Syn/anti rotamers

An ab initio calculation of vibrational wavenumbers was reported for fluoro-fluoroxydioxirane, (7), and other CF2O3 isomers. Matrix IR spectra were obtained for syn and anti rotamers of trifluoromethyl fluoroformate, CF30C(0)F, and syn,syn and syn,anti rotamers of perfluorodimethyl carbonate, (CF30)2C0. The assignments were helped by ab initio calculations, e.g. vC=0 1893, 1916 cm respectively for the former pair, and 1858, 1870 cm for the latter pair. ... 

One of the most important early findings, although the significance was not realized at the time, was that both syn and anti rotamers of W(CHSiMe3)(NAr) (OAr)2 could be observed (Eq. 13) and that they interconvert on the NMR time scale (AG =12 kcal mob1). (The alkylidene is required to lie in the N-W-C plane in order to form a W=C it bond.)... 

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). 

It should be noted that this adduct is chiral and that the opposite enantiomer would result upon addition of the phosphine to the other CNO face, which corresponds basically to addition of the phosphine to the other side of the M=C bond. Predictably, the base is bound most strongly when the alkoxide is electron-withdrawing (e.g., OCMe(CF3)2), and is more strongly bound to W than to Mo. A relatively stable CNO adduct of the syn rotamer forms first when OR is OCMe(CF3)2, as it is virtually the only rotamer present (Kei-103), and as was shown later (see below), the rate of conversion of the syn to the anti rotamer... 

Molybdenum catalysts that contain enantiomerically pure diolates are prime targets for asymmetric RCM (ARCM). Enantiomerically pure molybdenum catalysts have been prepared that contain a tartrate-based diolate [86], a binaph-tholate [87], or a diolate derived from a traris-1,2-disubstituted cyclopentane [89, 90], as mentioned in an earlier section. A catalyst that contains the diolate derived from a traris-1,2-disubstituted cyclopentane has been employed in an attempt to form cyclic alkenes asymmetrically via kinetic resolution (inter alia) of substrates A and B (Eqs. 45,46) where OR is acetate or a siloxide [89,90]. Reactions taken to -50% consumption yielded unreacted substrate that had an ee between 20% and 40%. When A (OR=acetate) was taken to 90% conversion, the ee of residual A was 84%. The relatively low enantioselectivity might be ascribed to the slow interconversion of syn and anti rotamers of the intermediates or to the relatively floppy nature of the diolate that forms a pseudo nine-membered ring containing the metal. 

N-Nitroso compounds have been found to exist as syn and anti rotamers [30, 31] due to restricted rotation of the N-N bond resulting from nitrogen lone-pair delocalization (Fig. 3.2). This delocalization causes the hydrogens at the a-carbons to become acidic as evident by their base-catalyzed reactions, such as exchange with deuterium... 

Intermediate molybdacyclobutane complexes have also been detected in the reactions of 7 with 21-24115. Only in the case of 21 is the ultimate product a long-chain polymer, but in all cases one may observe, at 0-60 °C, a clean first-order rearrangement of the initial metallacyclobutane complex to the first metal carbene adduct, consisting of an equilibrium mixture of syn and anti rotamers in the ratio 9 1 (see below). Except in the case of 21, the metal carbene complexes do not survive for very long. For 21, however, ROMP is propagated, and distinct H NMR signals are seen for the longer-chain metal carbene complexes in both syn and anti forms. 

This is interpreted in terms of a mechanism involving an equilibrium between the syn and anti rotamers of the initiator, in which the conversion of the dominant syn rotamer into the minor anti rotamer is rate-determining at high [M], with it = 6.1 x 10 5 s 1, while at low [M] the addition of M to the anti rotamer becomes rate-determining305. The value of k agrees well with the value determined directly in toluene by photochemical displacement of the equilibrium between the rotamers121,122. The tacticity of this polymer is mentioned in Section VIILA.5. 

A marked temperature dependence of the cis content of the polymer formed from 219 using 224 as initiator in THF (100% at — 35 °C, 24% at 60 °C) has been interpreted in terms of an equilibrium between syn and anti rotamers, with cis C=C formed mainly by addition to the THF-free syn rotamer, and trans C=C formed mainly by addition to the THF-free anti rotamer556 cf Section III.B.5. 

The reaction of 219 with 7 (R = Me) proceeds mainly via the less abundant but much more reactive anti rotamer of the initiating and propagating species. The first four propagating species may be distiguished in the H NMR spectrum for the longer chains the anti and syn rotamers are just resolved, in the ratio of 1 6. The corresponding 13C... 

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