Lewis bases carbonyl reductions

Lower yields of Na3[Mn(CO)4] were obtained from the direct reduction of Mn2(CO)10 in Na-HMPA, because the neutral dimer underwent slow disproportionation in this medium to form [Mn(HMPA)Jt][Mn(CO)5]2, in contrast to Re2(CO)10, which showed no tendency to react with HMPA at room temperature. Of all neutral binary carbonyl dimers known, Re2(CO)10 appears to be the most resistant toward Lewis base-promoted disproportionation reactions. The slightly lower yields of Na3[Re(CO)4], compared to those of Na3[Mn(CO)4], may have arisen from the fact that Re2(CO)10 does not cleanly reduce to [Re(CO)5] in HMPA or other solvents (22). It should... 

Finally, it is noteworthy that Lewis base adducts of gallane (I.GalL) reduce cyclic ketones, enones and w-haloketones to the corresponding alcohols in excellent yields254. These reagents show some promise as a new extension of the boron-type reductions of carbonyl compounds. 

Acrylonitrile has now been successfully hydroformylated (76). The j3-cyanopropionaldehyde which was formed, polymerized unless the cobalt carbonyls were decomposed before work-up. Yields were highest in Lewis base solvents, which were said to suppress reduction of acrylonitrile and give higher rates of hydroformylation than hydrocarbon solvents. 

For a-bromo ketones and related compounds, enhancement of the enantioselectivities has been achieved by the addition of simple Lewis acids (e.g., BF3, Cp2TiCl2, or magnesium salts, which presumably form Lewis acid-Lewis base complexes with the carbonyl functions of the substrates).2627 The stereochemical effect of some Lewis acid additives on the reduction of a racemic a-bromo ester, namely ethyl 2-bromo-2-phenylpropanoate, is summarized in Scheme 27.2. 

Of the numerous examples of asymmetric reactions catalyzed by Lewis bases, this chapter focuses mainly on the activation of silicon reagents and related processes. Various other types of Lewis basic (nucleophilic) activation, namely the Morita-Baylis-Hillman (MBH) reaction, acyl transfer, nucleophilic carbenes, and carbonyl reduction, are described in the other chapters of this book. 

Transition metal carbonyl derivatives of magnesium can be prepared readily by the reductive cleavage of the metal-metal bond in numerous dimeric transition metal carbonyl complexes or by the removal of halogen from transition metal carbonyl halide complexes with an excess of 1 % magnesium amalgam in the presence of a Lewis base. These preparations may be represented by the general equations... 

Extensive studies on the Lewis acid [( 7 " -C5H5)Re(NO)(PPh3)] have yielded detailed insight into transition metal Lewis acid-Lewis base interactions and the question of a or coordination of carbonyl groups [100]. The Re Lewis acid binds aldehydes via the TT-system whereas ketones coordinate to form M-O cr-complexes. Both react with Et4N CN to give Re-bound cyanohydrins. Diastereoselectivity ranges from 53 to 89 % de for additions to aldehydes and 71 to 99 % de for additions to ketones (Sch. 19) [101]. Diastereoselective reductions to primary and secondary alcohols have also been reported [102]. 

Silanes are widely recognized as efficient reagents for reduction of carbonyl and heterocarbonyl functionality. In the case of alkyl and arylsilanes, the reaction requires catalysis by Lewis acids or transition metal complexes 1, 3] however, with more Lewis acidic trichloro or trialkoxysilanes, an altemative metal free activation can be accomplished. Thus, it has been demonstrated that extracoordinate silicon hydrides, formed by the coordination of silanes to Lewis bases, such as tertiary amines 7a], DMF [7b] or MeCN, and so on [7], can serve as mild reagents for the reduction of imines to amines [8]. In the case of trichlorosilane, an inexpensive and relatively easy to handle reducing reagent, and DMF as a Lewis basic promoter, the intermediacy of hexacoordinate species has been confirmed by Si NMR spectro scopy [7b]. 

The reduction of carbonyl compounds by hydrosilylation is one of the most effective methods for the synthesis of alcohols [150]. The reactivity of organosil-icon reagents, such as trialkoxysilanes and trihalogenated silanes, in these reactions is enhanced by coordination with Lewis bases such as fluoride or DMF. Upon reaction with diols or aminoalcohols, these reagents can form pentacoor-... 

The cobalt carbonyls are prepared by the disproportionation reaction of [Co2(CO)g] in the presence of Lewis bases or by the reduction of cluster cobalt carbonyls with the alkali metals. The iridium compounds are obtained during reduction of [Ir4(CO)i2] with sodium in ether solution. The rhodium carbonyls are usually synthesized by reduction of [Rh2Cl2(CO)4] or [RhClg] " with carbon monoxide in basic medium or by nucleophilic attack of bases on the carbonyl group of carbonyl clusters (see preparation of [M4(CO)i2] and [M6(CO)i6]). 

Polynuclear anionic metal carbonyl compounds are usually prepared by reduction reactions of metal carbonyls M(CO) with such reducing agents as the alkali metals, NaBH4 in ethers, hydrocarbons, liquid ammonia, and similar solvents [see, for example, reactions (2.54), (2.55), (2.84), (2.89), (2.95>-(2.102), and (2.108)-(2.113)]. In alkali medium the metal carbonyls may be reduced by certain solvents (e.g., alcohols) or by the CO ligand itself, and in the presence of Lewis bases the carbonyls disproportionate to give anionic clusters. The mixed metal clusters containing platinum and rhodium are formed by reduction reactions of chloro complexes... 

Transition metal-free hydrosilylation of carbonyl compounds can be realized with the use of Brpnsted or Lewis acids as well as Lewis bases. Alkali or ammonium fiuorides (CsF, KF, TBAF, and TSAF) are highly effective catalysts for the reduction of aldehydes, ketones, esters, and carboxylic acids with H2SiPh2 or PMHS. Lithium methoxide promotes reduction of esters and ketones with trimethoxysilane. A generally accepted mechanism of Lewis base-catalyzed hydrosilylation of carbonyl compovmds involves the coordination of the nucleophile to the silicon atom to give a more reactive pentacoordinate species that is attacked by the carbonyl compound giving hexacoordinate silicon intermediates (or transition states), in which the hydride transfer takes place (Scheme 30) (235). 

A polyamino carbonyl ferrocene dmivative formed by coupling ferrocene carboxylic acid with a BOC protected polyamine is claimed to be a mammalian excitatory neurotransmitter antagonist. Ferrocenyltetrazolylhydrozones have been prqrared from the condensation reaction of ferrocenecarboxaldehyde with a series of tetrazolylhydrazines. The reaction of ferrocoie carboxaldehyde with CH2(COR)(COR ), R = Me, R = Me, OEt in ether containing a Lewis base results in the formation of FcCH=C(COR)COR (mixture of E and Z isomers the work is extendable to diferrocenyl derivatives. Further work on the reduction of fluorine substituted methyl acetylferrocenes has been reported. "... 

Carbon-heteroatom double bonds can also participate in this reaction. These include both carbonyl compounds (Scheme 11.37) and imines (Scheme 11.38). Addition to aldehydes is co-catalysed by tin(II) or indium(III) salts. Under these conditions, tetrahydrofiirans are obtained. The presence or absence of the co-catalyst can also switch the reaction from one mode to another (Scheme 11.39). An indium cocatalysed cycloaddition to a 7-pyrone aldehyde 11.117 was used in a synthesis of aureothin 11.122 and A-acetylaureothamine 11.123 (Scheme 11.40). Cross-metathesis of the exo-cyc ic alkene 11.118 allowed a subsequent Suzuki coupling with a gem-dibromide 11.120 that showed the expected selectivity (Section 2.1.4.2). This reaction required the use of thallium ethoxide as the Lewis base to suppress the formation of side products. A Negishi coupling completed the synthesis of aureothin 11.122. Reduction and acylation of the nitro group yielded A-acetylaureothamine 11.123. The latter compound is active digainst Helicobacter pylori, a bacterium behind stomach ulcers. 

Although the reduction of imines has been widely explored, as described above, the reduction of carbonyl groups has been less studied until now. Specifically, the reduction of ketones is more limited due to the low reactivity shown by these compounds. In this field, the pioneering works using trichlorosilane as reducing agent and a chiral Lewis base were reported by Matsumura and co-workers in 1999,... 

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