Masayoshi Oku

Solvolytic studies of tricyclo[4.2.0.02,4]octan-5-yl derivatives. Effect of cyclopropane and cyclobutane orbital alignments on reactivity and proclivity for rearrangement

The kinetics of hydrolysis of the four possible stereoisomeric tricyclo(4.2.0.02~ 4)octan-5-yl3,5-dinitroben- zoates and p-nitrobenzoates in 80 aqueous alcohol have been studied at several temperatures. A striking feature of the present results is the convergence of both pairs of isomers to two related but noninterconverting cations (51 and 52), only the latter of which is capable of cyclobutane ring opening or ring expansion for stereoelectronic rea- sons. Both la,2&4&6a-dinitrobenzoates gave identical product mixtures (after 10 half-lives) consisting of exo-cis- bicyclo(4.2.0)oct-2-en-5-ol (21a) and its in ternally returned ester 21b. Ionization in the la,Za,4a,6a series led to formation of the epimeric cis-bicyclo(3.3.0)oct-2-en-6-ols (26 and 27, 52-58 z), endo-cis-bicyclo(4.2.0)oct-2-en-5-ol (28, 33-41 %), and the alcohols of retained tricyclic structure (17 and 20, 7-973. The formation of the (4.2.0) bicyclic alcohols with hydroxyl orientation inextricably linked to a specific precursor geometry implicates a rigid cation geometry and highly directed discharge by solvent. Studies of the products obtained from hydrolysis of deuterium labeled ester 42 showed the absence of deuterium scrambling and excluded the operation of cyclopro- pylcarbinyl-cyclopropylcarbinyl rearrangement. Carbocationic leakage of the homoallylic -+ allylic type was also not in evidence. A connection is diaMiil beiweeii various i3.1.01 bicyclic systems and the reactivity features of these molecules are discussed. Further consideration is given the general questions of bisected bishomoallylic cat- ion intervention and of cyclobutane neighboring group involvement. tudy of the solvolytic reactions of cyclopropylcar- S binyl derivatives has over the years provided strong evidence for very large conjugative interaction between a three-membered ring and an adjacent sp2-hybridized carbon. In systems which are conformationally flexible, adoption of the bisected rather than perpen- dicular geometry usually obtain~.~ In more rigid mole- cules such as the endo-1- and exo-2-substituted bicyclo- (3.1 .O)hexanes (2),4 the situation is less clear. Accel- erated solvolytic behavior in accord with anchimeric assistance is seen in the two isomers, but the rates and products of reaction in both instances are essentially identical. This behavior was not entirely anticipated since initial conversion of 1 to 3 and of 2 to 4 was ex- pected by analogy. Although the initial intervention of these different delocalized cations is not presently disc~unted,~~ the data require that the 1 - 3 and 2 - 4 reactions have near comparable activation energies and that rapid equilibration of both to 5 occur prior to sol- vent attack. Alternatively, 1 and 2 could experience

Direct 1,5-cycloaddition of sulphur dioxide to cyclo-octatetraene. 9-Thiabarbaralane 9,9-dioxide

Reaction of cyclo-octatetraene with antimony pentafluoride in liquid sulphur dioxide leads directly to 9-thiatricyclo[3,3,1,02,8]nona-3,6-diene 9,9-dioxide by unprecedented 1,5 cycloaddition; 9-thiabicyclo[4,2,1]nona-2,4,7-triene 9,9-dioxide is formed concomitantly and is the thermodynamically more stable C8H8O2S isomer.