1‐[(E)‐2‐Arylethenyl]‐2,2‐diphenylcyclopropanes: Kinetics and Mechanism of Rearrangement to Cyclopentenes

Abstract

Dedicated to Paul von Ragué Schleyer, friend and mentor, on the occasion of his 80th birthday Kinetic measurements for the thermal rearrangement of 2,2-diphenyl-1-[(E)-styryl]cyclopropane (22a) to 3,4,4-triphenylcyclopent-1-ene (23a) in decalin furnished DH = isom ¼ 31.0 AE 1.2 kcal mol À1 and DS = isom ¼ À 6.0 AE 2.6 e.u. The lowering of DH = by 20 kcal mol À1, compared with the rearrangement of the vinylcyclopropane parent, is ascribed to the stabilization of a transition structure (TS) with allylic diradical character. The racemization of (þ)-(S)-22a proceeds with DH = rac ¼ 28.2 AE 0.8 kcal mol À1and DS = rac ¼ À 5 AE 2 e.u., and is at 1508 106 times faster than the rearrangement. Seven further 1-(2-arylethenyl)-2,2-diphenylcyclopropanes 22, (E)-and (Z)-isomers, were synthesized and characterized. The (E)-compounds showed only modest substituent influence in their k rac (at 119.48) and k isom (at 159.38) values. The lack of solvent dependence of rate opposes charge separation in the TS, but a linear relation of log k rac with log p.r.f., i.e., partial rate factors of radical phenylations of ArH, agrees with a diradical TS. The ring-opening of the preponderant s-trans-conformation of 22 gives rise to the 1-exophenylallyl radical 26 that bears the diphenylethyl radical in 3-exo-position, and is responsible for racemization. The 1-exo-3-endo-substituted allylic diradical 27 arises from the minor s-gaucheconformation of 22 and is capable of closing the three-or the five-membered ring, 22 or 23, respectively. The discussion centers on the question whether the allylic diradical is an intermediate or merely a TS. Quantum-chemical calculations by Houk et al. (1997) for the parent vinylcyclopropane reveal the lack of an intermediate. Can the conjugation of the allylic diradical with three Ph groups carve the well of an intermediate?

The Rearrangement of 2,2‐Diphenyl‐1‐[(E)‐2‐phenylethenyl]cyclopropane to 3,4,4‐Triphenylcyclopent‐1‐ene: a DFT Analysis

“…The introduction of an (E)-2-phenylethenyl group into the three-membered ring of 10 makes the geminal phenyl groups in 1 different, their twist angles, a(cis) and a(trans) included; 1 occurs in enantiomers [15]. The trans-Ph group has more rotational freedom than cis-Ph, and a(trans) is larger than a(cis) in s-trans-1 and s-gauche-1 ( Table 5 and Scheme 3).…”

“…Results and Discussion. To compare the results of calculation with the crystal structure of the starting material, Table 1 lists relevant bond lengths, bond angles, and dihedral angles of s-trans-1T (Scheme 1) and compares them with the data from the X-ray structure [15], which is characterized by the T-arrangement of the two Ph groups at C(2) 1 ). The general agreement is a check on the reliability of the calculations.…”

“…In our preceding publication [15], experimental details were given for the kinetics and mechanism of rearrangement of 2-[(E)-2-arylethenyl]-1,1-diphenylcyclopropanes to 4-aryl-3,3-diphenylcyclopentenes.…”

“…Interestingly, the X-ray-analysis of 8 [15] revealed two crystallographically independent conformations, both of which are trans-8. Possibly, packing forces in the crystal overcompensate the small calculated difference in DG of cis-8 and trans-8.…”

Ground and excited-state electron-transfer reactions of a few (anthracen-9-yl)methanamines: a comparative study

“…(Anthracen-9-yl)methanamines 1a-k were prepared by Leukart and Leukart-Wallach reactions. 15,35 (Anthracen-9-yl)methanamines 1a, 52,53 1b, 53 1c, 53,54 1d 53 and 1g 53 are known compounds. Newly synthesized compounds 1e,f,h-k were identified on the basis of spectroscopic and analytical data.…”