Handbook of Functionalized Organometallics Applications in S

11
Polyfunctional Metal Carbenes for Organic Synthesis
Karl Heinz Dötz, Alexander Koch, and Martin Werner
11.1
Introduction
Since the discovery of the first metal carbene complex by Fischer and Maasböl in
1964, the applications of transition-metal carbenes have been developed into a
powerful toolbox in the hand of synthetic working chemists. Their impressive
synthetical potential, based on the strongly electrophilic carbene atom, the a-CHacidity
of an alkylcarbene substituent and the template properties of a low-valent
metal center, allowed for the development of a broad variety of transformations
that ± in part ± are unprecedented in classical organic chemistry. In particular, the
metal-carbene fragment L nM=C turned out to be a versatile functionality that,
beyond the isolobal analogy with the carbonyl group O=C, may be involved in
reaction patterns characteristic for and restricted to organometallic processes. In
addition, a proper selection and control of the reaction conditions can be applied
to secure C±C bond formation with high chemo-, regio- and diastereoselectivity.
In this chapter, we present an overview of organometallic transformations of
Fischer-type carbene complexes relevant to organic synthesis with a specific focus
on chromium carbenes bearing multiple functionalities.
11.2
Chromium-Templated Cycloaddition Reactions
The group 6 Fischer carbene complexes are attractive reagents for the synthesis of
carbocycles and heterocycles [1]. Chromium carbenes are versatile starting materials
for cycloaddition and cyclization reactions affording small- and medium-sized
rings ranging from three- [2] and four-membered [3] rings up to seven- [4] and
eight-membered [5] rings; also larger ring systems are available [6]. While some
types of reactions are restricted to the carbene ligand exploiting the metal-carbene
unit as an activating organometallic functional group, the majority of processes
directly involves the metal as a template in stepwise C±C bond formation.
Organometallics. Paul Knochel
Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 3-527-31131-9
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