Bromine-induced aromatic alkylation and dienone-phenol rearrangements

摘要

1166 J. CHEM. SOC. PERKIN TRANS. I 1989 Bromine-induced Aromatic Alkylation and Dienone-phenol Rearrangements Xiaolian Shi,t Roberta Day, and Bernard Miller * Department ofChemistry, University of Massachusetts, Amherst, MA 0 7 003, U.S.A. The first examples of a 'Friedel-Crafts' alkylation resulting from bromination of an afkene and of a dienone-phenol rearrangement resulting from bromination of an enone are reported. Addition of bromine to isolated double bonds is one of the oldest and most reliable reactions in organic chemistry. Side reactions (aside from the intervention of nucleophiles other than bromide ion) are rare, but Wagner-Meerwein type migrations do occur during reaction of bromine with alkenes which are exceptionally prone to rearrangement, such as those in the pinene' or norbornene2 series or those from which p-methoxyphenonium ions may be formed as intermediates3 Despite this evidence that the addition of bromine to isolated double bonds can result in reactions typical of carbonium ions, no reaction has previously been reported in which such attack results in alkylation of an unactivated aromatic ring in a Friedel-Crafts-like process.This is not surprising in view of the very vigorous conditions normally necessary for alkylations of aromatic rings. We now report the first example of alkylation of an unactivated aromatic ring during reaction of bromine with a cycloalkene. Reaction of 1,l-dibenzyl-3,4-dihydronaphthalen-2(1H)-one with toluene-p-sulphonohydrazide(2.8 equiv.) and hydrochloric t Visiting Scholar from Zhejiang University, Hangzhou, PRC.acid (1.4 equiv.) in refluxing methanol for 36 h converted the ketone into its toluene-p-sulphonylhydrazone,m.p. 176 178 "C (decomp.). Slow addition of butyl-lithium (2.1 equiv.) to a suspension of the hydrazone in tetrahydrofuran at OOC, followed by stirring at room temperature for 2 h, yielded the hydrocarbon (1) as a yellow oil, which was purified by chromatography on neutral alumina, Slow addition of bromine (1.1 equiv.) to a solution of (1) in carbon tetrachloride at 0 "C resulted in rapid decolourization of the bromine, but hydrogen bromide was produced during the reaction. No evidence could be obtained for formation of the expected addition product.Instead, evaporation of the solvent yielded a single compound as colourless crystals (m.p. 164-165 "Cfrom absolute ethanol). J. CHEM. SOC. PERKIN TRANS. I 1989 X-Ray crystallographic analysis established the structure of the product as the bicycloC3.3. llnonane derivative (2).* Molecular models of (1) indicate that in the preferred conformation one benzyl group lies over one face of the nonaromatic ring. This assignment is supported by the n.m.r. spectrum of (l),which shows the C-4 methylene signal at 6 2.63 p.p.m., markedly upfield from the corresponding signal which appears at 6 3.35 p.p.m. in the spectrum of 1,l-dimethyl-1,4- dihydronaphthalene. A cyclic bromonium ion derived from (1) would be expected to have a similar conformation (3).In this conformation the 'endo' benzyl group would interfere with nucleophilic attack at C-2 and C-3, and also interfere with the abstraction of those hydrogens unti to the bromine atom, thus inhibiting both addition and substitution reactions. Ph Intermediate (3), however, seems well suited for a rearrangement process, since migration of a benzyl substituent would result in formation of a tertiary, benzylic carbonium ion. A similar migration process does occur during reaction of bromine with ketone (4), m.p. 121.5-122"C, which was prepared by oxidation of (1) with t-butyl chromate (1.4 equiv.). Unlike (l), ketone (4) did not react with bromine in carbon tetrachloride solution, but addition of bromine (1.2 equiv.) to a solution of (4) in acetonitrile at room temperature resulted in loss of the bromine colour.Hydrogen bromide was again * Crystal data: C,,H,,Br, M = 389.34. Triclinic space group Pi (No.2), a = 9.566(3), b = 12.097(4), c = 18.341(4) A, a = 71.41(2), = 75.10(2), y = 66.97(3), I/ = 1 825.6(9) A3, 2 = 4, D, = 1.413 g ~m-~, F(000) = 800, p(Mo-K,) = 22.3 cm-'. 'The crystal used for the study (cut to dimensions of 0.33 x 0.33 x 0.50 mm) was mounted in a thin- walled glass capillary tube which was sealed as a precaution against moisture sensitivity. Preliminary examination and data collection were performed with graphite monochromated Mo-K, radiation (A = 0.710 73 A) on an Enraf-Nonius CAD4 diffractometer at an ambient temperature of 23 -+_ 2 "C. A total of 4 181 unique reflections were measured (+h, amp; k, amp; 1; 0 -20 scan mode, 28,,,.= 43"). An empirical absorption correction based on psi scans was applied (0.767 to 1.00on I). The structure was solved by using Patterson and difference Fourier techniques and was refined by full-matrix least-squares methods function minimized: w(lF,,I -FJ2, w: = 2F0L,/0,. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in idealized positions as fixed isotropic scatterers. The final agreement factors were R = 0.035 and R, = 0.045 for the 2 964 reflections having I 2 30,. All computations were performed on a Microvax I1 computer using the Enraf-Nonius SDP system of programs. Tables of atomic co-ordinates and bond lengths and angles are available on request from the Director of the Cambridge Crystal- lographic Data Centre, University of Cambridge Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW.Any request should be accompanied by the full literature citation for this communication. produced during the reaction. The sole product obtained from the bromination was the dienone-phenol rearrangement product, 3,4-dibenzyl-2-bromo-l-naphthol(5) (m.p. 143.5- 145 "C from ethanol). The structure of (5) was established by elemental analysis, spectroscopy, and independent synthesis by bromination of 3,4-dibenzyl- 1 -naphthol obtained by rearrange-ment of (4) in sulphuric acid-acetic acid. Ph Q t)H (4) (5) Ketone (4) was recovered unchanged from solution in acetonitrile saturated with hydrogen bromide, demonstrating that rearrangement to (5)must have occurred during reaction with bromine.This is the first reported example of a dienone-phenol rearrangement occurring under such conditions; other amp;unsaturated ketones either simply add bromine to form dibromides or eliminate HBr to yield 2-bromo-2-en- 1-one derivatives. The formation of (5)therefore suggests that the endo benzyl group in the reactive intermediate arising from bromination of (4) can inhibit even the normally facile abstraction of a proton a to a carbonyl group. The 'Friedel-Crafts' process which occurs during bromination of (1) is not observed in the case of (4); this may be due in part to such a reaction requiring the generation of a higher degree of positive charge on the attacking carbon than is compatible with the presence of the carbonyl group at C-1.In addition, previous studies have demonstrated that benzyl is an excellent migrating group in dienone-phenol rearrangements: but is sometimes less effective in simple Wagner-Meerwein ~hifts.~,~ Nonetheless, it is surprising that migration of a benzyl group during the bromination of (1) should occur so much more slowly than the normally higher energy electrophilic attack on an unactivated aromatic ring. Acknowledgements We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for a grant which supported this work. References 1 0.Wallach, Liebigs Ann. Chem., 1891, 1,264. 2 J. D. Roberts, E. R. Trumbull, W. Bennett, and R. Armstrong, J. Am. Chem. SOC.,1950, 72, 3116; H. Kwart and L. Kaplan, ibid., 1954, 76, 4072; L. Kaplan, H. Kwart, and P. von R. Schleyer, ihid., 1960, 82, 2341; S. Winstein, ibid., 1961,83, 1516. 3 K. Otsuki and T. Irino, Chem. Pharm. Bull., 1975, 23, 646; P. R. R. Costa and J. A. Rabi, Tetrahedron Lett., 1975,4535. 4 B. Miller, J. Am. Chem. Soc., 1970,92, 6252; 1974,96, 7155. 5 P. Warrick, Jr., and W. H. Saunders, Jr., J. Am. Chem. Soc., 1962,84, 4095. 6 H. 0.House, E. J. Grubbs, and W. F. Gannon, J. Am. Chem. SOC., 1960, 82, 4099; J. R. Owen and W. H. Saunders, Jr., ibid., 1966, 88, 5809. Received 27th September 1988 (Accepted 3 1st January 1989); Paper 9/00518H