Effect of Double-Bond Substituents on the Rate of Cyclization of α‑Carbomethoxyhex-5-enyl Radicals

Rate constants have been calculated, and compared with experimental results, for the cyclizations of 1-carbomethoxy-1-methyl-5-hexenyl radicals (<b>2</b>) with various substituents on C6. The calculations have been done by DFT at the B3LYP/6-311++G** level of theory. They show considerable interaction between C5 and the radical centers even in the ground state of all of the radicals <b>2</b>. Experimentally, the radicals have been generated by H^• transfer to the corresponding acrylate esters <b>1</b> and the yields of cyclized products compared to the calculated rate constants. (The “cyclized products” include those from cyclohydrogenation, <b>4</b>, and those from cycloisomerization, <b>9</b>.) Two phenyl substituents on C6 (<b>2i</b>), or a phenyl and a methyl substituent (<b>2g</b>, <b>2h</b>), increase the rate of cyclization, but a <i>single</i> phenyl substituent on C6 produces a <i>greater</i> increase. The calculations show that the two phenyl substituents are twisted in the transition state for cyclization, while a single phenyl substituent remains flat in that transition state. A methyl substituent on C6 along with a single phenyl causes the phenyl to twist in the transition state and decreases the rate constant for cyclization below that of the H/Ph-substituted <b>2e</b>, <b>2f</b>

Kinetics and Mechanism of the Hydrogenation of the CpCr(CO)₃^•/[CpCr(CO)₃]₂ Equilibrium to CpCr(CO)₃H

The kinetics of the hydrogenation of 2 CpCr­(CO)₃^•/[CpCr­(CO)₃]₂ to CpCr­(CO)₃H has been investigated. The reaction is second-order in Cr and first-order in H₂, with a rate constant (if the rate law is written with [CpCr­(CO)₃^•]²) of 12(2) M^–2 s^–1 at 25 °C in benzene. DFT calculations rule out a <i>side-on</i> H₂ complex as an intermediate and suggest either (1) <i>homolytic</i> cleavage via a collinear Cr–H–H–Cr transition state or (2) <i>end-on</i> approach of H₂ to one Cr as charge is transferred to the other, followed by <i>heterolytic</i> cleavage of the coordinated H₂ between the first Cr and the O of a carbonyl ligand on the second Cr, and eventual isomerization of the resulting O-protonated intermediate to CpCr­(CO)₃H