J. Org. Chem. 1988,53, 5871-5875
5871
Reductive Coupling of Carbonyl Compounds with Zinc and Trimethylchlorosilane To Produce 0-Silylated Pinacols. Effect of Ultrasonic Waves Jeung-Ho So, Moon-Kyeu Park,' and Philip Boudjouk* Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105
Received M a y 23, 1988 Trimethylchlorosilanereacts with carbonyl compounds in the presence of zinc to give 0-silylated pinacols, vicinal bis(trimethylsiloxy)alkanes,in good yields via reductive dimerization. This is a very mild route to bis(siloxy)alkanes,which are easily converted to pinacols or pinacolones in excellent yields. Electron-donating groups accelerate coupling while electron-withdrawing groups have an inhibiting effect. Crowcoupling reactions yield a mixture of bis(si1oxy)alkanes. Ultrasonic irradiation of these reactions increases the yields up to 50% compared to stirring at the same temperature.
Introduction Reductive dimerization of carbonyl compounds is an important method for making carbon-carbon bonds. Alkali-metal-induced coupling of carboxylic acid derivatives, the acyloin condensation, is widely used; however, milder approaches have been developed.' For example, some carbonyl compounds are reductively dimerized t o give pinacolic products in the presence of amalgamated aluminum.' This method is efficient for aliphatic and cyclic ketones but not for aryl-substituted ketones and benzaldehyde. Amalgamated magnesium-titanium tetrachloride? cerium in the presence of iodine: samarium i ~ d i d e or , ~ ytterbium metal6 have also been used for reductive coupling of aromatic and aliphatic carbonyl compounds t o produce pinacols. For acetophenone and benzaldehyde, good yields of reductive coupling have been observed in t h e presence of trimethylchlorosilane and magnesium turnings in hexamethylphosphoric triamide under vigorous conditions (95 "C, 22 h).' Under mild conditions, tetrahydrofuran (THF) or 1,4-dioxane as solvent at 40 OC for 24 h, no reaction is observed? Chlorosilanes have also been used to transform ketones t o a l k e n e ~ , ~ Jorganic O sulfoxides t o thioethers," diols to alkenes)' aliphatic aldehydes to reductive coupling product^,'^ and aryl-substituted carbonyl compounds t o pinacolone products via carbenoid intermediate^.'^ Recently we reported t h a t ultrasonic irradiation improved the reductive silylation of some dicarbonyl compounds in the presence of trimethylchlorosilane and zinc t o give bis(trimethylsi10xy)alkenes.~~This procedure is a convenient modification of the acyloin condensation with the benefit of being applicable t o t h e preparation of unsymmetrical bis(si1oxy)alkenes. We have extended our investigation t o simple carbonyl compounds and report here a very mild method of producing vicinal bis(trimethylsiloxy)alkanes, which can be easily converted to pinacols or pinacolones.
Results and Discussion Aryl- or vinyl-substituted carbonyl compounds are readily dimerized in the presence of trimethylchlorosilane and zinc (carbonyl:Me3SiC1:Zn,1:1:5 mole ratio)16in ether solvents t o give 0-silylated pinacols in good yields. These 0-silylated pinacols are stable toward bases and can be purified by column chromatography. In t h e presence of a Lewis acid such as boron trifluoride etherate, however, they are readily hydrolyzed and rearranged t o the pinaf Visiting Professor from Department of Chemistry, Chungnam National University, Daejon, 300-31, Republic of Korea.
colone in excellent yields. Rearrangement can be avoided by hydrolyzing with tetrabutylammonium fluoride t o give nearly quantitative yields of the pinacols. With arylAr
I Ar-C-C-R I
k
0
MesSiO 1 BF
3 2.H20
OSiMe,
1 I Ar-C-C-Ar I
k
1
k
HO
OH
I I Ar-C-C-Ar H20 I I TBAF
R
R
substituted ketones, GC analysis of the reaction mixtures show mainly coupled products, a mixture of d, I , and meso stereoisomers. These stereoisomers were analysed by GC and l H NMR. T h e products and yields are summerized in Table I. Some +unsaturated carbonyl compounds were converted efficiently t o highly functionalized bis(trimethy1siloxy)&anes. For example, acrolein, crotonaldehyde, and 2-cyclohexenone gave the coupled products in high yields
(1) Bloomfield, J. J.; Owsley, D. C.; Nelke, J. M. Org. React. 1976,23, 259-403. ( 2 ) Schreibmann, A. A. P. Tetrahedron Lett. 1970, 4271-4272. (3) Corey, E. J.; Danheiser, R. L.; Chandrasekaran, S. J. Org. Chem. 1976,41, 260-265. ( 4 ) Imamoto, T.; Kusumoto, T.; Hatanaka, Y.; Yokoyama, M. Tetrahedron Lett. 1982,23, 1353-1356. (5) Namy, J. L.; Kagan, H. B. Tetrahedron Lett. 1983, 24, 765-766. (6)Hou, Z.; Takamine, K.; Fuziwara, Y.; Taniguchi, H. Chem. Lett. 1987, 2061-2064. (7) Chan, T. H.; Vinokur, E. Tetrahedron Lett. 1972,72-78. (8) Trimethylchlorosilane (1.23 mL, 9.6 mmol) was added at room temperature with stirring to the mixture of acetophenone (1 mL, 8 mmol) and magnesium (0.236 g, 9.7 mmol) in 10 mL of THF or dioxane. The mixture was stirred for 24 h at 50 OC. Analysis of the reaction mixture by GC showed no decrease of reactant and no coupling products. (9) Motherwell, W. B. J. Chem. SOC.,Chem. Commun. 1973, 935. (10) Hodge, P.; Khan, M. N. J. Chem. SOC.,Perkin Tram. 1 1975, 809-81 1. (11) (a) Schmit, A. H.; Russ, M. Chem. Ber. 1981,114, 822-824. (b) Olah, G. A.; Narang, S. C.; Gupta, B. G.; Malhotra, R. Synthesis 1979, 61-62. (12) (a) Barua, N. C.; Sharma, R. P. Tetrahedron Lett. 1982, 23, 1365-1366, (b) Sarma, J. C.; Barua, N. C.; Sharma, R. P.; Barua, J. N. Tetrahedron 1983,39, 2843-2846. (13) Rautenstrauch, V. Synthesis 1975, 787-788. (14) Smith, C. L.; Arnett, J.; Ezike, J. J. Chem. SOC.,Chem. Commun. 1980, 653-654. (15) Boudjouk, P.; So, J.-H. Synth. Commun. 1986,16,775-778. The effects of ultrasonic waves on heterogeneous reactions have been reviewed Boudjouk, P. High Energy Processes in Organometallic Processes; Suslick, Ed.; American Chemical Society Symposium Series, No. 333; American Chemical Society: Washington, DC, 1987; Chapter 13, pp 209-222. Boudjouk, P. J. Chem. Educ. 1986,63, 427-429. (16) The stoichiometry must be carefully controlled because exceas Zn and MesSiCl leads to deoxysilylation. So, J.-H.; Park, M.-K.;Boudjouk, P., unpublished results.
0022-3263/88/1953-5871$01.50/0 0 1988 American Chemical Society
5872
J. Org. Chem., Vol. 53, No. 25, 1988
So et al.
Table I. Reductive Coupling of Carbonyl Compounds in the Presence of Trimethylchlorosilane and Zinc (Ratio 1:1:5) for 2-h Reaction entry reactant yields," %; [stirring] sonication productsb
@P
1
1A; [N.R.] 56
0
1A 0
2
2A; [42] 56 2B; [trace] 15
@a,
H
2A
28 0
3
RO
3A; [66] 67
cl*L
OR
H
H
3A
4A; [54] 65
OR
RO
Me
Me
4A
5A; [63] 76
I
I
Me
Me
5A
6A; [50] 74
0
RO
OR
M e O a 8 - h b Me
Me
SA
7A; [48] 62
7
OR
RO
Me
Me
7A 0
8
8A; [81] 85
CI*!-Me
Me
Me
8A
9
9A; [65] 84
0
@A
"Dioxane solvent. No significant change observed in THF for runs 4 and 8. *R = Me3%,d , 1, and meso stereoisomers were not separated. (Table 11) but phenyl-substituted systems such as cinnamaldehyde and trans-4-phenyl-3-buten-2-one gave complex mixtures. MesSiO OSiMes 2R
R'
+
MesSiCl
+
Zn
-
I
R' R = H,Me R'= H
I
R-/T~-~LR R'
Reactions were monitored by GC by following the depletion of the reactant in the presence of a n internal standard. The results are shown in Table 111. Reactions times (trm)were recorded a t >99% completion and were usually
