Diphenylsilane-Tetrakis(triphenylphosphine)palladium(0)-Zinc Chloride1


[775-12-2]  · C12H12Si  · Diphenylsilane-Tetrakis(triphenylphosphine)palladium(0)-Zinc Chloride  · (MW 184.33) ((Ph3P)4Pd)

[14221-01-3]  · C72H60P4Pd  · Diphenylsilane-Tetrakis(triphenylphosphine)palladium(0)-Zinc Chloride  · (MW 1155.62) (ZnCl2)

[7646-85-7]  · Cl2Zn  · Diphenylsilane-Tetrakis(triphenylphosphine)palladium(0)-Zinc Chloride  · (MW 136.29)

(mild reducing agent for the reduction of a,b-unsaturated systems2 and allylic hetero substituents7)

Physical Data: Ph2SiH2: bp 95-97 °C/13 mmHg; d 0.993 g cm-3.

Solubility: Ph2SiH2: sol usual organic solvents; insol H2O.

Form Supplied in: Ph2SiH2: liquid; widely available.

Handling, Storage, and Precautions: Ph2SiH2: irritant; avoid inhalation, ingestion, or skin contact. Use in a fume hood.

1,4-Reduction of a,b-Unsaturated Aldehydes and Ketones.2

a,b-Unsaturated aldehydes and ketones are readily reduced using a combination of diphenylsilane, Tetrakis(triphenylphosphine)palladium(0), and Zinc Chloride in chloroform at ambient temperature.3 The reduction is highly chemoselective, occurring in a 1,4-fashion (eqs 1 and 2).4 Although a,b-unsaturated esters, amides, and nitriles are reduced under these conditions, the reaction is much slower, allowing the possibility of selective reduction of unsaturated aldehydes or ketones (eq 3).

Typical reaction conditions employ a slight excess of diphenylsilane, 1-2% of the palladium catalyst, and 10-50% of zinc chloride. No special precautions such as dry or oxygen free conditions are required; this allows commercial solvents and wet zinc chloride to be used as supplied without purification. Anhydrous conditions have in fact been found to result in slower reaction rates.

The stereoselectivity of the reduction has been examined by the use of diphenyldideuteriosilane, which has shown that deuterium is introduced from the least hindered face of the enone (eq 4).2,5

A variation on the above conditions involving the inclusion of a catalytic quantity of Copper(II) Chloride has been used by an Australian group to selectively reduce an a,b-unsaturated ketone in the presence of a vinylic phosphine oxide (eq 5).6

Allylic Reductions.

The reduction of allylic heterosubstituents can readily be achieved using the diphenylsilane-tetrakis(triphenylphosphine)palladium-zinc chloride system in THF (eq 6).7 Reduction also occurs with diphenylsilane and tetrakis(triphenylphosphine)palladium alone, conditions that will not reduce a,b-unsaturated ketones. This provides a complementary chemoselectivity to that of the corresponding Tri-n-butylstannane system which favors reduction of the enone (eq 7).8 The allylic displacement occurs with absolute inversion of configuration, consistent with a mechanism of palladium complexation from the opposite side to the leaving group followed by transfer of hydride to the ligand from palladium. An apparent exception to this was found when tri-O-acetylglucal was reduced under these conditions with retention of configuration. This has been explained by a neighboring group participation by the 4-acetoxy substituent.9

Related Reagents.

Palladium(II) Hydroxide; Tri-n-butylstannane.

1. Keinan, E.; Greenspoon, N. COS 1991, 8, 546.
2. Keinan, E.; Greenspoon, N. JACS 1986, 108, 7314.
3. (a) Archer, D. A.; Bromidge, S. M.; Sammes, P. G. JCS(P1) 1988, 3223. (b) Pietrusiewicz, K. M.; Salamonczyk, I. JOC 1988, 53, 2837. (c) Asaoka, M.; Sonoda, S.; Takei, H. CL 1989, 1847.
4. Kocovsky, P.; Stieborova, I. TL 1989, 30, 4295.
5. Keinan, E.; Greenspoon N. TL 1985, 26, 1353.
6. Haynes, R. K.; Stokes, J. P.; Hambley, T. W. JCS(C) 1991, 58.
7. Keinan, E.; Greenspoon, N. IJC 1984, 24, 82.
8. Keinan, E. PAC 1989, 61, 1737.
9. Keinan, E.; Greenspoon, N. JOC 1988, 53, 3723.

Malcolm Chandler

Glaxo Research & Development, Stevenage, UK

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