Stereochemistry.  XLV.  Zwitterionic transition states in epimerization reactions of substituted cyclopropanes

Howe, Norman E.; Yankee, Ernest W.; Cram, Donald J.

doi:10.1021/ja00794a018

Cited by 31 publications

(6 citation statements)

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“…The further reaction at 1508 corresponds to a carbocationic S n 1-type behavior, indicating an equilibrium with the zwitterion 39 (Scheme 12). The energy well of this intermediate is deep enough for interception by MeOH [54].…”

Section: Methodsmentioning

confidence: 99%

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

Mulzer

Huisgen

Arion

et al. 2011

Helvetica Chimica Acta

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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?

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Section: Methodsmentioning

confidence: 99%

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

Mulzer

Huisgen

Arion

et al. 2011

Helvetica Chimica Acta

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“…Recent ab initio calculations studied the potential surface for stereo mutation , and isomerization to propene . Much earlier Cram and co-workers − discussed the importance of zwitterions and diradicals as intermediates of ring opening reactions of substituted cyclopropanes. This influence of the solvent polarity on the activation and rate constant was measured for epimerization reactions. , Zwitterionic transition states were deduced from a large solvent polarity dependence on the rate.…”

Section: Trimethylenesmentioning

confidence: 99%

“…Much earlier Cram and co-workers − discussed the importance of zwitterions and diradicals as intermediates of ring opening reactions of substituted cyclopropanes. This influence of the solvent polarity on the activation and rate constant was measured for epimerization reactions. , Zwitterionic transition states were deduced from a large solvent polarity dependence on the rate. In contrast, an absence of isomerization rate dependence on solvent polarity was considered as indication for a diradical transition state.…”

Section: Trimethylenesmentioning

confidence: 99%

Solvation Effects on Diradicals and Zwitterions: Tetramethylenes and Trimethylenes

Jug

Kölle

1998

J. Phys. Chem. B

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A solution model previously implemented in the semiempirical molecular orbital method SINDO1 is applied to the study of trimethylenes and tetramethylenes in different solvents. It is shown that diradical trimethylenes and tetramethylenes are polarized by the solvent. This influence of the solvent can also lead to a zwitterionic state in cases where a diradical is predicted without the solvent. Solution effects are the reason for the discrepancy between the experimentally observed polymerization mechanisms of substituted tetramethylenes and gas phase calculations. For a prediction of the correct state, a geometry optimization in solution is important.

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“…As another small strained ring, cyclopropanes were found to form either polar P3 biradicals or zwitterions, depending on subtle changes in substituents, according to Cram and coworkers 132–135. Applying these compounds as initiators, ethyl 1‐cyano‐2‐( p ‐methoxyphenyl)cyclopropanecarboxylate when heated with acrylonitrile gave free radical homopolymer,136 but with NVCz gave cationic homopolymer 137.…”

Section: Introductionmentioning

confidence: 99%

A paradigm for the mechanisms and products of spontaneous polymerizations

Hall

Padías

2009

J. Polym. Sci. A Polym. Chem.

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Stereochemistry. XLV. Zwitterionic transition states in epimerization reactions of substituted cyclopropanes

Cited by 31 publications

References 0 publications

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

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

Solvation Effects on Diradicals and Zwitterions: Tetramethylenes and Trimethylenes

A paradigm for the mechanisms and products of spontaneous polymerizations

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