Carbenes unsaturated, from vinyl

Casey and coworicers have shown that ketone etiolates add efficiently to a,3-unsaturated vinyl carbene complexes (164), irrespective of 3,3-disubstitution on the complex or high substitution on the enol-ate 133 thus, contiguous 3 and y quaternary centers are easily assembled. When coupled with the ease of release of the carbene ligand from the complexes by either oxidation to the ester functionality1331 or elimination to the corresponding enol ether,133 the vinyl carbene complexes are synthetic equivalents for a,3-unsaturated esters or a,3-unsaturated aldehydes, respectively (Scheme 63). 

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

Cyclopropane formation occurs from reactions between diazo compounds and alkenes, catalyzed by a wide variety of transition-metal compounds [7-9], that involve the addition of a carbene entity to a C-C double bond. This transformation is stereospecific and generally occurs with electron-rich alkenes, including substituted olefins, dienes, and vinyl ethers, but not a,(J-unsaturated carbonyl compounds or nitriles [23,24], Relative reactivities portray a highly electrophilic intermediate and an early transition state for cyclopropanation reactions [15,25], accounting in part for the relative difficulty in controlling selectivity. For intermolecular reactions, the formation of geometrical isomers, regioisomers from reactions with dienes, and enantiomers must all be taken into account. 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

Nucleophilic attack on sulphur, resulting in S—O bond fission, has also been observed in work on the generation of unsaturated carbenes by base-induced a-elimination of the elements of CFa SO3H from primary vinyl triflates (see Scheme 70). Use of hard bases such as (MeaSOaNLi, PhaCNa, Bu"Li, and NaH in step... 

Although no cyclopropane formation was observed in the reactions of heteroatom-stabilized carbene complexes with simple alkenes such as cyclohexene or tetramethylethylene, cyclopropane formation has been observed both with electron-deficient a,) -unsaturated esters and with electron-rich vinyl ethers. The mechanisms involved in cyclopropane formation from these very different classes of alkenes may be substantially different. 

Stang, P.I. and Magnum, M.G., Unsaturated carbenes from primary vinyl triflates. The nature of vinylidene carbene intermediates, /. Am. Chem. Soc., 91, 6478, 1975. 

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