4440 J. Org. Chem., Vol. 44, No. 24, 1979 Notes from acetic acid mp 16142 "C; IR 2630 cm-' (brd, OH); 'H NMR Chart I 11.22 (brd s, 1 H, OH),7.72 (m, 2 H) and 7.47 (m, 3 H) (C6H5), 2.10 (5, 3 H, CH3). Anal. Calcd for Cloll,N20Cl: C, 57.57; H, 4.35; N, 13.42; C1, 16.99. Found: C, 57.84; H, 4.43; N, 13.26; C1, 17.02. Vilsmeier Reaction of la. Reaction of la under the conditions above gave, from the ether extract, 27% of 4a, mp 210-11 "C," which was identical with a sample prepared by chlorination of 1 2 3 3,5-diphenylpyrazole with S02C12.5 A = 5.80 A = 3.90 A = 5.34 Neutralization of the basic aqueous solution gave 48% of 3a: mp 191-93 "C (from acetic acid); IR 2650 cm-' (brd, OH), 'H NMR 8.50 (brd, 1 H, OH), 7.15-7.60 (m, 10 H, C6H5). Anal. Calcd for Cl!jH11N20C1: C, 66.55; H, 4.10; N, 10.35. Found: C, 66.67; H, 3.84; N, 10.28. Vilsmeier Reaction of 2b. A solution of 1.7 g (11 mmol) of POCl, in 10 mL of DMF was cooled below 10 "C and treated with 1.74 g (10 mmol) of 2bl. The solution was stirred at room tem- 4 5 perature for 150 min, poured into 100 mL of ice-water, and A = 4.67 A = 3.20 neutralized with NaHC03. The solid was collected, washed with water, and dried to give 1.49 g (77%) of 4b. ence of benzene annelation on homoaromatic systems Vilsmeier Reaction of 2a. Reaction using the same method seems to be of interest. In order to achieve maximum as for 2b gave 96% of 4a. overlap of the pa orbitals in these systems, the benzene 4-Chloro-3,5-diphe1nyl-4-methyl-4H-pyrazole1-Oxide (9). rings must tilt to enable homoaromatic delocalization and A solution of 0.85 g (5.5 mmol) of POC13 in 10 mL of DMF was at the Same time the aromatic delocalization in the benzene cooled below 10 "C and treated in portions with 1.31 g (5 mmol) rings must also be maintained. This distortion and the of 8. After 60 min at 0-10 "C the solution was poured into 50 requirement a mL of ice-water and extracted with three 50-mL portions of ether. to share common bonds between benzenoid The ether solution was washed with saturated NaC1, dried (Naz- and homoaromatic components may introduce difficulties SO4), and evaporated under reduced pressure without heating. in achieving homoaromatic overlap followed by a depres- The residue was crystallized from CHzClzwithout heating to yield sion of the homoaromatic character. 0.41 g (29%) of Sa8 The homotropylium cation (1) represents the most Reaction of lb with Acetyl Chloride. A mixture of 0.95 g widely investigated system in this seriese3 The different (5 mmol) of lb in 25 mL of dry benzene was treated over 30 min environment of the protons attached to the sp3-hybridized with 0.4 g (5 mmol) of acetyl chloride in 5 mL of benzene. After carbon bridge (Hi, H,) in 1 results in a difference (A) of 2 h at room temperature, the solution was washed with water, the chemical shifts of these two protons (A = 6 6~i).l*~ dried (Na2S04),and evaporated. Recrystallization from ethanol Several mono- and dibenzannelated homotropaiim cat- gave 0.57 g (55%) of 3b. ions (2-5) have been investigated in recent In Acknowledgment. Y.I.K. wishes to acknowledge the these cations, the comparison of the A values (ppm) of the support of Hong-Ik University and the Ministry of Edu- bridgehead protons showed decreased values attributed cation of the Republic of Korea during the course of this to benzene annelation (Chart I). However, inspection of work. the magnetic characteristics of the benzannelated homo- aromatic species 2-5 reveals two significant phenomena: Registry No. la, 59434-82-1; lb, 55026-66-9; 2a, 59434-85-4; 2b, (a) When benzene annelation is adjacent to the bridge, 59434-84-3; 3a, 71582-22-4; 3b, 71549-27-4; 4a, 71549-28-5; 4b, as 71549-29-6; 6b, 71549-30-9; 8, 17953-33-2; 9, 61355-02-0; 3,5-di- in 2 and 5, A is significantly decreased relative to A for L4J phenylpyrazole, 1145-01-3. On the other hand, a remote benzene annelation with respect to the bridge, as in 35 and 4,6decreases the mag- nitude of A to a lesser extent relative to A for 1. (b) In (12) A meltine Doint of 179-80 "C for this comDound has been re- homotropylium cations where benzene annelation is re- ported: Grandbeyg 1. I.; ECost, A. N. J. Gen. Chem. Lk3SR (Engl. Transl.) 1961, 31, 3454. mote with respect to the sp3 bridge, the protons attached at the a position to the bridge show an upfield shift relative to the shift of the other peripheral protons (6H. is 5.5 and 4.2 ppm in 3 and 4, respectively). It therefore seems that the location of the benzannelation relative to the bridge 1,2:6,7-Dibenz:o-1,7-homotropylium Cation. rather than the extent of benzannelation influences the Examination of Steric Effects Operating in homoaromatic overlap. Honioaromatic Overlap Results and Discussion Itamar Willner, Abrayham Minsky, and Mordecai Rabinovitz' To gain insight into the parameters operative in achiev- Department of Organic Chemistry, The Hebrew University ing homoaromatic overlap in benzannelated homo- of Jerusalem, Jerusalem, Israel tropylium species we studied the 1,2:6,7-dibenzo-l,'l-homo- tropylium cation (6). This system includes a double ben- Receiued April 17, 1979 (2) (a) Haddon, R. C. J. Am. Chem. SOC.1975,97,3608. Tetrahedron Monohomoaromatic systems include an sp3-hybridized Lett. 1974, 2797. (b) Goldstein, M. J. J. Am. Chem. SOC.1967,89,6357. carbon inserted into a cyclic framework of pa orbitals (c) Bischof, P.; Gleiter, R.; Heilbronner, E. Helo. Chim. Acta 1970, 53, containing 4n + 2 T electrons, with the result that the 1425. (d) Hehre, W. J. J. Am. Chem. SOC.1972,94,8908; 1973,95,5807; topology of the system is no longer planar.'I2 The influ- 1974, 96, 5207. (3) v. Rosenberg, J. L.; Mahler, J. E.; Pettit, R. J. Am. Chem. SOC. 1962, 84, 2842. (4) Merk, W.; Pettit, R. J. Am. Chem. SOC.1968, 90,814. (1) For general reviews see: (a) Winstein, S. Chem. SOC.,Spec. Publ. (5) Corver, H. A.; Childs, R. F. J. Am. Chem. SOC.1972, 94, 6201. 1967, No. 21, 5. (b) Hapood-Farmer, J. Chem. Reu. 1974, 74, 315. (c) (6) Childs, R. F.; Winstein, S. J. Am. Chem. SOC.1967, 89, 6348. Story, P. R.; Clark, Jr., B. C. Carbonium Ions 1972,3,1007. (d) Paquette, (7) Mattescu, G. D.; Nenitzescu, C. D.; Olah, G. A. J. Am. Chem. SOC. L. A. Angew. Chem., Int. Ed. Engl. 1978, 17, 106. 1968, 90, 6235. 0022-326317911944-4440$01.00/0 0 1979 American Chemical Society Notes J. Org. Chem., Vol. 44, 24, 1979 4441 Table I. Chemical Shifts" of 7 and the Homotropylium Cation 69311 7 6.80 5.70 4.95 7.10b 3.95 3.90 6 9.18 8.05 9.18 8.14, 8.05 7.70, 8.31 4.83 2.52 A(6 H~)" 2.38 2.35 4.23 1.04, 0.95 0.60, 1.21 0.88 4.38 " Relative to Me,Si ilexternal, capillary). Center of multiplet. = fiH1(6)-- SH,(7). Scheme I for the benzene ring protons, an A2B pattern of the pe- CI CI ripheral homoaromatic protons H,, H4, and H5, and an AB pattern, assigned to the bridgehead of protons Hi and H, (inner and outer protons) (Table I). In 6 proton H, is shifted paratropically (-1.38 ppm), and the two protons Hi and H, differ in their chemical shifts (A = 2.4 ppm) (Table I). These phenomena are rational- ized by the induced diamagnetic ring current resulting from a homoaromatic overlap of 67r electrons. It should be noted that the value for A of 6 is the lowest observed in the benzannelated homotropylium cations and is in line with the suggestion that adjacent benzannelation to the aromatic bridge reduces this value significantly. Further- more, it should be noted that this reduced value originates mainly from a reduced paratropic shift of H, (SHl is 2.52 ppm in 6, as compared with 1.50 and 0.55 ppm in 5 and 4, respectively). On the other hand, the peripheral protons H3, H4, and H5as well as the benzene ring protons (H,-Hd) LiCl - are significantly shifted to lower field (Table I). DMF To evaluate the effect of the developed induced ring current resulting from homoaromatic overlap on the chem- 12 13 ical shifts in 6, we compared its chemical shifts with those of the Huckeloid cation 15. This ion schematically rep- 14 7 zannelation adjacent to the homotropylium bridge and 15, R = H may shed light, on the influence of benzannelation upon 16, R = CI homoaromatic chara cter. resents an abstraction of the homoaromatic bridge from 6 and is related to it in the same sense as the tropylium cation is related to the homotropylium cation (1). Thus, the comparison between the peripheral protons (H3,H,, H5) of 6 and the respective protons of 15 may reveal the 7 6a effect of the induced ring current on the chemical shifts of the homoaromatic species.