Deuterium isotope effects carbon-alkene reactions

Reaction of atomic carbon with alkenes generally involves both DBA and vinyl C—H insertion. An interesting example is the reaction of C atoms with styrene in which the major products are phenylallene (21) and indene (22). The synthesis of a number of specifically deuterated styrenes and the measurement of the deuterium isotope effects on the 21/22 ratio led to the conclusion that 21 was formed by DBA followed by ring expansion and by C—H(D) insertion into and followed by rearrangement of the resultant frawi-vinylcarbene (23). The indene was formed by C—H(D) insertion into Xb followed by cyclization of the resultant cw-vinylcarbene (24) (Eq. 18). An examination of the product ratios and their label distributions when atoms are used leads to the conclusion that the ratio of C=C addition to C—H insertion is 0.72 1 in this case. 

If there is only one step, the reaction has to be second order first order in the peracid and first order in the alkene. The reaction rate has very little dependence upon the solvent, supporting a concerted mechanism with little charge developing at the transition state. The small charge development is also supported by the fact that the rates correlate with a Hammett parameter a ), but the p value is only -1.1 for p-XArCH=CH2. There are only small primary kinetic isotope effects. Values of Ath/Ato around 1.1 to 1.2 are found for the peracid [ROiHfD)]. This means that the hydrogen atom transfer shown in the electron pushing of Scheme 10.5 has to be either minimal at the transition state or almost complete (see Section 8.1.2). Secondary deuterium isotope effects on the alkene carbons are inverse, as may be expected for an sp- to sp transformation. 

This reaction has an isotope effect in that deuterium-carbon bonds are broken more slowly than C-H bonds. The order of reactivity of alkyl halides toward E2 elimination is 3 > 2 > 1 . It reflects the relative stabilities of the alkenes being formed. 

Solvent effects on, and products from, reaction of styrene with ethylene in the presence of di-)ti-chloro-dichlorobis(styrene)dipalladium(n), [Pd-(Ph CH—CH2)Cl2]2, indicate a mechanism similar to (i)->(iv) above, with the addition of a preliminary equilibrium between the dimer and solvated monomers. The mechanism of reaction of styrene with vinyl compounds, catalysed by the same chloride-bridged dipalladium complex, has been studied using isotopic tracer (H, D) experiments. Palladium-acetate-catalysed reaction of styrene with benzene, also investigated using deuterium tracer experiments, involves no hydride shift, in contrast to the rather closely related Wacker process. The importance of intermediates with palladium-carbon n-bonds in palladium(ii)-catalysed alkylation and arylation of alkenes has been demonstrated. 

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