J. Org. Chem. 2002, 67, 837-846
837
Mechanistic Investigation of a Novel Vitamin B12-Catalyzed Carbon-Carbon Bond Forming Reaction, the Reductive Dimerization of Arylalkenes Justin Shey, Chris M. McGinley, Kevin M. McCauley, Anthony S. Dearth, Brian T. Young, and Wilfred A. van der Donk* Department of Chemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Ave., Urbana, Illinois 61801
[email protected] Received August 22, 2001
In the presence of catalytic vitamin B12 and a reducing agent such as Ti(III)citrate or Zn, arylalkenes are dimerized with unusual regioselectivity forming a carbon-carbon bond between the benzylic carbons of each coupling partner. Dimerization products were obtained in good to excellent yields for mono- and 1,1-disubstituted alkenes. Dienes containing one aryl alkene underwent intramolecular cyclization in good yields. However, 1,2-disubstituted and trisubstituted alkenes were unreactive. Mechanistic investigations using radical traps suggest the involvement of benzylic radicals, and the lack of diastereoselectivity in the product distribution is consistent with dimerization of two such reactive intermediates. A strong reducing agent is required for the reaction and fulfills two roles. It returns the Co(II) form of the catalyst generated after the reaction to the active Co(I) state, and by removing Co(II) it also prevents the nonproductive recombination of alkyl radicals with cob(II)alamin. The mechanism of the formation of benzylic radicals from arylalkenes and cob(I)alamin poses an interesting problem. The results with a one-electron transfer probe indicate that radical generation is not likely to involve an electron transfer. Several alternative mechanisms are discussed. Introduction The properties and reactivity of vitamin B12 derivatives have been extensively investigated ever since it was shown to possess a cobalt-carbon bond in the biological cofactors adenosyl- and methylcobalamin.1-5 These versatile organometallic complexes are used in a variety of enzymes to catalyze radical rearrangements and methyl transfers. Moreover, vitamin B12 has found use in organic chemistry for carbon-carbon bond formations.6-11 In this study, we report a previously unknown B12-catalyzed reaction that forms a new carbon-carbon bond between two alkenes providing dimerization products 1 (eq 1). It is noteworthy that the regiochemistry of this coupling is opposite to that expected for radical chemistry involving alkenes. The transformation employs catalytic vitamin B12 and an auxiliary reductant such as Zn or Ti(III)citrate and can be carried out at 25 °C in a mixture of aqueous (1) Lenhert, P. G.; Hodgkin, D. C. Nature 1961, 192, 937-938. (2) Hodgkin, D. C. Science 1965, 150, 979-988. (3) Dolphin, D. B12; John Wiley & Sons: New York, 1982. (4) Kra¨utler, B.; Arigoni, D.; Golding, B. T. Vitamin B12 and B12Proteins; John Wiley & Sons: New York, 1998. (5) Banerjee, R. The Chemistry and Biochemistry of B12; Wiley: New York, 1999. (6) Scheffold, R.; Abrecht, S.; Ruf, H.-R.; Stamouli, P.; Tinembart, O.; Walder, L.; Weymuth, C. Pure Appl. Chem. 1987, 59, 363-372. (7) Ogoshi, H.; Kikuchi, Y.; Yamaguchi, T.; Toi, H.; Aoyama, Y. Organometallics 1987, 6, 2175-2178. (8) Pattenden, G. Chem. Soc. Rev. 1988, 17, 361-382. (9) Baldwin, D. A.; Betterton, E. A.; Chemaly, S. M.; Pratt, J. M. J. Chem. Soc., Dalton Trans. 1985, 1613-1618. (10) Baldwin, J. E.; Adlington, R. M.; Kang, T. W. Tetrahedron Lett. 1991, 48, 7093-7096. (11) Branchaud, B. P.; Friestad, G. K. Encyclopedia of Reagents for Organic Synthesis; Paquette, L., Ed.; Wiley: New York, 1995; pp 55115514.
buffer (pH 8) and polar organic solvents such as ethanol, methanol, or acetonitrile.12 Perhaps the most interesting aspect of this transformation involves the mechanism by which it takes place. We describe in this report the scope of the reaction and a detailed investigation of its mechanism. Results and Discussion Scope of B12-Catalyzed Dimerization of Alkenes. The range of substrates that can be coupled is shown in Table 1. Monosubstituted arylalkenes gave a 1:1 ratio of diastereomeric products, whereas the disubstituted substrates produced two quaternary carbon centers in high yields (entries 4-7). When either vitamin B12 or Ti(III)citrate was omitted from the reaction mixture, only starting alkenes were recovered. Other methods have been reported for the preparation of these types of compounds, but they have generally relied on relatively harsh reaction conditions13-18 or photochemical19 reac(12) The content of aqueous buffer is determined by the volume of aqueous Ti(III)citrate added. The reaction rates in these solvents systems follow the general trend: aqueous buffer > MeOH > EtOH ≈ MeCN. (13) Brown, W. G.; McClure, D. E. J. Org. Chem. 1970, 35, 20362037 and references therein. (14) McMurry, J. E.; Silvestri, M. J. Org. Chem. 1975, 40, 26872688. (15) Prostasiewicz, J.; Mendenhall, G. D. J. Org. Chem. 1985, 50, 3220-3222.
10.1021/jo0160470 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/12/2002
838
J. Org. Chem., Vol. 67, No. 3, 2002
Shey et al.
Table 1. Vitamin B12/Ti(III)-Mediated Dimerization of Alkenes
Scheme 1
yield (%)a
alkene entry
R1
R2
dimer
reduction product
1 2 3 4 5 6 7
Ph 4-MeC6H4 2,5-(Me)2C6H3 Ph 4-ClC6H4 4-FC6H4 Ph
H H H Me Me Me Ph
50 80 56b 85 80 70 90
4 13 7 7 3 0 5
a Typical reaction times for complete conversion varied from 8 to 12 h. b In aqueous tert-butyl alcohol.
Chart 1
tions of benzylhalides or benzyl alcohols.20 The B12catalyzed dimerization, on the other hand, utilizes readily available styrene derivatives, mild reaction conditions, and environmentally friendly solvent systems. Several 1,2-disubstituted and trisubstituted alkenes (Chart 1) as well as alkyl-substituted alkenes that were examined proved unreactive. Mechanistic Considerations: Testing the Intermediacy of Alkylcobalamins. The transformations in Table 1 have synthetic utility, but they are more interesting from a pure mechanistic viewpoint. A solution of Ti(III)citrate (10 equiv) and vitamin B12 displays the characteristic strong absorption around 385 nm indicative of Co(I),3 suggesting the highly nucleophilic cob(I)alamin (B12 in its Co(I) oxidation state) could be the reactive form of the catalyst. Cob(I)alamins and related cobalt complexes have been used previously by several groups for catalytic or stoichiometric carbon-carbon bond formations.21-28 In general, these reports describe cou(16) Bors, D. A.; Kaufman, M. J.; Streitwieser, J. A. J. Am. Chem. Soc. 1985, 107, 6975-6982. (17) Popielarz, R.; Arnold, D. R. J. Am. Chem. Soc. 1990, 112, 30683082. (18) Maslak, P.; Champan, J. W. H. J. Org. Chem. 1990, 55, 63346347. (19) Pasternak, M.; Morduchowitz, A. Tetrahedron Lett. 1983, 24, 4275-4278. (20) For methodology starting with aryl ketones and Grignard or organolithium reagents in the presence of vanadium or tungsten catalysts, see: (a) Kataoka, Y.; Akiyama, H.; Makihira, I.; Tani, K. J. Org. Chem. 1996, 61, 6094-6095. (b) Kauffmann, T.; Jordan, J.; Voss, K.-U. Angew. Chem., Intl. Ed. Engl. 1991, 30, 1138-1139. For other metal-catalyzed transformations, see also: Billington, D. C. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Pattenden, G., Eds.; Pergamon Press: New York, 1991; Vol. 3, Chapter 2.1, p 421. (21) For reviews, see refs 8, 11, and Ali, A.; Gill, G. B.; Pattenden, G.; Roan, G. A.; Kam, T. S. J. Chem. Soc., Perkin Trans. 1 1996, 10811093. (22) Okabe, M.; Tada, M. Bull. Chem. Soc. Jpn. 1982, 55, 14981503. (23) Baldwin, J. E.; Li, C.-S. J. Chem. Soc., Chem. Commun. 1987, 166-168. (24) Branchaud, B. P.; Detlefsen, W. D. Tetrahedron Lett. 1991, 32, 6273-6276. (25) Busato, S.; Tinembart, O.; Zhang, Z. D.; Scheffold, R. Tetrahedron 1990, 46, 3155-3166.
plings between alkylhalides and alkenes, or in some cases alkene polymerizations. In the former, an alkylhalide is reacted with a cobalt(I) complex to form an organocobalt intermediate, which in turn serves as a precursor for a reactive radical via homolytic cleavage of the weak Co-C bond (e.g. Scheme 1). This radical subsequently initiates either an inter- or intramolecular addition to an alkene to generate a product radical that is then trapped by Co(II) to form a second organocobalt complex that can be further elaborated. The structures of both starting materials and reaction products in Table 1 imply that they must be formed via pathways that are different from that in Scheme 1. If the catalyst were to act as a source of radicals that would add to the arylalkene, either the regioisomeric product radical 2 or a polymer would be produced instead of the symmetrical dimerization product 1. Cob(I)alamin has been reported to react with styrene at acidic pH to provide a product that was tentatively assigned as R-phenethylcobalamin.29 The complex was too reactive to be isolated, but the authors did not report formation of dimerization products. Although cob(I)alamin reportedly29,30 does not react with styrene under the slightly basic reaction conditions used here (pH 8.0), the regiochemistry of the coupling reactions in Table 1 suggests that substituted benzylcobalamins (3) may be intermediates along the reaction pathway. Cob(I)alamin is an exceptionally strong nucleophile that readily reacts with organohalides to form organocobalamins.31,32 We therefore tested for the intermediacy of alkylcobalamins by reaction of various benzyl halides under the reaction conditions used for the coupling of alkenes (Table 2). Dimerization products were obtained in good to excellent yields with identical regiochemistry as observed for the corresponding alkenes (e.g., compare entry 1, Table 1 (26) Hu, C.-M.; Qiu, Y.-L. J. Org. Chem. 1992, 57, 3339-3342. (27) Inokuchi, T.; Kawafuchi, H.; Aoki, K.; Yoshida, A.; Torii, S. Bull. Chem. Soc. Jpn. 1994, 67, 595-598. (28) Wayland, B. B.; Poszmik, G.; Mukerjee, S. L.; Fryd, M. J. Am. Chem. Soc. 1994, 116, 7943-7944. (29) Schrauzer, G. N.; Holland, R. J. J. Am. Chem. Soc. 1971, 93, 4060-4062. (30) Cob(I)alamin has been reported not to react with styrene under neutral or alkaline conditions, but cob(I)aloximes do react with styrene under these conditions to give R-phenethylcobaloxime. Schrauzer, G. N., Windgassen, R. J. J. Am. Chem. Soc. 1967, 89, 1999-2007. (31) Schrauzer, G. N.; Deutsch, E.; Windgassen, R. J. J. Am. Chem. Soc. 1968, 90, 2441-2442. (32) Schrauzer, G. N.; Deutsch, E. J. Am. Chem. Soc. 1969, 91, 3341-3350.
Reductive Dimerization of Arylalkenes Table 2. Vitamin B12/Ti(III)-Mediated Dimerization of Alkylhalides in Aqueous Ethanol
a Reactions were complete within 1 h. b Reaction run in acetonitrile/water.
with entry 2, Table 2).33 These observations suggest that benzylcobalamins 3 may indeed be common intermediates in the coupling of alkenes and benzylhalides, although the latter reactions required significantly shorter reaction times.34 Probing the Intermediacy of Alkyl Radicals. Alkylcobalamins have weak Co-C bonds, with the bond strengths decreasing with more bulky alkyl groups.35-39 Benzylcobalamin contains one of the weakest Co-C bonds (∼23 kcal mol-1) among alkylcobalamins for which the bond dissociation energies have been determined quantitatively.36,38-41 Therefore, if alkylcobalamins such as 3 are formed in the reaction of cob(I)alamin with arylalkenes, they may produce benzyl radicals 4 that can dimerize to provide the observed products 5 (Scheme 2). Such dimerization of radicals is supported by the lack of diastereoselectivity that was observed in the reaction products in Tables 1 and 2. If the coupling products were derived directly from alkylcobalamins, some stereoselectivity would be expected by virtue of the chirality of B12, as was previously found in various synthetic processes.7,42-46 (33) Reductive dimerization of benzylhalides using stoichiometric (PPh3)3CoCl has been reported. Momose, D.-I.; Iguchi, K.; Sugiyama, T.; Yamada, Y. Chem. Pharm. Bull. 1984, 32, 1840-1853. (34) This difference in reaction rates implies that the overall rates cannot be governed by a step after the formation of the common intermediate. Instead, the kinetics of formation of the intermediates must be different for the two classes of compounds. This is not unexpected given the anticipated difference in mechanism of alkylcobalamin formation from alkenes vs alkylhalides. (35) Chemaly, S. M.; Pratt, J. M. J. Chem. Soc., Dalton Trans. 1980, 2274-2281. (36) Schrauzer, G. N.; Grate, J. H. J. Am. Chem. Soc. 1981, 103, 541-546. (37) Kim, S.-H.; Chen, H. L.; Feilchenfeld, N.; Halpern, J. J. Am. Chem. Soc. 1988, 110, 3120-3126. (38) Brown, K. L.; Brooks, H. B. Inorg. Chem. 1991, 30, 3420-3430. (39) Waddington, M. D.; Finke, R. G. J. Am. Chem. Soc. 1993, 115, 4629-4640. (40) Nome, F.; Rezende, M. C.; Sabo´ia, C. M.; da Silva, A. C. Can. J. Chem. 1987, 65, 2095-2099. (41) Andruniow, T.; Zgierski, M. Z.; Kozlowski, P. M. J. Am. Chem. Soc. 2001, 123, 2679-2680. (42) Maihub, A.; Grate, J. W.; Hui Bi, X.; Schrauzer, G. N. Z. Naturforsch., B: Anorg. Chem., Org. Chem. 1983, 38B, 643-647. (43) Davies, A. G.; Golding, B. T.; Hay-Motherwell, R. S.; MwesigyeKibende, S.; Ramakrishna Rao, D. N.; Symons, M. C. R. J. Chem. Soc., Chem. Commun. 1988, 378-380. (44) Bonhoˆte, P.; Scheffold, R. Helv. Chim. Acta 1991, 74, 14251444.
J. Org. Chem., Vol. 67, No. 3, 2002 839 Scheme 2
Given the well-established radical polymerization of styrene, it appears unlikely at first glance that two benzylic free radicals 4, present in very low concentrations, would combine to give the observed products in the presence of excess alkene.47 However, a look at the rate constants of these competing processes shows that such a mechanism is feasible. If one takes the known rate coefficient for the propagation step in homopolymerizations of bulk styrene, 85 M-1 s-1 at 25 °C,48-51 as a reasonable approximation for the rate constant for addition of radical 4 to the alkene (kp), the rate of formation of products such as 2 from radical 4 would be 85 × [styrene][4]. On the other hand, the rate constant for the competing dimerization of two radicals 4 will be close to diffusion-controlled (kd ≈ 109 M-1s-1 at room temperature), giving a rate of 109 × [4]2 for the production of products 5 from 4. Thus, at the alkene concentrations used (4 mM initial) the radical concentration needs to exceed only ∼2 × 10-9 M to provide a >10-fold preference for dimerization over polymerization. For substituted styrene derivatives the concentration of radicals can be even lower as the propagation rate coefficient is even smaller (e.g., 0.62 M-1 s-1 for R-methylstyrene).52,53 Indeed, the higher yields obtained from R-substituted styrene derivatives (Table 1) may reflect this higher (45) Anderson, R. J.; Dixon, R. M.; Golding, B. T. J. Organomet. Chem. 1992, 437, 227-237. (46) Hisaeda, Y.; Nishioka, T.; Inoue, Y.; Asada, K.; Hayashi, T. Coord. Chem. Rev. 2000, 198, 21-37. (47) In this scenario, the observed R,R-dimer products 1 could form directly by dimerization of benzylic radicals or by rearrangement of initially formed R,para- and/or R,ortho-semibenzenes. (For a detailed discussion, see: Langhals, H.; Fischer, H. Chem. Ber. 1978, 111, 543553. Skinner, K. J.; Hochster, H. S.; McBride, J. M. J. Am. Chem. Soc. 1974, 96, 4301-4306.) In fact, small but significant amounts (3%) of the R,p-dimer were observed in entry 4, Table 1. When this reaction was stopped after a short reaction time (1 h,
