The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

Highlights

• Many examples were reported in 2014 where metal carbene/alkylidene complexes serve as catalysts and reactive intermediates for olefin metathesis, which is an important tool in polymer chemistry and organic synthesis.

• Several reports in 2014 featured metal carbyne/alkylidyne complexes as catalysts for alkyne metathesis, an emerging tool in organic synthesis and polymer chemistry.

• Many examples appeared in 2014 highlighting the facile generation of metal-carbene complexes from a variety of organic precursors, including alkynes, allenes, diazo compounds, triazoles, tosylhydrazones, and organolithium reagents, and have been instrumental in the design of reaction processes that lead to rapid gains in molecular complexity.

• In 2014 carbene complexes were reported for virtually every transition metal, and several examples of carbene complexes of lanthanides and actinides were reported as well.

Abstract

This is a review of papers published in the year 2014 that focus on the synthesis, reactivity, or properties of compounds containing a carbon-transition metal double or triple bond.

Introduction

This survey is intended to be a comprehensive synopsis of articles that report on the synthesis, reactivity, or properties of compounds featuring a multiple bond between carbon and a transition metal. Reactions that employ metal-carbene complexes as transient intermediates generated through well-established routes using conventional catalysts are not covered, unless there is some effort to characterize the carbene complex intermediate or a non-traditional reaction pathway is noted. Several reviews in this area appeared in 2014 [1], [2], [3], [4], [5], [6], [7], [8]. Although a determined effort has been made to include patents, in general, only patents listed in the sections “Organometallics and Organometallic Compounds” or “Catalysis, Reaction Kinetics, and Inorganic Reaction Mechanisms” of Chemical Abstracts have been included. Patents that appeared in 2014 editions of Chemical Abstracts have been included. Only compounds that feature a multiple bond between a single carbon atom and a single transition metal are discussed in this survey, thus bridging carbene and carbyne complexes are not covered unless there is a multiple bond to at least one transition metal. The complexes of N-heterocyclic (or Arduengo) carbenes with transition metals have not been included since the π-acceptor component of these complexes is usually minimal, and thus there is no formal carbon–metal multiple bond [9]. The chemistry of NHC complexes was reviewed several times in 2014 [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], and several in languages other than English [22], [23], [24]. Every effort has been made to include NHC complexes where significant π-acceptor ability of the NHC ligand is noted. This survey has been divided into two sections: metal-carbene (or alkylidene) complexes and metal carbyne (or alkylidyne) complexes; the carbene complex section represents the vast majority of this article. The metal-carbene section has been organized according to metal, starting from the left side of the Periodic Table. The Ionic Model [25] has been employed for the discussion of oxidation states and ligand electron count throughout this survey. A special section focusing on alkene metathesis has been included prior to the discussion of carbene complexes of individual metals. The metal carbyne section has been organized according to reaction type.

Abbreviations (see also the instructions for authors in the Journal of Organic Chemistry [26] and the list of ligand acronyms in the Strem Catalog [27] (Fig. 1)).

Unless otherwise indicated, all alkanes and alkyl groups are assumed to be the straight-chain (n) isomer.

See also Scheme 1 for abbreviations of distinct modes of olefin metathesis.