N.M.R.SPECTRA OF ARYLDIFLUOROMETHYL ETEERS
227
Nuclear Magnetic Resonance Spectra of Aryl Difluoromethyl Ethers
by John E. Baldwin and David J. Fenoglio Noyes Chemical Laboratory, Unhersity of Ill~noiS,Urbana, I l l h i e
(ReceCsd Aug& 6, 1966)
The proton and fluorine nuclear magnetic resonance spectra for a series of five para-aubstituted phenyl difluoromethyl ethers show a regular dependence of OCHF, and OCWI chemical shifts and the geminal spin-spin coupling constant JHFon the electronic character of substituents. The value of JBBdecreases while the fluorine resonance shifts to higher fields as para substituents on the phenyl group become more electronegative. This behavior, the reverse of some previous correlations for other series of compounds, is d&cussed in terms of different mechanisms controlling chemical shifts and spin-spin coupling.
Introduction Progress toward understanding the mechanisms through which nuclear spin-spin couplings and chemical shifts in nuclear magnetic resonance (n.m.r.) spectroscopy are influenced by molecular structure depends in part on investigations of series of closely related compounds. An ideal set of compounds would permit variation of only one aapect of electronic structure at a time, so that its influence on spin-spin couplings and chemical shifts might be determined. A carefully chosen set would approximate an ideal set and might make possible evaluations of the minor contributions to a physical effect that are lost when large perturbations in structure are introduced. For this particular investigation, directed toward further understanding of electronic influences on spinspin couplings and chemical shifts in the fluoromethylene group, a series of para-substituted phenyl difluoromethyl ethers (Ia-Ie) was selected. These compounds offer many attractive features for such a study. The para-substituent b six bonds removed from the carbon of the difluoromethyl group, thus assuring that steric influences about that group will be essentially constant. The difluoromethyl group in I gives a simple triplet in proton n.m.r. and a doublet in fluorine n.m.r. spectra uncomplicated by signilicant spin-spin couplings with ring protons, thus rendering spectral analysis by firstorder procedures possible. Also, the spin-spin coupling between proton and fluorines in the ditluoromethyl group is fairly large, making possible measurements of small variations in the geminal JHF. Finally, compounds Ia-Ie are easily synthesized.1-6
x-Q-o
--CHF~
Ia, X = OCHs b, CHa c, H d, Br e, NOa Other work on the n.m.r. spectroscopy of molecules containing a fluoromethylene group haa established certain generahatiom. In fluoromethanes6and fluoroethanes,’ increasing fluorine substitution for hydrogen leads to increased values in the geminal JHF,increased deshielding of fluorine (fluorine resonance at lower fields), and increased deshielding of hydrogen. I n some other types of molecules, however, fluorine chemical shifts and proton chemical shifts respond to the electronegativity of substituent p u p s in opposite dire~tions.~-~ Resonance considerationsa and a postu(1) J. Hme and J. J. Porter, J . Am.C h m . SOC.,79,6493 (1967),m d subsequent papers. (2) L. 2. SoborovskiY and N. F. Baina, Zh. Obsbh. Khim., 29, 1142, 1144 (1969). (3) T.Miller and J. T h a s s i , J . Org. Chem.,25, 2009 (1980). (4) T.Y. Shen, 9. Lucaa, and L. H. Sarett, Tetrahedron Letters, No. 2, 43 (1961). (6) R. Van Poucke, R. Pollet, and A. DeCat, ibid., No. 7, 403 (1966). (6) L. H. Meyer and H. 5. Gutowsky, J . Phys. Chem., 57, 481 (1963). (7) D. D.Elleman, L. C. Brown, and D. Williams, J . Mol. Spedry., 7,307 (1961). (8) J. A. Pople, W. G. Sohneider, and H. J. Bernatein, ‘‘Highresolution Nuclear Magnetic Resonance,” MoGraw-Hill Book Go., Inc., New York, N. Y., 1959,p. 322.
Vdum 70, Number 1 January 1966
JOHNE. BALDWIN AND DAVID J. FENOGLIO
228
lated steric "repulsive unshielding" of fluorine by adjacent groupsg have been used to rationalize these results. Examination of the proton n.m.r. spectra of a series of compounds containing the difluoromethyl group led to the conclusion that the geminal J H Fand proton deshielding increase as the electronegativity of the attached group increases,1oin agreement with previous st~dies.~?~ These c o r r e l a t i o n ~ ~among ~ ~ ~ ~the J ~electronegativities of substituents, geminal J H Fvalues, and chemical shifts for fluorine and hydrogen are all based on responses in n.m.r. parameters to rather drastic changes in molecular structure. Variations in bond lengths, trihedral angles, hybridization, and steric factors brought about by changing one substituent for another of different electronegativity are recognizeds-10 as probably significant influence on chemical shifts and J H Fin these compounds. The contribution of substituent electronegativity in itself is thus obscured. Aryl difluoromethyl ethers Ia-Ie offer a more favorable prospect for investigating the influences of electron-withdrawing or -releasing groups without concomitant drastic variations in other structural factors. Their n.m.r. spectra are discussed in the next section.
Results N.m.r. Spectral Data. The proton and fluorine n.m.r. spectra of the five aryl difluoromethyl ethers Ia-Ie provide the data summarized in Table I. The numerical values for spin-spin coupling constants and chemical shifts, averaged over repeated measurements, showed a precision of better than *0.3 C.P.S. except as noted. The positive sign for the geminal hydrogen-fluorine coupling constants in Table I is based on experimental ~
Table I: N.m.r. Data for Aryl Difluoromethyl Ethers Spin-epin coupling,
substie uent
Structure
-Chemical Proton," 0.p.s.
OCHa
Ia Ib IC Id Ie
380.0 380.8 386.6 385.5 400.1
para
CH:
H Br NOz
shifta in OCHFFluorine, C.P.8.
-14.3 -11.0 0.0 +36.7 +115.1
*1
J m in OCHFz, C.P.8.
+74.4 +74.2" +73.gd +73.2 +72.2
C.P.S. downfield from tetramethylshe at 60 Mc./sec. C.P.S. relative to fluorines in compound IC'at 56.4 Mc./sec. positive chemical shifts correspond to shifts to higher fields. Reference 10 reports J m 74.7 C.P.S. Reference 10 reports 73.8 C.P.S.
The J o u d of Physical Chemistry
work in other systems. The CIa-H coupling constant is positive,'l H - G H couplings are negative,I2 and vicinal H-GGH couplings in ethanes are p0sitive.'~114 Vicinal H - G G H and geminal F-GH couplings in fluoroethanes16J6and fluoropropanes16are of the same sign. Hence, the geminal J H Fis positive. The same conclusion may be reached by another route. In fluorodichloromethane, J c ~ and ~ HJ H Fhave the same sign,'? so geminal J H Bis positive.11 The salient trends in the data in Table I indicate deshielding of the OCHFz proton, shielding of OCHFz fluorines, and a decrease in the geminal spin-spin coupling constant as more electron-withdrawing groups are substituted at the para position of the phenyl group.
Discussion That the proton and fluorine resonances respond in opposite directions to variations in the electronwithdrawing capability of para substituents is in part a reflection of the different mechanisms determining chemical shifts for these two nuclei. For hydrogen, the dominant contribution to chemical shift involves local diamagnetic currents.'* Electron-withdrawing groups attached to carbons bearing a proton reduce the local diamagnetic screening at the proton and thus deshield it. This effect has been observed in many systems and requires no special comment here. For fluorine nuclei, the dominant mechanism determining chemical shift depends on paramagnetic screening.lS Other things being equal, increasing the electron-withdrawing power of a substituent on a carbon bearing a fluorine should decrease the polarity of the carbon-fluorine bond, make the electron distribution about fluorine less symmetrical, and increase the paramagnetic shielding. This would shift fluorine resonances to lower fields, as is commonly observed. In the aryl difluoromethyl ethers, some additional influence must be operative to cause the observed (9) G. V. D.Tiers, J . Am. Chem. SOC.,78, 2914 (1966). (10) B. H.Arkon, T. Y. Shen, and N. R. Trenner, J . Chem. Soc., 3828 (1962). (11) A. D. Buckingham and K. A. McLauchlan, PTOC.Chem. Soc., 144 (1963). (12) F. A. L. b e t , J. Am. Chem. SOC.,84,3767 (1962). (13) M.Karplus, ibid., 84, 2468 (1962). (14) P.C.Lauterbur and R. J. Eurland, 8id., 84, 3406 (1962). (16) M.Barfield and J. D. Bddeachwieler, J . MOL Spedl-y., 12, 23 (1964). (16) D.F.Evans, S. L. Manstt, and D. D. Elleman, J . Am. Chem. SOC.,85, 238 (19631,and references cited therein. (17) G.V. D. Tiers, J . Phy8. Chem., 67, 1373 (1963). (18) Cf.ref. 8,p. 176. (19) A. Saika and c. P. Slichter, J . Chem. Php., 22,26 (1964).
N.M.R.SPECTRAOF ARYLDIFLTJOROMETHYL ETHERS
reversal in correlation between electronegativity and fluorine chemical shift. The data and theoretical expectations can be reconciled through a mechanism in which a more electronegative substituent produces a more positive environment about the proton and a more symmetrical electronic environment about the fluorine atoms. One descriptive means for couching a plausible explanation would involve consideration of the ethers as resonance hybrids of different canonical forms including I1 and I11 P
F
F I1
I11
Substituents able to delocalize negative charge effectively would tend to increase the relative contribution of form I11 in a hybrid. Were the electronic distribution about fluorines more symmetrical in form I11 than in 11, the experimental data could be accommodated. This speculative rationale, of course, neglects certain factors such as possible variations in solvent, association, or conformational effects that could well be influences on the observed chemical shifts. The experimental results of Table I with respect to fluorine chemical shift values may be compared with the observations on fluoromethanes of Meyer and Gutowsky.6 There too, a reversal of expectations based on an electronegativity effect alone was observed in the chlorofluoromethanes, and changes in molecular geometry were clearly implicated as a probable cause for the results. The instances of “repulsive unshielding” of fluorine by adjacent groupsg similarly reflect the great sensitivity of fluorine chemical shifts to local electronic symmetry. The aryl difluoromethyl ethers exhibit this effect even though the perturbing influence is well removed and the cause cannot be related to considerations involving steric bulk or interelectronic repulsions between OCHF2 and the variable group. The distant perturbing influence in I effects an anisotropic modification of electronic character at the difluoromethyl group with, perhaps, attendant small changes in bond lengths and angles; the changes in chemical shift caused by changes in local electronic symmetry overcome contributions from a purely electron-withdrawal-mediated influence. The small but regular diminution in geminal fluorine-
229
hydrogen spin-spin coupling associated with increases in the electron-withdrawing power of the para substituent in I is consistent with theory; reducing the electron density at the difluoromethyl group should reduce the magnitude of the spin-spin interaction transmitted via electron spins.2o Other correlations between the electronegativity of attached groups and geminal J E F exhibiting the opposite trend (increasing JHF as substituent groups become more electron withdrawing)6#7v10 result from another cause: changes in hybridization brought about by altering substituents, the more electronegative substituents favoring greater s character in the G H bond, give rise to larger geminal J H Fvalues.
Conclusions The generalizationsthat electron-withdrawing groups favor increased geminal fluorinehydrogen coupling and that they lead to deshielding of fluorine are not accurate when applied to aryl difluoromethyl ethers. The influence of the symmetry of electron distribution about fluorine rather than total electron density about fluorine may, as in the present case, reverse th,p correlation for fluorine chemical shifts. When different electron-withdrawing groups leave carbon hybridization in a H-C-F fragment essentially constant, more powerful electronegative substituents can reduce the geminal J E Fcoupling.
Experimental Section21 Aryl dijluoromethyl ethers were prepared by the general procedure of Miller and Thanassi3 and identsed by their spectroscopic and physical properties. p Bromophenyl difluoromethyl ether had b.p. 59” (4 mm.), nZ2D 1.5003. Anal. Calcd. for C7H6BrF20: C, 37.70; H, 2.26; Br, 35.83. Found: C, 37.85; H, 2.18; Br, 35.39. The n.m.r. spectral parameters determined for the five ethers studied are summarized in Table I. (20) H. 8. Gutowsky, D.W. McCall, and PhyS., 21,279 (1953).
C. P. Slichter, J. C h a .
(21) Analyses are by J. Nemeth and associates, Urbana, Ill. Proton nuclear magnetic resonance (n.m.r.) spectra were obtained on a Varian A-60 spectrometer with approximately 30% solutions in carbon tetrachloride; tetramethylsilane served as an internal standard. Fluorine n.m.r. spectra were determined with a Varian V4300B instrument, equipped with a super stabilizer. Chemical shifts were measured with the sideband technique using a HewlettPackard 200CD audio oscillator and Hewlett-Packard 512C frequency counter.
Volume 70,Number 1 January 1966
