organic reaction mechanisms 1966

ORGANIC REACTION MECHANISMS 1966 An annual survey covering the literature dated December 1965 through November 1966

B. CAPON University of Leicester

M. J. PERKINS King’s College, University of London

C. W. REES University of Leicester

I N T E R S C I E N C E P U B L I S H E R S a division of

John Wiley & Sons London - New York - Sydney

ORGANIC REACTION MECHANISMS - 1966

ORGANIC REACTION MECHANISMS 1966 An annual survey covering the literature dated December 1965 through November 1966

B. CAPON University of Leicester

M. J. PERKINS King’s College, University of London

C. W. REES University of Leicester

I N T E R S C I E N C E P U B L I S H E R S a division of

John Wiley & Sons London - New York - Sydney

Copyright @ 1967 by John Wiley & Sons Ltd. All rights reserved Library of Congress Catalog Card Number 66-23143

Made and printed in Great Britain by Spottiswoode, Ballantyne & Co. Ltd., London and Colchester

Preface to the 1965 Volume

This book is a survey of the work on organic reaction mechanisms published in 1965. E’or convenience, the literature dated from December 1964 to November 1965, inclusive, was actually covered. The principal aim has been to scan all the chemical literature and to summarize the progress of work on organic reaction mechanism generally and fairly uniformly, and not just on selected topics. Therefore, certain of the sections are somewhat fragmentary and all are concise. Of the 2000 or so papers which have been reported, those which seemed a t the time to be the more significant are normally described and discussed, and the remainder are listed.

Our other major aim, second only to comprehensive coverage, has been early publication since we felt that the immediate value of such a survey as this, that of “current awareness”, would diminish rapidly with time. I n this we have been fortunate to have the expert cooperation of the London office of John Wiley and Sons.

If this book proves to be generally useful, we will continue these annual surveys, and then hope that the series will have some lasting value; some form of cumulative reporting or indexing may even be desirable.

It is not easy to deal rigidly and comprehensively with so ubiquitous and fundamental a subject as reaction mechanism. Any subdivision is a necessary encumbrance and our system, exemplified by the chapter headings, has been supplemented by cross-references and by the form of the subject index. We should welcome suggestions for improvements in future volumes.

February 1966 B.C. M.J.P. C.W.R.

Contents

1. Classical and Non-classical Carbonium Ions . Bicyclic Systems . Phenonium Ions . Participation by Double and Triple Bonds. Cyclopropyl Carbonium Ions . Cationic Opening of Cyclopropane and Cyclobutane Rings Other Stable Carbonium Ions and Their Reactions

Borderline Mechanisms and Ion-pair Phenomena Solvent Effects . Neighbouring-group Participation . Isotope Effects . Deaminations and Related Reactions . Fragmentation Reactions . Displacement Reactions a t Elements other than Carbon Ambident Nucleophiles . Other Reactions .

.

. 2. Nucleophilic Aliphatic Substitution .

.

3. Electrophilic Aliphatic Substitution . 4. Elimination Reactions . 5. Addition Reactions .

Electrophilic Additions . Additions of halogens and related reactions Addition of sulphenyl halides . Hydrations and related additions . Epoxidations .

Nucleophilic Additions . Radical Additions . Diels-Alder Reactions . Other Cycloaddition Reactions. .

6. Nucleophilic-aromatic Substitution . Meisenheimer and Related Complexes . Substitution in Polyfiuoro-aromatic Compounds . Heterocyclic Systems . Diazonium Decomposition . Other Reactions . Benzyne and Related Intermediates .

.

.

. 1

. 1

. 19

. 24

. 31

. 37

. 40

. 44

. 44

. 50

. 53

. 69

. 70

. 72

. 75

. 81

. 82

. 91

. 103

. 124

. 124

. 124

. 130

. 132

. 136

. 137

. 140

. 148 , 152 . 160 . 168 . 171 . 172 . 176 . 178 . 181

’ Contelzts ... Vlll

% *

7. Radical and Electrophilic Aromatic Substitution , . 148 Radical Substitution . . 188 Electrophilic Substitution . . 193

8. Molecular Rearrangements . . 209 Aromatic Rearrangements . . 209 Cope and Related Rearrangements : Valence-bond Isomerization . 217 Intramolecular Hydrogen Migrations and Related Reactions . ,225 Radical Rearrangements. . . 229 Heterocyclic Rearrangements . . 233 Other Rearrangements . . 239

9. Radical Reactions. . 246 Radical-forming Reactions . . 246 Reactions of Free Radicals . . 256

Radical abstraction and displacement processes . . 256 Oxygen radicals . . 263 Nitrogen radicals . . 264 Nitroxide radicals . . 265 Radical anions and cations . . 268 Miscellaneous data on free radicals . . 270

Electron-spin Resonance Data. . . 276 10. Carbenes and Nitrenes . . 279 11. Reactions of Aldehydes and Ketones and Their Derivatives . 307

Formation and Reactions of Acetals and Ketals . . 307 Reactions with Nitrogen Bases. . . 316 Enolization and Related Reactions . . 321 Other Reactions . . 334

12. Reactions of Acids and Their Derivatives . . 339 Carboxylic Acids . . 339

14. Oxidations and Reductions . . 399 Ozonolysis . . 399

Non-carboxylic Acids . . 360 13. Photochemistry . . 369

Oxidations by Metallic Ions . . 402 Other Oxidations . . 406 Reductions . . 412 Hydrogenations . . 417

Author Index . . 421 Subject Index . . 469 Errata for Organic Reaction Mechanisms, 1965 . . 481

CHAPTER 1

Classical and Non-classical Carbonium Ions

Bicyclic Systems This year there have been published three re~iewsl -~ and a “collection of reprints with ~ommentary)’~ dealing with non-classicalions. All three reviewers and the commentator support the view that exo-norbornyl compounds react via a non-classical ion. Brown, on the other hand, has restated’his arguments for believing that they do not.5

As a result of the discussion by Goering and Schewene6 and Brown and Tritle’ of the 2-norbornyl system reported last year,s it is now clear that just as the exo:endo rate ratio measures the difference only in free energy of activation for ionization of the exo- and endo-isomers, the exo :endo product ratio measures the difference only in free energy of the transition states for capture of the 2-norbornyl ion(s) in the exo- and the endo-direction and that these two differences are closely related. In the strictest sense, then, it is only valid to draw conclusions about the structures of the transition states from these kinds of result. One may then extrapolate to the structure of the intermediate ion(s), but this involves an assumption that this structure is closely related to that of the transition states. This is undoubtedly frequently valid but it should be remembered that it is an assumption and need not always be valid.

In our opinion the question that should now be asked about solvolysis reactions of 2-norbornyl systems is not “does the exo-compound react via a non-classical ion!” but “are the high exo:endo rate and product ratios the result of delocalization of the 1,6-bonding electrons in the transition state for ionisation of the exo-isomer and for capture of the intermediate ion! )’

Sargent in his review1 accepts Brown’s view that the high exo:endo rate ratios observed in the solvolyses of tertiary 2-norbornyl derivativesg are not

1 G. D. Sargent, Quart. Rev. (London), 20,301 (1966). 2 G. E. Gream, Rev. Pure. AppZ. Chem., 16,25 (1966). 3 C. A. Bunton in “Studies on Chemical Structure and Reactivity”, J. H. Ridd, ed., Methuen,

London, 1966, p. 73. 4 P. D. Bartlett, “Non-classical Ions: Reprints and Commentary,” W. A. Benjamin, New York,

N.Y. 1966. 5 H . C. Brown, Chem. Brit., 2, 199 (1966). 6 H. L. Goering and C. B. Schewene, J . Am. Chem. Soc., 87,3516 (1965). 7 H. C. Brown and G. L. Tritle, J . Am. Chem. L ~ O C . , 88,1320 (1966). * See Organic Reaction Xechanism, 1965, 13. 9 See Organic Reaction Mechanisms, 1965,s.

1

Organic Reaction Mechanisms 1966 Edited by B. Capon, M. J. Perkins, C. W. Rees Copyright © 1967 by John Wiley & Sons, Ltd.

2 Organic Reaction Mechanisms 1966

the result of participation in the reactions of the exo-isomers, but he rejects Brown's explanation that they result because the rates for the endo-isomers are low owing to steric hindrance to ionisation. Instead he prefers the explanation that they are caused by the release of steric strain in the transition states for the reactions of the exo-isomers arising from the movement away of the 2-methyl or 2-phenyl substituent from the 6-hydrogen atom, and he calculates the strain relieved (-3 kcal mole-l) to be in good quantitative agreement with the observed exo:endo rate ratios. On this view, then, the difference in activation energy for the solvolyses of secondary exo- and endo-derivatives, caused by participation in the reaction of the exo-isomer, is, by chance, almost identical with the difference for tertiary derivatives, which results from a quite different factor, namely, release of steric strain.

As pointed out by Rei and Brown,lo however, equilibration studies (Table 1) indicate that steric strain in exo- and endo-isomers must be approximately the same as, or a t most only slightly greater than, in the 1-norbornyl isomer which they considered to be strain-free as far as the substituents are concerned. It is, therefore, difficult to see how relief of steric strain as envisaged by Sargent could be the cause of the rate differences between tertiary exo- and endo- isomers.

Rei and Brownlo also studied the kinetics of the acid-catalysed conversion of 2-methyl-exo-norbornan-2-01 into 2-methyl-endo-norbornan-2-01 and 1-methyl-exo-norborbornan-2-01 and report that the former is formed twice as rapidly as the latter.

Brown and Takeuchill report that the rates of ethanolysis of 2-aryl-exo- norbornyl chlorides can be correlated by the u+ constants to yield a p-value (-4.3) similar to that observed with 1-arylcyclopentyl (-4.5) and 2-aryl-2- propyl chlorides (-4.9). It was thought that if there were participation of the 1,6-bonding electrons in the reactions of the exo-norbornyl chlorides the proportion of this should increase on going from the p-methoxyphenyl (extrapolated k = 2.5 x lo2 sec-l) to the p-nitrophenyl compound (k = 7.08 x

sec-l) and that this would lead to a curved Hammett plot. Apparently this does not occur and i t seems reasonable to suppose that there is no participation.

Brown and Muzzio12 have attempted to correlate the solvolysis rates of bicyclic arenesulphonates with rates of borohydride reduction of the corresponding ketones. Although a fast borohydride reduction tended to accompany a slow arenesulphonate solvolysis the plot of log (partial rate factor) for ketone reduction against log krel. for toluene-p-sulphonate solvolysis was not a straight line even when compounds believed to undergo solvolysis with

10 M-H. Rei and H. C. Brown, J . Am. Chem. SOC., 88,5335 (1966). 11 H. C. Brown and K. Takeuchi, J . Am. Chem. Soc., 88,5336 (1966). 12 H. C. Brown and J. Muzzio, J . Am. Chem. Soc., 88, 2811 (1966).

Classical and Non-classical Carboniurn Ions 3

Table 1. Percentages of exo- and endo-2-norbornyl and 1 -norbornyl derivatives present at equilibrium.

Conditions Ref.

I Me Me

5 70

OH 20

A M e h O H I Me OH

17 61

OAc 16

Acetone at 100-137" 14

60% Aqueous dioxan a t 25" 10

AcOH at 48.9' 6

participation were excluded. In particular, the rate constants for endo- derivatives were very poorly correlated. Thus the solvolysis of endo-norbornyl toluene-p-sulphonate is slower than that of cyclopentyl toluene-p-sulphonate and the reduction of 2-norbonanone from the endo-direction is also slower than the reduction of cyclopentanone. This breakdown in the quantitative correlation was attributed to unusual slowness of both attack from, and departure in, the endo-direction.

13 N. A. Belikova, A. F. Plat6, and Kh. 3. Sterin, J. Gen. Chem. U.S.S.R., 34, 125 (1964). 14 C. F. Wilcox, M. Sexton, and M. F. Wilcox, J . Org. Chem., 28, 1079 (1963); E. L. Eliel,

S. H. Schroeter, T. J. Brett, F. J. Biros, and J. C. Richer, J. Am. Chem. SOC., 88,3327 (1966).


Schleyer and his co-workers’ assignment16 of the slower solvolysis of 6,6-dimethyl-2-exo-norbornyl toluene-p-sulphonate than of exo-norbornyl toluene-p-sulphonate to an unfavourable steric interaction between the methyl groups and C(l) and C(z) in a non-classical transition state15 has been further substantiated by Berson, McRowe, and Bergman.ls These workers set out to test whether this slower rate was the result of an initial or transition- state energy difference by investigating the effect of methyl substituents on the site of capture by solvent of the norbornyl cation. In particular it was found that attack a t C(2) of cation (1) (written by the authors as non-classical)

SOH 8-10 1

(1) (2) (3)

occurred 8-10 times faster than attack at C,,). The transition state for the formation of the 6-endo-methyl-2-em-derivatives (3) is therefore of higher free energy than that for formation of the 6-exo-methyl-2-exo-derivative (2). Inasmuch, then, as capture of the ion by solvent is the microscopic reverse of ionization of the toluene-p-sulphonate, this result shows that a 6-endo-methyl group has a destabilizing effect on the transition state for the latter as well as for the former; Schleyer’s assignment of the decelerating effect of a 6,6- dimethyl substituent to a steric effect in the transition state is thus substantiated. It was suggested by Berson et al. that this originates from the interaction between the hydrogen a t Ct2) and the methyl group a t C,!). A syrt-methyl group a t C(7) was shown to favour substitution a t C(2), but with a methyl group a t C(5) and an anti-methyl group a t C(7) there were approximately equal amounts of substitution a t C(l) and C(z).

A 3-endo-phenyl substituent slightly accelerates the acetolysis of endo- norbornyl toluene-p-sulphonate but a 3-em-phenyl substituent causes a 260-fold rate decrease; this was attributed to steric hindrance to solvation. It was also found that a 3-exo-substituent causes a large (130-fold) decrease in the rate of acetolysis of exo-norbornyl toluene-p-sulphonate, possibly arising from the same cause17 (see also, ref. 26, p. 8 below).

Following the observations by Takeuchi, Oshika, and Koga reported last year1* that exo- and endo-5,6-trimethylene-exo-2-norbornyl toluene-p-

15 See Organic Reaction Mechankms, 1965, 17. 16 J. A. Berson, A. W. McRowe and R. G. Bergman, J . Am. Chem. SOC., 88,1067 (1966). 17 D. C. Kleinfelter, E. S. Trent, J. E. Mallory, and T. E. Dye,J. Am. Chem. SOC., 88,5350 (1966). 18 See Organic Reacticnt Mechanisms, 1965, 15.

Classical and Non-classical Carbonium Ions 5

sulphonate gave significantly different proportions of acetates on acetolysis, Cash and Wilderlga have now reported that decomposition of the corresponding chlorosulphites (4) and (7) yields strikingly different proportions of the chlorides (6) and (9). Typically, in cyclohexane a t 28" (4) yields 34% of (5) and 47% of (9)) while (7) yields 2.6% of (6) and 97.4% of (9). Ifthese reactions proceed via carbonium ions then they must involve trapping of the classical ions (5) and (8) before their interconversion or conversion into a non-classical ion.

The reaction of exo- and endo-norbornanol with thionyl chloride and decomposition of the resulting chlorosulphites has also been investigated. 2,3,3-Trideuterio-exo-norbornan-2-01 (10) yielded the deuterated norbornyl chlorides shown, indicating that there had been some Wagner-Meerwein and 2,6-hydride shift, but that the former had not occurred to the extent required by intervention of a non-classical ion as the sole intermediate.lgb

&:: OH soclt &:+D&cl+$+cl c1 H D D D H

(10) 71% 10% 19%

Total yield 34%

In a very interesting investigation Sauers, Parent, and Damle20 have studied the acetolysis of exo- and endo-tricyclo[3.2.1.0S.B]oct-2-yl toluene-p-

19' D. J. Cash and P. Wilder, Chem. Comm., 1966,662. 19' J. K. Stille and F. M. Sonnenberg, J . Am. Chem. SOC., 88,4915 (1966). 20 R. R. Sauers, R. A. Parent, and S. B. Darnle, J. Am. Chern. Soe., 88,2257 (1966).

6 Organic Reaction Mechaiaisms 1966

sulphonate ( l l a and l l b ) which both yield unrearranged exo-acetate. That of the exo-isomer proceeds about 200 times faster than that of the endo-isomer and about 500 times faster than was calculated by use of Schleyer's correlation. If the exo-isomer reacts by way of non-classical ion (12), this must then react with exclusive attack a t C(z).

Tritium distribution in the products of the acetolyses of exo- and endo-2- tritio-2-norbornyl p-bromobenzenesulphonates and of the formolysis of the exo-isomer have been determined.21 The extent of rearrangement was ex- pressed (see Table 2) as the percentage of the products with:

(i) all the carbon atoms equivalent, arising from 3,2- and 6,2(or 6,1)-hydride shifts and Wagner-Meerwein shifts;

(ii) C(l), C(z), and C(s) equivalent, arising from a 6,2(0r 6,l)-hydride shift and Wagner-Meerwein shifts ; and

(iii) C(l) and C(z) equivalent, arising from a Wagner-Meerwein shift only. Of particular interest is the very much greater proportion of 3,2-hydride shift

Table 2. Rearrangement in the products of acetolysis and formolysis of norbornyl p-bromobenzeiiesulphonates.

yo Contribution __

in acetolyses

from EXO from endo at 25' a t 45' reflux

Complete equivalence C,,,, C,,,, and C,,, equivalent 45 47 C,,, and C,,) equivalent 45 55 43

in formolysis

from ezo at 25"

0 36.2 21.6 43.2

21 C. C. Lee and L. K. M. Lam, J . Am. Chem. SOC., 88, 2831, 5355 (1966).


found with the formolysis than with the acetolysis of the exo-isomer, consistent with the carbonium ion’s having a longer lifetime in the former reaction.

Following Berson and Grubb’s work reported last year,22 Benjamin and Collins have now described several more examples of stereospecific 6 -+ 2 and 6 -+ 1 hydride shifts in norbornyl systems. Thus treatment of diol(l3) yielded ketone (15) which analysis by NMR showed to have less than 3% deuterium at the bridgehead.23 It was therefore concluded that migration of the ex0-6- deuterium of ion (14) occurred with a stereospecificity of a t least 94% and the reaction was formulated as shown in Scheme 1.

Ph (13)

Scheme 1

Further examples were found in the hydrolysis of toluene-p-sulphonate (16) which in aqueous acetone containing sodium carbonate yields (17) (60%) formed by a Wagner-Meerwein shift, (18) (25%) formed by stereospecific elimination of the 6-exoo-deuterium, and (19) (15%) formed by stereospecific ex0-6,l deuteride migration (20) -+ (21) -+ (22).24 In addition, a trace of (24) was formed and this consisted of 9 parts of (24b), formed by two stereospecific hydride shifts (20) --f (22) and (22) -+ (25)’ and 1 part of (24a), formed by endo-6,2-hydride shift (20) -+ (23). The formation of (24a and b) in this way was confirmed by showing that toluene-p-sulphonate (26) yields (27a and b) in an approximately 1 : 9 ratio.24

Other examples of exclusive ex0-2’3-shifts in norbonyl systems have also been reported. Pinch and V a ~ g h a n ~ ~ found that sulphonation of (+)-[S-14C]- camphor (28) yields racemic camphor rr-sulphonic acid (32) with half the label a t C(,,, and half a t C(9y Resolution of the product yielded (+)-camphor-r- sulphonic acid with all its label a t C(s). These results were explained by the

22 See Organic Reaction Mechanisms, 1965,22. 23 B. M. Benjamin and C. J . Collins, J . Am. Chem. SOC., 88, 1556 (1966). 24B. M. Benjamin, B. W. Ponder, and C. J . Collins, J . Am. Chem. SOC., 88, 1558 (1966);

B. M. Benjamin and C. J . Collins, Tetrahedron Letters, 1966,5477. 25 A. M. T. Finch and W. R. Vaughan, J . Am. Chem. SOC., 87,5520 (1965).


T f

D OTs

series of reactions given in Scheme 2 involving exclusive exo-2,3-methyl shifts which were considered to be compelling evidence for the intervention of non- classical ions. I n our opinion, however, the only definite conclusion that can be drawn is that the free energies of the transition states for exo-migration (29,30, and 31) are considerably lower than those of the transition states for endo-migration and that this may possibly be due to delocalization of the 1 ,&bonding electrons, but there could be other explanations.22

An unsuccessful attempt has been made to observe an endo-2,3-hydride shift in the 2-p-methoxyphenyl-3-exo-hydroxy-2-norbornyl cation (34).26 This cation was generated by dissolving the corresponding alcohol (33) in sulphuric acid and was considered to have a classical structure which, i t was thought, would be favourable for an endo-hydride shift. However, the only

26 D. C. Kleinfelter and T. E. Dye, J . Am. Chem. Soc., 88,3174 (1966).


Sulphonation X=SOsH 1

0’ .Is - (32) + (32) (31)

Scheme 2

non-sulphonated product isolated was 3-endo-p-methoxyphenyl-2-norbornanone (38), formed presumably via ions (35) to (37). The driving force for this rearrangement was considered to be the instability of ion (34), due to steric hindrance to solvation by the exo-hydroxyl group (see also ref. 17, p. 4 above).

(35) 1*


The exo-lactone (40) obtained by the action of sulphuric acid on tricyclo- ekasantalic acid (39; R = C02H) has been shown to result exclusively from an exo-3,2-methyl shift as shown.27

R = CHzCHzCOzH

Details have now been published2s of Gassman and Marshall’s investigation of the solvolyses of the exo- (41) and endo-toluene-p-sulphonate (42) of 2-hydroxybicyclo[2.2.l]heptan-7-one first reported last year.29 The acetolysis of the exo-isomer is slightly slower than that of the endo-isomer, while the ethanolysis is slightly faster. The acetolysis of the exo-isomer gave 71% of a yellow oil which contained 65% of exo-acetate, 20% of endo-acetate, and 15% of six other unidentified products. The endo-isomer (88.9% pure) yielded 59% of a yellow oil which contained 76% of exo-acetate, 3% of endo- acetate, and 21% of unidentified components. Possible explanations, sym- bolized by (43) and (44), for the relatively high rates for the endo-isomer were excluded and i t was concluded that the exo:endo rate ratios found here are those to be expected from norbornyl systems in the absence of participation by the 1,6-bonding electrons. Hanack and Dolde30 have reported that the

OTs

(41) (42) (44)

104kzOH 1.84 4.66

104kgOH 2.56 1.2

27 G. E. Gream and D. Wege, Tetrahedron, 22,2583 (1966). 28 P. G. Gassman and J. L. Marshall, J . Am. Chem. Soc., 88,2822 (1966). 29 See Organic Reaction Mechanisms, 1965,26. 30 M. Hanack and J. Dolde, Tetrahedron Lettera, 1966, 321.


solvolysis of a 73 : 27% mixture of the endo- and exo-toluene-p-sulphonates yields some cyclohex-3-ene-1-carboxylic acid.30

Another very interesting investigation of the solvolysis of a pair of exo- and endo-norbornyl derivatives with which there is no participation of the 1,6- bonding electrons in the reactions of the exo-isomer has been reported by Traylor and These workers measured the rates of acid-catalysed exchange of the exo- and endo-methoxyl groups of camphor dimethyl ketal(47) and 2-norbornanone dimethyl ketal(45) in deuteriomethanol and the relative rates of capture of the intermediate ion (46) from the exo- and endo-directions by methanol and by borohydride. With the norbornanone ketal the exo-

h % M e

@Me

H+

(45)

I COMe

(45)

&+ OMe

CDaOD/ /-D+

CDaOD

OMe I

OCDj

OMe

methoxyl group was exchanged 16 times more rapidly than the endo-group, and ion (46) was captured 20 and 24 times more rapidly from the --direction by methanol and borohydride, respectively. If then this reaction is a good model for how exo- and endo-norbornyl toluene-p-sulphonates would react in the absence of participation by the 1,6-bonding electrons these results indicate that only part of the 1600-fold rate enhancement found with the exo-isomer can be the result of steric hindrance to ionization and suggest that most of i t must result from participation, An even more striking effect was found with the camphor acetal(47) with which the exo-methoxyl group was exchanged 10 times more slowly than the endo-group and capture of the ion by methanol and borohydride was 10 and 8 times, respectively, more rapid from the endo- direction. Traylor and Perrin concluded then that the higher rates of solvolyses of exo-norbornyl toluene-p-sulphonate and of isobornyl chloride than those of their endo-isomers are in the main the result of participation, but they suggested that part of the driving force could arise from relief of steric strain on lengthening of the 1,6-bond in the transition state.

31 T. G . Traylor and C. L. Perrin, J . Am. Chem. Soc., 88,4934 (1966).


Me, ,Me

(47) (48) (49)

The validity of these conclusions depends on the assumption that ion (46) is a good model for a classical norbornyl cation. The most important contri- buting structure to ion (46) is, however, not the carbonium ion structure (48) but the oxonium ion structure (49) and the carbon-oxygen bond must have considerable double-bond character. This could mean that stereoelectronic factors play an important part in its formation but unfortunately our under- standing of these in reactions of acetals is slight. Before accepting completely Traylor and Perrin's conclusions it should be borne in mind that these authors assume the activating electronic effects of the exo- and endo-methoxyl groups to be identical.

According to Jensen and Beck, the best defined NMR spectra of the norbornyl cation32 are obtained from 2-norbornyl bromide with gallium bromide in liquid sulphur dioxide. At -80" spin-spin splitting was observed, the spectrum consisting of signals a t 6 = 5.2, 3.1, and 2.1 with relative intensities of 4:1:6 and observed multiplicities of 7, 1, and 6, respectively. It was considered that the slow rates of the 2,Shydride shifts (dG* 1: 11 kcal molep1) were not consistent with a classical structure for the ion, and the NMR spectrum was interpreted in terms of a series of equilibrating alkyl-bridged and protonated cyclopropane structures, as shown in Scheme 3.33

11

Scheme 3

32 See Organic Reaction Mechanism, 1965,23-25. 33 F. R. Jeneen and B. H. Beok, Tetrahedron Letkra, 1966,4287.


It has been calculated from their mass spectra that the rate of fragmentation of the ion C7HllBr+ from exo-norbornyl bromide to yield C7HI1+ is 10 times greater than that of the analogous ion from endo-norbornyl bromide.34

Decomposition of norbornan-2-one tosylhydrazone by hot alkoxide solutions leads to nortricyclene. In the aprotic medium, diglyme, the yield is greater than 99% and when the 6-exo- or 6-endo-deuterionorbornan-2-one derivative was used the tricyclene had the same deuterium content as the starting norbornanone. It was suggested that this reaction involved a carbene insertion with an intramolecular transfer of hydrogen or deuterium. In a protic medium, ethylene glycol, the yield of nortricyclene was still very high (92-93y0), but now deuterium was lost when 6-exo- or 6-endo-deuterionorbornan-2-one was starting material. Strikingly, however, the loss of 6-endo-deuterium, 52%, was larger than the loss of 6-exo-deuterium, 19%. It was concluded, therefore, that in the protic medium the reaction could not involve solely protonation of the carbene to a 2-norbornyl cation since this would be expected to be either a non-classical ion or a rapidly equilibrating pair of classical ions which would make the 6-endo- and 6-exo-positions equivalent. Instead, protonation of the intermediate diazoalkane (50) from the exo-direction to give an endo-diazonium ion (51) which undergoes a 1,3-elimination was suggested as an important pathway.35

Other investigations of norbornyl systems include studies of the solvolyses of 2-n0rbornyl,~~ 1,3,3-trimethyl-2-norbornyl ( f e n ~ h y l ) , ~ ~ and 1,5,5-trimethyl-2-norbornyl (isofenchyl) 37 toluene-p-sulphonates and of the lead tetra-acetate oxidation of norborn-5-ene-2-carboxylic acid in the presence of lithium chloride;38 see also p. 143.

The formation of different products, or of the same products in different proportions, from apparently identical carbonium ions that are derived by ring expansion of different starting materials has been designated a “memory effect.”39 This year another investigation of the acetolyses of syn-

34 D. C. DeJongh and S. R. Shrader, J . Am. Chem. SOC., 88,3881 (1966). 35 A. Nickon and N. H. Werstiuk, J . Am. Chem. Soc., 88,4543 (1966). 36 W. Hiickel and 0. Vogt, Ann. Chem., 695, 16 (1966). 37 W. Huckel and H.-J. Kern, Annulen, 687,40 (1965). 38 R. M. Moriarty, H. Gopal, and H. G. Walah, Tetrahedron Letters, 1966,4369. 39 J. A. Berson and M. S. Poonian, J . Am. Chem. Soc., 88,170 (1966).


(52) and artti-norborn-2-en-7-ylmethyl arenesulphonate (57) originally studied by Berson and Gajewski*O has been reported.41 The products of ring expansion may be divided into the L series, (61)-(63), previously shown by LeBel to result from carbonium ion reactions of exo-2-bicyclo[2.2.2]octen-5-yl derivatives, and the G series, (55)-(56), previously obtained by Goering

(53) (54)

AcO,

'& G Series

I OAc

from endo-2-bicyclo[2.2.2]octen-5-yl derivatives. The syn-p-bromobenzenesulphonate yields a mixture of L and G acetates in the ratio 1 :20-40 while the anti-isomer yields only L acetates. It appears then either that the bond

40 J. A. Berson and J. J. Gajewski, J . Am. Chem. Soc., 86,5020 (1964). 41 R. K. Bly and R. S. Bly, J . Org. Chern., 31, 1577 (1966).

Classical and Non-classical Carbolzium Ions 15

migrations, (52) -+ (53) -+ (54), and (57) -+ (58) -+ (59) + (60), are largely concerted or that the conformationally isomeric carbonium ions (53) and (58) (the vacant p-orbitals of which lie along the broken lines) are formed as the first intermediates in the ring expansions and are trapped before they can interconvert. Delocalization of the l,7-bond of ion (53) leads to products of the G-series and delocalization of the 1,6-bond or of the 5,6--rr-electrons of ion (58) leads to the L series. That the L:G ratio in the products derived from anti-p-bromobenzenesulphonate is greater than the G :L ratio in those derived from syn-p-bromobenzenesulphonate indicates that -rr-trapping in carbonium ion (58) is more eEcient than a-trapping in ion (53).

Another example of a “memory effect” was reported by Berson and P ~ o n i a n . ~ ~ Solvolysis of 7-norbornylmethyl p-bromobenzenesulphonate (64) and deamination of the corresponding amine yield mixtures of 2-exo- and 2-endo-bicyclo[3.2.l]octyl and 2-bicyclo[2.2.2]octyl derivatives as well as unrearranged products. The exo-bicyclo[3.2.1]octyl derivatives (67) are formed by a double carbonium ion rearrangement and the second of these, (65) -+ (66), was shown by specific deuterium labelling to proceed with preferential migration of the anti-bridge (65, type x) rather than of the syn-bridge (65, type y). The ratio of anti to syn-migration was larger in the deamination (5.7-6.6) than in the solvolysis of the p-bromobenzenesulphonate (1.4-1.6).

The ring expansion of 2-norbornylmethyl derivatives has also been in- v e ~ t i g a t e d . ~ ~

Wiberg and Ashe’s investigation of the acetolysis of exo- and endo-6- bicyclo[3.1 .O]hexanylmethyl toluene-p-s~lphonates~~ has been supplemented by an investigation of the deamination of the corresponding amines (68) and (69).44

42 W. Kraus and P. Schmutte, Chem. Ber., 99,2259 (1966). 43 See Organic Reaction Mechanisms, 1965, 28. 44 F. T. Bond and L. Scerbo, Tetrahedron Letters, 1965,4255.


I

(69)

CHzNHz

+ Olefins

Similar products were obtained but in different proportions. In particular, the deamination reactions yielded a much higher proportion of the unrearranged alcohols. It was suggested that the initially formed cyclopropyl- methyl cation is trapped before i t can rearrange. The exo-bicyclo[4.1.0]heptylmethyl system was also investigated. Again solvolysis of the toluene-p- sulphonate (70) proceeded with more rearrangement than did deamination of the amine (71).44

( i ) ENOI-ACOH + 60% + Olefin m 34%

CHzNHz

(71)

The kinetics and products of the acetolysis of the bridged cyclobutylmethyl toluene-p-sulphonates (72)-(75) have been investigated and compared with those for cyclobutylmethyl toluene-p-sulphonate (76) itself .45 Unrearranged acetate from optically active e.ndo-5-bicyclo[2.1.1]hexyl[~-2H]methyl toluene- p-sulphonate was formed with complete inversion of configuration, so this

45 K. B. Wiberg and B. A. Hem, J . Am. Chem. ~ o c . , 88,4433 (1966).


L k H --3 L$H + &0Ac H

CHzOTs CH20Ac

I

CHaOTs

I

CHzOAc 16% 22% 25% 37%

H

H

~ 0 . 4 % 40% 45% 15%


and the other unrearranged acetates were considered to be formed by S,2 processes. Prom the product ratios the observed rate constants were dissected into constants for S,2 and S,1 solvolyses. The constants for the acetolyses of the em-isomers (73) and (75) were very similar to that observed for cyclobutylmethyl toluene-p-sulphonate itself, but those for the endo-isomers (72) and (74) were about 100 times slower. It was suggested that in the transition states for the endo-isomers there was an unfavourable non-bonding 1,3- interaction between one of the methyl-hydrogen atoms and the axial or pseudoaxial hydrogen atoms on the same side of the ring. It is also of interest that the rearranged products from mo-6-bicyclo[3.l.l]heptylmethyl toluene- p-sulphonate (75) are the same as those from e~-6-bicyclo[3.2.l]octyl p- bromobenzenesulphonate.

The acetolyses of the cis- and trans-3-bicyclo[3.2.0]heptyl toluene-p- sulphonates (77), (78), and (79) have been studied. It was thought that with

&OTs

OTs

the trans-fused compound (77) the cyclopentane ring would be held in a half-chair conformation and with the cis-fused compounds (78 and 79) in an envelope conformation. The reactions yielded unrearranged acetates which with the cis-fused compounds were shown to be predominantly those from inversion of configuration. The rates were all lower than that for cyclopentyl toluene-p-sulphonate itself, that for the trans-fused compound (77) (the slowest) being 55 times less.46

Oxidation of exo-bicyclo[2.2.0]hexan-2-ol(80) with aluminium tert-butoxide and “quinone ether” did not yield the expected bicyclo[2.2.0]hexan-2-one, but instead gave the mixture of products shown.47 Two possible mechanisms

+

were considered for the rearrangement. The iirst, ( l ) , involving dissociation of the aluminium alkoxide (81) into an ion pair, was thought to be unlikely

46 J. Meinwald, P. Anderson, and J. J. Tufariello, J. Ana. Chem. SOC., 88, 1301 (1966). 47 R. N. McDonald and C. E. Reineke, Tetrahedron Letters, 1966,2739.


since no bicyclo[Z. 1 .l]hexand-one was obtained with aluminium tert-butoxide alone. The second, (a), was therefore favoured, and to explain the absence of a norbornyl type of rearrangement48 i t was suggested that the hydride transfer occurred simultaneously with the migration of the 1,4-bridge.

Other reactions of bicyclic and polycyclic systems which have received attention include the solvolytic rearrangement of substituted bicyclo[4.3.1]- decyl methanesulphonates to bicycl0[5.3.O]decanes,~~ of piny1 toluene-p- su lphona te~ ,~~ of 1,l'-bishomocubyl methanesulphonate, and of homocubyl- methyl toluene-p-~ulphonate,"~ and the reaction of bicyclo[2.2.2]octane-2- carboxylic acid with bromine to yield 2-(axial)bromobicyclo[3.2.l]octane-l- carboxylic acid.52

Phenonium Ions There has been relatively little work this year on phenonium ions or on equilibrating phenethyl

Cram, Montgomery, and Knox have investigated the acetolysis of [8]paracyclophan-3-yl toluene-p-sulphonate (82).53b The rate is compared with that of other paracyclophane toluene-p-sulphonates in Table 3. The products were [8]paracyclophan-4-ene (83) (60%) and two other hydrocarbons (39% and lye), the major one being either (85) or (86). These are shown in Scheme 3 as

48 See Organic Reaction Mechanisms, 1965,30. 49 J. A. Marshall and J. J. Partridge, Tetrahedron Letters, 1966,2545. 50 W. Huckel and D. Holzwarth, Ann. Chem., 697, 69 (1966); H. Schmidt, M. Miihlstadt, and

5 1 W. G. Dauben and D. L. Whalen, J. Am. Chem. Soc., 88,4739 (1966). 52 A. W. Chow, D. R. Jakas, and J. R. E. Hoover, Tetrahedron Letters, 1966,5427. 53'See, however, M. Brookhart, F. A. L. Anet, and S. Winstein, J. Ant. Chem. Soc., 88, 5657

(1966); M. Brookhart, F. A. L. Anet, D. J. Cram, and S. Winstein, ibid., p. 5659; G. A. Olah, C. U. Pittman, E. Namanworth, and M. B. Comisarow, ibid., p. 5571.

P. Son, Chem. Ber., 99,2736 (1966).

53' D. J. Cram, C. S. Montgomery, and 0. R. Knox, J . Am. Chem. Soc., 88,515 (1966).


being formed through a series of classical ions, although in the original paper it was suggested that the intermediate was a composite bridged ion (84) “in which a proton is imbedded in the two r-clouds, that of the incipient olefm and that of the benzene ring”.

Table 3. Relative rates of acetolysis of cyclic toluene-p-sulphonates at 50”.

Toluene-p-sulphonate Relative rate

Cyclo hex yl 1 Cyclodecyl 539

[9]Paracyclophan-3-y1 60

[ 9lParacyclophan-4-yl 1800

[ 10]Paracyclophan-5-y1 15



[2.2]Paracyclophan-l-ylmethyl toluene-p-sulphonate (87) undergoes acetolysis with concurrent rearrangement to the [2.3]paracyclophanyl acetate (88) and toluene-p-sulphonate (89).54 The total rate of ionization is only 28 times greater than that of /I-methylphenethyl toluene-p-sulphonate

54 E. Hedaya and L. M. Kyle, J . Am. Chem. SOC., 88,3667 (1966).

organic reaction mechanisms 1966

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