Reactivity of 1,6-bis(trimethylsilyl)-hexa-3-ene-1,5-diynes towards triethylborane, triallylborane, and 1-boraadamantane: first molecular structure of a 4-methylene-3-borahomoadamantane derivative, and the first 6,8-diborabicyclo[2.2.2]oct-2-ene derivative.

Compounds (E)- (1) and (Z)-1,6-bis(trimethylsilyl)-hexa-3-ene-1,5-diyne (2) react with triethylborane (3) by 1,1-ethylboration in a 1:1 or 1:2 molar ratio (in the case of 1), whereas in the case of 2 only the 1:1 product is formed. The analogous reactions of 1 or 2 with triallylborane (4) are more complex because of competition between 1,1-allyl- and 1,2-allylboration. Again, compound 2 reacts only with one equivalent of 4. In the case of 1-boraadamantane (5), 1,1-organoboration of 1 and 2 takes place either at one or at both C[triple bond]C bonds leading to compounds containing the 4-methylene-3-borahomoadamantane unit(s). The product of the reaction of 1 with two equivalents of 5 was characterized by X-ray structure analysis. The primary products of the reaction of 2 with 5 rearrange upon heating by deorganoboration and organoboration to give finally a tetracyclic compound 24 that contains an exocyclic allenylidene group. The product of the 1:2 reaction of 2 with 5 rearranges to give the 6,8-dibora-bicyclo[2.2.2]oct-2-ene derivative 25. All reactions were monitored by (1)H, (11)B, (13)C, and (29)Si NMR spectroscopy.