Aluminum carbonyl reduction

The chemical reduction of enamines by hydride again depends upon the prior generation of an imonium salt (111,225). Thus an equivalent of acid, such as perchloric acid, must be added to the enamine in reductions with lithium aluminum hydride. Studies of the steric course (537) of lithium aluminum hydride reductions of imonium salts indicate less stereoselectivity in comparison with the analogous carbonyl compounds, where an equatorial alcohol usually predominates in the reduction products of six-membered ring ketones. 

The homology between 22 and 21 is obviously very close. After lithium aluminum hydride reduction of the ethoxycarbonyl function in 22, oxidation of the resultant primary alcohol with PCC furnishes aldehyde 34. Subjection of 34 to sequential carbonyl addition, oxidation, and deprotection reactions then provides ketone 21 (31% overall yield from (—)-33). By virtue of its symmetry, the dextrorotatory monobenzyl ether, (/ )-(+)-33, can also be converted to compound 21, with the same absolute configuration as that derived from (S)-(-)-33, by using a synthetic route that differs only slightly from the one already described. 

Two other functionally substituted 17-cyclopentadienyl titanium dicarbonyl complexes prepared by Rausch and co-workers include the vinyl Cp compound (i7-C5H4CH=CH2)CpTi(CO)2 (81) and the carbomethoxy Cp compound (rj-C5H4C02Me)CpTi(C0)2 (82). Both were synthesized via the aluminum-induced reductive carbonylation of the corresponding dichloride derivatives. 

An important example of this type of reaction is the formation of esters, which was discussed previously in connection with the reactions of alcohols in Section 15-4D. Similar addition-elimination mechanisms occur in many reactions at the carbonyl groups of acid derivatives. A less obvious example of addition to carboxyl groups involves hydride ion (H 0) and takes place in lithium aluminum hydride reduction of carboxylic acids (Sections 16-4E and 18-3C). 

Liquid injection molding, for silicone rubbers, 3, 674—675 Liquid ligands, in metal vapor synthesis, 1, 229 Liquid-phase catalysis, supported, for green olefin hydroformylation, 12, 855 Lithiacarbaboranes, preparation, 3, 114 Lithiation, arene chromium tricarbonyls, 5, 236 Lithium aluminum amides, reactions, 3, 282 Lithium aluminum hydride, for alcohol reductions, 3, 279 Lithium borohydride, in hydroborations, 9, 158 Lithium gallium hydride, in reduction reactions, 9, 738 Lithium indium hydride, in carbonyl reductions, 9, 713—714... 

Based on the accompanying kinetic data and an observation that lithium cation is essential in the lithium aluminum hydride reduction,5 Ashby and Boone proposed that the reduction would occur via a six-membered transition state in which the lithium cation is involved4 (Scheme 4.II). Because the aluminum in the boat transition state TS-boat is proximal to the carbonyl oxygen, the boat transition state might be of lower energy than the chairlike transition state TS-chair. Furthermore, the boatlike transition state would be a favored states as it results in direct formation of the lithium alkoxyaluminum hydride intermediate. 

Axial attack has been rather difficult to attain. The treatment of the ketone with 3 equiv. of MeaAl has been known to give equatorial alcohol predominantly, although low selectivity was obtained using 1 equiv. of reagent. Methylation of the MeaAl-complexed carbonyl was suggested. This methodology, however, was not applicable to ethyl- or butyl-aluminum because of the competitive carbonyl reduction. ... 

Lund, H. Aluminum isopropylate as a reducing agent. A general method for carbonyl reduction. Ber. 1937, 70B, 1520-1525. 

The product of a lithium aluminum hydride reduction of a cinnamic acid or ester 1 is highly dependent on the solvent. The product of exclusive carbonyl reduction, cinnamyl alcohol 2, was obtained in hydrocarbon solvents (pentane, hexane or benzene) even under prolonged reflux. 3-Phenylpropanol 3, resulting from carbonyl and C-C double bond reduction was produced in diethyl ether. However, phenylcyclopropane 4 was obtained in tetrahy-drofuran or 1,2-dimethoxyethane, notably after prolonged reflux. 

The lithium aluminum hydride reductive method to cleave tosyl groups is also convenient and will likewise reduce any carbonyl groups present in the macrocycle at the same time. Isolation of the products of this cleavage reaction is relatively easy. 

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