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Titolo:
Chiral diamines 4: A computational study of the enantioselective deprotonation of Boc-pyrrolidine with an alkyllithium in the presence of a chiral diamine
Autore:
Wiberg, KB; Bailey, WF;
Indirizzi:
Yale Univ, Dept Chem, New Haven, CT 06520 USA Yale Univ New Haven CT USA 06520 Univ, Dept Chem, New Haven, CT 06520 USA Univ Connecticut, Dept Chem, Storrs, CT 06269 USA Univ Connecticut StorrsCT USA 06269 cut, Dept Chem, Storrs, CT 06269 USA
Titolo Testata:
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
fascicolo: 34, volume: 123, anno: 2001,
pagine: 8231 - 8238
SICI:
0002-7863(20010829)123:34<8231:CD4ACS>2.0.ZU;2-4
Fonte:
ISI
Lingua:
ENG
Soggetto:
MOLECULAR-DYNAMICS SIMULATIONS; ASYMMETRIC DEPROTONATION; FORCE-FIELD; GEOMETRY OPTIMIZATION; TRANSITION-STATES; LIGAND STRUCTURE; (-)-SPARTEINE; LITHIATION; GENERATION; COMPLEXES;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Life Sciences
Physical, Chemical & Earth Sciences
Citazioni:
39
Recensione:
Indirizzi per estratti:
Indirizzo: Wiberg, KB Yale Univ, Dept Chem, 225 Prospect St, New Haven, CT 06520 USA Yale Univ 225 Prospect St New Haven CT USA 06520 , CT 06520 USA
Citazione:
K.B. Wiberg e W.F. Bailey, "Chiral diamines 4: A computational study of the enantioselective deprotonation of Boc-pyrrolidine with an alkyllithium in the presence of a chiral diamine", J AM CHEM S, 123(34), 2001, pp. 8231-8238

Abstract

The enantioselective deprotonation of N-Boc-pyrrolidine (1) with i-PrLi-(-)-sparteine has been studied at theoretical levels up through B3P86/6-31G*. Four low-energy intermediate complexes involving i-PrLi-(-)-sparteine and I were located via geometry optimizations; two of these complexes would lead to abstraction of the pro-S hydrogen from 1, and the other two complexes would lead to loss of the pro-R hydrogen. The lowest-energy intermediate complex was found to lead to loss of the pro-S hydrogen as observed experimentally. Transition states for the deprotonations were located using the synchronous transit-guided quasi-Newton method. The calculated activation enthalpy for transfer of the pro-S hydrogen within the lowest-energy intermediate complex, 10.8 kcal/mol, is reasonable for a reaction that occurs at a relatively low temperature, and the calculated kinetic hydrogen isotope effectis in agreement with experimental data. The lower enantioselectivity observed experimentally for deprotonation of I using t-BuLi-(-)-sparteine is attributed to a transition-state effect due to increased steric interaction engendered by the bulky t-BuLi. Replacement of the tert-butoxycarbonyl group in I by a methoxycarbonyl is predicted to result in a slower deprotonation with somewhat decreased enantioselectivity. Asymmetric deprotonation of I using i-PrLi in combination with the C-2-symmetric diamine, (SS)- 1,2-bis(N,N-dimethylamino)cyclohexane, was calculated to be much less selective than is the deprotonation mediated by (-)-sparteine as observed experimentally. The relative energies of the intermediate complexes were fairly well-reproduced by ONIUM calculations in which the sparteine ligand less its nitrogen atoms was treated by molecular mechanics and the remainder of the complex was treated by quantum mechanics.

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Documento generato il 19/01/20 alle ore 20:29:19