Alkylidenes rotation

EXSY experiments were performed at -87 °C (186 K) to determine the rate of exchange between the cis and trans stereoisomers of metallacycles 26a-c and 27a-c (Table 8.2). These rate constants incorporate the rates of metallacycle cycloreversion, alkylidene rotation, and cycloaddition to interconvert the two species. Given that propene, 1-butene, and 1-hexene are all Type I olefins for cross-metathesis [42], it is not unexpected that these results correlate with each other. 

In virtually all other W(CHR)(NAr)(OR )2 complexes only the syn alkylidene ro-tamer is observed readily [63]. It was not clear at the time why rotamers could be observed in this particular case and why they interconverted readily. Later it was shown that the reactivities of certain syn and anti species could differ by many orders of magnitude and that the rates of their interconversion also could differ by many orders of magnitude as OR was changed from O-t-Bu to OC-Me(CF3)2. Therefore in any system of this general type two different alkylidene rotamers could be accessible (although both may not be observable), either by rotation about the M=C bond, or as a consequence of the metathesis reaction itself. The presence of syn and anti rotamers further complicates the metathesis reaction at a molecular level, and at least in ROMP reactions (see below) in important ways. The apparent ease of interconversion of syn and anti rotamers in phenoxide complexes could be an important feature of systems in which access to both syn and anti rotamers must be assured (see later). 

A pathway that can lead to loss of specificity involves hydronation of the vinyl ligand at the carbon atom remote from the metal. This gives rise to an alkylidene species and consequent formation of a carbon-carbon single bond. Of course, dehydronation can occur to regenerate the vinyl species, but free rotation about this carbon-carbon single bond in the alkylidene would be expected to lead to a mixture of cis- and irans-vinyl species. Surprisingly, this need not be the case. 

In this mechanism the destruction of the stereospecificity can only come about if the second protonation occurs at the carbon atom remote from the metal. This will result in the formation of an alkylidene and even if deprotonation can occur, the rapid rotation about the C-C single bond would result in the loss of stereospecificity. 

As mentioned earlier, initiator 2 has rotational isomers in which the alkylidene substituent is turned either towards the second multiple bond (5y ) or away from it anti)-, eqn. (11). In this case the equilibrium position is very much in favour of the syn rotamer K= 1450 in toluene at 25°C), making it difficult to detect the anti rotamer in routine spectra. However, the equilibrium can be displaced by UV irradiation (366 nm) of the solution for several hours at — 80°C to yield a mixture containing about 33% of the anti rotamer as determined from the resonances syn, 6 12.11,7ch = 120.3 Hz anti, 5 13.30, Jch= 153.3 Hz (Oskam 1992, 1993a). On adding 0.33 equivalents of 2,3-bis(trifluoromethyl)norbomadiene to this solution and running the spectrum again at — 80°C it is found that the anti rotamer has been completely consumed, giving the syn first-addition product, eqn. (12), while the syn rotamer has scarcely reacted at all. It is estimated that the... 

In spite of the successful detection of 3a by its mmw spectrum, we have not as yet been able to record its FTIR spectrum. While the FTIR technique is certainly less sensitive than both TDL and pure rotational experiments, the wide and continuous scan range makes FTIR spectra extremely valuable. FTIR spectroscopy has, however, only been applied in very few cases to study transient molecules.Recently, we have been able to observe both the high-resolution FTIR and mmw rotational spectra of short-lived monomeric chalcogenophosphines FP=0 (14a) and FP=S (14b).It is instructive to compare effective lifetimes of alkylidene- and chalcogenosilanes with those of corresponding transient phosphines (Scheme 6.4). 

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