Electronic metallacyclobutane intermediates

Crowe proposed that benzylidene 6 would be stabilised, relative to alkylidene 8, by conjugation of the a-aryl substituent with the electron-rich metal-carbon bond. Formation of metallacyclobutane 10, rather than 9, should then be favoured by the smaller size and greater nucleophilicity of an incoming alkyl-substituted alkene. Electron-deficient alkyl-substituents would stabilise the competing alkylidene 8, leading to increased production of the self-metathesis product. The high trans selectivity observed was attributed to the greater stability of a fra s- ,p-disubstituted metallacyclobutane intermediate. 

For a cis alkene to be formed the reaction would have to proceed through a czs-a,p-disubstituted metallacyclobutane intermediate (cis isomer of 10). Although it was unclear why there was a preference for forming a cis metallacycle, which leads to the thermodynamically less stable product, it was probably related to the small size or the electron-withdrawing properties of the nitrile group. 

Electronic Structure of Metallacyclobutane Intermediates for the Molecular Catalysts... 

The transition state for metallacyclobutane formation is lower in energy for the second-generation Grubbs catalysts due to increased stabilization of the active conformation of the olefin complex. In addition, the metallacyclobutane intermediate is further stabilized in the second-generation Grubbs system by a combination of steric and electronic effects. 

If the metal already has 18 valence electrons, which is the case for all the Fischer carbenes, the electrophilic carbene carbon is attacked by the olefin to develop a zwitterionic intermediate before ring closure.From the 18-electron metallacyclobutane, p-elimination, that requires a free coordination site, cannot occur. Among the above reactions, only the metathesis and reductive elimination of the metallacyclobutane to cyclopropane ean be observed, as in the two following examples ... 

The retention of stereoselectivity can be explained by considering the energy levels of the two possible metallacycles leading to cis or trans isomers. The ligands of the precursor complexes do not govern the stereoselectivity simply by their own steric requirement, but also by their electronic effects on the stabihties of the metallacyclobutane intermediates produced. 

The possibility of being involved in olefin metathesis is one of the most important properties of Fischer carbene complexes. [2+2] Cycloaddition between the electron-rich alkene 11 and the carbene complex 12 leads to the intermediate metallacyclobutane 13, which undergoes [2+2] cycloreversion to give a new carbene complex 15 and a new alkene 14 [19]. The (methoxy)phenylcar-benetungsten complex is less reactive in this mode than the corresponding chromium and molybdenum analogs (Scheme 3). 

The metal-carbenoid intermediates, especially ones derived from a-diazocarbonyl compounds, are electrophilic, and electron-rich olefins in general react more easily with the carbenoid intermediates than electron-deficient olefins. For the interaction of metal carbenoid and olefin, three different mechanisms have been proposed, based on the stereochemistry of the reactions and the reactivity of the substrates (Figure 12) 21 (i) a nonconcerted, two-step process via a metallacyclobutane 226,264... 

Over the past 15 years the understanding of the mechanism of these reactions has been greatly enhanced through the preparation of metal carbene complexes, particularly of Mo, W and Ru, that are both electronically unsaturated (<18e) and coordinatively unsaturated (usually <6 ligands), and which can act directly as initiators of olefin metathesis reactions. The intermediate metallacyclobutane complexes can also occasionally be observed. Furthermore, certain metallacyclobutane complexes can be used as initiators. 

The reactions between an alkylidene and a terminal olefin are routinely employed to prepare new alkylldenes, usually from a neopentylidene or neophylidene. In all such reactions, the intermediate that leads to the new alkylidene, an a,a -disubstituted metallacyclobutane (Eq. (1.6)), must be formed, but it is rarely stable enough to be observed. However, a 14-electron Mo vinylalkylidene MAP complex, sy -Mo(NAr)(CHCH=CMe2)(Me2Pyr)(OHMT), was successfully prepared and isolated by treating Mo(NAr)(CH-f-Bu)(Me2Pyr)(OHMT) with excess4-methyl-l,3-pentadiene [14]. 

In order to improve the catalytic performance of ruthenium-based metathesis complexes, extensive theoretical [9,10] and experimental [11,12] studies were conducted. The generally accepted mechanism involves the formation of a highly active and unstable 14-electron complex through the dissociation of one of the phosphine ligands (Scheme 11.1) [13-17]. This actual catalytic species allows the binding of incoming olefin to form the Chauvin metallacyclobutane [18] intermediate and eventually the reaction products. 

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