Titanium-induced carbonyl coupling

Table 9-1 Effect of the ring size on intramolecular, low-valent titanium-induced carbonyl coupling reactions.

Yue et al. reported on the total synthesis of / -(—)-cembrene A by using the mod-ihed Wittig reaction and titanium-induced intramolecular carbonyl coupling as key steps the overall yield is 29% starting from -geranylacetone (164) and R-(+)-limonene (168) (Scheme 6-15). ... 

An alternative strategy to prepare ( )-cembrene A (59) by this group is described below. The key step here is a titanium-induced intramolecular carbonyl coupling reaction starting from -geranylacetone and geraniol (137) (Scheme 6-16). ... 

Pinacols With zinc in the presence of pivalonitrile both aromatic and aliphatic ketones are induced to couple at 0° to ambient temperature (11 examples, 61-97%). The moderate coordination of the nitrile to the low-valent Ti species appears important other additives such as pyridine, HMPA coordinate too strongly to permit the titanium species to interact with the carbonyl substrates. 

Titanium-induced Intramolecular Carbonyl Coupling Reactions... 

Clerici, A. Porta, O. Riva, M. (1981) Mixed carbonyl coupling induced by aqueous titanium trichloride. Some comments on the mechanism. Tetrahedron Lett., 22,1043-6. 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

This reductive coupling involves two steps. The coupling is induced by single electron transfer to the carbonyl groups from alkali metal, followed by deoxygenation of the 1,2-diol with low-valent titanium to yield the alkene. 

Deoxygenative coupling of carbonyl compounds to alkenes induced by low-valent titanium ... 

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