Potential routes to cyclobuta[1,2-C] cyclopropabenzene

The CpCo(CO)₂-catalyzed cyclization of 1-trimethylsilyl-1, 5-hexadiyne (4) with propargyltetrahy-dropyranylether (5 leads to the formation of both possible regioisomers of the benzocyclobutene product: 3-trimethylsilyl-4-tetrahydropyranyloxymethylbenzocyclobutene (6) and the corresponding 5-tetrahydropyranyloxymethyl-isomer 7. On the other hand, cyclization of the same diyne with 3-(trimethylsilyloxy) propyne (10) gave only the 3,4-disubstituted product as the free alcohol 11. Compound 6 was iododesilylated, the ether hydrolyzed, and the alcohol tosylated to give 4-iodo-5-tosyloxymethylbenzocyclobutene 13, also obtainable more directly from 11 by treatment with ICI and tosylation. Iodide 13 could be reacted with n-butyllithium to give a variety of products incorporating the n-butylgroup and derived from crosscoupling of the carbon skeleton of starting material. The title compound 1 may be an intermediate en route to the observed products, but was not detected. Several thermal elimination routes to 1 from appropriately 4,5-substituted benzocyclobutenes were unsuccessful. Flash vacuum thermolysis of 11 furnished the silyloxyderivative 21 through elimination of methane.