CARBON-CARBON BOND FORMATION BY REDUCTIVE COUPLING WITH TITANIUM (II) CHLORIDE BIS(TETRAHYDROFURAN)

Titanium(II) bis(tetrahydrofuran) 1, generated by the treatm ent of TiCl₄ in THF with two equivalents of n-butyllithium at -78 °C, has been found to form carbon-carbon bonds with a variety of organic substrates by reductive coupling. Diphenylacetylene is dimerized to exclusively (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene; benzyl bromide and 9-bromofluorene give their coupled products, bibenzyl and 9,9'-bifluorenyl, as do benzal chloride and benzotrichloride yield the 1,2-dichloro-1,2-diphenylethanes and 1,1,2,2-tetrachloro-1,2-diphenylethane, respectively. Styrene oxide and and cis-stilbene oxide undergo deoxygenation to styrene and fra/«-stilbene, while benzyl alcohol and benzopinacol are coupled to bibenzyl and to a mixture of tetraphenylethylene and 1,1,2,2-tetraphenylethane. Both aliphatic and aromatic ketones are smoothly reductively coupled to a mixture of pinacols and/or olefins in varying proportions. By a choice of experimental conditions either the pinacol or the olefin could be made the predom inant product in certain cases. The reaction has been carried out with heptanal, cyclohexanone, benzonitrile, benzaldehyde, furfural, acetophenone, benzophenone and 9-fluorenone. In a remarkable, multiple reductive coupling, benzoyl chloride is converted into 2,3,4,5-tetraphenylfuran in almost 50% yield. The stereochemical course of two such couplings, that of diphenylacetylene to yield exclusively (E,E)-1,2,3,4-tetraphenyl-l,3-butadiene and that of acetophenone to produce only racemic-2,3-diphenyl-2,3-butanediol, is interpreted to conclude that the couplings proceed via two electron transfer pathways (TET) involving titanium (IV) cyclic intermediates of the titanirene and the oxatitanacyclopropane type, respectively.