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7953045 
Journal Article 
Enantioselective deprotonation of alkyl carbamates by means of (R,R)-1,2-Bis(N,N-dimethylamino)cyclohexane/sec-butyllithium - Theory and experiment 
Wurthwein, EU; Behrens, K; Hoppe, D 
1999 
Yes 
Chemistry: A European Journal
ISSN: 0947-6539
EISSN: 1521-3765 
5 
12 
3459-3463 
English 
WOS:000084163400004 
The O-ethyl-, O-isobutyl-, and O-neopentyl carbamates 1a-c were subjected to the conditions of chiral diamine-assisted deprotonation. The efficiencies were compared for (-)-sparteine (7), title compound 8 and (-)-α-isosparteine (9). The degree of enantiotopos-differentiation in the deprotonation step was concluded from the enantiomeric ratio in the carboxylation products 6/ent-6. The deprotonation of carbamates 1a and 1b by sec-butyllithium/7 proceeds with very good selectivity (>95% ee in favor for the pro-S-H), but is not successful with 1c. (-)-α-Isosparteine (9) does not support the deprotonation of alkyl carbamates at all. The medium efficiency of diamine 8 is enhanced by increasing bulk in the β-position (1a > 1b > 1c). Quantum-chemical calculations (PM3, ab initio methods) on several models for the competing diastereomorphic transition states of the deprotonation under the influence of 8 reflect well the sense and the magnitude of the observed chiral induction and, in addition, give the first insight in the effects which determine the stereochemical outcome of these reactions. 
chiral diamines; lithiation; quantum-chemical calculations; (-)-sparteine; transition states