696
(14) (15) (16)
(17) (18) (19) (20)
(21) (22)
J Org. Chem., Vol. 43, No. 4, 1978
Zupan and Sket
determined hydrogendonor abilities of organosilanes are substantially the same as those reported for organosilane reductions of 4-fert-butylcyclohexanone" and for olefin and alcohol reduction^.^^^^^ M. P. Doyle and C. T. West, J. Org. Chem., 40, 3829 (1975). D. N. Kusanov, Z. N. Parnes, G. I. Bassova, N. M. Loim, and V. I. Zdanovich, Tetrahedron, 23, 2235 ('1967). The reduction of A%lO)-octalinby triethylsilane represents the only possible exception to this general consideration. Addition of triethylsilane to 94ecalyl trifluoroacetate in trifluoroacetic acid produced decahydronaphthalene in a 33:67 cis:trans product ratio. The difference between this result and that reported in Table I may be due to partial direct ionic hydrogenation of A%'O)-octalin and reflect a distinctly different stereochemical requirement for that process. (a) S . Winstein and W. J. Holness, J. Am. Chem. Soc., 77, 5562 (1955); (b) C . A. Grob, W.Schwarz. and H. P. Fischer, Helv. Chim. Acta, 47, 1385 (1964). A1(*)-and A2(3)-Octalinunderwent slow addition of trifluoroacetic acid, but neither the olefin nor its addition product was reduced by triethylsilane. L. H. Sommer. "Stereochemistry, Mechanism, and Silicon", McGraw-Hill. New York, N.Y., 1965. This argument outlines the directional influence of the nucleophile (leaving group) on hydride transfer and does not assume that hydride transfer occurs with retention of configuration at silicon.~~pl A similar stereospecificity has been observed in reactions of cis- and trans-1-phenyl-4-tert-butylcyclohexanol with hydrogen chloride: K. D. Berlin, R. 0. Lyerla, and D. E. Gibbs, J. Org. Chem., 37, 528 (1972). L. H. Sommer and D. L. Hauman, J. Am. Chem. SOC., 91, 7076 (1989). This interpretation stggesrs that water increases the rate of hydride transfer
(23) (24) (25) (28) (27) (28) (29) (30) (31) (32)
from silicon in reductions of trifluoroacetate derivatives in trifluoroacetic acid. Indeed, the addition of an equimolar amount of water (based on original alkene) to 1-methylcyclohexyl trifluoroacetate in 4 molar equiv of trifluoroacetic acid increased the rate for hydride transfer from n-butylsilane by a factor of 20. M. P. Doyle and C. T. West, "Stereoselective Reductions", Dowden, Hutchinson and Ross, Stroudsburg, Pa., 1976, Part Ill. The ratio of % cisdecahydronaphthalene to YO cis4-tert-butyl-I-phenylcyclohexane (from organosilane reductions of the more reactive 4-tertbutyl-1-phenylcyclohe~anols~~) is between 1.2 and 1.4. R. M. Carlson and R. K. Hill, J. Org. Chem., 34, 4178 (1969). T. R . B. Mitchell, J. Chem. SOC., B, 823 (1970). S . Siege1 and B. Dmuchovsky, J. Am. Chem. SOC.,84, 3132 (1962). M. P. Doyle and C . T. West, J. Am. Chem. Soc., 97, 3777 (1975). E. M. Dexheimer, L. Spialter, and L. D. Smithson, J. Organomet. Chem., 102, 21 (1975). W. G. Dauben, E. C . Martin, and G. J. Fonken. J. Org. Chem., 23, 1205 (1958). A. L. Tumolo. U S . Patent 3 579 604 (1971); Chem. Abstr., 75, 48772 (1971). Analysis for A"lO)-octalin provides a minimum estimate of the contribution by this isomer. E. V. Couch, J. A. Landgrebe, arid E. T. Casteneda. J. Org. Chem., 40, 1529 11975) ~__._,_
(33) H. 0.House and W. L. Respess, J. Org. Chem., 30, 301 (1965). (34) S . Mitsui, K. Gohke, H. Saito. A. Nanbu, and Y. Senda, Tetrahedron, 29, 1523 11973). (35) Theseassignmentsare in agreement with those for the 9decalyl acetates and ethyl ethers.lob
Fluorination with Xenon Difluoride. Stereochemistry of Fluorine Addition to Phenyl-Substituted Cycloalkenes Marko Zupan* and Boris Sket Department of Chemistry and "J. Stefan" Institute, University of Ljubljana, Ljubljana Yugoslavia Received June 28,1977
Acid-catalyzed liquid-phase fluorine addition with xenon difluoride to some phenyl-substituted cycloalkenes, Le., I-phenylcyclopentene, 1-phenylcyclohexene, and 1-phenylcycloheptene,results in the formation of vincinal difluorides in high yield. The ratio of syn and anti addition depends on ring magnitude. The stereochemistryof fluorine addition to aryl-substituted cyclohexenes also depends on the substituent in the phenyl ring. The mechanisms of electrophilic addition of halogens have been widely investigated, both from the kinetic and stereochemical points of view.' Apart from the relative importance of the various kinetically significant processes, it is now known that the nature of the intermediates of the addition depends on the structure of the substrate, on the halogen, and on the reaction medium, ranging from strongly bridged ions (type C), to weakly bridged species (type B), or to open ions like A (Scheme I). If the cation is of the open structure A (X = F), a mixture of cis and trans adducts is generally expected. However, ion-pairing phenomena can cause preferential formation of the cis adduct and electronic, steric, or conformational effects can cause attack at one or the other side of the carbonium p orbital of A to be favored. On the other hand, the intermediate can have a bridged structure (C, X = Br), which will be presumably opened stereospecifically to form a trans adduct. Recently we have observed that xenon difluoride readily adds fluorine to phenyl-substituted olefins to give the corresponding 1,2-difluoroph1enylalkanesin high yield and under mild conditions.2 The ratios of dl-erythro and dl-threo difluorides formed by fluorination of alkyl- or phenyl-substituted olefins are nearly independent of the starting olefin, and in the trans series anti addition of fluorine predominates. We have suggested the formation of an open P-fluorocarbonium ion intermediate.3 The intermediate from the trans olefin collapses preferentially to an anti adduct, while the cis olefin intermediate can freely rotate about the newly formed single bond, thus assuming a sterically more favorable conformation identical with that of the trans intermediate. As an extension
Scheme I
A B C of our research, we therefore chose some phenyl-substituted cycloalkenes, i.e., 1-phenylcyclopentene, 1-phenylcyclohexene, and 1-phenylcycloheptene, as substrates in the acid-catalyzed liquid-phase fluorination reaction with xenon difluoride. We extended our studies to cycloolefins so as to eliminate a complexity which exists in an acyclic system, in which there is a possibility of rotation about the carbon-carbon single bond in the P-fluorocarbonium ion, depending on its lifetime and the energy barrier resisting free rotation about the newly formed single bond. From the data obtained it should be possible to get information about the influence of the ring magnitude on the stereochemistry of the fluorine addition. The variation of the substituent on the phenyl ring (X = H, p-OCH3, m-Cl) in 1-phenylcyclohexene will give us further information about the effect of the stability of P-fluorocarbonium ions on the stereochemistry of fluorine addition.
Results and Discussion The preparation of fluoroalkanes presents a different problem from that of other haloalkanes and necessitates a specific method of f l ~ o r i n a t i o nThe . ~ acid-catalyzed liquidphase fluorination of organic substrates with XeF2 avoids some experimental difficulties, e.g., low temperature, high-
0022-3263/78/1943-0696$01.00/0 @ 1978 American Chemical Society
J . Org. Chem., Vol. 43, No. 4 , 1978 697
Fluorene Addition to Phenyl-Substituted Cycloalkenes
Table I
3
2
n = l
n=2
n=3
n=l
3 n=2
-157.9 -186.4 4.8 52 7.5 5
-178.4 -212.0 4.81 49.5
-159.4 -189.4 4.63 45
