RESUMO
Alkylative carbocyclization reactions of omega-iodoalkynyl tosylates with alkynyllithium compounds to give products with incorporated iodine atoms are described. Slow addition of 2-(3-iodoprop-2-ynyloxy)ethyl tosylates to 1-alkynyllithium compounds in tetrahydrofuran at 40 degrees C followed by additional stirring at this temperature gives (Z)-3-(1-iodoprop-2-ynylidene)tetrahydrofurans stereoselectively in good to moderate yields. Under similar conditions at 0 degrees C, 4-iodobut-1-ynyl tosylates react with 1-alkynyllithium compounds to give (1-iodoprop-2-ynylidene)cyclopropanes. The carbocyclization reactions are proposed to proceed through a new carbenoid-chain process involving the exo cyclization of a lithium acetylide intermediate and the vinylic substitution of the resulting TsO,Li-cycloalkylidenecarbenoids (Ts=tosyl) by 1-alkynyllithium compounds.
RESUMO
Atom-economical carbocyclization reactions of omega-iodo-1-alkynes and 1,omega-diiodo-1-alkynes to give products with incorporation of iodine atoms is described. Cycloisomerization of 2-(2-propynyloxy)ethyl iodides is initiated by a catalytic amount of LDA to give 3-(iodomethylene)tetrahydrofurans in high yields. Upon treatment of with a catalytic amount of 1-hexynyllithium, 1,omega-diiodo-1-alkynes efficiently undergo cycloisomerization to give (diiodomethylene)cycloalkanes. The diiodomethylene products are also obtained by iodine atom-transfer-type cyclization of omega-iodo-1-alkynes, using 1-iodo-1-hexyne as an external iodine atom source. Bromine atom-transfer and proton-transfer cyclization proceed as well by employing 1-bromo-1-octyne and 1-octyne, respectively. These reactions are proposed to proceed through a carbenoid-chain process involving exo-cyclization of the lithium acetylide intermediates to give Li,I-alkylidene carbenoids. It is shown that the exo-cyclization proceeded stereospecifically through inversion of the stereochemistry at the electrophilic carbon.
