Carbene-hydride complex

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... 

The bispyrazolyl-methane complex (Bp 0Rh(CO)(pyridine) was prepared and reacted with CH3I yielding the Rh carbene hydride complex HB(Me2pz)2Rh(H)(I)(C5H5N)(C(0)Me) 295, resulting from formal addition of CH3I across the Rh-C bond concomitant with hydride transfer from B to Rh. ... 

Ruthenium hydride complexes, e.g., the dimer 34, have been used by Hofmann et al. for the preparation of ruthenium carbene complexes [19]. Reaction of 34 with two equivalents of propargyl chloride 35 gives carbene complex 36 with a chelating diphosphane ligand (Eq. 3). Complex 36 is a remarkable example because its phosphine ligands are, in contrast to the other ruthenium carbene complexes described so far, arranged in a fixed cis stereochemistry. Although 36 was found to be less active than conventional metathesis catalysts, it catalyzes the ROMP of norbornene or cyclopentene. 

As a final example in this section, a contribution by Grubbs et al. is discussed. The chloride-free ruthenium hydride complex [RuH2(H2)2(PCy3)2] (37) is believed to react, in the presence of alkenes, to form an unidentified ruthenium(O) species which undergoes oxidative additions with dihalo compounds, e.g., 38, to give the corresponding ruthenium carbene complex 9 (Eq. 4) [20]. 

When dicarbene complexes of the form 21 were tested for 1-bntene or propene dimerisation, npon activation with AfEt Cl or MAO in tolnene, rapid deactivation took place yielding Ni(0) [25]. It was shown that this decomposition did indeed involve carbene-hydride and carbene-aUcyl rednctive elimination. Some dimerisation was evident at -15°C (TON = 50), however decomposition of the intermediate Ni species seemed too rapid for effective catalysis. In contrast, when the complexes were... 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

Z,Z)-l,4-Dialkoxy-l,3-dienes can be readily prepared from propargyl ethers and molybdenum carbene complexes (equation 185)307. High stereoselectivity in this reaction may be due to the formation of stable vinyl hydride complex with the enol ether. 

A mononuclear nickel hydride complex with three N-heterocyclic carbene ligands has been reported the compound was formed by oxidative addition of an imidazolium salt to the Ni(0) bis(carbene) complex [19]. The hydride signal of this nickel(II) complex appears at -15 ppm. 

Recendy, we found that A -allyl-o-vii rlaniline 44 gave 1,2-dihydroquinoline 45 by normal RCM and developed silyl enol ether-ene metathesis for the novel synthesis of 4-siloxy-1,2-dihydroquinoline and demonstrated a convenient entry to quinolines and 1,2,3,4-tetrahydroquinoline [13], We also have found a novel selective isomerization of terminal olefin to give the corresponding enamide 46 using rathenium carbene catalyst [Ru] and silyl enol ether [14], which represented a new synthetic route to a series of substituted indoles 47 [12], We also succeeded an unambiguous characterization of mthenium hydride complex [RuH] with ACheterocyclic carbene... 

In spite of the successful use of NHCs in a number of palladium-catalyzed reactions, no system for hydrogenation was reported until 2005. This can be easily explained as it had been observed that hydridopalladium-carbene species decompose due to attack of the hydride on the carbene, which results in its reductive elimination to yield the corresponding imidazolium salt [ 190]. However, Cavell and co-workers recently showed that the oxidative addition of imidazolium salts to bis-carbenic palladium complexes leads to isolable NHC-hydridopalladium complexes [191]. This elegant work evidenced the remarkable stabilizing effect of NHC ligands in otherwise reactive species and led to the development of the first NHC-palladium catalyst for hydrogenation. 

Careful treatment of 1,3-disubstituted imidazolium salts (e.g., 350) with sodium hydride (in some cases addition of a small amount of potassium /< r/-butoxide is also recommended) in tetrahydrofuran gives stable carbenes of type 351 (see also Section 2.4.4.2.3). They behave as rather strong bases (p , 24 in DMSO) and nucleophiles. Thus, carbene 351 and salt 350 form a crystalline bis(carbene)proton complex 352 which is a rare example of an asymmetrical hydrogen bridge between two carbon centers. By the action of BF3, B2H6 or AlH3NMe3 stable mesoionic adducts of type 353 are obtained. 

Four years after Grundemann et al. s original observation [49], Peris and coworkers [40] proved that steric strain is not the decisive factor in the abnormal coordination of the carbene in these particular iridium(ni) hydride complexes. We may well suspect that differences in... 

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). 

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 carbenes [85]. Complexes (26) (Scheme 9, M - Cr, W), which contain a pyridinium ylide, 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... 

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