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How do analogous α-chloroenamides and α-iodoenamides give different product distributions in 5-Endo radical cyclizations?

Curran, DP and Guthrie, DB and Geib, SJ (2008) How do analogous α-chloroenamides and α-iodoenamides give different product distributions in 5-Endo radical cyclizations? Journal of the American Chemical Society, 130 (26). 8437 - 8445. ISSN 0002-7863

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5-Endo cyclizations of N-alkenyl carbamoylmethyl radicals provide γ-lactam radicals, which in turn evolve to reduced or non-reduced (alkene) products depending on reagents and reaction conditions. Several groups have made surprising observations that chlorides are better radical precursors than iodides in such cyclizations. Here is described a detailed study of tin and silicon hydride-mediated radical cyclizations of N-benzyl-2-halo-N-cyclohex-1- enylacetamides. The ratios of directly reduced, cyclized/reduced, and cyclized/non-reduced products depend not only on the reaction conditions and reducing reagent but also on the precursor. Prior explanations for the precursor-dependent product ratios based on amide rotamer effects are ruled out. The precursor-dependent behavior is further dissected into two different effects: (1) the ratio of cyclized/reduced products to cyclized/non-reduced products depends on the ability of the radical precursor to react with the product γ-lactam radical in competition with tin hydride (iodides can compete, chlorides cannot), and (2) the occurrence of large amounts of directly reduced (noncyclized) products in the case of iodides is attributed to a competing ionic chain reaction by which the precursor is reductively deiodinated with HI. This side reaction is not available to chlorides, thereby explaining why the chlorides are better precursors in such reactions. The ability of the iodides to provide cyclized products can be largely restored by adding base. The chlorides and iodides then become complementary precursors, with chlorides giving largely cyclized/reduced products and iodides giving largely cyclized/non-reduced products. © 2008 American Chemical Society.


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Item Type: Article
Status: Published
CreatorsEmailPitt UsernameORCID
Curran, DPcurran@pitt.eduCURRAN
Guthrie, DB
Geib, SJgeib@pitt.eduGEIB0000-0002-9160-7857
Date: 2 July 2008
Date Type: Publication
Journal or Publication Title: Journal of the American Chemical Society
Volume: 130
Number: 26
Page Range: 8437 - 8445
DOI or Unique Handle: 10.1021/ja8012962
Schools and Programs: Dietrich School of Arts and Sciences > Chemistry
Refereed: Yes
ISSN: 0002-7863
PubMed ID: 18540600
Date Deposited: 08 Apr 2013 17:33
Last Modified: 02 Feb 2019 15:58


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