3066
J . Org. Chem. 1988,53, 3066-3073
Acknowledgment. This study was supported by NSF Grant CHE-8518801 and in part by the UC Riverside Intramural Research Fund. H.E. also acknowledges receipt of a Chancellor’s Patent Fund grant from the UC Riverside Graduate Division. Badische-Anilin und Soda Fabrik (Ludwigshafen) generously provided several chemicals utilized in this investigation. Dr. R. Lee, Dr. A. de Lera, and Ms. P. Theobald provided valuable advice during the course of this study.
114582-59-1;( & ) - 1 4 ~114613-31-9; , (*)-14d, 114582-60-4;18,7977-6; ( f ) - 2 0 a , 114582-47-7; (*)-20b, 114582-49-9; (*)-2Oc, 114582-51-3;(*)-20d, 114582-53-5;(*)-23a, 114582-61-5;(*)-23b, 114582-62-6;( f ) - 2 4 a , 94369-97-8;(&)-24b,114582-63-7;(f)-24c, 114594-79-5; 25a, 114582-64-8; 25b, 114582-67-1; (f)-26a, 114582-65-9; (&)-26b, 114582-66-0; 27a, 114582-68-2; 27b, 114582-70-6;28a, 114582-69-3;28b, 114582-71-7;HCECMe, 7499-7; HCSCEt, 107-00-6; HCECPr-i, 598-23-2; HCECBu-t, 917-92-0.
Registry No. (&)-9a,114582-48-8;(*)-9b, 114582-50-2;(&)-9c, 114582-52-4; (&)-sa,114582-54-6; l l a , 114582-55-7; l l b , 114582-56-8; 1 2 ~ 114582-57-9; , ( f ) - 1 3 ~ ,114582-58-0; (f)-13d,
Supplementary Material Available: Spectral and analytical data (31 pages). Ordering information is given on any current masthead page.
Rearrangements of 6-Tricycl0[3.3.0.0~~~]octyl Cations. Factors Influencing the Relative Stabilities of Bridged Carbocations T. William Bentley Department of Chemistry, University College of Swansea, Singleton Park, Swansea, SA2 8PP Wales, United Kingdom
Bernhard Goer and Wolfgang Kirmse* Fakultut fur Chemie, Ruhr- Universitat Bochum, 0-4630 Bochum, Federal Republic of Germany
Received December 18, 1987 The objective of this work was to explore the effect of ring strain on u delocalization of carbocations. The
6-tricycl0[3.3.0.6~~]0ctylcation (3) incorporates 2-norbornyl and 2-bicyclo[2.l.l]hexyl structures in a highly strained molecular framework. Solvolyses of the epimeric brosylates 22 and 23, as well as nitrous acid deaminations of the analogous amines, 24 and 21, served to generate 3. The exo:endo rate ratios of the brosylates and the exo:endo (19, 20) are close to unity. Product distributions and kinetic product ratios of the tricyclo[3.3.0.02~7]octan-6-ols data suggest a weak k , contribution at least for the endo brosylate 23. Several nondegenerate rearrangements of 3 were elucidated: Migration of C-2 from C-7 to C-6 (3 28) is followed, in part, by fragmentation (28 31). A minor fraction of 3 undergoes 4,g-hydride shifts (3 37 s 38). The degeneracy of 3 was probed with the aid of a 6-2Hlabel. Migration of C-8 from C-7 to C-6 was found to be rapid, as compared to nucleophilic capture, whereas the norbornyl-type Wagner-Meerwein rearrangement (migration of C-4) was not observed. Product and label distributions indicate that the bridged structure (involving C-6, -7, and -8) 3c is nearly isoenergetic (k0.5 kcal/mol) with the unsymmetrical ion 3a while products from the norbornyl-type delocalized ion (3b) are not observed, so 3b must be less stable by a t least 3 kcal/mol. The exceptional order of relative stabilities is explained in terms of “olefinic strain”, i.e., the additional strain resulting from contraction of the basal bond in bridged carbocations.
--
Many carbocations are known in which the charge is delocalized in two-electron three-center b0nds.l By Olah’s terminology these are carbonium ions as opposed to the charge-localized carbenium ions.2 These terms actually refer to limiting cases; there can be a continuum of electron delocalization in carbocation~.~Electronic effects on u delocalization have been thoroughly studied. For instance, the classical (C,) structure of the 2-norbornyl cation (la) was found to be favored by charge-stabilizing substituents (1)For recent reviews, see: (a) Vogel, P. Carbocation Chemistry; Elsevier: Amsterdam, 1985. (b) Olah, G.; Prakash, G. K. S.; Sommer, J. In Superacids; Wiley: New York, 1985;Chapter 3.5. (c) Shubin, V. G. Top. Curr. Chem. 1984,116,267.(d) Saunders, M.; Chandrasekhar, J.; Schleyer, P. v. R. In Rearrangements in Ground and Excited States; de Mayo, P., Ed.; Academic: New York, 1980;Vol. 1, p 1. (e) Kirmse, W. Top. Curr. Chem. 1979,80,125. (0 Brown, H. C. The Nonclassical Ion Problem (with comments by Schleyer, P. v. R.);Plenum: New York, 1977. (2)(a) Olah, G.A. J. Am. Chem. SOC. 1972,94, 808. (b) Olah, G. A. Angew. Chem. 1973,85,183;Angew. Chem., Int. Ed. Engl. 1973,12,173. (c) Olah, G. A. Top. Curr. Chem. 1979,80,211. (d) Olah, G. A. Chem. Scr. 1981,18, 97. (3)(a) Rates: Grob, C. A. Acc. Chem. Res. 1983,16,426.(b) Isotopic perturbation: Saunders, M.: Kates. M. R. J.Am. Chem. SOC. 1980,102, 6867;1983,105,3571.(c) X-ray: Laube, T. Angew. Chem. 1987,99,580; Angew. Chem.,Int. Ed. Engl. 1987,26,561.Montgomery, L. K.: Grendze, M. P.; Huffman, J. C. J . Am. Chem. SOC.1987,109, 4749
0022-3263/88/ 1953-3066$01.50/0
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at C-1 and C-2,184as well as by electron-withdrawing groups a t C-6.3a95 The influence of ring strain has received much less attention. Recent solvolytic6 and computational studies‘ of the 2-bicyclo[2.l.l]hexy1cation (2) indicate that the delocalized structure 2b should be about 3 kcal/mol more stable than 2a (the exchange of the methylene groups of 2b must proceed via 2a). Estimates of the stabilization energy of the 2-norbornyl cation due to bridging (1b vs la) are higher: 6-8 kcal/mol from exo:endo rate ratios1y4and from heats of ionization;s 10-11 kcal/mol from gas phase hydride affinitie~.~ However, such estimates depend on the choice of appropriate models.’O Any conclusion from (4)Sargent, G. D.In Carbonium Ions; Olah, G. A,, Schleyer, P. v. R., Eds.; Wiley: New York, 1972;Vol. 111, p 1099. (5)Kirmse, W.; Mrotzeck, U.; Siegfried, R. Angew. Chem. 1985,97,51; Angew. Chem., Int. Ed. Engl. 1985; 24, 55. (6)Kirmse, W.; Zellmer, V.; Goer, B. J. Am. Chem. SOC.1986,108, 4912. (7)Schleyer, P. v. R.; Laidig, K.; Wiberg, K. B.; Saunders, M.; Schindler, M. J. Am. Chem. SOC. 1988,110,300. (8)(a) Amett, E.M.; Petro, C. J. Am. Chem. SOC.1978, 100, 2563, 1980, 5408. (b) Arnett, E. M.; Pienta, N.; Petro, C. J . Am. Chem. SOC. 102, 398. (9)(a) Lossing, F.P.; Holmes, J. L. J. Am. Chem. SOC.1984,106,6917. (b) Blanchette, M. C.; Holmes, J. L.; Lossing, F. P. J . Am. Chem. SOC. 1987,109,1392.
0 1988 American Chemical Society
J. Org. Chem., Vol. 53, No. 13, 1988 3067
Relative Stabilities of Bridged Carbocations these data, concerning the effect of ring strain on u delocalization, would be premature.
Scheme I
SBS 5
L
la
Ib
3a
2a
2b
3b
3c
For further insight, we have studied the 6-tricyclo[3.3.0.02,']octyl cation (3). This ion incorporates the structural elements of both 1 and 2. Neither the degeneracy of the Wagner-Meerwein rearrangements nor the symmetry of delocalized intermediates (3b,c) is disturbed by the additional bridge. On the other hand, the strain energy of tricyclo[3.3.0.02~7]octane(48 kcal/mol) is significantly higher than that of norbornane (17 kcal/mol) and bicyclo[ 2.1.11hexane (41 kcal/mol) ,11 The only previous report on 6-tricyclo[3.3.0.02J]octyl cations refers to the 1-methyl derivative S.12 Acetolysis of anti-7-methyl-2-norbornene-syn-7-carbinyl brosylate (4) gave a product mixture which contained, in addition to unrearranged (5) and ring-expanded acetates (derived from 6), a set of five tricyclic products (ca. 45%). Independent syntheses identified two of the tricyclic acetates as 10-OAc and 11-OAc. The same products, albeit in different ratios, were obtained by acetolysis of the tosylate 7, derived from 10-OH (Scheme I). The efficient cyclization of 4 is to be contrasted with the behavior of the 7-unsubstituted analogue 9, which gave no detectable cyclization p r 0 d ~ c t s . l ~ The available data did not shed light on the potential degeneracy and stereoselectivity of 8. Extended studies (now reported), particularly of the parent system 3, promised a significant advance. Results Preparation of Substrates. Tricyclo[3.3.0.02~7]octan6-one (17), a key intermediate, has been prepared previously by base-induced cyclization of 13.14 The route leading to 13, however, is a rather elaborate, multistep sequence. We obtained 17 (16% yield) by intramolecular photocycloadditionof 14, which is readily accessible in two steps from 3-chlorocyclopentene and acrolein. A related approach starts from 2,6-cyclooctadien-l-one (12) and generates 17 in two sequential photoreactions,15 but this alternative proved inferior to ours with regard to yield and accessibility of the precursor (Scheme 11). The stereoselectivity of 17 toward lithium aluminum hydride was similar to that of 2-norbornanone: the alcohols 19 and 20, separable by HPLC, were obtained in a 1:9 ratio. The 'H NMR spectra of all tricyclo[3.3.0.02~7]octane derivatives contain a sharp doublet of endo 8-H (J8n,8x ca. 8 Hz). In the spectrum of 20 (CDC13),the doublet is located a t 6 1.12, but because of the proximity of the OH (10)For a criticism of nonclassical stabilization, see: Brown, H. C. Acc. Chem. Res. 1983, 16,432. (11)Engler, E. M.;Andose, J. D.; Schleyer, P. v. R. J.Am. Chem. SOC. 1973. ~. -, 95. 8006. (12) Berson, J. A.; Donald, D. S.; Libbey, W. J. J. Am. Chem. SOC. 1969,91, 5580. (13)(a) Bly, R.K.; Bly, R. S. J. Org. Chem. 1966,31,1577.(b) Benon, J. A.; Gajewski, J. J.; Donald, D. S. J. Am. Chem. SOC.1969.91. 5550. (14)Nakazaki, M.;Naemura, K.; Kadowaki, H. J. Org. Chem: 1976, 41,3125. (15)(a) Cantrell, T.S.; Solomon, J. S. J. Am. Chem. SOC.1970,92, 4656. (b) Noyori, B.; Inoue, H.; Kato, M. J. Chem. SOC.D 1970,1695.
--.
-
+
* . I
6
1
CG i? BOR ... OR
10
9
.+
+
11
Scheme I1
12
I
ss
hJ
0
15
13
& N-NHTS
16
/ I GQ
I fzQ
19
20
21
22
23
&ou
OH
08s
NH2
2L
group, it is shifted downfield to 6 1.79 in the spectrum of 19, thus confirming the configurational assignment. Moreover, 6-H of 19 absorbs a t higher field (6 3.82) as compared with 6-H of 20 (6 4.11), in agreement with ero(6 3.75) and endo-2-norbornanol (6 4.20). The alcohols were converted to the analogous brosylates, 22 and 23. Inverting displacement of the endo OBs group with hexadecyltributylphosphonium azide,16followed by LiA1H4 reduction, afforded the exo amine 24. The endo isomer 21 was obtained by Pt-catalyzed hydrogenation of the oxime 18. The tosylhydrazone 16 was prepared as another convenient source of diazonium ions (Scheme 11). Product Studies. Solvolyses of the brosylates 22 and 23 were performed in 70% aqueous dioxane in the presence of 2,6-lutidine. The diazonium ions 25 and 26 were generated by nitrous acid diazotization of the amines 24 and 21, respectively, and by photolysis of tosylhydrazone 16 (16) (a) Banert, K.; K i m , W. J.Am. Chem. SOC.1982,104,3766.(b) Banert, K. Chem. Ber. 1985,118,1564. (c) Kirmse, W.; Streu, J. J. Org. Chem. 1985,50,4187.
3068 J. Org. Chem., Vol. 53, No. 13, 1988
Bentley et al.
Table I. Product Distributions Obtained f r o m Tricyclo[3.3.0.0*~']oct-6-y1and Related Substratesn uroducts ( % I precursor conditions 19 20 29 32 33 35 39 43.1 8.7 15.6 22.6 3.0 3.0 22 70% aqueous dioxane, 80 "C, 20 h 0.8 35.6 24.2 14.3 23 70% aqueous dioxane, 80 "C, 20 h 9.9 2.0 1.3 2.5 12.4 23.1 15.2 35.7 4.2 3.4 1.0 24 HzO/HC104, pH 3.5,NaNOz 32.6 44.2 7.1 9.3 1.0 0.8 0.5 21 HzO/HC1O4, p H 3.5,NaNOZ 33.2 11.0 22.4 23.2 2.9 2.2 0.9 16 0.2 N NaOH, hv (Pyrex) 27 0.2 N NaOH, hu (PyrexIb 47.3 41.6 5.3 4.4 30 0.2 N NaOH, hu (Pyrex)' 67.8 8.7 12.1 36 1 N H2S04,70% aqueous dioxane, 60 "C, 3 days 17
40 3.2 10.2 5.0 4.5 4.3 83
Alcohols normalized t o 100%. T h e fraction of bicyclo[3.3.0]octadieneswas
