Carbenes, hydrido

Section III.C A Hydrido(methyl)carbene Complex of Platinum(IV) (223) and Methyl(hydrido)platinum(IV) Complexes with Flexible Tridentate Nitrogen-Donor Ligands (224) are structurally related to the system shown in Scheme 13 and give additional information on how steric and electronic factors influence the stability of platinum(IV) methyl hydrides. 

Arduengo-type carbenes have also been successfully employed for complexation of group 13 trialkyls (Me3Ga- CNfPflCzMezNfPr1) 13091) and mixed hydrido/alkyls (Cp 2(H)Ga- CN(Me)C2Me2N(Me) 13192). Adduct 131... 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

First attempts to isolate monocarbene-hydrido complexes by oxidative addition of A -(2-pyridyl)imidazolium cations to Pd° with utilization of the chelate effect of the donor-functionalized carbene ligand failed and only the dicarbene complexes such as 29 were isolated [112]. The iridium hydrido complex 30 was obtained in the oxidative addition of an W-(2-pyridylmethyl)imidazolium cation to iridium(I) (Fig. 11) [113]. This reaction proceeds most likely via the initial coordination of the nitrogen donor which brings the imidazolium C2-H bond in close proximity to the metal center. No reaction was observed with Rh under these conditions. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

While the reductive elimination is a major pathway for the deactivation of catalytically active NHC complexes [127, 128], it can also be utilized for selective transformations. Cavell et al. [135] described an interesting combination of oxidative addition and reductive elimination for the preparation of C2-alkylated imida-zohum salts. The in situ generated nickel catalyst [Ni(PPh3)2] oxidatively added the C2-H bond of an imidazolium salt to form a Ni hydrido complex. This complex reacts under alkene insertion into the Ni-H bond followed by reductive elimination of the 2-alkylimidazolium salt 39 (Fig. 14). Treatment of N-alkenyl functionalized azolium salts with [NiL2] (L = carbene or phosphine) resulted in the formation of five- and six-membered ring-fused azolium (type 40) and thiazolium salts [136, 137]. 

The most important difference between Chauvin s mechanism for olefin metathesis and the mechanism for alkane metathesis is that the latter applies itself to the reverse reaction of cleavage of alkanes by methane (which has no single C-C bond, see below) and, especially, it is based on a metal hydrido-carbene in equi-Ubrium with a metal-alkyl. 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

Finally a carbene compound with a U=C bond is Cp3U=CHPMe3.61 Various alkyl and hydrido Cp species can catalyze oligomerization of terminal acetylenes, activate C-H bonds, hydrogenate alkenes, and so on.62... 

A Fischer-type dithiocarbene complex (447) which undergoes protonation at the carbene carbon was reported by Le Bozec et al (265). The reaction, which produces 448, proceeds via formation of a /rans-hydrido-carbene intermediate, isomerization to the cis isomer, and a metal to carbene carbon hydrogen shift. 

The Italian group has examined a series of Pt(II) trifluoromethyl compounds that contain peroxo, hydrido, and alkoxy ligands along with the CF3 group (27,74). In several cases the CF3 (75) [or CO (76)] ligands have been converted into Fischer-type carbenes as... 

In at least one case, there is an example of ethene polymerization using a Ta-carbene complex in which there is strong evidence for a metathesis-based mechanism. See H. W. Turner and R. R. Schrock, J. Am. Chem. Soc., 1982,104, 2331, a paper that describes oligomerization of up to 35 ethene units in the presence of Ta[=CH(f-Bu)](H)(PMe3)3l2. It seems clear that although the Cossee mechanism is operative when polymerization occurs in the presence of Z-N-type catalysts, some polymerizations may involve metathesis, especially when hydrido-metal carbenes can form readily. 

A cationic hydrido-methyl complex, HPtCHs ", is formed upon the almost barrierless insertion of a Pt ion into a C-H bond of methane and then 1,2-migration of a hydrogen atom and subsequent elimination of H2 from the transition-metal center gives the platinum carbene cation PtCHi"" with an apparent rate constant of k - 8.2 x 10" cm molecule" s. The reaction of ground-state platinum atoms produced by the photodissociation of [PtMe3-(CsHiMe)] follows a termolecular mechanism [4c] ... 

Unexpected 2,1-addition of Ru-H bond across the C=C bonds of ethyl vinyl ether [61], vinyl acetate [62] and vinyl chloride [63] followed by a-hydrido, a-acetato and a-chloro eliminations, respectively, to give carbene complexes has been also known. 

The debate on the mechanism of polymerization, whether an insertion mechanism (Cossee-Arlman) [6], or a metathesis-type mechanism initiated by a-H elimination from the alkyl complex to give a hydrido-carbene intermediate (Green-Rooney) [108], was solved in favor of the former on the basis of the absence of isotope effect on the rates of insertion, and on the stereochemistry of alkene intramolecular insertion, when a-D alkyls were used in the cyclizafion reaction shown in Eq. 6.21 [109]. 

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