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The Ring Closure of Cyclopenta‐1, 3‐dien‐5‐yl‐carbene to Benzvalene. A mechanistic study of an unusual carbene reaction
Author(s) -
Burger Ulrich,
Gandillon Gérard,
Mareda Jiri
Publication year - 1981
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19810640323
Subject(s) - carbene , chemistry , intramolecular force , substituent , ring (chemistry) , transition metal carbene complex , cyclopropanation , medicinal chemistry , singlet state , stereochemistry , photochemistry , catalysis , organic chemistry , excited state , physics , nuclear physics
The base‐induced α‐elimination of hydrogen chloride from 5‐chloromethyl‐5‐methylcyclopenta‐1, 3‐diene ( 19 ) produces 1‐methyltricyclo [3.1.0.0 2,6 ]hexene‐3 (1‐methylbenzvalene) ( 21 ) together with toluene and spiro [4.2]heptadiene ( 23 ). A common intermediate, 5‐methylcyclopenta‐1, 3‐dien‐5‐yl‐carbene ( 20 ), accounts for these results by intramolecular 1, 4‐carbene addition, 1, 2‐carbon shift and CH‐insertion, respectively. Independent synthesis of 2‐methylbenzvalene ( 24 ) allows us to show that the classic intramolecular cyclopropanation is completely suppressed by the linear cheletropic ring closure. MINDO/3 predicts the key carbene to have a bisected conformation in its singlet ground state. This ideally fulfills the stereoelectronic conditions for a carbene reaction of least motion. The influence of the methyl substituent upon that process is discussed.