carbonate such as Na2CO3, which delivered lower yields (see Table S4 in the Supporting Information). Using the optimized conditions, we have evaluated the substrate scope of the reaction. In contrast to aldehydes and ketones, alkenes bearing electron-donating substituents (2-methyl, 3-methy, 4-methyl, and 4-methoxy) afforded the corresponding boronate esters with greater regioselectivity and conversions (6b–f in Table ).

Table 7

Substrate Scope for the Catalyzed Hydroboration of Alkenesa

Isolated yields.

The reaction was conducted for 12 h.

α-Methylstyrene in which the methyl group is located at the benzylic position was highly regioselective but afforded the product in relatively low yield (6g). 4-Acetoxy styrene bearing an ester functional group, and the 2-vinylpyridine substrates did not undergo hydroboration under this condition (6h and 6l). Styrenes containing electron-withdrawing groups afforded the products with slightly reduced regioselectivity (6i–j). Alkenes substituted with the polyaromatic group and the aliphatic chain also displayed similar reactivity as the halogen-bearing styrenes (6k and 6n), while allylbenzene afforded the product with exclusive regioselectivity (6m).

Conclusions

An efficient protocol for catalyzed hydroboration of aldehydes and ketones with the commercially available potassium carbonate (K2CO3) is reported. K2CO3 successfully catalyzed the hydroboration of various aldehydes and ketones with stoichiometric conversions at low catalyst loading. In addition, aldehydes were reduced chemoselectively in the presence of ketones using the developed catalytic system. Further, synthetically important alkyl boronates were synthesized by K2CO3-catalyzed hydroboration of variedly substituted alkenes. The developed green catalyzed hydroboration methodology displays a broad substrate scope and functional group tolerance. The catalyst is inexpensive, very mild, and readily available and generates simple, nontoxic byproducts and has potential applications in large-scale hydroborations compared to toxic metals and strong bases.

Experimental Section

General Information

All glassware used was dried thoroughly in an oven, assembled hot, and cooled under a stream of dry nitrogen prior to use. All reactions and manipulations of air- and moisture-sensitive materials were carried out using standard techniques for the handling of such materials. All the chemicals were commercial products of the highest purity, which were further purified before use by using standard methods. HBpin, aldehydes, ketones, alkenes were purchased from Aldrich Chemical Company, Alfa Aesar, and Tokyo Chemical Industry Company (TCI). 1H NMR spectra were measured at 400 MHz with CDCl3 as a solvent at ambient temperature unless otherwise indicated and the chemical shifts were recorded in parts per million downfield from tetramethylsilane (δ = 0 ppm) or based on residual CDCl3 (δ = 7.26 ppm) as the internal standard. 13C NMR spectra were recorded at 100 MHz with CDCl3 as a solvent and referenced to the central line of the solvent (δ = 77.0 ppm). The coupling constants (J) are reported in hertz. Analytical thin-layer chromatography was performed on glass precoated with silica gel (Merck, silica gel 60 F254). Column chromatography was carried out using 70–230 mesh silica gel (Merck) at normal pressure. Gas chromatography (GC) analyses were performed on a Younglin Acme 6100M and 6500 GC FID chromatography, using an HP-5 capillary column (30 m). All GC yields were determined with the use of naphthalene as the internal standard and the authentic sample.[25]

General Procedure for the Hydroboration of Aldehydes (2a–o)

In a nitrogen atmosphere glovebox, powder K2CO3 was (0.5 mol %, 0.0007 g) weighed, charged into a 25 mL round-bottom flask, sealed with septum, and was taken out from the glovebox. To this benzaldehyde (0.10 mL, 1.0 mmol), THF (1 mL) and HBpin (0.22 mL, 1.5 mmol) were added at room temperature and stirred for 30 min. Reaction was terminated by the addition of two drops of water. The conversion of boronate ester was confirmed by GC and the crude mixture was hydrolyzed to alcohol by adding 1 N aqueous NaOH solution (2 mL). After stirring for 30 min, diethyl ether (5 mL) and NaCl were added until solution become supersaturated. The organic layer was extracted, dried over MgSO4, and the yield was calculated using 1H NMR.

General Procedure for the Hydroboration of Ketones (4a–m)

In a nitrogen-atmosphere glovebox, powder K2CO3 was (5 mol %, 0.0069 g) weighed, charged into a 25 mL round-bottom flask, sealed with septum, and was taken out from the glovebox. To this acetophenone (0.12 mL, 1.0 mmol), THF (1 mL) and HBpin (0.22 mL, 1.5 mmol) were added at room temperature and stirred for 12 h. Reaction was terminated by the addition of two drops of water. The conversion of boronate ester was confirmed by GC and the crude mixture was hydrolyzed to alcohol by adding 1 N aqueous NaOH solution (2 mL). After stirring for 30 min, NaCl was added until supersaturated, and the organic layer was extracted, dried over MgSO4, and the yield was calculated using 1H NMR.

General Procedure for the Chemoselective Hydroboration of Aldehydes in the Presence of Ketones

In a nitrogen atmosphere glovebox, powder K2CO3 was (0.5 mol %, 0.0007 g) weighed, charged into a 25 mL round-bottom flask, sealed with septum, and was taken out from the glove box. To this benzaldehyde (0.10 mL, 1.0 mmol), acetophenone (0.12 mL, 1.0 mmol), THF (1 mL) and HBpin (0.22 mL, 1.5 mmol) were added at room temperature and stirred for 30 min. Reaction was terminated by the addition of two drops of water. Crude mixture was hydrolyzed to alcohol by adding 1 N aqueous NaOH solution (2 mL). After stirring for 30 min, diethyl ether (5 mL) and NaCl were added until solution become supersaturated. The organic layer was extracted, dried over MgSO4 and the yield was calculated using 1H NMR.

Procedure for the Gram-Scale Synthesis of 4,4,5,5-Tetramethyl-2-(benzyloxy)-1,3,2-dioxaborolane

In a nitrogen atmosphere glovebox, powder K2CO3 was (0.5 mol %, 0.007 g) weighed, charged into a 25 mL round bottom flask, sealed with septum, and was taken out from the glove box. To this benzaldehyde (1.0 mL, 10 mmol), and HBpin (2.2 mL, 15 mmol) were added at room temperature and stirred under neat condition for 30 min. Reaction was terminated by the addition of two drops of water. The conversion of boronate ester was confirmed by GC (93%) and the crude mixture was hydrolyzed to alcohol by adding 1 N aqueous NaOH solution (5 mL). After stirring for 30 min, diethyl ether (5 mL) and NaCl were added until solution become supersaturated. The organic layer was extracted, dried over MgSO4, and the crude mixture was subjected to column chromatography with ethyl acetate and hexane (1:10), and benzyl alcohol was obtained (0.95 g, 88%) as colorless liquid.

General Procedure for the Hydroboration of Alkenes (6a–l)

A 25 mL test tube was charged with potassium carbonate powder (0.0069 g, 5 mol %), sealed, and filled with argon. To these alkenes (1.0 mmol), HBpin (0.29 mL, 2.0 mmol) were added at room temperature. The reaction mixture was stirred for 3 h at 110 °C. After cooling to room temperature, the reaction mixture was filtered through a short pad of celite using dichloromethane (10 mL) and the volatiles were removed under vacuum. Conversion of the product was calculated by 1H NMR using 1,3,5-trimethoxybenzene (0.0185 g, 0.11 mmol) as the internal standard.

Spectroscopic Data for Isolated Products

4,4,5,5-Tetramethyl-2-(benzyloxy)-1,3,2-dioxaborolane (2a)

It is obtained as colorless oil in 97% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.25 (s, 12H), 4.92 (s, 2H), 7.25–7.27 (m, 1H), 7.30–7.35 (m, 4H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.72, 66.74, 83.08, 126.79, 127.47, 128.39, 139.27. NMR data was in accordance with reported literature.[13a]

4,4,5,5-Tetramethyl-2-((4-methylbenzyl)oxy)-1,3,2-dioxaborolane (2b)

It is obtained as colorless oil in 83% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.25 (s, 12H), 2.32 (s, 3H), 4.88 (s, 2H), 7.13 (d, J = 8.0 Hz, 2H), 7.23 (d, J = 8.0 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 21.27, 24.72, 66.66, 83.02, 126.92, 129.05, 136.26, 137.11. NMR data was in accordance with reported literature.[13a]

2-((4-Methoxybenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2c)

It is obtained as colorless oil in 95% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.24 (s, 12H), 3.78 (s, 3H), 4.83 (s, 2H), 6.85 (d, J = 8.4 Hz, 2H), 7.26 (d, J = 8.28 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.71, 55.35, 66.51, 83.00, 113.72, 128.66, 131.49, 159.05. NMR data was in accordance with reported literature.[13a]

2-((4-Fluorobenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2d)

It is obtained as colorless oil in 84% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.25 (s, 12H), 4.86 (s, 2H), 6.97–7.03 (m, 2H), 7.28–7.32 (m, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.70, 66.14, 83.15, 115.20 (d, J = 21.5 Hz), 128.70 (d, J = 8.1 Hz), 135.02 (d, J = 3.2 Hz), 162.26 (d, J = 245.0 Hz) NMR data was in accordance with reported literature.[13a]

2-((4-Chlorobenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2e)

It is obtained as colorless oil in 99% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.24 (s, 12H), 4.86 (s, 2H), 7.25–7.30 (m, 4H) 13C NMR (100 MHz, CDCl3): δ (ppm) 24.69, 66.02, 83.20, 128.17, 128.51, 133.16, 137.75. NMR data was in accordance with reported literature.[15]

2-((2-Bromobenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2f)

It is obtained as colorless oil in 99% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.27 (s, 12H), 4.96 (s, 2H), 7.15–7.08 (m, 1H), 7.34–7.27 (m, 1H), 7.50 (dd, J = 8.3, 0.7 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.71, 66.37, 83.26, 121.58, 127.44, 127.84, 128.69, 132.33, 138.39. NMR data was in accordance with reported literature.[12]

2-((3-Bromobenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2g)

It is obtained as colorless oil in 96% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.25 (s, 12H), 4.88 (s, 2H), 7.16–7.25 (m, 2H), 7.38 (d, J = 7.8 Hz, 1H), 7.51 (s, 1H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.70, 65.90, 83.27, 122.52, 125.21, 129.77, 129.96, 130.50, 141.54. NMR data was in accordance with reported literature.[16]

2-((4-Bromobenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2h)

It is obtained as colorless oil in 82% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.24 (s, 12H), 4.85 (s, 2H), 7.19–7.22 (m, 1H), 7.24–7.45 (m, 1H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.70, 66.04, 83.22, 121.29, 128.48, 131.46, 138.27. NMR data was in accordance with reported literature.[15]

4,4,5,5-Tetramethyl-2-((4-nitrobenzyl)oxy)-1,3,2-dioxaborolane (2i)

It is obtained as yellow solid in 99% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.25 (s, 12H), 5.01 (s, 2H), 7.49 (d, J = 8.6 Hz, 1H), 8.18 (d, J = 8.7 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.70, 65.62, 83.50, 123.70, 126.91, 146.68, 147.28. NMR data was in accordance with reported literature.[15]

2-(Furan-2-ylmethoxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2j)

It is obtained as yellow oil in 93% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.25 (s, 12H), 4.81 (s, 2H), 6.28–6.31 (m, 2H), 7.36 (m, 1H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.69, 59.27, 83.17, 108.44, 110.34, 142.57, 152.49. NMR data was in accordance with reported literature.[12]

4,4,5,5-Tetramethyl-2-(naphthalen-2-ylmethoxy)-1,3,2-dioxaborolane (2k)

It is obtained as colorless oil in 99% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.27 (s, 12H), 5.09 (s, 2H), 7.43–7.48 (m, 3H), 7.79–7.82 (m, 4H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.73, 66.82, 83.16, 83.17, 124.93, 125.21, 125.81, 126.12, 127.76, 128.01, 128.08, 132.88, 133.39, 136.76. NMR data was in accordance with reported literature.[13a]

2-(Cinnamyloxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2l)

It is obtained as colorless oil in 94% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.26 (s, 12H), 4.52–4.53 (d, 2H), 6.24–6.31 (m, 1H), 6.59–6.63 (d, J = 15.9 Hz, 1H), 7.20–7.25 (dd, J = 11.0, 4.2 Hz, 1H), 7.25–7.32 (t, J = 7.6 Hz, 2H), 7.35–7.37 (d, J = 7.7 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.71, 65.36, 83.05, 126.52, 126.84, 127.60, 128.62, 130.63, 136.91. NMR data was in accordance with reported literature.[13a]

4,4,5,5-Tetramethyl-2-(naphthalen-2-ylmethoxy)-1,3,2-dioxaborolane (2m)

It is obtained as yellow solid in 99% yield; 1H NMR (400 MHz, CDCl3) δ: (ppm) 1.25 (s, 12H), 4.10 (t, J = 1.9 Hz, 2H), 4.13 (s, 5H), 4.63 (s, 2H), 4.22 (t, J = 1.9 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.65, 63.27, 68.28, 68.53, 82.92, 85.39. NMR data was in accordance with reported literature.[13a]

2-(Hexyloxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2n)

It is obtained as colorless oil in 97% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 0.84–0.87 (t, J = 6.7 Hz, 2H), 1.23 (s, 12H), 1.25 (m, 6H), 1.50–1.57 (m, 2H), 3.81 (t, J = 6.6 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ (ppm) 14.15, 22.73, 24.67, 25.36, 31.50, 31.61, 65.09, 82.68. NMR data was in accordance with reported literature.[17]

2-(Cyclohexylmethoxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2o)

It is obtained as colorless oil in 99% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 0.85–0.95 (m, 2H), 1.06–1.19 (m, 3H), 1.22 (s, 12H), 1.44–1.54 (m, 1H), 1.60–1.65 (m, 1H), 1.66–1.71 (m, 4H), 3.62 (d, J = 6.5 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 24.63, 25.86, 26.61, 29.39, 39.37, 70.47, 82.67. NMR data was in accordance with reported literature.[13a]

1-Phenylethanol (4a)

It is obtained as colorless oil in 96% yield; 1H NMR (400 MHz, CDCl3): δ (ppm) 1.42 (d, J = 6.5 Hz, 3H), 3.30 (s, 1H), 4.79 (d, J = 6.5 Hz, 1H), 7.23–7.32 (m, 5H). NMR data was in accordance with reported literature.[13a]

1-(p-Tolyl)ethanol (4b)

It is obtained as colorless oil in 93% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.44 (d, J = 6.5 Hz, 3H), 2.31 (s, 3H), 2.42 (s, 1H), 4.79–4.83 (m, 1H), 7.12 (d, J = 6.5 Hz, 2H), 7.23 (d, J = 8.0 Hz, 2H). NMR data was in accordance with reported literature.[13a]

1-(4-Methoxyphenyl)ethanol (4c)

It is obtained as yellow oil in 95% yield. 1H NMR (400 MHz, CDCl3) δ: (ppm) 1.45 (d, J = 6.5 Hz, 3H), 2.05 (s, 1H), 3.78 (s, 3H), 4.82 (q, J = 6.3 Hz, 1H), 6.86 (d, J = 8.8 Hz, 2H), 7.27 (d, J = 8.4 Hz, 2H). NMR data was in accordance with reported literature.[13a]

1-(4-Fluorophenyl)ethanol (4d)

It is obtained as yellow oil in 86% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.44 (d, J = 6.5 Hz, 3H), 2.20 (s, 1H), 4.82–4.87 (m, 1H), 6.96–7.01 (m, 2H), 7.29–7.32 (m, 2H). NMR data was in accordance with reported literature.[13a]

1-(4-Chlorophenyl)ethanol (4e)

It is obtained as yellow oil in 94% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.43 (d, J = 6.5 Hz, 3H), 2.35 (s, 1H), 4.81–4.86 (m, 1H), 7.25–7.27 (m, 4H). NMR data was in accordance with reported literature.[13a]

1-(4-Bromophenyl)ethanol (4f)

It is obtained as yellow solid in 98% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.44 (d, J = 6.5 Hz, 2H), 2.04 (s, 1H), 4.81–4.84 (m, 1H), 7.21–7.25 (m, 2H), 7.43–7.46 (m, 2H). NMR data was in accordance with reported literature.[13a]

1-(4-Nitrophenyl)ethanol (4g)

It is obtained as yellow solid in 99% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.48 (d, J = 6.5 Hz, 3H), 2.45 (s, 1H), 4.97–5.00 (m, 1H), 7.51 (d, J = 8.9 Hz, 2H), 8.16 (d, J = 8.8 Hz, 2H). NMR data was in accordance with reported literature.[13a]

4-(1-Hydroxyethyl)benzonitrile (4h)

It is obtained as yellow oil in 97% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.44 (d, J = 6.5 Hz, 3H), 2.67 (s, 1H), 4.90 (q, J = 6.5 Hz, 1H), 7.44 (d, J = 8.2 Hz, 2H), 7.58 (d, J = 8.4 Hz, 2H). NMR data was in accordance with reported literature.[18]

1-(Naphthalen-2-yl)ethanol (4i)

It is obtained as white solid in 89% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.55 (d, J = 6.4 Hz, 3H), 2.40 (s, 1H), 5.00–5.03 (m, 1H), 7.43–7.49 (m, 3H), 7.78–7.82 (m, 4H). NMR data was in accordance with reported literature.[19]

(E)-4-Phenylbut-3-en-2-ol (4j)

It is obtained as colorless oil in 91% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.33 (t, J = 6.4 Hz, 3H), 4.08 (q, J = 6.4 Hz, 1H), 6.23 (dd, J = 15.9, 6.6 Hz, 1H), 6.52 (d, J = 15.9 Hz, 1H), 7.18–7.35 (m, 5H). NMR data was in accordance with reported literature.[20]

Heptan-2-ol (4k)

It is obtained as colorless oil in 92% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 0.85 (t, J = 6.9 Hz, 3H), 1.14 (d, J = 6.2 Hz, 3H), 1.21–1.22 (m, 2H), 1.23–1.29 (m, 4H), 1.35–1.44 (m, 2H), 2.22 (s, 1H), 3.69–3.79 (m, 1H). NMR data was in accordance with reported literature.[19]

1-Cyclohexylethanol (4l)

It is obtained as yellow oil in 87% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 0.86–1.01 (m, 2H), 1.07–1.15 (m, 5H), 1.61–1.83 (m, 7H), 2.25 (s, 1H), 3.47–3.53 (m, 1H). NMR data was in accordance with reported literature.[21]

Diphenylmethanol (4m)

It is obtained as white solid in 87% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) 2.61 (s, 1H), 5.81 (s, 1H), 7.31–0.38 (m, 10H). NMR data was in accordance with reported literature.[13a]

4,4,5,5-Tetramethyl-2-phenethyl-1,3,2-dioxaborolane (6a)

It is obtained as colorless oil in 86% yield. (0.86 mmol, 0.200 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.13–1.17 (m, 2H), 1.22 (s, 12H), 2.63–2.78 (m, 2H), 7.14–7.18 (m, 1H), 7.21–7.29 (m, 4H). 13C NMR (101 MHz, CDCl3): δ (ppm) 24.93, 30.07, 83.21, 125.63, 128.12, 129.22, 144.50. NMR data was in accordance with reported literature.[5g]

4,4,5,5-Tetramethyl-2-(2-methylphenethyl)-1,3,2-dioxaborolane (6b)

It is obtained as colorless oil in 88% yield. (0.88 mmol, 0.216 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.10–1.14 (m, 2H), 1.25 (s, 12H), 2.33 (s, 3H), 2.71–2.75 (m, 2H), 7.06–7.15 (m, 3H), 7.20 (dd, J = 6.5, 2.7 Hz, 1H). 13C NMR (101 MHz, CDCl3): δ (ppm) 19.46, 24.95, 27.30, 83.22, 125.75, 125.98, 128.17, 130.08, 135.91, 142.61. NMR data was in accordance with reported literature.[22]

4,4,5,5-Tetramethyl-2-(3-methylphenethyl)-1,3,2-dioxaborolane (6c)

It is obtained as colorless oil in 85% yield. (0.85 mmol, 0.208 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.13–1.17 (m, 2H), 1.24 (s, 12H), 2.33 (s, 3H), 2.71–2.75 (m, 2H), 6.99 (d, J = 7.4 Hz, 1H), 7.02–7.06 (m, 2H), 7.17 (t, J = 7.5 Hz, 1H). 13C NMR (101 MHz, CDCl3): δ (ppm) 21.59, 24.94, 30.01, 83.19, 125.14, 126.36, 128.25, 128.99, 137.76, 144.49. NMR data was in accordance with reported literature.[22]

4,4,5,5-Tetramethyl-2-(4-methylphenethyl)-1,3,2-dioxaborolane (6d)

It is obtained as colorless oil in 87% yield. (0.87 mmol, 0.216 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.11–1.15 (m, 2H), 1.24 (s, 12H), 2.31 (s, 3H), 2.69–2.73 (m, 2H), 7.10 (q, J = 8.1 Hz, 4H). 13C NMR (101 MHz, CDCl3): δ (ppm) 21.14, 24.94, 29.64, 83.19, 127.96, 129.00, 134.98, 141.50. NMR data was in accordance with reported literature.[22]

2-(4-(tert-Butyl)phenethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6e)

It is obtained as colorless solid in 70% yield. (0.70 mmol, 0.202 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.10–1.14 (m, 2H), 1.21 (s, 12H), 1.29 (s, 9H), 2.68–2.72 (m, 2H), 7.14 (d, J = 8.1 Hz, 2H), 7.28 (d, J = 8.1 Hz, 2H). 13C NMR (101 MHz, CDCl3): δ (ppm) 24.90, 29.46, 31.52, 34.41, 83.17, 125.17, 127.71, 141.45, 148.32. NMR data was in accordance with reported literature.[5g]

2-(4-Methoxyphenethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6f)

It is obtained as colorless oil in 85% yield. (0.85 mmol, 0.223 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.10–1.13 (m, 2H), 1.21 (s, 12H), 2.66–2.70 (m, 2H), 3.76 (s, 3H), 6.80 (d, J = 8.6 Hz, 2H), 7.13 (d, J = 8.3 Hz, 2H). 13C NMR (101 MHz, CDCl3): δ (ppm) 24.92, 29.15, 55.32, 83.17, 113.63, 128.97, 136.62, 157.58. NMR data was in accordance with reported literature.[5g]

4,4,5,5-Tetramethyl-2-(2-phenylpropyl)-1,3,2-dioxaborolane (6g)

It is obtained as colorless oil in 39% yield. (0.39 mmol, 0.095 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.10–1.14 (m, 2H), 1.15 (s, 12H), 1.27 (d, J = 6.8, 3H), 2.98–3.07 (m, 1H), 7.12–7.16 (m, 1H), 7.22–7.78 (m, 4H). 13C NMR (101 MHz, CDCl3): δ (ppm) 24.78, 24.87, 25.05, 35.91, 83.09, 125.79, 126.71, 128.29, 149.29. NMR data was in accordance with reported literature.[5g]

2-(4-Bromophenethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6h)

It is obtained as colorless solid in 47% yield. (0.47 mmol, 0.146 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.07–1.11 (m, 2H), 1.20 (s, 12H), 2.66–2.70 (m, 2H), 7.07 (d, J = 8.6 Hz, 2H), 7.35 (d, J = 8.4 Hz, 2H). 13C NMR (101 MHz, CDCl3): δ (ppm) 24.90, 29.48, 83.30, 119.26, 129.92, 131.28, 143.42. NMR data was in accordance with reported literature.[23]

2-(4-Chlorophenethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6i)

It is obtained as colorless solid in 70% yield. (0.70 mmol, 0.187 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.08–1.12 (m, 2H), 1.20 (s, 12H), 2.67–2.71 (m, 2H), 7.12 (d, J = 8.2 Hz, 2H), 7.20 (d, J = 8.4 Hz, 2H). 13C NMR (101 MHz, CDCl3): δ (ppm) 24.90, 29.42, 83.29, 128.33, 129.48, 131.21, 142.89. NMR data was in accordance with reported literature.[23]

4,4,5,5-Tetramethyl-2-(2-(naphthalen-2-yl)ethyl)-1,3,2-dioxaborolane (6j)

It is obtained as colorless solid in 68% yield. (0.68 mmol, 0.193 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.20 (m, 2H), 1.21 (s, 12H), 2.88–2.92 (m, 2H), 7.35–7.44 (m, 3H), 7.36 (m, 1H), 7.33–7.99 (m, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm) 24.93, 30.22, 83.26, 125.02, 125.76, 125.83, 127.37, 127.52, 127.66, 127.79, 131.94, 133.67, 142.05. NMR data was in accordance with reported literature.[22]

4,4,5,5-Tetramethyl-2-(3-phenylpropyl)-1,3,2-dioxaborolane (6k)

It is obtained as colorless oil in 80% yield. (0.80 mmol, 0.196 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 0.88–0.86 (m, 2H), 1.25 (s, 12H), 1.70–1.78 (m, 2H), 2.60–2.63 (m, 2H), 7.15–7.19 (m, 3H), 7.25–7.29 (m, 2H). 13C NMR (101 MHz, CDCl3): δ (ppm) 24.96, 26.31, 38.74, 83.05, 125.70, 128.30, 128.69, 142.79. NMR data was in accordance with reported literature.[5g]

2-Heptyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6l)

It is obtained as colorless oil in 72% yield. (0.72 mmol, 0.163 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 0.72–0.76 (m, 2H), 0.84 (t, J = 6.9 Hz, 3H), 1.21 (s, 12H), 1.23–1.30 (m, 8H), 1.33–1.40 (m, 2H). 13C NMR (101 MHz, CDCl3): δ (ppm) 14.23, 22.77, 24.11, 24.89, 29.20, 31.90, 32.51, 82.90. NMR data was in accordance with reported literature.[24]