Regioselective 1,4-trifluoromethylation of α,β-unsaturated ketones via a S-(trifluoromethyl)diphenylsulfonium salts/copper system.

Regioselective conjugate 1,4-trifluoromethylation of α,β-unsaturated ketones by the use of shelf-stable electrophilic trifluoromethylating reagents, S-(trifluoromethyl)diphenylsulfonium salts and copper under mild conditions is described. A wide range of acyclic aryl-aryl-enones and aryl-alkyl-enones were converted into β-trifluoromethylated ketones in low to moderate yields.

Introduction

One of the challenges in synthetic organic chemistry is the nucleophilic 1,4-addition of the trifluoromethyl (CF3) group into electron-deficient internal alkenes as represented by the Michael addition reaction, even in a racemic, non-stereoselective fashion [1-5]. The nucleophilic trifluoromethylation to conjugated alkenes essentially occurs solely via a 1,2-addition [1-11], not a 1,4-addition (Scheme 1), with the exception of non-general examples of 1,4-additive trifluoromethylation of (trifluoromethyl)trimethylsilane (Me3SiCF3, Ruppert–Prakash reagent) to very specific substrates such as trans-1-benzoyl-2-(dimethylamino)ethylene [12], 2-polyfluoroalkylchromones [13-14], isoxazoles with a nitro group at the 4-position [15], and Morita–Baylis–Hillman adducts (via SN2’ [16] or successive SN2’/SN2’ mode [17]).

Scheme 1

Trifluoromethylation of α,β-unsaturated ketones.

Sevenard and co-workers reported the nucleophilic 1,4-trifluoromethylation to chromones, coumarins and cyclohex-2-enone using the Ruppert–Prakash reagent, which was achieved by blocking the carbonyl moiety of the substrates with a bulky aluminium-centered Lewis acid with low to moderate yields [18]. Dilman and co-workers partially overcame this problem by using highly electrophilic alkenes bearing either Meldrum’ acids [19], or two geminal nitrile groups [20]. However, direct 1,4-trifluoromethylation to conventional α,β-unsaturated ketones such as chalcone is very tough, presumably due to the hardness of the CF3 anion. Recently, we reported the copper-mediated trifluoromethylation at the benzylic position by using shelf-stable electrophilic trifluoromethylating reagents, S-(trifluoromethyl)diphenylsulfonium salts, in good to high yields under mild conditions [21]. In this reaction, a bromide at the benzylic position would be replaced by a CF3 anion mediated by a copper via SET process, although the reaction mechanism is not clear. We envisaged that the system could be applicable to the conjugated 1,4-trifluoromethylation to simple chalcones. During the preparation of this article, the Nicewicz group showed a single example of conjugate trifluoromethylation of chalcone with sodium trifluoromethanesulfinate salt in the presence of N-methyl-9-mesitylacridinium as a photoredox catalyst resulting in a low product yield of 31% as a mixture of regioisomers (C2/C3 1.1:1) [22]. We disclose herein the regioselective 1,4-addition of the CF3 group into simple conjugated acyclic enones including chalcones using S-(trifluoromethyl)diphenylsulfonium salt 3 and a copper system in 11–37% yields (12 examples).

Results and Discussion

We initiated our investigation with the reaction of chalcone (1a) using a series of electrophilic trifluoromethylating reagents 3 [23-26] in the presence of copper in DMF at 60 °C (Table 1), based on previously reported conditions [21]. First, the trifluoromethylation of 1a with S-(trifluoromethyl)diphenylsulfonium salt 3a was attempted, and a desired product 2a was obtained in only 4% yield (Table 1, entry 1). Next the solvent was screened for yield improvement. We attempted the same reaction using NMP and DMSO, and the desired product 2a was obtained in 6% and 11% yields (Table 1, entries 2 and 3, respectively). Interestingly, adding water (DMF/H2O 1:1) effectively improved the yield to 23% (Table 1, entry 4). Reactions mediated by other metals, such as Ni and Zn, either gave poor yields (Table 1, entries 5 and 6). The use of larger excesses of S-(trifluoromethyl)diphenylsulfonium salt 3a (4.0 equiv) and Cu (6.0 equiv) in DMF/H2O (1:1) led to an increase in the yield of 2a (Table 1, entry 9). The best result was obtained by treating 1a at 60 °C in DMSO/H2O (1:1) in the presence of S-(trifluoromethyl)diphenylsulfonium salt 3a (4.0 equiv) and Cu (6.0 equiv), leading to the isolation of 2a in 37% yield (Table 1, entry 10). Using 4.0 equiv of Umemoto’s reagent 3b instead of 3a gave the product 2a in 27% yield (Table 1, entry 12). S-(Trifluoromethyl)benzothiophenium salt 3c [24], trifluoromethylsulfoxinium salt 3d [25], and hypervalent iodine(III) CF3 reagent 3e [26] did not proceed or provided only a trace amount of the desired product 2a under the same reaction conditions (Table 1, entries 13–15). No reaction was observed using Ruppert–Prakash reagent in the presence of Cu under the same conditions (Table 1, entry 16). In all the cases, the reaction was regioselective and a trace amount of regioisomers and/or byproducts was detected in a crude mixture analyzed by 19F NMR. Under the best conditions shown in entry 10, we re-examined the reaction, but in the presence of TEMPO. The product formation was inhibited by TEMPO and O-trifluoromethylated TEMPO was detected in 2% by 19F NMR analysis (Table 1, entry 17).

Table 1

Optimization of CF3 reagents, metal, and solvent for copper-mediated conjugate trifluoromethylation of chalcone (1a).a

Entry	CF3 reagent	metal	solvent	Yield (%)b
1	3a	Cu	DMF	4
2	3a	Cu	NMP	6
3	3a	Cu	DMSO	11
4	3a	Cu	DMF/H2O (1:1)	23
5	3a	Ni	DMF/H2O (1:1)	9
6	3a	Zn	DMF/H2O (1:1)	trace
7	3a	Cu	NMP/H2O (1:1)	trace
8	3a	Cu	DMSO/H2O (1:1)	5
9c	3a	Cu	DMF/H2O (1:1)	25
10c	3a	Cu	DMSO/H2O (1:1)	37
11	3b	Cu	DMSO/H2O (1:1)	24
12c	3b	Cu	DMSO/H2O (1:1)	27
13	3c	Cu	DMSO/H2O (1:1)	trace
14	3d	Cu	DMSO/H2O (1:1)	0
15	3e	Cu	DMSO/H2O (1:1)	trace
16	Me3SiCF3	Cu	DMSO/H2O (1:1)	0
17c,d	3a	Cu	DMSO/H2O (1:1)	tracee

aThe reaction of 1a with 3 (2.0 equiv) was carried out in the presence of metal (3.0 equiv) at 60 °C. bIsolated yield. c3 (4.0 equiv) and metal (6.0 equiv) were used. dThe reaction was performed in the presence of TEMPO (4.0 equiv). eO-Trifluoromethylated TEMPO was detected in 2% by 19F NMR.

With suitable conditions in hand, the scope of copper-mediated conjugate trifluoromethylation of α,β-unsaturated ketones 1 with 3a was explored with a variety of substrates selected in order to establish the generality of the process (Table 2). With respect to the aryl ketone group, aromatic rings substituted with either electron-donating or -withdrawing substituents, such as methyl, methoxy, fluoro and chloro were tolerated (Table 2, entries 2–4). A heteroaromatic, furanyl-substituted enone was compatible with the same reaction conditions (Table 2, entry 6). We next examined substrates differing in the nature of the β-aryl substituents under the same reaction conditions. A series of compounds with aromatic rings substituted with either electron-donating or -withdrawing substituents, such as methyl, methoxy, fluoro and chloro were also acceptable. Furthermore, the β-alkyl-substituted enone also produced the desired product 2l (Table 2, entry 12).

Table 2

Copper-mediated conjugate trifluoromethylation of α,β-unsaturated ketones 1 with 3a.a

Entry	1	Ar	R	2	Yield (%)b
1	1a	Ph	Ph	2a	37
2	1b	4-MeC6H4	Ph	2b	20
3	1c	4-MeOC6H4	Ph	2c	11
4	1d	4-FC6H4	Ph	2d	13
5	1e	4-ClC6H4	Ph	2e	22
6	1f	2-furanyl	Ph	2f	13
7	1g	Ph	4-MeC6H4	2g	12
8	1h	Ph	4-MeOC6H4	2h	12
9	1i	Ph	4-FC6H4	2i	17
10	1j	Ph	3-ClC6H4	2j	18
11	1k	Ph	4-ClC6H4	2k	13
12	1l	Ph	Me	2l	36

aThe reaction of 1a with 3a (4.0 equiv) was carried out in the presence of Cu (6.0 equiv) at 60 °C. bIsolated yield.

Based on these results, we hypothesized the reaction mechanism as shown in Scheme 2. First, the conjugate trifluoromethylation of α,β-unsaturated ketones would be initiated by a single-electron transfer between S-(trifluoromethyl)diphenylsulfonium salt 3a and copper. The intermediate 4 decomposes to give the CF3 radical whose generation is supported by the TEMPO inhibition experiment. Ph2S was formed and checked by the 1H NMR spectroscopy. The resulting CF3 radical reacted directly with α,β-unsaturated ketones 1 and/or through the formation of CuCF3 species to provide the 1,4-adduct 2 in low to moderate yield. Although the true reactive species including CF3 radical and/or CuCF3 are not clear, the naked CF3 radical should be ruled out since high regioselectivity was observed, otherwise, a 1:1 mixture of regioisomers (C2/C3) should be detected like in the photoredox trifluoromethylation reaction [22].

Scheme 2

Proposed mechanism for the conjugate trifluoromethylation of α,β-unsaturated ketones by S-(trifluoromethyl)diphenylsulfonium salt and copper.

Conclusion

We developed for the first time the copper-mediated conjugate trifluoromethylation of simple α,β-unsaturated ketones through the use of shelf-stable electrophilic trifluoromethylating reagent 3a under mild conditions. Although the yields are low, wide substrate generality was observed. Getting higher yields for this chemistry [27-29] and extension to asymmetric conjugate trifluoromethylation to simple α,β-unsaturated ketones are both our subsequent challenges and we are currently working in these directions.

Experimental section.