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“Dimers” and “Trimers” of Tetrahydroindenes and Hexahydroazulenes, Respectively Generated from [2‐(1‐Cycloalkenyl)ethynyl]carbene Complexes (M=W, Cr) by Cascade Cyclization/Cycloaddition Reactions
Author(s) -
Wu HePing,
Aumann Rudolf,
Fröhlich Roland,
Saarenketo Pauli
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20010202)7:3<700::aid-chem700>3.0.co;2-d
Subject(s) - carbene , chemistry , cycloaddition , medicinal chemistry , cyclopentadiene , nucleophile , stereochemistry , catalysis , organic chemistry
A cascade of cyclization/cycloaddition reactions was triggered by addition of protic oxygen nucleophiles ROH 2 (RO=CH 3 CO 2 , PhCO 2 , PhO) to [2‐(1‐cyclohexenyl)ethynyl]carbene complexes 1 b and 1 c (M=W, Cr, respectively), affording highly strained “dimers” 11 / 11′ and “trimers” 12 of the carbene ligand. The first reaction step involved the formation of 1‐metalla‐1,3,5‐hexatrienes 7 , which readily gave tetrahydroindenes 8 by π cyclization and extrusion of the metal unit. “Dimers” 11/11′ were generated from tetrahydroindenes 8 by a highly exo selective [4+2] cycloaddition of compounds 1 b and 1 c to afford 1‐metalla‐1,3,5‐hexatriene intermediates 9 , and a spontaneous π cyclization of the latter compounds involving the disengagement of the metal unit. Propenylidene cyclohexenes 13 / 13′ were formed in “ene”‐type side reactions to the π 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 π‐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‐metalla‐1,3,5‐hexatriene intermediates 16 and 17 were isolated and characterized. Alkynyl benzene derivatives 19 were obtained by an unprecedented ring‐expansion of a cyclopentadiene unit of “dimers” 11 a and 11 c , involving the insertion 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 1 d .