Science of Synthesis

Gandon, V.; Thorimbert, S.; Malacria, M., Science of Synthesis, (2009) 46, 186.

A problem inherent in the syntheses of cyclohexa-1,3-dienes from simple alkynes is competitive [2 + 2 + 2] cyclotrimerizations that give benzenes.[‌53‌,‌55‌,‌57‌,‌58‌,‌61‌–‌65‌] A solution to this problem is found in the use of a titanacyclopentadiene [obtained by the sodium amalgam reduction of a bis(aryloxy)dichlorotitanium in the presence of an alkyne] to catalyze the chemo- and stereoselective cocyclizations of unelaborated alkynes such as hex-3-yne, 3,3-dimethylbut-1-yne, and (trimethylsilyl)acetylene with simple alkenes including ethene, trimethyl(vinyl)silane, and styrene. For instance, metallacycle ‌48‌ can be prepared from complex ‌47‌ and hex-3-yne, and this catalyst may then promote the stereoselective cocy­clization between hex-3-yne and ethene to give (5R*,6R*)-1,2,5,6-tetraethylcyclohexa-1,3-diene (‌49‌) (‌Scheme 19‌); note that the diene framework of this product is shifted compared to that of the expected isomer.[‌66‌] Despite the fact that this method favors alkyne/alkene cocyclization over alkyne cyclotrimerization, mixtures of regioisomers are often formed.

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References