1400 J . Org. Chem., Vol. 43, No. 7, 1978
Battistini e t al.
Nucleophilic Step of Ring-Opening Reactions of Cyclopropanes with Electrophiles. Electronic Substituent Effects on Stereoselectivity of Reactions of Some l-Arylbicyclo[4.1.0]heptanes with Mercuric Salts C. Battistini, P. Crotti, B. Macchia, and F. Macchia* Istituti di Chimica Organica e Chimica Farnaceutica, Uniuersitci d i Pisa, 56100 Pisa, Italy C. H. DePuy Department of Chemistry, Uniuersity of Colorado, Boulder, Colorado 80302 Receiued July 19, I977 The stereochemistry of the nucleophilic step of the ring-opening reactions of 1-(p-tolyl)-(lb) and 1-(m-chlorophenyl)bicyclo[4.1.0]heptane( I C ) with mercuric salts has been investigated and compared with that of the corresponding phenylcyclopropane (la) in order to verify how a substituent on the phenyl of la, modifying the conjugative ability of the aromatic system, could influence the stereochemical results. The mercuration reactions of l b and I C have a behavior parallel to that of la: the stereoselectivity changes markedly with the salt and with the reaction conditions; however, all the percentages of syn products arising from the reactions of IC and most of those obtained in the ring opening of l b are slightly lower than the corresponding values obtained for the unsubstituted cyclopropane la. Possible explanations of the observed stereochemical results have been given on the basis of a mechanism, modified from that previously suggested for la, implying intermediate structures with a high degree of carbocationic character. T h e ring opening of cyclopropanes by electrophiles occurs in the direction of the more stable carbocation with either retention or inversion of configuration a t the site of the electrophilic attack, depending on the nature and configuration of the ring substituents, whereas the stereochemistry of the nucleophilic step takes place with complete or strongly predominant inversion of configuration.1-4 Recently it has been shown5 t h a t the ring opening of phenylcyclopropane la with mercuric salts occurs, according to e ~ p e c t a t i o n , ~ , *by ~ ~attack ~6 of the electrophile on the least-substituted carbon, in the direction of the benzylic carbon. However, the stereochemistry of the nucleophilic step was highly variable, ranging from almost complete inversion t o markedly predominant retention of configuration depending on the type of mercuric salt and on the s01vent.~T h e results ~ b t a i n e d ,in~ agreement with kinetic results of the mercuration of arylcyclopropanes,6pointed to transition states or intermediates with a high degree of positive charge on the benzylic carbon. In order to justify the results a mechanism was suggested (see Scheme I ) j analogous to the one previously postulated to rationalize the similar stereochemical behavior of the acid-catalyzed ring opening of 1 - a r y l ~ x i r a n e sAttack .~ Scheme I a
1"9
\
a, Ar
(y CHzOH
9
x2
Ar
3 =
Chart Aa
'OCWH 11
1 10
ti
8
C,HI; b. Ar =p-CH,C,H,; c, Ar = m-ClC,H,
0022-3263/78/1943-1400$01.00/0
-
A
r
CH,OH
,H9..
H
7
of the mercury (as HgX# on the least-hindered c a r b ~ n l ~ ~ ~ ~ of la leads t o a corner-mercurated intermediate which can evolve through an incipient carbenium ion (like 2) t o selectively interacting ion-nucleophile pairs (3 for reaction in nonnucleophilic solvents and 4 for reactions in nucleophilic solvents). According t o the mechanistic scheme p r ~ p o s e d , ~ the anti adducts 5 or 6 should have arisen by attack of the nucleophile (X- or S O H ) a t the stage of the incipient carbenium ion 2, whereas collapse of the ion-nucleophile pairs should have afforded the syn compounds 7 or 8. Evidently in such a mechanism every factor affecting the relative stability of structures 2,3, or 4 and their reactivity should also modify the syn/anti ratio; factors favoring the development of the positive charge on the benzylic carbon should increase this ratio. I t was therefore of interest t o study how a substituent on
%I 1978 American Chemical Society
e CH,OTs
12
I
J. Org. Chem., Vol. 43, No. 7, 1978 1401
1-Arylbicyclo[4.:1.O] heptanes
Table I. Stereochemistry of the Nucleophilic Step of the Mercuration of Cyclopropane 1 Cyclopropane
la b C
a b
Registry no.
Mercuric salt
Registry no.
Solvent
2415-82-9 64705-88-0 64705-89-1
Hg(OOCCH3)z
1600-27-7
HzO
Hg(OOCCF3)z
13257-51-7
Hz0
Cis (13)/trans (14) ratio 13.5:86.5" 11.2:88.8 9.2:90.8 19.5:80.5= 2 2.5W.5 15.3:84.7 22.5:77.5a 21.4:78.6 16.3:83.7 23.O:77.Oa 22.3:77.7 18.8:81.2 25.5:74.5a 18.k81.9 23.4:76.6 28.5:7 1.5 22.9:77.1 28.3:71.7 58.0:42.0" 59.k40.9 52.3:47.7 75.O:25.Oa 67.3:32.7 74.0:26.0
C
a b C
a b C
a b
THF-H20 (1:l)
C
a b
THF-HzO (1:l)
C
a b
CHzClz
C
a b
CHZC12
C a
Reference 5.
the phenyl of la, modifying the conjugative ability of the aromatic system, could influence the stereochemistry of the nucleophi!ic step of the cyclopropane ring opening with mercuric salts. T h e present investigation deals with t h e synthesis a n d the study of the mercuration reactions of cyclopropanes l b and IC.T h e p - m e t h y l in lb, because of its electron-donating properties, should stabilize a benzylic electron-deficient center,TC,swhereas the overall electron-withdrawing effect of the m-chloro group in IC should have an opposite Cyclopropanes l b and IChave been obtained by the Simmons-Smith reaction of the corresponding olefins 15b and 15c and have been purified by treatment with ozone followed by chromatography. Analogously to the unsubstituted cyclopropane la,5 the hydroxymercuration of l b and I C with mercuric acetate and mercuric trifluoroacetate in water yielded mixtures of the two corresponding organomercurials cis-8 a n d trans-6, in which the latter predominated (see below) and was obtained in a pure state by crystallization. Reductive demercuriation of the trans organomercurials 6b and 6c (X = OOCCH3 and OOCCF3) gave the corresponding practically pure trans alcohols 14b and 14c. T h e diastereoisomeric cis alcohols 13b and 13c have been prepared in a pure form through unequivocal stereospecific syntheses. T h e reaction of 2-hydroxymethylcyclohexanone (9) with an excess of the suitable arylmagnesium bromide afforded mixtures consisting mainly of the cis diols 10b and l0clo accompanied by small amounts of their corresponding trans isomers, from which the former were obtained by crystallization. T h e diols 10b and 1Oc were transformed into their corresponding primary monotosylates 12b and 1212, which on reduction with LiAlH4 afforded the alcohols 13b and 13c having the same relative configuration as t h e starting diols 12b and 12c. Pure alcohol 13b has been also obtained by t h e reaction of 2methylcyclohexanone with p -tolylmagnesium bromide, followed by column chromatography; in this reaction the trans isomer 14b is practically absent (
