Organic synthesis via transition metal complexes, part 106 - "Dimers" and "trimers" of tetrahydroindenes and hexahydroazulenes, respectively generated from [2-(1-cycloalkenyl)ethynyl]carbene complexes (M = W, Cr) by cascade cyclization/cycloaddition reactions

A cascade of cyclization/cycloaddition reactions was triggered by addition of protic oxygen nucleophiles ROH 2 (RO = CH3CO2, PhCO2, PhO) to [2-(1-cycl ohexenyl)ethynyl]carbene complexes 1b and 1c (M = W, Cr, respectively), aff ording highly strained "dimers'' 11/11' and "trimers" 12 of the carbene lig and. The first reaction step involved the formation of 1-metalla-1,3.5-hexa trienes 7, which readily gave tetrahydroindenes 8 by pi cyclization and ext rusion of the metal unit. "Dimers" 11/11' were generated from tetrahydroind enes 8 by a highly exo selective [4+2] cycloaddition of compounds 1b and 1c to afford 1-metalla-1.3,5-hexatriene intermediates 9, and a spontaneous pi cyclization of the latter compounds involving the disengagement of the met al unit. Propenylidene cyclohexenes 13/13' were formed in "ene"-type side r eactions to the pi cyclization of 1-metalla-1,3,5-hexatrienes 7, by loss of the metal unit. "Dimers" 11 were transformed into "trimers" 12 by a [4+2] cycloaddition and subsequent pi -cyclization of the resulting 1-metalla-1,3 ,5-hexatriene system. The course of the reaction was elucidated by means of model reactions with (2-phenylethynyl)carbene complex 14, in which 1-metal la-1,3,5-hexatriene intermediates 16 and 17 were isolated and characterized . Alkynyl benzene derivatives 19 were obtained by an unprecedented ring-exp ansion of a cyclopentadiene unit of "dimers" 11a and 11c, involving the ins ertion of a carbene carbon atom of compound 14 into a C=C bond. A reaction cascade leading to "dimers" 24/24' could also be triggered by treatment of compounds 2 with [2-(1-cycloheptenyl)ethynyl]carbene tungsten complex 1d.