1-Boraadamantane: reactivity towards mono-1-alkynyltin, -germanium and -silicon compounds — synthesis of 4-methylene-3-borahomoadamantanes

The reaction of 1-boraadamantane 1 with 1-alkynyltin (3), -germanium (4), and -silicon compounds (5) leads to enlargement of the tricyclic system by formation of 4-methylene-3-borahomoadamantanes (6–9). These are 1,1-organoboration reactions which proceed by cleavage of the MC bond (M=Sn, Ge, Si). There is evidence for 1,1-deorganoboration which apparently take place much more readily than for non-cyclic analogues, most likely as the result of the strained tricyclic system. When 2-ethyl-1-boraadamantane (2) is used, again 3-borahomoadamantanes are formed, the isomers 15–18. The product distribution is sensitive to steric effects. However, it appears that the BC(H)Et bond in 2 is slightly more reactive than the BCH2 bonds. All products were characterised by 1H-, 11B-, 13C-, 29Si- and 119Sn-NMR.