Dissolving metal cleavage

Monti and his co-workers have successfully demonstrated not only the efficiency of intramolecular ketocarbene capture in the production of tricyclo[3.3.0.02-8 ]-octan-3-one (27), but also the marked stereoselectivity of its dissolving metal cleavage and ring opening under acidic conditions (Scheme 7).ss Prevailing stereoelec-tronic and thermodynamic factors favor predominant conversion to bicyclo[3.3.0]-octyl end products. 

Na or Li and ammonia, excellent yields. " A dissolving metal reduction can be effected without cleavage of a sulfur-carbon bond. Note also the unusual selectivity in the cleavage illustrated below. This was attributed to steric compression. ... 

The dissolving metal reduction is not suitable, because it effects a cleavage ol the benzylic C-N bond. b The cleavage of the N-N bond by hydrogenolysis is affected by some racemization. 

The central bond in 1 undergoes reductive cleavage with dissolving metals. For instance, reaction of la and lo with lithium 4,4 -di- -butylbiphenyl in dimethyl ether yields poorly soluble and incompletely characterized organometallic derivatives in moderate yields, presumably the corresponding l,3-dilithiobicylo[l.l.l]pentanes (64)95 (equation 24). The... 

Reactions of 1,2,4-thiadiazoles with radicals and electron deficient species are virtually unknown. Catalytic and dissolving metal reductions usually result in S—N bond cleavage. For example, the 5-anilino-3-hydroxy derivative (51) gives a good yield of l-phenyl-2-thiobiuret (52) on Zn-HCl reduction (Scheme 27). Reduction of the diamino derivative (53) gives amidinothiourea (54) from which it may be prepared by oxidation (Scheme 28). Under similar conditions, cleavage of the 3,5-diphenyl derivative (55) results in loss of sulfur and formation of benzylbenzamidine (56 Scheme 29). Reduction of 5-alkylamino-or 5-arylamino-3-alkylthio derivatives (57) with H2S in pyridine-triethylamine or sodium in liquid ammonia yields 1-substituted dithiobiurets (58 Scheme 30). 

C—S bonds can be cleaved reductively by dissolving metals, by processes which depend on the ability of sulfur to accept electrons.1,3-Dithiolanes tend strongly to suffer cleavage of both C—S bonds with sodium in liquid ammonia when C-2 is unsubstituted or carries a phenyl group isopropylidene derivatives (70), however, give single cleavage (equation 39). Bond rupture requires formation of a relatively stable carbanion. 

The dissolving metal reduction of benzo[b]furans and dibenzofurans is a classical technique and, depending on the conditions, can give rise to a variety of products.Several examples are summarized in equations (104-108). " The cleavage of dibenzofurans to afford 2-hydroxy biphenyls (equation 107) is a particularly valuable route to these compounds. The reduction of 3-carbomethoxybenzofu-rans with Mg/MeOH (equation 108) is reminiscent of the indole reduction reported earlier (equation 51). 

Reactions of carbon-carbon % bonds dissolving metal reductions, conversions to alcohols and enantiomerically pure alcohols, chemo- and enantioselective epoxidations, procedures for cleavage of carbon-carbon double bonds, and reactions of carbon-carbon triple bonds... 

While sulfonamides are robust protecting groups for amines and anilines, their cleavage often calls for dissolving metal chemistry. 

Little is known about the behavior of hydantoins in dissolving metal reductions. The reaction of hydantoin (132) with 5 equivalents of lithium in tert-butyl alcohol and liquid ammonia gives the 4-imidazolin-2-one 133. The use of methanol instead of r rt-butyl alcohol gives a mixture of reduced products, probably originated by ring cleavage as well as by reduction.325 ... 

This is all the more surprising as the history of the oxo process actually prescribed the use of aqueous catalysts and catalyst precursors (aqueous cobalt salts as precursors of the earlier Diaden process [21], the at least partially aqueous cycle of the BASF and Kuhlmann process [22b], or the cleavage of solvent-soluble by-products and heavy ends of the oxo process with the aid of water-dissolved metal salts [22c, 23]). 

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