Bridged species equilibrating

Cotton has discussed this phenomenon extensively (37). For complementary sets of more than two carbonyl bridges, the equilibration is also probably concerted. All of the carbonyls in Rh4(CO)u appear magnetically equivalent at room temperature, with bridge terminal interconversion occurring via the formation of an unbridged species whose structure is analogous to Ir4(CO)12 (39). 

Space does not permit a detailed discussion of the entire topic. Fortunately, a full discussion has recently appeared11. Consequently, the present review will consider only the 2-norbornyl system. Is it better represented as an equilibrating pair of classical cations equivalent to the Meerwein proposal1, or is it better represented as a a-bridged species equivalent to the Wilson proposal2 ... 

This is a fascinating hypothesis. It possesses the advantages of simplicity in replacing two or more equilibrating classical cations by a single o-bridged species. 

Indeed, many examples are now known of such rapidly equilibrating carbocations under stable ion conditions (see Table 13.1, Ref.11)). The question to be resolved is whether the behavior of the 2-norbornyl cation under solvolytic conditions is best interpreted in terms of such a pair of rapidly equilibrating classical carbocations or ionpairs, or as the stabilized a-bridged species. 

In these early studies equilibration was not correlated with an independent measure of ion pair return. If initial ionization leads to an ion pair whose cationic fragment is either an achiral bridged species or a rapidly equilibrating pair of enantiomers,... 

The optically active exo-products from the exo-amine may result either from a chiral norbornyl cation, but, more likely, as emphasized by Collins (1975), by asymmetry in the diazonium-carboxylate ion pairs, which give carbocation-carboxylate ion pairs (see Scheme 7-17 in Sect. 7.3). The authors emphasized, however, that these product studies cannot distinguish a rapidly equilibrating and highly exo-selective classical norbornyl cation from the bridged species 7.107 and 7.108 as intermediates (Scheme 7-35). 

With Fourier transform capability, it is possible to observe every carbon in the system temperature dependence yields information on bridged vs. equilibrating nature of species. Difficulty may arise in calculating chemical shifts. Ji3c H allows calculation of hybridization. 

Reactions of acyclic derivatives with carbon electrophiles have also been examined.33,34 An illustrative reaction involving methylation of the unsubstituted complex [MnCr 4-butadiene)(CO)3], (19), is shown in Scheme 16. Again, the reaction is presumed to occur via a methylmanganese species (20) and after methyl migration the unsaturated metal center is stabilized by formation of a Mn—H—C bridge (isomers 21a and 21b). Deprotonation of equilibrating (21a and 21b) yields the [Mn(l-methylbutadiene)(CO>3]-complex (22), which has exclusively trans stereochemistry.34 This sequence represents alkylation of the terminal carbon of butadiene and complements the iron carbonyl chemistry, where terminal acylation has been achieved as described above. Unpublished results indicate that a second methylation of (22) occurs... 

The unsymmetrically bridged ions 506 would be undistinguishable from the symmetrically bridged system 504 in solution NMR experiments. (However, see subsequent discussion of solid-state low-temperature 13C NMR as well as ESCA studies.) It is important to recognize that equilibrating open classical cations 505 cannot explain the NMR data and thus cannot be involved as populated species. 

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