Cyclobutyl tosylates, solvolysis

Although under solvolytic conditions carbocations are formed only as short-lived (transient) species, it is of interest to discuss here some recent work involving substituted C4H7+ cations which are relevant to the general discussions in the review. Wiberg and coworkers have carried out extensive studies on solvolysis reactions involving 3-substituted cyclobutyl tosylates 12 (equation 18), and rationalized the observed products by ab initio molecular orbital calculations at the MP2/6-31G level46,47. 

They have postulated that the solvolysis of 3-substituted cyclobutyl tosylates 12 (X = Alkyl, Ar, Cl, OEt, SiR 3) proceeds through the initial formation of bicyclobutonium ion in the rate determining step, which rearranges stereospecifically to the cyclopropyl-... 

A set of rapidly equilibrating carbenium ions might account for the rearrangements and the label scrambling but this cannot be the correct explanation, for cyclopropylcarbinyl, cyclobutyl, and allylcarbinyl systems all solvolyze much more rapidly than would be expected from model compounds. Thus, for example, the rate of solvolysis of cyclopropylcarbinyl tosylate is 10 times that of the solvent-assisted solvolysis of isobutyl tosylate.77 Cyclobutyl tosylate solvolyzes 11 times... 

Coates, R. M., and W. H. Robinson Solvolysis of /ra/i5-2,2-Dimethyl-3-(2 -methyl-propenyl)-cyclobutyl Tosylate. Model Reactions Relevant to Squalene Biosynthesis. J. Amer. Chem. Soc. 94, 5920 (1972). 

Furthermore, radiolabeling studies have revealed that the solvolysis of cyclopropylcarbinyl diazonium (the first reactant given in Eq. 11.42) results in partial scrambling of the carbon attached to the diazonium group into all of the carbons of all three products. Hence, there must be an intermediate where several of the carbons become equivalent. Another facet to this puzzle is that cyclopropylcarbinyl, cyclobutyl, and homoallyl derivatives all undergo solvolysis faster than analogous structures. For example, cyclopropylcarbinyl to-sylate undergoes solvolysis approximately 10 times faster than isobutyl tosylate. 

A sequence of Cu(I)-catalyzed [2 + 2]-photocycloaddition-cyclobutyl carbinyl cation rearrangement has been employed for the construction of novel carbocyclic compounds. Thus, cycloaddition of the dienol 40 to the hydroxymethyl cyclobutane 41 followed by solvolysis of its tosylate 62 afforded the triquinane 63 after reduction (Scheme 19). An efficient route to ex -l,2-polymethylene-7-hydroxy norbornanes 66 has also been developed based on this protocol. Photocycloaddition of an epimeric mixture of the dienols 64 to 2-hydroxybicyclo[3.2.0]heptanes 65 followed by treatment with trifluoro-acetic acid (TFA) affords the 7-hydroxynorbomanes 66 in overall good yields. 

It is significant that trans-cyc obuty tosylate 298-OTs is much more reactive in solvolysis than its cis isomer (k trans l ds = 250) (380, 381) as is commonly the case with 3-substituted cyclobutyl sulfonates (76). The... 

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