The cation radical vinylcyclopropane→cyclopentene rearrangement: reaction mechanism and periselectivity

Tcrmhedron Lcrreo. Vol. 36. No. 41. pp. 14%7410.

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The Cation

Radical VinylcyclopropaneAIyclopentene Rearrangement: Reaction Mechanism and Periselectivity J. P. Dinnocenzo* and D. A. Cordon Department of Chemistry University of Rochester Rochester, NY 14627-0216

Abstract: The vinylcyclopropane+cyciopentene rearrangement catalyzed by oneelectron oxidation has been shown to be most consistent with a stepwise, cation radical mechanism. It was also discovered that the format rearrangement periselectivity of vinylcyclopropanes with cis-alkyl substituents can be changed by one-electron oxidation.

The rearrangement of vinylcyclopropanes to cyclopentenes has been shown to be gready accelerated by triarylaminium ion cata1ysis.t Current evidence favors a cation radical mechanism but the precise origin of the rate enhancement is not clear. Mechanistically, it might be explained by a odd-electron pericyclic process or by a stepwise mechanism. We describe herein experimental evidence that distinguishes between these two possibilities. In addition, we demonstrate that ont-elcctron oxidation can alter the formal perkselectivity of vinylcyclopropane rearrangements. This permits ring expansion reactions that are difficult to achieve by putely thermal means. Finally. we introduce two alternate catalytic methods for promoting rearrangement that provide further evidence for the intermediacy of cation radicals and provide variants that should be useful for the preparative utilization of these reactions. To distinguish between pericyclic and stepwise ring expansion mechanisms, the rearrangement stereochemistry was investigated by using vinylcyclopropanes 1 and 2. These compounds were prepared from ketones 3 and 4 using the ylid derived from P.Pdiethyldibenzophospholium 1 or 2 with 17 mol% p-CIPhsNt cyclopentenes 5 and 6 (7.oM.2: P-h

T-

H

X

iodide.2 Rearrangement of either

gbF6- in acetonitrile at 20 “C for 15 set gave the same mixtun of

I, 80%

CH,

overall yield).3

Control experiments showed that the identical product

H

CH3

W

P-h I

CH3

1: X = E-CHCH, 3:x=0

p-An

?

X

CH3

2: X = E-CHCH3 4:x=0

7415

HI

‘0:

CH3 CH3

5: trans 6: cis

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ratios obtained from 1 and 2 were not due to their prior isomerization. nor due to subsequent isomerixation of S or 6. The former

was

excluded by examining the reaction at partial conversion which revealed that the ring

expansion was 8- 12 times faster than interconversion of cyclopropanes 1 and 2. The latter was excluded by showing that cyclopentenes 5 and 6 were stable to the reaction conditions. Essentially identical stereochemical results were obtained in acetonitrile when vinylcyclopropanes 1 and 2 were isomerized using 10 mol% Fe( 1, IO-phenanthroline)33+ (PF,j-)3 as the catalyst.4 In this case the 56

ratio was 6.8d~O.4:I. Fe(phen)33+ is a more traditional outer-sphere oxidant and provides further evidence that the aminium ion salts are functioning as one-electron oxidants. The iron salt has the added benefit that it simplifies product isolation, since simple dilution of the reaction mixture with ether followed by aqueous extraction removes the reduced form of the oxidant from the organic phase. Finally, ring expansion of vinylcyclopropanes 1 and 2 under photoinduced electron transfer conditions5 also provided cyclopentenes 5 and 6. Photolysis of acetonitrile solutions of 1 or 2 (102 M) containing 1,4dicyanonaphthalene (I@ M) as the electron acceptor in a Rayonet reactor equipped with sixteen 350 nm bulbs for ca. 10 min. with or without biphenyl cosensitization,Sa provided 5 and 6 in a ratio of 6.7H.5: I (80% yield). The combined stereochemical experiments are most consistent with a s~epwise. cation radical ring expansion mechanism. The fact that the ring expansions of vinylcyclopropanes 1 and 2 are faster than their interconversion further requires that the ring openings of It and 2t be rate determining. Therefore the origin of the rate acceleration of the cation radical ring expansion reactions must be due to lower activation barriers for C-C bond cleavage. This is consistent with thermochemical data which reveal that the Cl-C2 one-electron bond energies in It and 2t are ca. 23 kcal/moi lower than in their neutral molecules.6 In addition to resulting in large rate accelerations for ring expansion, we have found that one-electron oxidation can change the formal periselectivity of vinylcyclopropane rearrangements. It is well known that the thermolysis of vinylcyclopropanes with alkyl groups cis to the vinyl group results in the formation of I ,Cdienes (via a retro-ene reaction) rather than ring expansion to fotm cyclopentenes.as9 We have found that one-electron oxidation can reverse this selectivity. Thus, thermolysis of vinylcyclopropane 7*O(cis/trans mixture) at 160 “C for 6h provided predominantly the expected diene 9 along with a minor amount of cyclopentene 8 (92:8). In contrast, treatment of 7 with 15 mol% of Fe(phen)$+ for IO min at 60 “C in acetonitrile provided cyclopentene 8 as the exclusive product (86%). Similar results were obtained with aminium ion catalysis and by photoinduced electron transfer.

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The formal rearrangement periselectivity of 7t can be rationalized within the framework of a stepwise ring expansion mechanism.

Ring-opening of 7t gives a distonic cation radical intermediate, HI?, which

prefers to ring close to St instead of forming 9t. This selectivity may have a stereoelectronic origin. As depicted in

ll?, the transition state for hydrogen atom abstraction to form 9? may be unfavorable due to poor

orbital overlap of the C-H bond with the adjacent carbocationic center.

In summary, the vinylcyclopropane+cyclopentene

rearrangements catalyzed by triarylaminium

ions, by

Fe(phen)ss+ salts, and by excited state electron acceptors are most consistent with a stepwise. cation radical mechanism.1 t The relative ease of the cation radical rearrangement can be explained by the much weaker oneelectron bonds in the cyclopropane cation radicals. Finally, one-electron oxidation can change the formal periselectivity of rearrangement. This provides a new method for ring expanding vinylcyclopropanes that normally undergo retro-ene reactions upon thermolysis. Acknowledgment.

We are grateful to Prof. E. Vedcjs (Univ. Wisconsin) for suggestions regarding the

preparation of 1 and 2. Research support was provided by the National Science Foundation (CHE-9312460) and by the donors of the Petroleum Research Foundation, administered by the American Chemical Society.

References

and

Notes J. P.; Conlon, D. A. 1. Am. Chem. Sot. 1988. I IO, 2324.

(1)

Dinnocenzo,

(2)

(a) Vedejs, E.; Marth, C. F.; Ruggeri, R. J. Am. Chem. Sec. 1988,110,

3940. (b) Vedejs, E.;

Marth. C. F. J. Am. Chem. Sot. 1988, I IO, 3948.

(3)

Like Eberson,3ab we have found that salts of the more commonly used triarylaminium

ion, p-BrPhsN?.

have a limited shelf life. In contrast, p-CIPhsN? forms much more stable salts.3C (a) Eberson, L; Larsson. B. Acfu Chcm. Sand..

Ser. B 1986,840.210.

Scan&

(c) Rowland, R. G. Ph.D. Dissertation, University

Ser. B 1987. B41, 367.

Rochester, NY.

1989.

(b) Eberson, L; Larsson. B. Acru Chem. of Rochester,

7418

(4)

Prepared according

to:

Schlesener.

C.; Kochi, J. K. /. Am. Chem. Sm. 1984,106,

C. J.; Amatore.

3567. (5)

For

general references see: (a) Mattes, S. L.; Farid. S. In “Organic Photochemistry”;

Marcel

Dekker:

New York.

425. (c) Kavarnos,

1983; Vol. 6, p 233.

N. J. Chem. Rev. 1986. 86.401.

G. J.; Turro,

Padwa, A., Ed.;

(b) Juthard, M.; Chanon. M. Chem.

1983.83,

Rev.

J. Synthesis

(d) Mattay,

1989,

233. (6)

The difference

in the Cl-C2

bond dissociation

from the equation, ABDE = BDE(neutral) oxidation

oxidation

potential

The oxidation

potentials

by the

radical, -0.02 V vs. SCE (D. D. M. Wayner,

AESDE= 23.3and 22.6 kcal/mol for 1 and 2, respectively.

(Et,) of 1 (0.99V) and 2 (0.96 V) were obtained by cyclic voltammetry

mV/sec) at a platinum disk electrode in acetonitrile hexafluorophosphate

where EP is the

potential of the trimethylene biradical

bond in 1 or 2. The latter was approximated

of the I-(4-methoxyphenyl)ethyl Accordingly,

ABDE, were calculated

radical) = EP - E,&I’B),

is the oxidation

cleavage of the Cl-C2

private communication). (7)

- BDE(cation

potential of 1 or 2’ and E&B)

produced by homolytic

energies of l(2) and lt(2?),

as the supporting electrolyte

(150

with ca. 0.1 M tetra-n-butylammonium and were irreversible.

The potentials were referenced

to internal ferrocene (0.3 I V vs. SCE). (8)

For reviews see: (a) Ramaiah, M. Synthesis 1984, 529. M. Org. React. 1985,A?, 247. (d)

(c) Goldschmidt.

Wong, H. N. C.; Hon. M.-Y.;

Tse, C.-W.;

(b) Hudlicky,

2.; Crammer,

T.; Kutchan.

T. M.; Naqvi.

S.

B. Chem. Sec. Rev. 1988.17, 229.

Yip, Y.-C.; Tanko. J.; Hudlicky,

T. Gem.

Rev.

1989.89, 165. (9)

Only under conditions achieved.

where the retro-ene reaction is reversible can ring expansion sometimes be

The experimental

(a) Roth, W. R.; K&rig. A. F. Tefrrrhe&on

conditions

1. Justus Lie&s

Prepared by heating the tosylhydraxone

(11)

Interestingly,

I, I .3.3-tctramcthylguanidine the closely r&ted

T.; Koszyk.

ofp-mcthoxyacetophcnonc

28. (b) Jorgenson. F. J. Tetrahedron

Cf.

M. J.; Thacher.

Lett. 1980. 2487.

with 2,3-dimcthyl-1,3-butadienc

in

(72 “C. 18h). cation radical vinylcyclobutane-icyclohexene

shown to occur in a stereoselective 6. K.; Bauld.

however, are usually scvcre (e.g. 600 “C).

Ann. Chem. 1965.688.

Lerr. 1969.4651. (c) Hudlicky.

(10)

H.-S.; Marsh,

for reversibility.

manner in several casts (Reynolds,

N. L. 1. Ploys. Org. Chem. 1989,2.

rearrangement has been

D. W.; Harirchian.

57).

(Received in USA 3 August 1995; revised 1I August 1995; accepted 14 August 1995)

B.; Chiou.