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On the Enantioselectivity of Transition Metal‐Catalyzed 1,3‐Cycloadditions of 2‐Diazocyclohexane‐1,3‐diones
Author(s) -
Müller Paul,
Allenbach Yves F.,
Bernardinelli Gérald
Publication year - 2003
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.200390257
Subject(s) - chemistry , cycloaddition , diazo , enantioselective synthesis , selectivity , catalysis , reactivity (psychology) , medicinal chemistry , furan , stereochemistry , asymmetric induction , transition state , enol , organic chemistry , medicine , alternative medicine , pathology
The formal 1,3‐cycloaddition of 2‐diazocyclohexane‐1,3‐diones 1a –1 d to acyclic and cyclic enol ethers in the presence of Rh II ‐catalysts to afford dihydrofurans has been investigated. Reaction with a cis/trans mixture of 1‐ethoxyprop‐1‐ene ( 13a ) yielded the dihydrofuran 14a with a cis / trans ratio of 85 : 15, while that with ( Z )‐1‐ethoxy‐3,3,3‐trifluoroprop‐1‐ene ( 13b ) gave the cis ‐product 14b exclusively. The stereochemical outcome of the reaction is consistent with a concerted rather than stepwise mechanism for cycloaddition. The asymmetric cycloaddition of 2‐diazocyclohexane‐1,3‐dione ( 1a ) or 2‐diazodimedone (=2‐diazo‐5,5‐dimethylcyclohexane‐1,3‐dione; 1b ) to furan and dihydrofuran was investigated with a representative selection of chiral, nonracemic Rh II catalysts, but no significant enantioselectivity was observed, and the reported enantioselective cycloadditions of these diazo compounds could not be reproduced. The absence of enantioselectivity in the cycloadditions of 2‐diazocyclohexane‐1,3‐diones is tentatively explained in terms of the Hammond postulate. The transition state for the cycloaddition occurs early on the reaction coordinate owing to the high reactivity of the intermediate metallocarbene. An early transition state is associated with low selectivity. In contrast, the transition state for transfer of stabilized metallocarbenes occurs later, and the reactions exhibit higher selectivity.