Cyclopropenes metathesis

The most common method for the generation of the metal alkylidene species seems to be a-elimination from an intermediate dialkyl-metal species. This procedure gives the most active catalysts. Above we mentioned the addition of other carbene precursors, which leads to active catalysts. Other methods to generate the metal alkylidene species involve alkylidene transfer from phosphoranes [16] or ring-opening of cyclopropenes [17], In Chapter 16.4 we will describe a few compounds that are active by themselves as metathesis catalysts. 

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

This stoichiometric reaction constitutes a new contribution to vinylidene chemistry and a novel method to generate alkenylcarbene ligand from simple propargyl alkyl ethers rather than via activation of cyclopropenes [4] or by stoichiometric activation of butadiene [6[. When linked to a suitable metal-ligand moiety this carbene constitutes an alkene metathesis initiator. 

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

Disubstituted cyclopropenes undergo ring-opening cross-metathesis with certain alkenes under the influence of 6 to produce (3S)-1,4-alkadienes. ... 

Acetylenes may be regarded as the first members of the series their polymerization using olefin metathesis catalysts is described in Ch. 10. There is no recorded attempt to polymerize cyclopropene with metathesis catalysts the product would probably be cyclohexa-1,4-diene rather than polymer. 

Singh, R. (2008) Cyclopropene Polymerization and Enyne Metathesis Catalyzed by High Oxidation State Molybdenum Alkylidenes. PhD thesis. Massachusetts Institute of Technology. 

Metal vinylidenes easily form by isomerization of metal-alkynecomplexes, which is important for instance in alkyne polymerization by the metathesis mechanism. Synthesis of Grubbs ruthenium vinylidene complexes [Ru(PR3)2Cl2(=C=CHPh)] originally proceeded by reaction of the ruthenium precursor with diphenyl-cyclopropene, but a more modern method leading to the parent vinylidene complex involves benzylidene exchange for the vinylidene in the metathesis with 1,3-butadiene ... 

Since metathesis is a reversible reaction, monomers must be chosen so that the monomer should be more reactive than the double bond in the polymer. If this is not the case, the polymer will compete with the monomer for reaction and will result in a chain transfer reaction. Consequently, norbomenes have been used in most cases, although other less readily available strained monomers such as cyclobutenes, cyclopropenes, and transcyclooctenes have also been used in living ROMP. ... 

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