Convenient Synthesis of Deuterosilanes by Direct H/D Exchange Mediated by Easily Accessible Pt(0) Complexes.

Easily accessible, simple phosphino-platinum(0) complexes catalyze (0.1-1 mol % equivalent) the deuteration of silanes in good yields under mild conditions (60 °C, 1 atm). The catalysis is mediated by platinum(II) deuteride/hydride complexes that are in equilibrium with the precursor Pt(0) complexes. The Pt(II) complexes can also be inserted into the Si-H bond of silanes to give intermediate Pt(IV) complexes. The proposed mechanism for catalysis is supported by density functional theory calculations.

Introduction

Deuterium-labeled compounds are widely used to elucidate reaction mechanisms in a wide variety of chemical reactions, such as in drug-discovery processes and metabolism research.[1] The rising need for deuterium-labeled compounds has stimulated the search for convenient, selective, preferably catalytic methods for their preparation.[1,2] Deuterium-labeled carbon compounds are usually prepared by the reduction of carbon–halogen or of multiple bonds with metal deuteride derivatives.[3] These methods have several drawbacks: (a) use of stoichiometric amounts of hazardous air- and moisture-sensitive metallodeuteride reagents, (b) limited tolerance toward functional groups, (c) low selectivity, and (d) generation of metal-containing waste products, which is a critical concern in the pharmaceutical industry. Direct H/D exchange taking place at carbon, which is both selective and atom efficient, has therefore recently witnessed a surge in activity.[2b]

Hydridosilanes, R3SiH, are widely used reagents in academia and industry.[4−6] Silanes are environmentally friendly and are desired alternatives to toxic metallohydrides or tin hydrides, because of their air and moisture tolerance and low toxicity. Hydridosilanes add to a variety of unsaturated bonds (hydrosilylation) and reduce carbon–halogen bonds.[4−6] By replacing hydrosilanes, R3SiH, with their deuterium analogs, R3SiD, a wide range of deuterium-labeled carbon compounds can be synthesized. Despite these advantages, use of deuterated silanes as isotopic labeling reagents is still limited, most probably owing to the scarcity of methods for efficient catalytic direct H/D exchange reactions at silicon centers, which leaves NaBD4 or LiAlD4 reduction of halosilanes as the commonly used method of synthesis of deuterated silanes.[7] Recently, several novel methods for the catalytic deuteration of silanes with D2 using transition metal catalysts has been reported.[8−12] However, all of these catalysts have relatively elaborate ligands (i.e., a, b, c, and d in Chart ), which are less easily accessible than the simple phosphines used here; for example, a cationic (η5-C5Me5)Rh(III) complex (a)[9] and an iridium complex with N-heterocyclic carbene ligands (b)[10] have been recently reported to catalyze the direct deuteration of silanes using D2. In 2014, the ruthenium complex (c) and the binuclear Ni(0) complex (d) were reported to catalyze the deuteration of silanes with D2[11] and C6D6,[12] respectively.

Chart 1

Catalysts Reported for the Deuteration of Silanes with D2 (a–c) and C6D6 (d)

Results and Discussion

In this paper, we report a novel convenient (60 °C) high-yield synthesis of deuterosilanes from the corresponding hydrosilanes and D2 (1 atm) via direct H/D exchange catalyzed by 0.1–1 mol % equivalents of easily accessible simple phosphino-Pt(0) complexes.

Reaction of a variety of silanes, R3SiH, where R = alkyl, Ph, Me3Si, OEt, and H, with D2 under mild conditions (1 atm D2, 60 °C) with minor catalytic amounts (1 mol %) of simple phosphino-platinum(0) complexes, either Pt(PEt3)3 (1) or (dmpe)Pt(PEt3)2 (dmpe = 1,2-bis(dimethylphosphino)ethane) (2), produces in high yields (≥90%) the corresponding deuterosilanes (eq and Table ). The commercially available Pt(PPh3)4 also catalyzes H/D exchange effectively. The more bulky (dtbpe)Pt(PEt3) (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) (3) is a less effective catalyst than 1 or 2 (Table ). No reaction occurs between R3SiH and D2 without the presence of one of these Pt(0) catalysts.

Table 1

H/D Exchange in the Reaction of Hydrosilanes with D2 Catalyzed by 1–3 and Pt(PPh3)4a

entry	silane	catalyst	loading (mol %)	temp (°C)	time (h)	yield (%)
1	Et3SiH	1	1	60	6	≥99
2	Et3SiH	1	0.1	60	12	≥99
3	Et3SiH	2	1	60	4	≥99
4	Et3SiH	3	1	60	4	60e
5	Et3SiH	Pt(PPh3)4	1	60	4	95
6	PhMe2SiH	1	1	60	6	95
7	(EtO)3SiH	1	1	60	6	≥99
8	(Me3Si)3SiH	1	1	60	6	55e
9	(Me3Si)3SiHb	1	10	60	12	90
10	PhMeSiH2b	1	1	40c	2	90f
11	PhSiH3b	1	1	rtc	12	90g
12	ClMe2SiH	1	1	60	2	0
13	Me3SiSiMe2H	1	1	rtd	24	50e

Reactions were carried out in hexane in a 100 mL Schlenk flask under 1 atmosphere (atm) of D2. To ensure full deuteration of the silane, three D2 loadings (each includes 5 mol equiv of D2) are used.

One D2 loading at 3 atm was used.

A complex mixture of products is observed at higher temperatures.

At 60 °C, deuterolysis of the Si–Si bond occurs to yield Me2SiD2 and Me3SiD.

The other compound present is the precursor hydridosilane.

The product is PhMeSiD2.

The product is PhSiD3.

As can be seen in Table , deuteration of the studied silanes occurs in high yields in a few hours at 60 °C. H/D exchange occurs effectively with Et3SiH (entries 1–5), PhMe2SiH (entry 6), (EtO)3SiH (entry 7), and (Me3Si)3SiH (entries 8 and 9). Multiple H/D exchange takes place when the silane carries more than one hydrogen, for example, with PhMeSiH2 and PhSiH3 (entries 10 and 11). H/D exchange does not occur with ClMe2SiH (entry 12). Interestingly with Me3SiSiMe2H, H/D exchange occurs at 25 °C, but at 60 °C, the cleavage of the Si–Si bond also occurs, yielding Me3SiD and Me2SiD2 (entry 13). It is also worth mentioning that the deuterated silane is obtained without the need for further separation and purification from waste products (unlike their synthesis using metallohydrides such as LiAlD4).

How does this efficient deuteration reaction (eq ) occur? Activation of Si–H bonds by platinum complexes was extensively studied.[4b,13] It is also known that H2 is activated by phosphino-Pt(0) complexes, yielding platinum hydride complexes.[8a,14] There is one report suggesting that platinum dihydride complexes, obtained by reacting H2 with L2Pt(C2H4) (L = PMe3, PEt3), catalyze the deuteration of Et3SiH with D2, but no details on the catalyst loading or the yield of the deuterosilane were reported.[8a]

To better understand how reaction occurs, we first studied the activation of D2 by Pt(0) complexes 1–3. No reaction is observable [using nuclear magnetic resonance (NMR) spectroscopy] when 1 or 2 reacts with D2, in contrast to L2Pt(C2H4) (L = PMe3, PEt3), which yields the platinum dihydride complex.[8a] However, when 3 is reacted in toluene at rt with an excess of D2, immediate full conversion to (dtbpe)PtD2 (4) is observed using NMR spectroscopy (eq ). Furthermore, reaction is reversible, reaching Keq = 1.0 (ΔG° = 0), which is in good agreement with the ΔG°(eq ) of 0.73 kcal mol–1 calculated using density functional theory (DFT)[15] [at B3LYP-3D[16]/6-311+G(d,p) (for all atoms except Pt), LANL2DZ[17] for Pt[18]].[19] According to these DFT calculations, a similar equilibrium (eq ) exists also for 1 and 2, but in these cases, the equilibrium is strongly shifted toward the precursor Pt(0) complexes (Keq = 6.62 × 10–5 and ΔG° = 5.67 kcal mol–1 for 1; Keq = 1.76 × 10–6 and ΔG° = 7.81 kcal mol–1 for 2),[18,19] which explains why for 1 and 2 the platinum dideuteride complex is not observed by NMR spectroscopy. In hexane, (dtbpe)PtD2 (4) (or its H-analog) precipitates from the reaction mixture as a white powder, driving reaction to full conversion. (dtbpe)PtH2 was previously obtained in 68% yield by the less convenient reaction of (dtbpe)PtCl2 with 1% Hg/Na amalgam.[20] Addition of 4 (20% mol equiv) to Et3SiH under a D2 atmosphere (at 60 °C, 2 h) yields quantitative deuteration of the silane, supporting the suggestion that platinum deuteride complexes are responsible for the deuteration of silanes in reaction .

It is experimentally known that platinum(0) complexes, including 1, insert into Si–H bonds to yield silylated platinum hydride complexes.[21] Our DFT calculations[15,19] also show that the insertion of phosphino-platinum complexes into either Si–H or H–H bonds is energetically plausible. The calculated energies [at B3LYP-3D[16]/6-311+G(d,p) (for all atoms except Pt), LANL2DZ[17] for Pt[18]], for reactions of Pt(PMe3)3, a model for 1, with Me3SiH and D2 are presented in Scheme and Figure .[18] On the basis of the computational results and the experimental evidence[14,21] that platinum complexes can activate both Si–H and D–D bonds, we suggest that the H/D exchange reaction of silanes (eq ) occurs via the mechanism presented in Scheme . The proposed mechanism involves the following steps: In the first step, a phosphine ligand dissociates, yielding a dicoordinated platinum(0) complex having a free coordination site at Pt (Scheme , step a). This step is endoergic (ΔG) by only 2.35 kcal mol–1. In the second step, an oxidative addition of D2 to the Pt(0) complex yields a platinum dideuteride complex 5 (Scheme , step b).[22a] This step is endoergic (ΔG) by only 0.78 kcal mol–1 with a barrier of ΔG‡ = 16.82 kcal mol–1. This step is followed by a second oxidative addition of the platinum deuteride complex 5 into the Si–H bond, yielding an intermediate platinum(IV) species 6 (Scheme , step c).[22b,22c] This step is endoergic by 11.01 kcal mol–1 with a barrier of ΔG‡ = 25.42 kcal mol–1. The deuterated silane is obtained by reductive elimination from platinum(IV) complex 6 (this is the reverse of step c), yielding the H, D, mixed Pt complex 7 (Scheme , step d) in an exoergic step (ΔG = −11.01 kcal mol–1; ΔG‡ = 14.40 kcal mol–1). Finally, HD (which is observed in the reaction mixture using NMR spectroscopy) is released to produce the dicoordinated (R3P)2Pt(0) complex (Scheme , step e) in an exoergic step (ΔG = −0.78 kcal mol–1; ΔG‡ = 16.03 kcal mol–1). Alternatively, it is also possible that the initial step is oxidative addition of the dicoordinated (R3P)2Pt(0) complex to a Si–H bond to produce a silylated platinum hydride complex 8 with ΔG = −0.78 kcal mol–1 and ΔG‡ = 14.63 kcal mol–1 (Scheme , step f).[22d] Then, D2 coordinates with 8, forming the sigma complex 9 (Scheme , step g). This step has a barrier of ΔG‡ = 27.20 kcal mol–1 (ΔG = 26.45 kcal mol–1) (Scheme , step g). In the next step, platinum inserts into the D–D bond to yield the platinum(IV) complex 6 (ΔG = −13.86 kcal mol–1; ΔG‡ = 0.50 kcal mol–1) (Scheme , step h). Mechanisms of the type shown in Scheme were previously proposed for reactions of platinum hydrides[8a,23] and other transition metals[8b,24] with R3EH (E = Si or Sn), but our study provides the first-ever computational support of this proposed catalytic cycle. The catalytic scrambling of H2 and D2 to produce HD by Pt hydride complexes was also previously reported.[8a]

Scheme 1

Proposed Mechanism for the Catalytic R3SiH to R3SiD Exchange Including Calculated Energies (at 25 °C) of the Various Steps (eq )

ΔH, ΔG, and ΔG‡ values are given in kcal mol–1.

Figure 1

(a) Free energy profile for the reaction of (Me3P)2Pt(0) first with D2 (step b) and then with Me3SiH (step c). (b) Free energy profile for the reaction of (Me3P)2Pt(0) first with Me3SiH (step f) and then with D2 (steps g and h).

The calculations show that both reaction sequences have similar highest activation free energies: that is, along the sequence a → b(5) → c(6), the rate-determining step is the insertion of 5 into Si–H, ΔG‡ = 25.42 kcal mol–1 (the free energy of the transition state is 26.21 kcal mol–1 relative to [(Me3P)2Pt + Me3SiH + D2] Figure ). Along the alternative sequence a → f(8) → g(9) → h(6), the rate-determining step is the insertion of 8 into D–D, ΔG‡ = 27.20 kcal mol–1 (the free energy of the transition state is 26.41 kcal mol–1 relative to the precursors Figure ). Therefore, both catalytic sequences are comparable in energy.

Conclusions

In conclusion, we report that the simple phosphine-substituted platinum(0) complexes 1–3 (all are easily prepared) and Pt(PPh3)4 (commercially available) effectively catalyze the direct deuteration of silanes with D2 at 1 atm pressure under mild conditions in excellent to good yields. The catalysis is mediated by platinum deuteride/hydride intermediates, which are in equilibrium with the phosphino Pt(0) catalyst. We are continuing to study the scope and mechanism of this intriguing reaction.

Experimental Section

Standard Schlenk techniques were used for all syntheses and all sample manipulations. Solvents were dried and kept over anhydrous CaCl2, filtered, degassed, kept on t-BuLi in vacuum, and distilled before use. All commercially available reagents were degassed from air and used without additional purification. (Ph3P)4Pt was purchased from Stream Chemicals.

General Procedure for the Isotopic Exchange of Silanes

Neat silane (0.8 mmol) was added to a 100 mL Schlenk flask. The flask was cooled down in a liquid nitrogen bath until the silane was frozen and vacuum was applied to remove the air. To the degassed silane 1 mol % of the catalyst (0.008 mmol) dissolved in 1 mL of hexane was added. The flask was charged with 5 mol equivalents of D2 relative to the silane (100 mL, 1 atm, 4 mmol), and the reaction was stirred at 60 °C for the time indicated in Table . To ensure full deuteration, after each 2 h, the reaction mixture was vacuumed and an additional 4 mmol portion of D2 was added. The addition of D2 was repeated two more times. The reaction yield was determined using 1H and 29Si NMR spectroscopies.