Alkylidene groups

Exchange of alkylidene groups of alkenes—metathesis of olefins... 

When alkenes are treated with certain catalysts (most often tungsten, molybdenum, or rhenium complexes), they are converted to other alkenes in a reaction in which the alkylidene groups (R RX=) have become interchanged by a process schematically illustrated by the equation ... 

Very recently, Eisch and co-workers have developed new alkylidene-group IV metal complexes such as methylidene titanium dichloride 67, readily accessible from titanium(iv) chloride and an excess of methyllithium at low temperature (Scheme 24).53 The new methylenating agent 67 can easily convert benzophenone at low temperature into 1,1-diphenylethylene in quantitative yield. 

Other Carbonyl Compounds. The reaction of alkylidenetriphenylphosphoranes with lactones (Scheme 8) affords betaines (32), which can be thermally decomposed to eliminate triphenylphosphine, giving lactones in which the alkylidene grouping of the starting ylide is incorporated into the ring.34... 

Metathesis of enynes is another intriguing application in the laboratory. It would seem from the outcome, that it is a completely intramolecular reaction, but if the mechanism involves a metal alkylidene this is not true and the alkylidene group moves on from one substrate molecule to the next, see Figure 16.25. The methylene group moves to the next ring-closed diene. This is a useful tool in organic synthesis [47],... 

Evidence for the dissociative mechanism was recently summarized. Key data include kinetics studies, tandem ESI-MS observations, computational studies, and interception of an intermediate in which a phosphine ligand of RuCl2(PCy3)2(=CHPh) is replaced by an olefin subtended by the alkylidene group. ... 

Vinylcyclobutanes substituted by an alkylidene group in the 3-position undergo [1,3] and [3,3] shifts (see Table 2).25 31 138 139 Sometimes the products of both rearrangements are identical, but if the substitution pattern allows a distinction, usually mixtures are found. If the substituents R1 and R2 differ strongly, just one product may be formed selectively. 

During the past decade a very considerable literature has developed concerning the generation and reactivity of alkyl and alkylidene groups adsorbed on metal single-crystal surfaces produced via the photochemical or thermal decomposition of adsorbed alkyl halides or nitrogen-substituted alkanes. In this review, we concentrate on publications which exhibit VEEL or RAIR spectra of the hydrocarbon groupings that can be used as reference spectra for the identification of such species in spectra of species derived from hydrocarbon chemisorption. Reviews of such work cover the kinetic as well as spectroscopic aspects of this area of research (142-144). 

The coverage of this chapter emphasizes category (i) reactions in which a simple methylene group is transferred, or added, to an alkene substrate. Also covered is transfer of nonfunctionalized alkylidene groups when these reactions may be regarded as simple extensions of the methylene transfer reactions. Whenever appropriate, aspects of the stereoselectivity, enantioselectivity, regioselectivity and chemose-lectivity of these reactions will be emphasized by means of specific examples. 

Most often, the starting materials bear additional substituents, especially electronegative groups, on the prospective carbene centers. Consequently, there are only a small number of examples in which this approach has been used to add a simple alkylidene group to an alkene (Table 4). 

Geminal dihaloalkanes are very commonly used reagents for the addition of alkylidene groups to alkenes. The simpler dihalides, especially the dihalomethanes, are readily available and are certainly much less hazardous to use than the corresponding diazoalkanes. Nevertheless, any reactive alkylating agent should be regarded as a toxic substance. 

The most generally useful caibene complexes for transfer of methylene and other simple alkylidene groups are the cationic complexes of the general types shown in equations (12) and (13). In particular, the complexes containing the cyclopentadienyldicarbonyliron group, commonly written as Cp(CO)2Fe or simply Fp, have been the most useful to date. Also, derivatives of this system have been studied in which at least one of the carbonyl ligands has been replaced by another ligand such as a phosphine or a phosphite. 

A large number of useful synthetic methods are available for the addition of methylene and other simple alkylidene groups to alkenes to give cyclopropanes. The methods that are most commonly used with alkenes of normal electronic characteristics are based upon the intermediacy of carbenes and related species. Very electron rich alkenes bearing electron-donating substituents are also good substrates for many of these carbene-based methods. Some of these same methods may be used with electron deficient alkenes, but the carbanion-based methods have been developed more specifically for these latter substrates. 

The olefin metathesis reaction is a relatively recent development in the chemistry of olefins. This reaction is in essence the catalyzed redistribution of alkylidene groups between olefins (54, 55). 

In the chemistry of (3-lactams, the silyl group at the 3-position on the ring, has been used as a good precursor for the introduction of an alkylidene group through a Peterson-type reaction. Protodesilylation of this position is also possible.122... 

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