J. Org. Chem. 1986,51,1987-1991 g). All showed an IR absorption at 2100 cm-I (azido group).
Acknowledgment. We thank Dr. D. A. Wilson of University College, Cardiff, for the I3C and 360-MHz 'H NMR spectra, and USIC of Madurai Kamaraj University for IH NMR and IR spectra. One of us (S.A.) thanks the UGC for the award of a Junior Research Fellowship. Registry No. 2-Azido-l-iodo-l-phenylcyclohexane, 10147103-8; 2-azido-l-iodo-(4-methylphenyl)cyclohexane, 101471-04-9; 2-azido-1-iodo-(3,4-dimethylphenyl)cyclohexane, 101471-05-0; 2-azido-l-iodo-l-(4-methoxyphenyl)cyclohexane, 101471-06-1; 2-azido-l-iodo-l-(4-tert-butylphenyl)cyclohexane, 101471-07-2;
1987
3-azido-2-phenylcyclohexene, 101471-08-3; 3-azido-2-(4-methylphenyl)cyclohexene, 101471-09-4; 3-azido-2-(3,4-dimethylphenyl)cyclohexene,101471-10-7; 3-azido-2-(4-methoxyphenyl)cyclohexene, 101471-11-8;3-azido-2-(4-tert-butylphenyl)cyclohexene, 101471-12-9; 1-phenylcyclohexene, 771-98-2; 1-(4methylpheny1)-cyclohexene, 1821-23-4; 1-(3,4-dimethylphenyl)cyclohexene, 101471-13-0; l-(4-methoxyphenyl)cyclohexene, 20758-60-5; l-(4-tert-butylphenyl)cyclohexene,60652-09-7; 1azido-2-iodo-l-phenylcyclohexane, 25022-21-3; 1-azido-2-iodol-(4-methylphenyl)cyclohexane,101471-14-1;l-azido-2-iodo-l(3,4-dimethylphenyl)cyclohexane,101471-15-2;1-azido-2-iodol-(4-methoxyphenylcyclohexane,101471-16-3; 1-azido-2-iodol-(4-tert-butylphenyl)cyclohexane,101471-17-4; iodine azide, 14696-82-3.
Crystal Structures of Isomeric (2,6-Dioxacyclohexy1)phenols: Models for Preassociation Complexes in Acid-Catalyzed Solvolysis of Acetals' Richard D. Gandour,* Julian Tirado-Rives, and Frank R. Fronczek Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1804
Received July 22, 1985 The crystal structures of 2-, 3-, and 4-(2,6-dioxacyclohexyl)phenol are reported. The crystals of the 2-isomer are monoclinic, space group R1/c, with four molecules in the unit cell with dimensions a = 7.520 (2) A, b = 15.593 (3) A, c = 8.550 (2) A, and B = 113.13 (2)O. The crystals of the 3-isomer are orthorhombic, space group P212,2,, with four molecules in the unit cell with dimensions a = 5.617 (3) A, b = 12.127 (3) A, c = 13.394 (4) A. The crystals of the 4-isomer are monoclinic, space grou R 1 / c , with eight molecules in the unit cell with dimensions a = 18.558 (2) A, b = 5.991 (1) A, c = 18.910 (1) and 6 = 118.19 ( 1 ) O . For the 2-isomer R = 0.033 for 1041 observed reflections, for the 3-isomer R = 0.050 for 740 reflections, and for the Cisomer R = 0.040 for 1676 reflections. The analyses establish that there is intermolecular 0-Ha-0 hydrogen bonding in the 2-isomer (01-H-.02 2.761 A), the 3-isomer (01-H--02 2.730 A), and the 4-isomer (01-H-.02' 2.720 A and 01'-H-.02 2.770 A). There is no intramolecular hydrogen bonding in the 2-isomer. Differences in the lengths of the two C-0 bonds in the acetal group are sensitive to hydrogen bonding, with the longer C-O bond always containing the hydrogen-bonded oxygen. As such, the structures are models of preassociation complexes that are proposed for acid-catalyzed acetal solvolysis. Hydrogen bonding does lead to a lengthening of the C-0 bond, and the orientation of this hydrogen bond appears to control the amount of lengthening. In the 2- and 4-isomers, the lengthened bond is clinal to the aromatic ring. This is consistent with the idea that maximum overlap, hence stabilization, would occur with an electron donor group. Finally, the failure to observe intramolecular catalysi~'~ when the phenolic group is adjacent to the acetal because of the conflict between optimal orientation for hydrogen bonding and for stabilization of the incipient oxocarbenium ion is easily visualized in the crystal structure.
1,
Introduction Crystallography has risen to prominence in recent years as a major tool for understanding chemical dynamics and reaction mechanisms. The pioneering work of Dunitz and Burgi2 has inspired many physical organic chemists to search for correlations between molecular structure and reactivity. Indeed, several groupsm have reported corre(1) This work was supported by Grant GM 29128 from the National Institutes of Health. (2) For a summary and leading references see: Burgi, H. B.; Dunitz, J. D. Acc. Chem. Res. 1983,16, 153-161. (3) (a) Jones, P. G.; Kirby, A. J. J. C h m . SOC.,Chem. Commun. 1979, 288-289. (b) Jones, P. G.; Sheldrick, G. M.; Kirby,A. J.;Briggs, A. J. Acta Crystallogr., Sect. C 1984, C40,1061-1065. (c) Allen, F. H.; Kirby, A. J. J. Am. Chem. SOC.1984, 106, 6197-6200. (d) Brigg, A. J.; Glenn, R.; 1984, I%, Jones, P. G.; Kirby, A. J.; Ramaswamy, P. J. Am. Chem. SOC. 6200-6206. (e) Jones, P. G.; Kirby, A. J. J. Am. Chem. SOC. 1984, 106, 6207-6212. (4) Kanagasabapathy, V. M.; Sawyer, J. F.; Tidwell, T. T. J. Org. Chem. 1986,50,503-509. (5) Bartlett, P. D.; Combs, G. L., Jr. J. Org. Chem. 1984,49,625-630.
Bartlett, P. D.; Combs, G. L., Jr.; Le, A.-X. T.; Wataon, W. H. J. Am. Chem. SOC.1982.104. 3131-3138.
(6) Schneider,'H.-J.; Schmidt, G.; Thomas, F. J. J. Am. Chem. SOC. 1983,105, 3556-3563.
lations for a variety of structures. Crystallography is attractive for the exploration of stereoelectroniclOJ1and orientation effects in molecules that undergo catalytic reactions. Jones and Kirby,$ have shown a linear relationship between bond length and reactivity based on correlationsof crystal structural data with hydrolytic rate constants of tetrahydropyranyl acetals. The structural dataMreveal that the C-OR bond lengthens and the endocyclic 0-C bond shortens with decreases in the pK, of the conjugate acid of the leaving group. These structural changes are greater with axial rather than equatorial OR groups, suggesting that stereoelectronic factors control the magnitude of these changes. These (7) Harano, K.; Okamoto, Y.; Yasuda, M.; Ueyama, K.; Kanematasu, K. J. Org. Chem. 1983,48, 2728-2733. (8) Seeman, J. I.; Viers, J. W.; Schug, J. C.; Stovall,M. D. J. Am. Chem. SOC. 1984,106,143-151. (9) Rtichardt, C.; Beckhaus, H.-D. Angew. Chem., Int. Ed. Engl. 1980, 19.42~40. .., .__ .- -. (10) Deslongchamps, P. Stereoelectronic Effects in Organic Chemistry, Pergamon: Oxford, 1983. (11) Kirby, A. J. The Anomeric Effect and Related Stereoelectronic Effects at Oxygen; Springer-Verlag: Berlin, 1983.
0022-3263/86/1951-1987$01.50/00 1986 American Chemical Society
1988
J. Org. Chem., Vol. 51, No. 11, 1986
Gandour e t al.
acetals solvolyze by an SN1mechanism.
F
Q
( a ) HOCH2CH2CH20H, HCON(CH3)z , S 0 2 ( 0 C H 3 ) 2 , C H z C f z
This report describes single-crystal X-ray analyses of isomeric 2-, 3-, and 4-(2,6-dioxacyclohexyl)phenols,prepared12 in one step from the appropriate hydroxybenzaldehyde. These molecules contain a weak acid (phenolic 0-H) and an acetal that solvolyzes by the more common acid-catalyzed me~hanism.'~Intermolecular hydrogen bonding occurs in all isomers between the 0-H and an acetal 0. Hence these crystal structures serve as models for 'preassociation' ~omplexes'~ in acid-catalyzed acetal hydrolysis. A mechanistic study15 on acid-catalyzed hydrolysis of (2,6-dioxacyclohexyl)areneshas clearly delineated the reaction steps involved. In most cases, proton transfer occurs in an equilibrium step prior to rate-limiting C-O cleavage. Protonation of the acetal oxygen weakens the C-0 bond and facilitates cleavage. +HA
e
A
