Modeling the 1,3-dipolar cycloaddition of nitrones to vinylboranes in competition with boration, cyclization, and oxidation reactions

Structures and energetics of reactants, reactant complexes, concerted trans ition structures, and products of the cycloaddition of the prototypical nit rone with vinylborane have been produced and discussed. Structure optimizat ions have been performed at the B3LYP/6-31G(d) and B3LYP/AUG-cc-pVDZ levels of approximation, and single-point calculations on the B3LYP geometries ha ve been carried out at the MP4(SDTQ) level with the same basis sets. Kineti c contributions to standard enthalpies, entropies, and free enthalpies have been computed at the same levels of geometry optimizations. The effects of methyl and chloro substitution on the BH2 group and of methyl substitution on the vinyl moiety has been also explicitly considered. The most striking theoretical features of this cycloaddition are (i) the formation of reacta nt complexes where the nitrone oxygen is strictly bound up to the boron ato m (B . . .O interactions), (ii) their persistence in the endo/exo transitio n structures, and (iii) energy profiles suggesting very high reaction rates , regiospecificity (5-borylisoxazolidines) and complete endo-stereoselectiv ity. The BH2 (BX2) substituent appears to induce a sort of intramolecular c atalysis which is also largely selective in favor of the endo reaction path . Possible competitive reaction paths such as cyclization, organoboration, and oxidation have equally been investigated, on the same grounds, both wit h prototypical reagents and with dimethylvinylborane, dichlorovinylborane, 2-methyl-1-propenylborane, and 2-methyl-1-propenyl-dichloroborane. The tran sition structures for these reaction paths are significantly higher in ener gy than those of the corresponding 1,3-dipolar cycloadditions in the sequen ce oxidation much greater than cyclization > boration > cycloaddition, wher eas the resulting reaction products show the reversed sequence. Polar solve nts appear to increase the competition of boration although maintaining its character of secondary reaction. As expected, the reaction rate of 1,3-dip olar cycloaddition is lowered by dimethyl substitution on the vinyl CH2 rea cting center (i.e., for the reaction of 2-methyl-1-propenylborane and 2-met hyl-1-propenyldichloroborane) whereas the reaction rate of boration is incr eased, the boration results being significantly competitive even in the gas phase. Experiments for the control of the above predictions are not yet av ailable.