Loading…

Catalytic Cyclization of o-Alkynylbenzaldehyde Acetals and Thioacetals. Unprecedented Activation of the Platinum Catalyst by Olefins. Scope and Mechanism of the Reaction

A general protocol for the synthesis of functionalized indenes from o-alkynylbenzaldehyde acetals and thioacetals has been elaborated. Acetals uniformly give cyclization products having the alkyl group from the starting acetylene migrated to the α-position, whereas the cyclization of the correspondi...

Full description

Saved in:
Bibliographic Details
Published in:Journal of the American Chemical Society 2004-12, Vol.126 (47), p.15423-15430
Main Authors: Nakamura, Itaru, Bajracharya, Gan B, Wu, Huanyou, Oishi, Kengo, Mizushima, Yuya, Gridnev, Ilya D, Yamamoto, Yoshinori
Format: Article
Language:English
Subjects:
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:A general protocol for the synthesis of functionalized indenes from o-alkynylbenzaldehyde acetals and thioacetals has been elaborated. Acetals uniformly give cyclization products having the alkyl group from the starting acetylene migrated to the α-position, whereas the cyclization of the corresponding thioacetals proceeds without alkyl migration. Optimization of the catalytic system for the cyclization of o-alkynylbenzaldehyde acetals revealed an unknown activation effect:  PtCl2 was found to be a better catalyst for the cyclization of acetals in the presence of olefins than without. A similar catalytic system (PtCl2/benzoquinone) has been found to be appropriate for the cyclization of cyclic acetals, whereas the optimal catalyst for the reaction of thioacetals is PdI2. NMR monitoring of two reactions, acetal 3a + Pd(CH3CN)Cl2 in CD3CN and thioacetal 5j + PdI2 in CD2Cl2, revealed that in both reactions similar cationic species are formed at the early stage of the transformation. Computational data (B3LYP/SDD level of theory) suggest that the difference in the reaction pathways for acetals and thioacetals can be rationalized by taking into account the relative stabilities of the corresponding vinylpalladium intermediates (22 vs 20 and 19 vs 21), which suggests a reversible thermodynamically controlled alkyl migration in the intermediate vinylcationic species.
ISSN:0002-7863
1520-5126
DOI:10.1021/ja044603c