摘要:
CHEMICAL COMMUNICATIONS, 1968 Enhancement of Equilibrium Acidities of Hydrocarbons by Polyfiuoroaryl Substitution? By ROBERT FILLER* and CHEN-SHEN WANG (Departmeat of Chernistyy, Illinois Institute of Technology, Chicago, Illinois 60616) WE have prepared a number of pentafluoro- and tetrafluoro-phenyl-substituted hydrocarbons and have evaluated the influence of these fluorophenyl groups on the equilibrium acidities of these com- pounds. Friedel-Crafts alkylation of pentafluorobenzene by CH,Cl, or CHC1, gives (C,F,),CH, (I) and (C,F,) ,CH (11) , respectively.1 Similarly, (p-MeC,F,) ,CH (111) and ($-MeOC,F,) ,CH (IV) are readily prepared from the corresponding tetra- fluorobenzene. Compound (11) can also be pre- pared in 70% yield by reaction of tris(penta- fluoropheny1)methanol and phosphorus tri- bromide.2 The formation of bromine after a 1 hr. induction period, suggests that reaction proceeds via the bromide, which is not isolated (equation 1).6-6f HBr (CtjF5) 3COH PBr, + [(C,F,),C-Br] (C,F,),CH + Br, + HBr + POBr (1) (11) The polarization of the C-Br bond to give some “positive” bromine character is likely, as three pentafluorophenyl groups are electron-attracting, and finds experimental support in the unusual stability of the tris(pentafluorophenyl)carbanion, When (C,F,),CHBr is treated with n-butyl- lithium at - 70°, sym-tetrakis(pentafluoropheny1)- ethane (V) is obtained in 35-40% yield (equation (C6F5) 3‘-* 2). - 70” (C,F,) ,CHBr + n-BuLi - (C,F5) ,CH- + Gr -/- (C6F,) ,CH-CH(C,F,) , c (C,F,) ,CHBr (2) (V) The equilibrium acidities of compounds (1)-(V) were determined spectrophotometrically using Streitwieser’s technique3 [lithium cyclohexylamide (LiCHA) in cyclohexylamine] .It was not possible to me 9-phenylfluorene as the reference hydro- carbon, since its anion absorbs strongly in the same region (ca. 420mp) as the carbanions (lithium 287 carbanide ion pairs) of the polyfluoroaryl hydro- carbons. However, fluorene (pK, 22433)3 proved to be a very satisfactory reference acid. The average values (four or five runs) of the equilibrium constants between pairs of compounds competing for a limited amount of base are shown in Table 1. From these data an acidity order for compounds (1)-(V) toward LiCHA can be derived (Table 2). The acidities of all five compounds lie in the pK range 15.8-22.7.TABLE 1 Average values of equilibrium constants in reaction with LiCHA HA HB K Log K (I) Fluorene 38 f 6.6 1.68&0-06 (V) Fluorene 1*46& 0.02 0*16&0*006 33.5 f 4.9 1.63f0.06 109 f19.5 2.04f0.10 99.8 2-3 1-99,tO.Ol 25.4 f 2.5 1*40*0-04 (1) (V) (11) (111) (IV) (1) (111) (IV) TABLE 2 Acidity order of jluorohydyocarbons toward LiCHA Relative Compound log K PK Fluorene . . .. .. 0 (22.83) (C,F,),CHCH(C,F6)2 . . . . 0.16f0.006 22.6, .. . . 1*68+0.06 21.2, (p-MeOC,F,),CH . . (C8F5)3CH * * .. . . 7*01&0*21 15.8, . . 3.57f0.07 19.26 (C6F6)2CH% (p-MeC,F,),CH . . . . 4.97f0.11 17-86 Comparison with the equilibrium pK,’s of triphenylmethane (3 1 -48) and diphenylmethane (33.1)3 shows that substitution of each phenyl group by a pentafluorophenyl group results in an enhancement of the acidity by 5-6 pK units.Electron-releasing substituents in (111) (p-Me : inductive) and (IV) (p-OMe: mesomeric) de- stabilize the carbanion and cause a diminution of acidity relative to (11). Streitwieser has concluded from kinetic acidity studies4 that more than half of the enhanced acidity of triphenylmethane over that of saturated hydro- carbons is due to the inductive effect of the benzene I. Presented, in part, a t the 154th meeting of the American Chemical Society, Chicago, Illinois, September, 1967.288 CHEMICAL COMMUNICATIONS, 1968 rings. Less than half can be attributed to reson- ance-stabilization of the carbanion, owing to steric restraints imposed by three aromatic rings, which cause significant departure from coplanarity.There is relatively little difference (A pK = 1.62) between triphenylmethane and diphenylmethane, since in the latter, resonance-stabilization of the carbanion makes a more important contribution. In (C,F,),C-, steric restraints should be even more demanding than in (C,H,) &-. Moreover, relief of C-F dipole-dipole repulsions should cause a further departure from coplanarity. Therefore, we ascribe the remarkable enhance- ment of acidity in polyfluoroaryl hydrocarbons primarily to the strong inductive influence of the C,F, and fi-RC,F4 groups. The large difference in acidity between (I) and (11) (ApK = 5.4) is indicative of this effect. We thank the National Institutes of Health for a grant. (Received, JanNary 29ad, 1968; Corn. 012.) W. F. Beckert and J. U. Lowe, jun., J . Org. Chem., 1967,32, 582. R. Filler, C. S. Wang, M. A. McKinney, and F. N. Miller, J . Amer. Chem. SOC., 1967,89, 1026. A. Streitwieser, jun., R. A. Caldwell, and M. R. Granger, J . Amer. Chem. SOC., 1964,86, 3578. * A . Streitwieser, jun., J. H. Hammons, E. Ciuffarin, and J. I. Brauman, J . Amer. Chem. SOL, 1967, 89, 59; A. Streitwieser, jun., E. Ciuffarin, and J. H. Hammons, J . Amer. Chem. Soc., 1967, 89, 63.
ISSN:0009-241X
DOI:10.1039/C19680000287
出版商:RSC
年代:1968
数据来源: RSC