Metal-carbene-hydride complexes

When Y in Scheme 7.21B is hydrogen, the reverse process yielding a metal carbene-hydride complex is c -hydrogen elimination process. It provides an important route leading to decomposition of metal alkyls beside the more often encountered -hydrogen elimination pathway giving metal hydride coordinated with an olefin. 

It might be interesting to note that the proponents of the carbene mechanism (mentioned earlier), point out that this is also consistent with their mechanism [254, 255], The reaction can consist of (a) an insertion of a metal into an a-CH bond of a metal alkyl to form a metal-carbene hydride complex. This is followed by (b) reaction of the metal-carbene unit with an alkene to form a metal-cyclobutane-hydride intermediate. The final step (c), is a reductive elimination of hydride and alkyl groups to produce a chain-lengthened metal alkyls. This assures that a chiral metal environment is maintained [254]. It is generally believed [258], however, that stereospecific propagation comes from concerted, multicentered reactions, as was shown in the Cossee-Arlman mechanism. The initiator is coordinated... 

Carbene Complexes.— Protonation of the chelated iron-carbene complex (59) in the presence of ethylene yields (60), the first example of a metal-carbene-olefin complex of the type postulated as intermediates in olefin metathesis reactions. Hydride abstraction from (61) affords (62). The new carbene complexes (63), (64), and (65) have been prepared by treatment of [Fe(jj-C5H5)(SnPh3)-(CO)(CS)l, [Fe( -C6H6) C(S)OPh (CO)3], and [Fe( -CS3) P(OMe)3 (CO)3] with ethylenediamine, methylfluorosulphonate (followed by methanol and PF e ions),... 

However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. 

We have previously stated that an ylide could be considered the coupling product of a singlet carbene with a nucleophile. Therefore, it seems logical that the reaction of a metallic carbene with a nucleophile would give a metal bonded ylide and, in fact, this is a quite useful method to prepare metallated ylides. Even more, in some cases coordinated ylides have been used as masked caibenes [85]. Complexes (26) (Scheme 9, M = Cr, W), which contain a pyridinium yhde, are conveniently prepared by reaction of the corresponding carbenes [(CO)5M=C(OEt) R] with 1,2- or 1,4-dihydropyridines. During the reaction an unprecedented hydride... 

A similar mechanism might operate in the activation of an azolium salt by a transition metal compound forming the metal carbene complex. However, since a basic substituent on the metal (acetate, alkoxide, hydride) usually reacts with the H -proton, the proton is removed from the reaction as the conjugate acid and reductive elimination does not occur. 

Once the hydroxy functionalised imidazolium salt is formed, it can be deprotonised and reacted with various metal complexes to form (transition) metal carbene complexes. The hydroxy group ensures that the ligand can be coordinated even to metals that are normally reluctant to form stable carbene complexes. A good example is the deprotonation of a hydroxyethyl functionalised imidazolium salt with potassium hydride [36]. The potassium cation coordinates to the oxygen atom of the alkoxide sidechain and forms cubes as structural elements (see Figure 4.6). The carbene end then coordinates to the respective... 

An attack of a nucleophilic transition metal hydride on coordinated CO has also been observed by Labinger and Wong, who treated carbonyls of Ct, Mo, W, Mn, Fe, Ku and Co with Cpi Nbllj. which is of a lower hydridic nature than the main group hydrides or group IV hydridic complexes (54]. Small amounts of C C3 hydrocarbons are formed. However, when the reaction of CpjNbH with Cr(CO)(( is carried out under H], only ethane t formed. Carbene intermediates are proposed to account for this selectivity (Scheme 14). 

In general, the termination reactions of these polymerizations are not well understood but, depending upon the metal and the monomer, reductive coupling of the metal carbene fragments to give alkene and reduced metal complexes is one possibility. Another termination reaction appears to be initiated by -Hydride Elimination from the carbene complex. These mechanisms have been observed in well-defined catalyst systems, and are possible in the ill-defined systems also. The fact that most catalysts are sensitive to oxygen and moisture (or other proton sources) means that termination of the polymer chain by added or adventitious sources of water is a common problem, especially for the ill-defined catalysts. 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

Similar a-hydride elimination reactions resulting in metal carbenes were studied extensively in the case of tungsten [Eq. (6.77)] and tantalum [Eq. (6.78)] complexes. An a-hydrogen bond interaction of this type was proposed to rationalize the stereospecificity of reactions catalyzed by cobalt-containing coenzyme B12. ... 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

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