THE KINETICS OF GRIGNARD REAGENT ADDITION TO A CYCLOPROPENE

The kinetics of the addition of various Grignard reagents to spiro{2.4}hept-1-ene were studied in tetrahydrofuran at 35.47(DEGREES)C. The addition of diethylmagnesium was syn and yielded, upon hydrolysis, 1-ethylspiro{2.4}heptane as the major product. The reaction was first-order in each reagent and gave a second-order rate constant of 1.20 x 10('5) M('-1) s('-1). Other reactions such as the metallation of the cyclopropene and the formation of high-molecular-weight products were also observed, but were considerably less important than the addition reaction yielding 1-ethylspiro{2.4}heptane.;The rate of addition of pure ethylmagnesium bromide in tetrahydrofuran was calculated to be about 2.17 x 10('-5) M('-1) s('-1), approximately twice the rate with pure diethylmagnesium in tetrahydrofuran.;Iron(III) 2,4-pentanedionate increased the rate of addition of diethylmagnesium and ethylmagnesium bromide. Also additional products (spiro{2.4}heptane and 1-ethylspiro{2.4}hept-1-ene) were tentatively identified when iron was purposefully added to kinetic solutions. To avoid these problems, special equipment and techniques were designed to transfer and contain air-sensitive solutions while avoiding transition-metal contamination.;A polar 4-centered transition state is proposed to account for the bimolecular reaction and the syn addition of the Grignard reagent. The interaction of an empty p orbital of magnesium with a cyclopropene double bond is proposed as an early step along the reaction coordinate for the polar process. Although such a complex was not observed between dimethylmagnesium and spiro{2.4}hept-1-ene, its binding strength might be weak or at low concentrations or both and might not be observed by spectral means.;Second-order reactions were assumed for all remaining studies. The rates of addition of dimethylmagnesium and diisopropylmagnesium to spiro{2.4}hept-1-ene were 1.16 x 10('-6) M('-1) s('-1) and 3.67 x 10('-5) M('-1) s('-1). Thus the rate ratios for R(,2)Mg were 1:10:32 for R = Me, Et, and i-Pr.;A radical mechanism is proposed for the iron-catalyzed reaction of Grignard reagents with cyclopropenes.;The redistribution of diethylmagnesium and dicyclopropylmagnesium to ethylcyclopropylmagnesium was found to be fast and its equilibrium constant was near the statistical value of four.