Conjugate reduction group 14 metal hydrides

Ketenes react at the carbonyl group of 215a to eliminate carbon dioxide and produce the more extended conjugated system (as in 216 obtained from a cycloheptatrienylidene ketene.)316 Reduction with metal hydride removes the carbonyl oxygen and induces coupling of two residues at the 4-position.317 Addition of dienophiles is not observed in the parent system, but the furan part of the dibenzotropone (217) readily adds quinones and other such addenda.318... 

The inertness of ordinary double bonds toward metallic hydrides is quite useful, since it permits reduction of, say, a carbonyl or nitro group, without disturbing a double bond in the same molecule (see Chapter 19 for a discussion of selectivity in reduction reactions). Sodium in liquid ammonia also does not reduce ordinary double bonds,275 although it does reduce alkynes, allenes, conjugated dienes,276 and aromatic rings (5-10). 

A similar type of reaction has been observed in the reactions of iron-caibene complexes with 1,3-dienes. In this case the direction of reductive elimination in the metal hydride intermediate corresponding to (193) is constrained to that which generates a conjugated 1,3-diene however, two isomeric products are also obtained from this reaction which are epimers about the face of the diene to which the iron tricarbonyl group is attached. This reaction produces highly functionalized 1,3-dienyl complexes of iron in high yield under relatively mild conditions and will likely play a role in the development of the... 

Stork and Danheiser have developed a highly useful procedure for the synthesis of 4-alkylcyclohex-2-enones, which involves a -alkylations of cross-conjugated lithium dienolates of 3-alkoxycyclohex-2-enones, followed by metal hydride reduction of the carbonyl group and hydrolysis (Scheme 30). Numerous applications of this procedure have been reported.Two different alkyl groups may be introduced at the 6-position of a cyclohex-2-enone derivative without difficulty. While dialkylation is generally not a problem in alkylations of cross-conjugated dienolates of cyclohex-2-enones, it was observed when relatively acidic 3-chlorocyclohex-2-enones were employed. ... 

The reduction of unsaturated carbonyl compounds by metal hydrides, and the reaction of organometallic nucleophiles with them, is a complicated story.87 It is more common than not, in each case, to get direct attack at the carbonyl group, but reaction in the conjugate position is well known. Conjugate reduction of a/i-unsaturated ketones by metal hydrides increases88 in the sequences Bu2iAlH < LiAlH4 < LiAlH(OMe)3 < LiAlH(OBu )3 and... 

The second strategy involves addition of a group 14 metal hydride to palladium. For example, a combination of either tributyltin hydride or various silicon hydrides and palladium catalyst are efficient systems that affect the conjugate reduction of enones with high chemoselectivity and regioselectivity. 

---