Transition metal complexes carbene synthesis

Despite the fact that transition metal complexes have found wide application in the synthesis of carbo- and heterocycles, [3+3] cyclisation reactions mediated or assisted by transition metals remain almost unexplored [3, 86]. However, a few examples involving Fischer carbene complexes have been reported. In all cases, this complex is a,/J-unsaturated in order to act as a C3-synthon and it reacts with different types of substrates acting as C3-synthons as well. 

The first propadienylidene complexes were reported in 1976 (16,17), and examples of terminal and bridging ligands of this type are now known their chemistry is now beginning to be understood. This article is concerned with the synthesis, properties, and chemistry of transition metal complexes containing these unsaturated carbene ligands. 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

The coordination of amido functionalised carbene ligands, where the amido function is introduced by the acetylation of an amino functionalised imidazolium salt, to transition metals has already been discussed in Section 3.1. Here it suffices to mention that the synthesis is similar to other carbonyl functionalised NHC ligands and their transition metal complexes. A suitable example is given in Rivera and Crabtree [226]. 

Figure 3.111 Synthesis of chiral bis-carbenes based on a 9,10-ethanoanthracene scaffold and their transition metal complexes.

Figure 3.117 Synthesis of transition metal complexes of bis-carbene cyciophane ligands with a xylene backbone.

Figure 3.121 Synthesis of Janus (back-to-back) bis-carbene transition metal complexes.

Pugh et al. [486] published an equally facile synthesis for the transition metal pincer carbene complexes of titanium(IV), vanadium(ll), chromium(ll), manganese(II), niobium(III) and uranium(IV). It consists of treating the free carbene with the respective transition metal halide or the pincer imidazolium salt with the respective metal bis-trimethylsilylamide (see Figure 3.160). 

P-Heterocycles, synthesis from transition metal complexes of phosphi-nides as carbene analogues 87AG285. 

The synthesis and reactivity of titanoxo units as fragments of transition metal Fischer carbene complexes have been reviewed.1349 Other reviews have appeareed covering structurally characterized organometallic hydroxo complexes of transition metals including mono- and bis-Gp titanium derivatives.809... 

The persistent radical effect must always play a role when transient and persistent radicals are formed with equal or nearly equal rates. It leads to the formation of the mutual reaction products in high yields and to the virtual absence of the self-termination reactions. In the few examples given earlier, the persistent species were radicals and transition metal complexes, but other reaction partners such as molecular ions and even normal molecules may take their place. Furthermore, the phenomenon can also work with other transient species, such as carbenes, nitrenes, and molecules in electronically excited states. A literature search would probably reveal a large variety of diverse reactions that exhibit the effect to some degree, although this went unnoticed, so far. Here, we restrict the survey to evident cases. A few of the reactions have even been designed to exploit the persistent radical effect in synthesis. 

N-Heterocyclic carbenes in transition-metal-complex synthesis 00JOM(600)12. 

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