“Dimers” and “Trimers” of Tetrahydroindenes and Hexahydroazulenes, Respectively Generated from [2-(1-Cycloalkenyl)ethynyl]carbene Complexes (M=W, Cr) by Cascade Cyclization/Cycloaddition Reactions

Abstract

A cascade of cyclization/cycloaddition reactions was triggered by addition of protic oxygen nucleophiles ROH 2 (RO = CH3CO2, PhCO2, PhO) to [2-(1-cyclohexenyl)ethynyl]carbene complexes 1b and 1c (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-metalla1,3,5-hexatrienes 7, which readily gave tetrahydroindenes 8 by pi 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 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 metal unit. Propenylidene cyclohexenes 13/13' were formed in "ene"-type side reactions 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-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" 11a and 11c, 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 1d.

Synthesis of New Polycyclic Oxazol-2-one Derivatives by a Tandem [4 + 2] Cycloaddition/Cyclopentannulation/1,5-Sigmatropic Rearrangement Process of Fischer (Arylalkynyl)(alkoxy)carbenes and exo-2-Oxazolidinone Dienes

“…In particular, Fischer carbene complexes of group 6 are of interest, due to the fact that these organometallic compounds offer the opportunity to access heterocyclic and carbocyclic rings, not easily available through conventional routes. In this context, α,β-unsaturated carbene complexes involving the Dötz [3 + 2 +1] and Diels−Alder [4 + 2] cycloadditions are particularly important. Among these processes, the Diels−Alder cycloaddition of alkynylcarbene complexes with dienes, which often involves cascade reactions, has proven a useful route for the synthesis of a wide variety of polycyclic compounds from simple synthetic building blocks. , It is well-known that (arylalkynyl)carbene complexes can be employed as dienophiles in Diels−Alder reactions.…”

“…In this context, α,β-unsaturated carbene complexes involving the Dötz [3 + 2 +1] and Diels−Alder [4 + 2] cycloadditions are particularly important. Among these processes, the Diels−Alder cycloaddition of alkynylcarbene complexes with dienes, which often involves cascade reactions, has proven a useful route for the synthesis of a wide variety of polycyclic compounds from simple synthetic building blocks. , It is well-known that (arylalkynyl)carbene complexes can be employed as dienophiles in Diels−Alder reactions. The resultant cis -1,3,5-metallahexatriene adducts can react further to form cyclopentadienes spontaneously, thermally, or photolytically to produce phenol derivatives .…”

Highly Selective Formation of [4+2] and [4+3] Cycloadducts of Tetrahydroindenes Generated in Situ from a (1-Alkynyl)carbene Tungsten Complex by the Metalla-1,3,5-hexatriene Route

“…This approach has been recently extended to include reactions of 1‐tungsta‐1,3‐butadienes, which were found to experience a strong reactivity enhancement under the influence of rhodium or copper catalysts 1, 7. Furthermore, a most versatile procedure for the anellation of cyclopentadiene rings was found; this involves formation of 1‐metalla‐1,3,5‐hexatrienes from [2‐(1‐cycloalkenyl)ethynyl]carbene complexes by addition of protic nucleophiles NuH,9, 15 for example, R′(RCO)CH 2 ,8 R 2 NH,9, 10 R 2 PH,9 RCOOH and ROH,11, 12 RCXSH (X=O, NH, NR),13 and RSH 14…”

“…. This last latter approach is suitable for the generation of highly reactive bicyclic cyclopentadienes that can be trapped in situ by cycloaddition6b, 12 and also by rearrangement reactions 10, 14…”

Organic Syntheses via Transition-Metal Complexes. 110.1 A Convenient Regio- and Stereoselective Approach to the Angular Allylation of Bicyclic Cyclopentadienes Generated by the (1-Alkynyl)carbene Complex Route

“…This latter feature is illustrated by the completely different reaction courses observed for cyclopentenyl and cyclohexenyl derivatives 1a and 1b, respectively. 10,11 free cyclopentadienes, addition of secondary amines to [2-(1-cyclopentenyl)ethynyl]carbene complex 1a gave cyclopentadiene complexes 6. The latter could be isolated in the form of zwitterionic η 1 -tetrahydropentalene iminium pentacarbonyl tungstates (Scheme 1).…”