Cyclobutenes alkylidene

The sterically bulky ruthenium alkylidene 42, formed via ring-opening of the cyclobutene, should react more rapidly with the terminal alkene than with a second molecule of the cyclobutene. This preference for reacting with the acyclic alkene is probably due to a combination of the greater steric hindrance of the cyclic alkene and the ability of the reaction with the terminal alkene to proceed... 

Reductive dimerization of propargyl chlorides.2 Treatment of propargyl chlorides with this nickel(O) complex effects cyclodimerization to 3,4-bis(alkylidene)-cyclobutenes. 

As shown above, insertion of alkylidene carbenes is highly regioselective. However, when the normal 1,5-C-H insertion pathway is blocked, 1,4- or 1,6-C-H insertion takes place [Eq. (109)]. Thus, the cyclobutene 121 [192] and the six-membered enol ether 123 [193] were obtained in modest yields. Intramolecular insertion into carbon-carbon double bond provides a method for synthesis of cyclopenten-annulated dihydropyrrole 124, which results from homolytic scission of a methylenecyclopropane intermediate [194]. 

A cyclobutene was recently obtained by a related reaction of dimethyl acetylenedicarboxylate with ethylene in the presence of a cationic ruthenium alkylidene catalyst precursor [43] (Eq. 35). 

The intrinsic reactivity of strained cycloalkenes such as norbornene and cyclobutene ensures that they react as desired, and simple homogeneous metal halide catalysts are often effective for this transformation. However for less strained cyclic substrates, manipulation of catalyst activity/selectivity by means of modifying ligands is required. This is where the well-defined alkylidene catalysts pioneered by Grubbs and Schrock have come to the fore. An interesting example illustrating the range of catalyst reactivity is provided by the... 

The use of diazoalkanes instead of 3,3-diphenylcyclopropene provides a series of air-stable alkylidene complexes, Ru(=CHR)(Cl)2(PPh3)2 and Ru(=CHR) (Cl)2(PCy3)2 (R = Me, Et, Ph, p-CgHtCl), which are very efficient catalysts for the ROMP of norbomene and substituted cyclobutenes. Thus, for the Ru(=CHPh)(Cl)2(PPh3)2-initiated ROMP of norbomene, kj/kp = 9 (see Table 11.4). Reaction of Ru(=CHPh)(Cl)2(PCy3)2 with excess ethene gives Ru(==CH2)(Cl)2(PCy3)2, the first methylidene complex which has been isolated and shown to be an active metathesis catalyst (Schwab 1995, 1996). 

Cyclobutene derivatives (66) have been synthesized from a diyne and an alkene via a novel Au(I)-catalysed reaction. A highly active vinylidene intermediate (67), formed by a dual Au(I)-mediated activation of the diyne precursor, is believed to act as an alkylidene Au(I)-carbenoid to effect stereospecific cyclopropanation of the alkene the resulting methylenecyclopropane (68) converts to (66) via an Au(I)-catalysed ring-expansion cascade. 

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