5632 in the tertiary system due to the conformation of the aryl ring [for example, see N. L. Allinger and M. T. Tribble, Tetrahedron Lett.,3259 (1971)], or some other factor. (13) D. J. Raber. R. C. Bingham, J. M. Harris, J. L. Fry, and P. v. R. Schleyer, J. Am. Chem. SOC.,92, 5977 (1970);J. K. Fry, C. L. Lancelot, L. K. M. Lam, J. M. Harris, R. C. Bingharn. D. J. Raber, R . E. Hall, and P. v. R. Schleyer, ibid., 92, 2538 (1970); J. M. Fry, J. M. Harris, R. C. Bingham, and P. v. R. Schlever, ibid.. 92. 2542 (1970). (a) F. G.Bordwell, P. F. W'iley, and T. G. Mecca, J. Am. Chem. Soc.,97, 132 (1975); (b) A. Miotti and A. Fava, ibid., 88, 4274 (1966). S.Winstein, 6. K. Morse, E. Grunwald, K. C. Shreiber, and J. Corse, J. Am. Chem. SOC., 74, 1113 (1952). S.Winstein. C. R. Lindearen, H. Marshall, and L. L. ingraham, J. Am. Chem. Soc.. 75. 147 (1953). For a.review, see P. k. Story and B. C. Clark in "Carbonium Ions" (ref 5c), Chapter 23. E. N. Peters and H. C. Brown, J. Am. Chem. Soc., 97, 7454 (1975). (a)P. D. Bartlettand M. R. Rice, J. Org. Chem., 28, 3351 (1963); (b) 6. A. Hess, Jr., J. Am. Chem. SOC., 93, 1000 (1971). S. Winstein, H. Walborsky, and K. C. Schreiber, J. Am. Chem. Soc., 72, 5795 (1950). E. N. Peters and H. C. Brown, J. Am. Chem. SOC.,94, 7920 (1972). E. N. Peters and H. C. Brown, J. Am. Chem. SOC., 95, 2398 (1973). H. C. Brown and E. N. Peters, J. Am. Chem. Soc.. 97, 7442 (1975). E. N. Peters and H. C. Brown, J. Am. Chem. SOC., 94, 5899 (1972j. (a) H. C. Brown, E. N. Peters, and M. Ravindranathan, J. Am. Cbem. SOC., 97,2900 (1975);(b) R. A. Sneen, hid., 80,3982 (1958);(c) P. G. Gassrnan and D. S. Patton, bid., 91, 2160 (1969). (26) H. C. Brown, E. N. Peters, and M. Ravindranathan, J. Am. Chem. Soc., 97, 7449 (1975).
(27) H. C. Brown, S. Ikegarni, and K.-T. Liu, J. Am. Chem. Soc., 91, 5909 (1969). (26) H. C. Brownand K.-T. Liu, J. Am. Chem. SOC.,91, 5911 (1969). (29) H. C. Brown and E. N. Peters, J. Am. Chem. Soc., 95, 2400 (1973). (30) For recent reviews, see (a) H. G. Richey, Jr., in "Carbonium Ions" (ref 5c), Chapter 25; (b) K. 6. Wiberg. B. A. Hess, Jr., and A. J. Ashe. ibid., Chapter 26. (31) E. N. Peters and H. C. Brown, J. Am. Chem. SOC., 95, 2397 (1973). (32) (a)J. E. Baldwin and W. D. Foglesong, J. Am. Chern. Soc., 90, 4303 (1968); (b) E. C. Friedrich. M. A. Saleh, and S.Winstein, J. Org. Chem., 38, 860 (1973). (33) J. D. Roberts and R. H. Mazur, J. Am. Chem. SOC.,73,3542 (1951);R. H. Mazur et al., ibid., 81, 4390 (1959). (34) Data for 8 taken from H. C. Brown and G. Ham, J. Am. Chem.Soc., 78,2735 (1956),and corrected for difference in solvent and leaving group (i.e., 7.42 x 10-13 S-1). (35) For example, 'H NMR data are consistent with clasicial I-phenylcyclobutyl cation; see G. A. Olah, C. L. Jeuell, D. P. Kelly, and R. D. Porter, J. Am. Chem. Soc., 94, 146 (1972). (36) H. C. Brown, Acc. Chem. Res., 8, 377 (1973); (b) H. C. Brown and E. N. Peters, Roc. Natl. Acad. Sci. U.S.A., 71, 132 (1974). (37) H. C. Brown, M. Ravindranathan, K. Takeuchi, and E. N. Peters, J. Am. Chem. Soc., 97, 2899 (1975). (38) S. Winstein and D. Trifan, J. Am. Chem. Soc.,74, 1154 (1952). (39) S.Winstein, E. Clippinger, R. Howe, and E. Vogelfanger, J. Am. chem. Soc.,, 87 376 1965) (40) (a{H. L. L e r i n g and J. V. Clevenger, J. Am. Chem. Soc., 94, 1010 (1972); (b) H. L. Goering and K. Humski, ibid., 90,6213 (1968). (c) For benzhydryl pnitrobenzoates, see H. L. Goering and H. Hopf, ibid., 93, 1224 (1971). (41) A. Fentiman, Jr., Ph.D. Thesis, The Ohio State University, 1969.
Vinylic Cations from Solvolysis. 24. Degenerate ,&Aryl Rearrangements during the Solvolysis of 1,2-Dianisyl-2-phenylvinylBromides and Trianisylvinyl Bromide. Free Vinyl Cations as Intermediates Yoram Houminer, Etta Noy, and Zvi RapPoport* Contributionf r o m the Department of Organic Chemistry, The Hebrew University, Jerusalem, Israel. Received December 31, 1975
Abstract: The degenerate 0-phenyl rearrangement in the 1,2-dianisyl-2-phenylvinylcation (8) and the P-anisyl rearrangement A In AcOH, in the trianisylvinyl cation (13) were studied by using the precursor bromides labeled by B - C D ~ O C ~ Hgroups. Me3CCOOH, or aqueous EtOH, 8 does not rearrange at all or only to a small extent, while 53 f 6% of P-phenyl rearrangement was found in TFE. Ion 13 shows 11.5,35,0, and 100% P-anisyl rearrangement in 60% EtOH, AcOH, MejCCOOH, and TFE, respectively, and it can be captured by Br- before rearrangement in AcOH and TFE. It is suggested that the rearrangement proceeds via free open vinyl cations, whose selectivity is not due to a rapid migration of the @-arylgroup between a pair of degenerate vinyl cations. The direction and extent of rearrangement in phenyl- and anisyl-substituted triarylvinyl cations are determined by the better charge dispersal ability of the anisyl group, either in the ground or transition state. Low nucleophilicity and high dissociation power of the solvent favor the rearrangement, making T F E the best medium for these reactions. Relative rate constants for reactions of triarylvinyl cations (capture by Br- ( k g , ) , capture by the solvent ( ~ s o H )0-phenyl , migration (kr(ph)). and 0-anisyl migration ( k r ( ~ " ) )were ) evaluated. E.g., k g , (1 M [ B T - ] ) : k r ( ~ n ) : k=~ 78:25:1 ~~ for 13 in TFE, and k s r ( 1 M [ B r - ] ) : k ~ ~ ~ : k ~= ( p21:1:=c/
An
SOH
SOH
Ph-C=C\
6a
-
-
+
An-C=C
sa
7a
Ph,
5*
10.Br
An
/Ph
Ph-C=C\
An
An/
An
\c=c’
SOH _t
6*
5
An d
(2)
\Ph
C=C-Ph
4-Br /Ph
/Ph*
An-C=C
Ph*\
Products
2*
2
Ph* / Ph \C=C An/ ‘Br 3-Br
SOH
\Ph
1-OTf
An
/Ph
+*
Ph-C=C
Ph’
OTf
An
-Ph
‘Br 12-Br
An d
An* \C=b-An / An 13 OTf = OSO,CF,
P-anisyl-@-phenylderivatives. There are four possible 8-anisyl rearrangements and four P-phenyl rearrangements, and two of each type are degenerate rearrangements. A study of the extents of these reactions in different solvents will be useful in elucidating the nature of the rearrangement reaction. All the reactions of Scheme I were investigated either by Lee et aL8 or by US.^.^ However, while systems 1,3,4, and 6 were studied extensively, the degenerate rearrangements of the 1,2-dianisyl-2-phenylvinyl(lO-Br, 11-Br) and the trianisylvinyl (12-Br) bromides were studied by Oka and Lee only in carboxylic acid media,xc.dand additional evidence concerning the nature of the intermediates is required. W e then used a different labeling pattern and studied these two systems in a variety of solvents in an attempt to answer five questions: (a) What is the nature of the cationoid intermediates in these reactions, and is there a correlation between the nature of the a substituent and the type of the cationoid intermediate in rearrangement across the double bond? (b) Is the selectivity of triarylvinyl cation^^.^ due to a combination of steric and electronic effects which operate in “static” vinyl cations, or is there a “dynamic” equilibrium between two degenerate cations that accounts for this selectivity? (c) What are the effects of the a- and P-aryl groups, the migrating group, and the solvent on the extent of the rearrangement? (d) What are the relative reactivities of these vinyl cations in rearrangement, capture by the solvent, and external ionreturn processes?
-An
*
f
w An-C=C,
/An
‘An
SOH
products
(9)
13* SOH = Solvent
(e) Can the heterolysis rate in nonsolvolytic media be evaluated from the extent of the degenerate rearrangement in these media? In addition, we wanted to evaluate the usefulness of the mass spectral and the proton N M R analytic methods for the study of degenerate rearrangements in the triarylvinyl systems.
Results For the degenerate rearrangements we used compounds 10-Br, 11-Br, and 12-Br labeled by a 8-p-(trideuteriomethoxy)phenyl group. The synthetic pathways are given in Scheme 11. p-Hydroxy-a-p-methoxyphenylacetophenone was obtained by condensation of p-methoxyphenylacetic acid with phenol in polyphosphoric acid. This method gives only a 7% yield, but it has an advantage over the demethylation of deoxyani~oin,~ which produces and requires separation from the isomeric a-p-hydroxyphenyl-p-methoxyacetophenone. The other reactions follow the usual pattern of preparation of triarylvinyl bromide^.^ Stereospecific labeling is not expected in the formation of labeled trianisylvinyl bromide. Its N M R showed three methoxy signals in a 1:2:1ratio, indicating the formation of a 1:l mixture of the two isomeric bromides, where the larger signal corresponds to the a-anisyl group; and the mixture was designated 12-Br. A mixture of 10-Br and 11-Br was obtained, and complete separation by repeated crystallization could not be achieved on our preparative scale. A 5: 1 mixture of 10-Br-11-Br was mostly used. Solvolysis and Rearrangement of 2-p-(TrideuteriomHouminer, Noy, Rappoport
/ Vinylic Cations from Solvolysis
5634 Scheme I1 AnCH,COOH
+ PhOH 3
CD IIEtO-
AnCH,COCGH,OH-p 3 EtOH
AnCH,COAn*
AnMgBrJ
OH
I
YMgBr OH I
AnCH?&(Ph)An *
AnCH,&An)An*
1
1HJQ
Hm4
AnCH =C(An)An
*
AnCH=C(Ph)An*
1
Br,/CCI,
An
\c=c
*/ An
/C=C
Ph
+
An* \ /C=C\
/An
An*\
/An \&
Ph
/-,=I
An
/An
*/\C=C
Br
An-
12.Br
‘ Br
10-Br
\.Br 11-Br
0
An*CH(Ph)-C-An 14
AnCH(Ph1- C14*
II
An*
AnCOPh 15a
15
Mass spectral analysis was conducted on the crude reaction mixture. Metastable peak analysis on the precursor ion corresponding to 14-14* showed that the daughter peaks are the anisoyl and the p-methoxybenzhydryl signals. W e expected to get the same extent of rearrangement from the pairs of ions a t m/e 135, 138 and 197,200 (eq 10). However, different results were obtained from each of these pairs, as well as from the signals a t m/e 212 and 215, which correspond to the ions derived from 15 and 15a. Moreover, the signal ratio in each pair was strongly dependent on the fragmentation temperature (cf. Table VI in the Experimental Section). AnCOCH(An* )Ph
An*COCH(An)Ph
+ i? i AnCO’ m/e
135
+J?j An*zHPh An*ChPh 200
215
/An ‘OCH2CH, lO-OCH,CH,
Ph/
0
AnC6Ph
An*CO+ An6HPh
a2
138
T
19; (10)
We attribute this discrepancy to two reasons. The small peaks observed for 15 and 15a may be formed either from the Journal of the American Chemical Society
/
98:18
/OCH,CH,
‘C=C
ethoxy)phenyl-1-p-methoxypheny1-2-phenylvinyIBromides (IO-Br and 11-Br). (a) In 60% EtOH. A 5:l mixture of 10Br-11-Br was kept in 60% E t O H buffered by 2,6-lutidine for 17 h at 125 O C . The only products obtained were the ketones 1 4 and 14*. From the 96:4 ratio of the methoxy signals a t 6 3.70 (for 14) to that a t 6 3.67 (for 14*) in CDC13 a value of 8 f 2% was calculated for the 0-phenyl rearrangement via the open free ion.I0 This is an upper limit, since the position of the 6 3.67 signal was slightly concentration dependent, and its presence might be attributed either completely or partially to the methoxy signal of a small amount of p-methoxybenzophenones 15 and 15a which may be formed during the solvolysis.
An*COPh
A$,
An*
An* = p-CD,OC,H,-
II
cleavage of 14-14* during the reaction or the mass spectral cleavage of 14-14*. A metastable ion spectra of the precursor ion a t m/e 2 12 (15a) shows daughter peaks a t m/e 135 (main peak) and 105. Hence, the ions a t m/e 135 and 138 may be formed from 15-15a, thus invalidating any conclusion based on these fragments, as demonstrated in eq 10. The analysis of the benzhydryl signals seems a priori more reliable, but the temperature dependence indicates complications with these peaks too. The mass spectra of 14-14* show a small fragment a t m/e 307, formed by a loss of a CO molecule. Metastable ion spectra of this precursor ion showed broad daughter peaks a t m/e ca. 273 ( M - MeO, M - OCD3), 230 (An(An*)CH+), and 197-200 (An(Ph)CH+ and An*(Ph)CH+). The intensity of the fragment at m/e 230 greatly exceeds that of m/e 197200 and this cleavage apparently contributes little to the overall formation of the p-methoxybenzhydryl peak. Hence, the value based on the benzhydryl fragment should be considered as an upper limit for the extent of the rearrangement. (b) In 80%EtOH. Solvolysis under the same conditions in 80% EtOH gives the ketones 14-14* and small quantities of the ethers 10-OCH2CH3 and ll-OCHzCH3. The overlap of
An*
’ ‘C=C
Ph
/An
‘OAc IO-OAc
/ C 4 \ Ph
An 11-OCH2CH3
r,c=c\
An
,OAc
Ph
An 11-OAc
the signals of the methylene quartet and the methoxy signals of the ethers and ketones resulted in a very approximate value of 14 f 3% of 14* in the ketone fraction. The mass spectrum showed the same signals as in 60% EtOH, in addition to the base peak of the ethers a t m/e 363. From the benzhydryl signals a value of 2.0 f 0.7% for 14* in the ketone mixture was evaluated (Table VI). This is an upper limit not only for the reasons stated above, but also because fragmentation of the ethers can also contribute to these signals. (c) In AcOH. Reaction of a 5:l mixture of 10-Br-11-Br with 2 mol equiv of NaOAc in AcOH for 187 h at 120 O C gave 72% of a 1 : I mixture of the acetates 10-OAc and 11-OAc, and 28% of 14-14*. In the acetate fraction only two methoxy signals of equal intensities a t 6 3.68 and 3.70 appear in the expanded 100-MHz spectrum. Since the unlabeled cis acetate has methoxy signals at 6 3.67 and 3.68 and the unlabeled trans acetate at 6 3.70 and 3.76, 4bwe concluded that no rearrangement took place within the detection limits ( f 3 % ) . The mass spectra showed a main signal for triarylethanone a t M - 42. The relative intensities of the pairs of aroyl, substituted benzhydryl, and substituted benzophenone fragments which are apparently derived from it were strongly temperature dependent (Table VI). Based on the benzhydryl fragment, the rearranged acetates consist of 6 3 . 5 f 1% of the total vinyl acetates. However, most of these fragments may be derived from the ion a t m/e 307, which is also formed from the vinyl acetates. Indeed, this was verified by a metastable ion spectra of this ion. When a similar mixture of 10-Br and 11-Br was solvolyzed in the presence of 0.077 M Bu4NBr and 0.01 1 M NaOAc for 6 h (1.2 solvolytic half-lives) a t 120 OC, isomerization of the vinyl bromides to their equilibrium mixture (54% 10-Br:46% ll-Br)2a took place. Only the methoxy signals of the a-anisyl groups were observed a t 6 (CDC13) 3.67 (10-Br) and 3.80 (11-Br), and none of those of the 0-anisyl groups at 6 3.70 (10-Br) and 3.73 (11-Br). T h e extent of rearrangement is therefore 93% rearranged, and the vinyl bromides were 7 5 f 15% rearranged. By using data on the trifluoroethanolysis of 12-Br a t 90 O C I I and the activation energy for its solvolysis in 80% EtOH,4a we estimated that 14 min are four solvolytic half-lives a t 120 O C in the absence of common-ion rate depression. Hence, the vinyl bromide which participated in the reaction is >SO% rearranged. Solvolysis for 14 min in the presence of 0.8 M Bu4NBr gave only 6 1 5 % of the ethers. The recovered vinyl bromides were 56 f 3% rearranged. When the solvolysis was conducted under the same conditions in the presence of 3.6 M Bu4NBr, only 12-Br was detected. The data are summarized in Table I.
Journal of the American Chemical Society
/
98:18
(g) In Trifluoroacetic Acid. Solvolysis of 12-Br in CF3COOH/CF3COONa a t 50 "C gave the triarylethanones 16-16*. Mass spectral analysis according to eq 12 showed 33 f 4% of 16* in the mixture. This value corresponds to complete rearrangement via the open ion.I0
Discussion Evaluation of the Mass Spectral and the NMR Methods. In a previous work on the degenerate rearrangements of the 2anisyl- 1,2-diphenylvinyl cation,' the mass spectral analysis of the solvolysis products was shown to be a rapid, accurate, and reproducible method, and therefore more convenient than methods which require degradation of the products. The analysis was supplemented by N M R analysis. Application of the same analytical methods in the study of the degenerate rearrangements which accompany the solvolysis of 10-Br11-Br and 12-Br revealed three types of limitations which severely hinder the use of these methods for accurate determination of the extent of rearrangement: (a) side reactions in the solvolysis; (b) instability of the solvolysis products; and (c) complications due to mass spectral fragmentation patterns. (a) In basic aqueous ethanol, the triarylethanones can be cleaved during the reaction to substituted benzophenones.
%
Ar3Ar2CHCOAr' Ar3Ar2C0 The propensity for such cleavage increases with the number of anisyl groups in the ethanone. Benzophenone 17a is the main product of the long-time solvolysis when Ar' = Ar2 = Ar3 = An? while 15a is a minor product when Ar2 = Ph, Ar' = Ar3 = An,4band it is formed in negligible amounts, if a t all, when Ar2 = An, Ar' = Ar3 = Ph. This cleavage complicates the N M R analysis of methoxylabeled compounds by introducing an additional signal into the methoxy region. Acetolysis produces mainly the vinyl acetates and some triarylethanone. While the mechanism of formation of the latter is not yet clear,12 it probably occurs without rearrangement. This complicates the methoxy region of the N M R , but does not affect the mass spectral analysis, since triarylethanone is formed anyway in fragmentation of the vinyl acetates. Formation of the vinyl acetates, vinyl formates, and ketone in AcOH-HCOOH makes accurate analysis even more difficult. Formation of both the triarylethanone and vinyl ethers in 80% EtOH makes the mass spectral analysis of the mixture useless (see below) and complicates the methoxy region in the N M R analysis. These complications are not unique to our analytical methods. A radioactive determinationsa of the extent of rearrangement will not give better results when applied to mixtures of the various products. The I3C N M R method which was applied successfully for systems 10-Br- 12-Br in RCOOH8b-d will be free of these complications provided that the signals of the various products do not overlap. (b) Degenerate rearrangement of the solvolysis products in the reaction medium can hinder the determination of the extent of rearrangement via vinyl cations. Ketone 14 rearranges partially in 1:l AcOH-HCOOH to its isomer 14*. Similar acid-catalyzed ketone-ketone rearrangements are known for analogous ketones, such as labeled 1,2,2-triphenyIethanone~,'~ and possible mechanisms were discussed.14 Consequently, neither this solvent nor more acidic solvents such as CF3COOH are suitable for studying the extent of the rearrangement. Moreover, similar reservation applies for systems substituted with more anisyl groups, such as the trianisylvinyl system. We found complete scrambling in the trifluoroacetolysis of 12-Br, which leads to the trianisylethanone, and complete scrambling was previously observed in the same reaction in CF3COOH/
/ September 1, 1976
Scheme 111 Ar(Ar’)CH
+
Ar(Ar”)CH
+
-
mentation a b. While for R = Ac or C H O the fragmentation commenced via a , further fragmentation via c leads to aroyl signals of the opposite labeling pattern than those obtained via b. Moreover, cleavage e leads to scrambling of all the aryl groups prior to formation of the benzhydryl peaks, and when A r = Ar’ = Ar” it will exaggerate the extent of rearrangement. When R = Et or CH2CF3,the base peak in the mass spectra is the triarylmethane fragment. Whether it is formed via g or a and e, it renders the mass spectral method useless. The cleavage patterns of Scheme I11 were verified by metastable peak analysis on the precursor ions corresponding to the triarylethanone, triarylmethyl, and benzophenone ions which show daughter peaks corresponding to routes b-f. The temperature dependence is therefore due to competition between routes b, c, and e, which apparently have different activation energies. This complexity requires that the mass spectral method should be evaluated individually for each reaction. For each reaction of the systems studied in this work and related systems, it should be checked in regard to the temperature dependence of the fragmentation and internal consistency. In the present work it was usually found to be inferior to the N M R method. Table I1 gives the results of the extent of rearrangement based on the two methods. The more reliable value (as justified above) is given in italics. Free Vinyl Cation as a Rearrangement Intermediate. Evidence against Contribution of a Windshield Wiper Effect to the Selectivity of Triarylvinyl Cations. The high selectivities of triarylvinyl cations, manifested by the common-ion rate depression in the solvolysis of the triarylvinyl bromide^,^.^^^^ was attributed to shielding of the cationic orbital by the cy- and 0-aryl substituents in a “static” vinyl An alternative is that a very rapid degenerate &aryl migration between “degenerate” vinyl cations leads to “dynamic” equilibrium
Ar’(Ar”)CH
tf Ar(Ar’) Ar”CH
OR I Ar( Ar’)C=C--Ar’’
0
A Ar ( Ar’ )CH -C
II
-Ar
5631
”
AgOOCCF3, which leads to trianisylvinyl trifluoroacetate.sc Although a high extent of scrambling is reasonable in this low-nucleophilic medium, some of the observed scrambling may take place in the ethanone or in the sp3-hybridized ion which is formed by initial protonation of trianisylvinyl bromide or t r i f l u ~ r o a c e t a t e .Such ’ ~ protonation fits the development of a purple color when 12-Br is dissolved in CF3COOH.8C (c) The strength of the mass spectral method is the formation of two pairs of fragments, where the relative abundance of the labeled and unlabeled peaks gives directly both the extent of rearrangement and an internal consistency test for the reliability of the results. In the present work, the temperature dependence of the relative intensities within each pair of peaks and the inconsistency of the results of the two pairs in several occasions (Table VI) makes the mass spectral method only a guide for an upper limit for the extent of the rearrangement. W e ascribe this behavior to multiple fragmentation patterns (Scheme 111). The mass spectral method is based on frag-
Table 11. Degenerate Rearrangements during the Solvolysis of Triarylvinyl Bromide@ % rearrangementb
Compd 10-Br- 11-Br (5:l)
Base 2,6-Lutidine
80% ErOH
2,6-Lutidine
115
28
AcOH AcOH
NaOAc NaOAc
120 120
< 3c -
AcOH Me,CCOOH
125
Bu,NBr AgOAc
from NMR 8 t 2 i
6
2,6-Lutidine NaOAc NaOAc
120 95 95
53 i 6 68 f 4 56 i 4
60% EtOH AcOH
NaOAc NaOAc
120 120
11.5 i 2
AcOH
NaOAc
AcOH 1:1 AcOH-HCOOH
CF ,COOH CF,COOH Me,CCOOH TF E TF E MeCN
AgOAc
Reflux
NaOAc NaOCOCF, AgOCOCF, NaOCOCMe, 2,6-Lutidine 2,6-Lutidine 2,6-Lutidine
Bu,NBrg (50 h) (160 h)
95 50 Room temp. 160 120 120 120 120
+_
1.5
kSOH’Ikr
38
i
9
4 8 r 20 265 >100e
1:l AcOH-HCOOH 1:l AcOH-HCOOH
120
4
kSOH. Since complete scrambling requires several forth and back rearrangements, it could be deduced that the windshield wiper effect might contribute to the selectivity only when k r ( ~ , ,>> ) k~,[Br-] with added high concentration of [Br-1. Under these conditions the products will be a 2:l mixture of 12-Br-I2*-Br at high [Br-1, since kef[&-] > kSOH. If the windshield wiper effect is unimportant, we expect that a t low [Br-] the firstorder rearrangement will be faster than the second-order reaction with Br-, Le., k r ( ~ , ,>) ker[Br-]. The products will then be partially rearranged (Le., 1 2 - 0 s 12*-OS) and will be accompanied by partially rearranged starting materials (12-Br 12*-Br). At high [Br-] the capture by Br- will be dominant, Le., ker[Br-] > kr(An) > kSOH, and only the unrearranged starting material 12-Br will be recovered. Table I shows that complete and nearly complete rearranged trifluoroethyl ethers were obtained in T F E after 8 h and 14 min, respectively. However, the vinyl bromide is regenerated in the presence of a large excess of [Br-1, as expected from the extensive common-ion rate depression observed for 12-Br in A C O H ~and ~ .TFE.” ~ The extent of rearrangement in the vinyl bromide decreases on increasing [Br-1, until only nonrearranged 12-Br is regenerated in the presence of 3.6 M [Br-I! The condition ker[Br-] >> kr(~,,)> k s o is~ therefore fulfilled and the selectivity of the ions is not due to a windshield wiper effect. The similar retardation of the rearrangement by Brin AcOH leads to a similar conclusion concerning the reaction in AcOH. The absence or the low extent of anisyl migration in the ion
+
+
Journal of the American Chemical Society
1
hriAn)
J. k.0,
+
hriAnj
e An--“6=CAnZ
8 in most solvents, combined with the formation of ca. 1:l cis-trans p r o d ~ c t s , * ,excludes ~ ~ . ~ ~ the windshield wiper effect for 8 in most of these solvents. Such a n effect is not unequivocally excluded in TFE, where the rearrangement is considerable, but it is excluded by analogy with the behavior of the ion 13. The suppression of the rearrangement by the addition of the common bromide ion is a clear and strong evidence that the reaction proceeds via the free triarylvinyl cation in both T F E and AcOH. Additional strong evidence for the intermediacy of an open ion in the acetolysis of trianisylvinyl bromide arises from comparison of our data with that of Oka and Lee.8c Our labeling in the anisyl group leads to scrambling of the three anisyl groups a t complete rearrangement via the free ion (Scheme IV). In Oka and Lee’s experiment triani~ylvinyl-2-~~C bromide was studied and the two vinylic carbons were scrambled at complete rearrangement (Scheme V ) . Both experiments together amount to a “double labeling” experiment of the type used by Collins and Bonner” to establish the involvement of an “open” cation in the reactions forming the 1,2,2-triphenylethyl cation. When the steady-state treatment or an analogous treatment of Bonner and C o l l i n ~ ”is~applied to the cationic intermediates of Schemes IV and V and isotope effects and capture by Br- (which does not affect the product distribution at infinity) are neglected, the following relationships are obtained.I8 [12-os]/[12*-os] = 2[1
+ (kSOH/kr(An))l
[ 1 ~ - O S l / [ l ~ * - O S=l 1 + (kSOH/kr(An))
(15) (16)
In AcOH/AgOAc, the [ 18-OS]/ [ 18*-OS] ratio is 4.0 f 0.25,8c Le., from Scheme V kSOH/kr(An) = 3.0 f 0.25. The [ 12-OS]/ [ 12*-OS] ratio is 7.6 f 0.49, i.e., from Scheme IV kSOH/kr(An) = 2.8 f 0.25. The identity, within the experimental error, of the two values for compounds labeled differently, argues that the assumption on which Schemes IV and V are based, i.e., that free vinyl cation is the intermediate, is correct, Ease of Migration as a Function of the Solvent, the Migrating Group, the Migration Origin, and the Migration Terminus. Data concerning the different rearrangements across the double bond in phenyl- and anisyl-substituted triarylvinyl cations are now available. W e can therefore assess the effect of the solvent, the migrating group, and the groups a t the migration origin and terminus on the ease of rearrangement. The transition states for these rearrangements have structures 19-24, where 19,21,23, and 24 represent those for degenerate rearrangements, and 20 and 22 may be obtained in two ways, either when the migration origin is substituted by a phenyl or an anisyl group.
‘
Ph-
Ph ,’I +‘‘\
C=C
19
An /+‘\ An-C=C-Ph I
\
22
/ 98:18 / September 1, 1976
Ph + ‘\ An-C= C20 I ,
-Ph
I
Ph ,
i + ‘\
An-C-C-An 23
An ,‘+ ‘\ Ph -C=C-Ph I
IC
Ph
\
21
An
I \
i+ \
An-C=C-An 24
5639 Table 111. Extent of p-Aryl Rearrangements in the Solvolysis of 1,2,2-TriarylvinylSystems (A) and the Deaminations of 1,2,2-Triarylethylamines(B)Q ~
~~
Transition state (A) Solvent
TFE AcOH 60% EtOH Me,CCOOH
19
20
21
22
23
24
Ref
>13.4b
85 (0)c -0
lOOb
100 ( 0 ) C 100 100 100
53
100 35 11.5 0
1,5,8a
13.4
5 (0)C
93 89
22 in TFE, >23 in AcOH, and 3.3 of T F E as a rearrangement medium has precedents in satuin 60% EtOH for the 23,24 pair, while values of 76 (TFE) and rated systems.27Its low nucleophilicity (Le., low k S O H ) enables 120 (AcOH) were previously obtained for the 19, 21 pair.’ kr to take over in our systems. This effect is emphasized in the These values (excluding that in 60% EtOH)28 are not 144 for 12-Br. These ratios one pair of data is sufficiently close for a valid comparison.
1
1
+
+
-
-
+
Houminer, Noy, Rappoport
/ Vinylic Cationsfrom Solvolysis
5640 Table IV. Relative Rate Constants for Several Processes Involving Triarylvinyl Cations ~~
~
~~~
~
Relative rate constant Solvent
Ion
T,"C
kBra
TFE
120
91
60% EtOH
Ref
kr ( A n )
kSOH
kr(Ph)
1
5 .I
160
1
0.053
4
1, 5b
AcOH
117
1
0.053
6.3
1, 5b
TFE
120
1
0.56
This work
80% EtOH
115
1
0.021
2b, this work
AcOH
120
21
1
TFE
120
78
1
AcOH
120
21
1
An
\
2=C+ Pi
5b
-Ph
5
/ Ph
3.1
8
An
