Synthesis 2005(7): 1109-1124  
DOI: 10.1055/s-2005-861864
PAPER
© Georg Thieme Verlag Stuttgart · New York

A Novel Approach to Functionalized Heterohelicenes via Chromium-Templated Benzannulation Reactions [1]

Jochen F. Schneidera, Martin Niegerb, Kalle Nättinenc, Karl Heinz Dötz*a
a Kekulé-Institut für Organische Chemie und Biochemie, Rheinische Friedrich-Wilhelms-Universität, 53121 Bonn, Germany
Fax: +49(228)735813; e-Mail: doetz@uni-bonn.de ;
b Institut für Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universität, 53121 Bonn, Germany
c Department of Chemistry, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland
Further Information

Publication History

Received 10 December 2004
Publication Date:
10 March 2005 (online)

Abstract

A series of functionalized oxa- and thiaheterohelicenes has been prepared by a new progressive annulation technique based on the chromium-templated [3+2+1]benzannulation reaction of hetero[5]helicene carbene complexes with various alkynes partly in competition of angular and linear annulation. Oxidative work-up with CAN yields the respective quinones along with small amounts of nitrated products or a nitrate ester byproduct. Thermal recomplexation of an acylated monobenzannulation product by Cr(CO)6 affords the corresponding, thermodynamically favored η6-oxa[6]helicene-Cr(CO)3 complex. Bidirectional benzannulation reveals a more delicate regioselectivity and yields three different isomers including a bisquinone with an unprecedented bislinear annulation pattern. Molecular structures determined by X-ray analysis indicate a decreasing helical distortion as a result of progressive annulation, steric demand of the quinone substitution pattern (Ph>Pr>Et), or the nature of the embedded heterocycle (SO2>S>O).

33

Preliminary quantum chemical calculations on electrostatic charge differences in chromium carbene starting materials and σ,π-allyl complex intermediates derived therefrom which have been performed with the semi-empirical PM3 method reveal the proper trend but were not fully conclusive. A more thorough DFT study is envisaged.