Amino carbene metal complexes

The reaction between a lithium amide and Cr(CO)s was first noted to give attack on a coordinated carbonyl and provide entry into amino-substituted carbene-metal complexes (equation 86).228,229... 

The first route to 2H-azaphosphirene complexes XIII, published by Streubel et al. in 1994 [22], used a triethylamine-induced condensation-rearrangement cascade starting from amino(aryl)carbene metal complexes VIII and [bis-(trimethylsilyl)methylene]halophosphanes IX (X=Cl,Br) (Schemes) the yields are generally good (50 - 85%) [19,23]. Experimental details and scope of this method as well as progress in this field were summarized quite recently [24]. 

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

In Figure 2.2 the most important synthetic approaches to alkoxy- or (acy-loxy)carbene complexes from non-carbene precursors are sketched. Some of these strategies can also be used to prepare amino- and thiocarbene complexes. These procedures will be discussed in detail in the following sections. In addition to the methods sketched in Figure 2.2, many complexes of this type have been prepared by chemical transformation of other heteroatom-substituted carbene complexes. Because of the high stability of most of these compounds, many different reactions can be used to modify the substituents at C without degrading the carbon-metal double bond. The generation of heteroatom-substituted carbene complexes from other carbene complexes will be discussed in Section 2.2. 

The second example shows that such a reaction need not always lead to an uncharged system. If, instead of thiocarbene complexes, amino-carbene complexes are treated with hydrogen halides, products of saltlike character are isolable (66). One finds the halogen at the metal and the hydrogen at the removed carbene ligand, and one obtains iminium halo-genopentacarbonylmetalates ... 

In an extremely short period of time, Lappert s group prepared more than 100 bis(amino)carbene complexes of almost all transition metals of groups VI, VIII, IX, X, and XI [60-62]. With regard to the reaction conditions, the general observation was that the V-arylolefins 31b were less reactive than the V-alkyl... 

The cyclic variety of MACs are called cyclic alkyl amino carbenes (CAACs) and have an excellent steric element [198]. With a cyclohexyl wingtip group on the carbon end of the carbene - known as a wall of protection - they can sterically block one side of the transition metal centre in a transition metal carbene complex effectively introducing outstanding regioselectivity and stereoselectivity in catalytic processes [198] (see Figure 3.70). 

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]. 

Having arrived at uranium as the coordinated metal, we are already past the lanthanides. It is necessary to take a break here and turn towards rare earth metal complexes with amino functionalised carbenes. Amongst the earliest examples are samarium and yttrium complexes of our already familiar amino functionalised imidazolium zwitteiion from Figure 4.28. 

Aminolysis of methoxy carbene metal pentacarbonyl complexes of the chromium group yields amino carbene compounds. If the aminolysis is performed with amino acid esters, amino carbene derivatives of amino acids are obtained [162,163]. The metal carbene is relatively stable under a variety of conditions but can be removed with TFA. It may therefore be regarded as an amino protecting group in peptide synthesis. The metal amino carbene amino acid may be activated and coupled to other amino acid esters. Following this idea, Weiss and Fischer prepared various dipeptides, the tripeptide (OC)5Cr(Ph)-Ala-Ala-Ala-OMe 69 [162] and the tetrapeptide (OC)5Cr(Ph)-Gly-Gly-Pro-Gly-OMe 70 (Scheme 5.35) [163]. Treatment with cone. TFA at 20 °C for 10 min. removes the metal carbene group and furnishes the free peptide esters along with Cr(CO)g. Removal of the metal carbene is also possible with 80% acetic acid (80 °C, 30 min.), which leaves Boc... 

---