Butadiynes isomerization

The composition of the mixture of products of different structures depends on the diacetylene diazomethane ratio (68LA124). With a 1 1 ratio of butadiyne and diazomethane, 3(5)-ethynylpyrazole dominates (55%). The yields of isomeric 3- and 5-ethynyl-l-methylpyrazoles are 8 and 11%, respectively. The double excess of diazomethane leads mainly to a mixture of N-methylated isomers (81%), 10% of 3(5)-ethynylpyrazole, and a small amount (3%) of bipyrazole (68LA124) (Scheme 3). 

Recently, detailed mechanistic studies have been made of the photolysis of l-aryl-4-(pentamethyldisilanyl)-l,3-butadiynes 96 in various solvents. Photolyses in the presence of methanol led to the isomeric products 97 and 98, derived from solvolytic ring opening of the initially formed silacyclopropene, 99, resulting in turn from 1,2-silyl migration48, while photolyses in acetone led to the products 100 and 101 arising from two-atom insertion into the three-membered ring49 (Scheme 14). 

The stereoselectivity of addition depends on the nature of the alcohol and the conditions, including the nature of the solvent" . Thus, in the presence of 2% KOH in excess alcohol as solvent, methanol or ethanol adds exclusively anti to butadiyne giving 94 (R = Me, Et). A sample of the syn adduct 95 (R = Et) failed to isomerize when it was heated at 150 °C with dilute alkali, thus ruling out the possibility of syn addition followed by isomerization. 

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