Aluminum amalgam reductive dimerization

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. 

Aluminum amalgam in reduction of oximinomalononitrile, 48, 2 Aluminum chloride, 46, 109 in conversion of tetramethy 1-1,3-cyclobutanedione to dimethyl-ketene d-lactone dimer, 48, 72 with lithium aluminum hydride, in reduction of l,4-dioxaspiro[4.5 -decane, 47, 37... 

The pinacol (94) was also obtained by reductive dimerization of retinaldehyde (2) with zinc amalgam (Reedy, 1968) or in the presence of chromium(III) salts (Sopher and Utley, 1979). With a mixture of lithium aluminum hydride and titanium chloride in anhydrous tetrahydrofuran, retinaldehyde (2) underwent reductive coupling to give p-carotene (Akiyama et al., 1979 Mukaiyama, 1977 McMurry et al., 1978 Ishida and Mukaiyama, 1977). 

It is alloyed with about 4% A1 and 0.02% Mg. The aluminum strengthens the zinc and also prevents the molten alloy from attacking the steel pressure casting dies. Zinc readily reacts with mercury or will displace mercury from a mercury(II) salt to form an amalgam that is usefril for reductions, as in the preparation of compounds of the lower oxidation states of transition metals and lanthanides (e.g. Cr , V , Eu°, dimeric Mo ) and in analytical chemistry (e.g. in the Jones reductor see Analytical Chemistry of the Transition Elements). 

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