Six- vs Seven-Membered Ring Formation from the 1-Bicyclo[4.1.0]heptanylmethyl Radical:  Synthetic and ab Initio Studies

Abstract

The viability of utilizing 1-bicyclo[4.1.0]heptanylmethyl radical (3) to serve as a progenitor of seven-membered carbocycles was examined. Rate constants for the rearrangement of this radical to 3-methylenecycloheptyl radical (4) and 2-methylenecyclohexyl-1-methyl radical (6) were measured using the competition method of 3 with thiophenol over the temperature range of −75 to 59 °C. Arrhenius functions were calculated for the conversions of 3 to 4 and 3 to 6 and found to be log(k/s-1) = (12.38 ± 0.20) − (5.63 ± 0.23)/θ and log(k/s-1) = (11.54 ± 0.32) − (5.26 ± 0.37)/θ, respectively. The rate constants for these conversions at 25 °C are 1.86 × 108 s-1 and 5.11 × 107 s-1, respectively. Hence, the seven-membered ring-expanded carbocycle is formed 3.6 times faster at 25 °C than the nonexpanded species. This suggests that the 1-bicyclo[4.1.0]heptanylmethyl radical system may be synthetically useful in seven-membered ring-forming methodology. Preliminary theoretical examination of this radical system qualitatively predicted the experimentally determined energies of activation: PMP4/6-31G*//HF/6-31G* ΔE a (3 → 6 − 3 → 4) = 3.0 kcal/mol with zero point energy correction. The HF/6-31G* optimized reaction coordinate stationary points suggest cyclopropyl substituent eclipsing interactions play an important role in determining the kinetic outcome of these rearrangements.

Relative Stability for the Structural Isomers of Producing Fused-Ring Systems from Acyclic Substrates : Ab Initio Study

“…While, in the 8-membered ring, the acetals with an allylsilane group only cyclized to methylcycloctene. Our results for the relative stability are in line with the experimental [1][2][3][4][5][6][7][8][9][10] and theoretical [12][13][14][15][16] results. Conclusionally, the relative potential energies of the 6-and 7-membered cyclic products with an exocyclic double bond are similar to those of the methylcycloalkene products with a double bond on the cyclic frame.…”

Cleavage of cyclopropyl ketones mediated by alkylmercury(ii) hydridesA preliminary account of part of this work was presented at the 12th National Symposium of Organic Chemistry (XII SINAQO), Córdoba, Argentina, November 1999. Abstract published in Molecules (online computer file), 2000, 5, 447.Electronic supplementary information (ESI) available: UHF/6-31G**-calculated structures, spin-density surfaces, cartesian coordinates, total atomic spin densities and Fermi-contact data for simplified models of radi

“…Ab initio calculations (UHF/6-31G**) 13 on a simplified model of radical 18 (derived from 6a) using Fermi contact analysis data as a gauge of the molecular-spin distribution 3 show, as expected, most of the radical character at C-11 (unpaired-spin value of 0.306 au, Fig. 1a).…”

“…Thus the bond that will be preferentially cleaved is determined by the lowest-energy transition state in which maximum overlap occurs. 3,12 For ketones 1a and 3a, the intermediate radicals (16 and 17) have their radical center exo to the ring system. The increased conformational mobility of the radical-bearing center around the C-17-C-20 bond allows SOMO overlap with either the β-cyclopropyl bond orbitals.…”

“…Several methods have been applied to the cyclopropylcarbinyl homoallyl radical transformation and a detailed study has been published. 3 In the case of cyclopropyl ketones, many examples of radical addition and electrontransfer reactions have been documented, most of them generating an organometallic oxycyclopropyl radical intermediate. 4 We considered the possibility of generating an acetoxycarbinyl radical next to a cyclopropane ring from a cyclopropyl ketone under the mild conditions of the organomercury chemistry developed by Giese, 5 by mercuriation of the hydrazone with Hg(OAc) 2 -HgO (to give the α-substituted organomercury salt) followed by reduction to the alkylmercury() hydride with a hydrogen donor like Bu 3 SnH or NaBH 4 .…”

Synthesis, Structure, and Physicochemical Characteristics of Thiols

“…The homolysis of the SH bond in thiols can be performed by photochemical [19,33], radiation [19], thermal [19,31], and chemical [19,34] effect. The chemical procedure is the most widely applied.…”