Document Detail


Kinetic studies on the stability and reactivity of beta-amino alkylzinc iodides derived from amino acids.
MedLine Citation:
PMID:  18855456     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
Beta-amino alkylzinc iodides are intrinsically unstable toward beta-elimination and protonation. The aim of this study was to determine the rates of these processes and also to understand how the reactivity of a range of beta-amino alkylzinc iodides in Negishi cross-coupling reactions is influenced by the presence of functional groups within the zinc reagent. Decomposition of beta-benzamido alkylzinc iodides occurs by protonation, and the first-order rate constant for the self-protonation of the carbon-zinc bond in reagent 4b was determined to be 5.2 x 10(-6) s(-1) (at 291 K). In contrast, the carbamate derivative 2 decomposes by a first-order elimination process. The homologous reagent 3, derived from glutamic acid, decomposes more quickly by beta-elimination, with a first-order rate constant of 24 x 10(-6) s(-1) (at 291 K). Reagents 23 and 25, in which the Boc group has been replaced with a trifluoroacetyl group, are more stable toward beta-elimination than the corresponding reagents 2 and 3, a striking outcome given that the trifluoroacetamido group is a better leaving group. Moreover, this replacement also changes the mechanism of the elimination to a second order process. Pseudo-second-order rate constants for the Negishi cross-coupling of reagents 2, 3, 23, and 25 with iodobenzene have been determined, revealing the higher reactivity of the glutamic acid-derived reagents 3 and 25. The main factor influencing reactivity, therefore, is determined to be the proximity of the ester group, rather than the nature of the nitrogen protecting group. Finally, beta-amino alkylzinc iodides 46-48 containing Weinreb amides have been prepared, rate constants for their decomposition through elimination determined, and their synthetic potential for the preparation of beta-amino ketones established.
Authors:
Ian Rilatt; Richard F W Jackson
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't     Date:  2008-10-15
Journal Detail:
Title:  The Journal of organic chemistry     Volume:  73     ISSN:  1520-6904     ISO Abbreviation:  J. Org. Chem.     Publication Date:  2008 Nov 
Date Detail:
Created Date:  2008-11-14     Completed Date:  2009-01-09     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  2985193R     Medline TA:  J Org Chem     Country:  United States    
Other Details:
Languages:  eng     Pagination:  8694-704     Citation Subset:  IM    
Affiliation:
Department of Chemistry, The University of Sheffield, Dainton Building, Sheffield S3 7HF, United Kingdom.
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MeSH Terms
Descriptor/Qualifier:
Amines / chemistry*
Amino Acids / chemistry*
Benzamides / chemistry
Carboxylic Acids / chemistry
Indicators and Reagents / chemistry
Iodine / chemistry*
Kinetics
Organometallic Compounds / chemistry*
Zinc / chemistry*
Chemical
Reg. No./Substance:
0/Amines; 0/Amino Acids; 0/Benzamides; 0/Carboxylic Acids; 0/Indicators and Reagents; 0/Organometallic Compounds; 7440-66-6/Zinc; 7553-56-2/Iodine

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