Carbene compounds

Their precursors must be the tricarbonyl o-allenyls with the uncoordinated C=C bonds. Neither an allylic rearrangement nor cis-trans isomerization has been observed in the reaction of CpMo(CO)3(cw-CH2CH=CHMe) with PPhj, the product being CpMo(CO)2(PPh3)(cw-COCH2CH=CHMe) (81). The interesting reaction leading to the formation of cationic carbene compounds was mentioned earlier [Eq. (17) and Section V] (78). 

The majority of gold(I) carbene complexes are pure organometallic compounds and the are out of the scope of this work. Some halide or triphenylphosphine carbene complexes are known and they will be considered here. Nucleophilic addition of alcohols or amines to gold-coordinated isocyanides is one of the best-established methods to obtain gold carbene derivatives. The reaction of H[Au(CN)2] with propene oxide and estirene oxide yields (cyano)carbene complexes (380) avoiding the intermediate step.2257 A cyclic carbene compound is obtained by reaction of a dinuclear isocyanide with amine (Scheme 32).2258... 

Development of the chemistry of metal carbenes commenced with the characterization of complexes such as (CO)sW=C(OMe)Ph by Fischer and his co-workers (e.g., Ref. 14 and references therein). The series of hydrocarbon-substituted carbene compounds discovered later by the Schrock group (e.g., Ref. 15 and references therein) appeared to be so different from the other known carbene complexes that they were placed in a different class altogether. Overemphasis of this distinction has persisted in the literature of carbene complexes, even to the present day. 

Alkylation of an anionic acylmetallate is the second step in the classic Fischer synthesis of carbene complexes. The same type of reaction can be performed with stable neutral acyl complexes, producing cationic carbene compounds. Compound 17 has been prepared from a ruthenium acetyl complex by direct methylation and by protonation with subsequent methylenation (45) ... 

Carbenes compounds in which carbon forms only two bonds. 

A prerequisite for the a-elimination is the absence of (3-hydrogen atoms in the alkyl groups and this was successfully achieved by using the neopentyl substituents at the metal centre. The nature of the double bond between the metal and carbon was established by its bond length and the occurrence of stereoisomers [13], Typical feature of the Schrock carbenes is that they contain an electrophilic, high-valent metal atom and an electron rich carbene carbon atom. The reverse is true for the older, Fischer carbene compounds, such as the one mentioned, (OC)5W=CPh2. 

Metal Complexes of Stable Carbenes compounds best described as Lewis acid-Lewis base adducts. 

When only one heteroatom of the dinucleophile possesses a hydrogen substituent, the reactions lead instead to alkenyl complexes rather than carbene compounds. Effectively, treatment of diphenylallenylidenes 1 and 6 with pyrazoles yields the heterocyclic derivatives 61 (Scheme 2.25) [76]. Interestingly, the dissymmetric 3-methylpyrazole (R=H, R = Me) provides only one regioisomer, in which the methyl group points towards the metal. This process, which formally corresponds to the addition of two nitrogen nuclei at C and Cy and a hydrogen atom at Cp, is assumed to take place through an initial nucleophilic attack at the Ca position. 

Examples of complexes that crystallize as dinudear aggregates which do not contain tertiary phosphines include the carbene compound [Au C(NHMe)NEt2 (SCN)] [44]. The change of the substituent in complexes [AuCl(l,3-R2-bimy)]... 

Ruthenium catalysts, coordinated with an N-heterocyclic carbene allowed for the ROMP of low-strain cyclopentene and substituted cyclopentenes (10,23). Suitable ruthenium and osmium carbene compounds may be synthesized using diazo compounds, by neutral electron donor ligand exchange, by cross metathesis, using acetylene, cumulated olefins, and in an one-pot method using diazo compounds and neutral electron donors (24). The route via diazo compounds is shown in Figure 1.7. 

Since the ruthenium and osmium carbene compounds of the type shown in Figure 1.7 are stable in the presence of a variety of functional groups, the olefins involved in the polymerization reactions may optionally be substituted with various functional groups. 

Figure 1.7 Synthesis of Ruthenium Carbene Compounds via Diazo Compounds (24)...

The UPS of each of the nine carbene compounds studied (Table VIII and Fig. 14) show a band at low IE that can be ascribed to electron ejection from the predominantly Cr(3z/) orbitals of symmetries a (2) and a". The band often exhibits shoulders and can be curve-fitted with two or three Gaussian peaks. The... 

Wittig-type alkenation of the carbonyl group is possible with Ti carbene compounds [56], The reaction is explained by the formation of nucleophilic carbene complexes of Ti, although they are not isolated. In the carbonyl alkenation, the oxametallacyclo-butane intermediate 182 is formed by [2+2] cycloaddition of the carbene complex 181 with the carbonyl group. This intermediate is converted to the new alkene 183 and the Ti(IV) oxo species 184, which is a stable compound, and hence the carbonyl alkenation requires a stoichiometric amount of the Ti complex. Also, ester 185 is converted to the enol ether 187 via 186. 

The main synthetic technique for obtaining carbyne complexes is through the reaction of carbene compounds with metal halides, preferably with those of Group IIIA [259c] as, for example, through reaction (2.2) ... 

Stromberg, J.R., Wnuk, J.D., Pinlac, R.A.F. and Meyer, G.J. (2006) Multielectron transfer at heme-functionalized nanocrystalline TiCU Reductive dechlorination of DDT and CCI4 forms stable carbene compounds. Nanoletters 6, 1284-1286. 

Finally, while on the topic of non-classical carbon ligands, the d° yttrium N-heterocyclic carbene compound mer-Y(OCMe2CH2[C N(CHCH)NPr1 ]3 [9] was prepared and structurally characterized.8... 

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... 

Two types of transition metal carbene compounds are traditionally referred to by the names of the scientists who first made them, namely E. O. Fischer [27] and R. R. Schrock [30]. The discovery of the first transition metal NHC complexes by Ofele [2] and WanzUck [3] falls in between the other two, but did not receive the same amount of recognition at the... 

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