5855
Stable Carbocations. CLXXIII.' Carbon-13 Nuclear Magnetic Resonance Studies of Alkynylcarbenium Ions and Alkynoyl Cations: the Relative Importance of Mesomeric Vinylic (Allenylic) Cation Forms George A. Olah,* Robert J. Spear,2 Phillip W. Westerman,2 and Jean-Marc Denis2 Contribution f r o m the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106. Received April 5, 1974
series of alkyl- and aryl-substituted alkynylcarbenium ions and alkynoyl cations have been studied by cmr and pmr spectroscopy. In the alkynylcarbenium ions, it is concluded, on the basis of the cmr chemical shifts, that the positive charge is extensively delocalized and the mesomeric allenyl cations, which are vinyl cations, contribute extensively to the total ion structure. In the alkynoyl cations, the charge is mainly localized on the oxygen atom, i.e., the oxonium ion form, although in the phenylpropynoyl cation, some charge is delocalized into the aromatic ring. On the basis of these results, the trends for competitive charge localization in oxonium ions, tertiary carbenium ions, and vinyl cations have been deduced.
Abstract: A
T
he structural properties of vinyl cations (1-111) have presented chemists with an intriguing problem in recent years.3 The large volume of experimental data which has been gathered so far is mainly indirect evidence for the intermediacy of one (or more) of the vinylic ions 1-111 in solvolyses of vinyl halide~,~-bvinyl triflate~,~b53~ and related system^,^ or in the addition of electrophilic reagents to alkyne^.^^^ Most of these data indicate that the vinyl cation intermediates in these
cated, on the basis of pmr and uv spectroscopic data, that the resonance form V was an important contributor to the total structure of these ions. Allenyl cations have recently been shown, by Schiavelli and coworkers, lo to be intermediates in the solvolysis of haloallenes, and the solvolytic data indicate that there is extensive delocalization involving the alkynylcarbenium ion forms. y..T
xd'
c,-c,=c.,-z
11, C, sp hybridized
Pa + ', R1-C,=C,-R, #'
III reactions are either of type I1 or 111. Vinyl cations have not, as yet, been experimentally observed under stable, long-lived ion conditions, despite extensive efforts in many laboratories, including ours. Alkynylcarbenium ions (IV), however, can also exist in the mesomeric allenyl cation forms (V) and therefore serve as convenient models for vinyl cations of type 11. Previous studies of alkynylcarbenium ion^^^,^,^ indi(1) Part CLXXII: G . A. Olah, G . Liang, and G. D . Mateescu, J . O r g . Chem., in press. (2) Postdoctoral research associates. (3) For reviews on vinyl cations, see (a) H. G . Richey, Jr., and J. M. Richey, "Carbonium Ions," Vol. 2, G . A. Olah and P. v. R . Schleyer, Ed., Interscience, New York, N. Y., 1970; (b) M. Hanack, Accounts Chem. Res., 3, 209 (1970); (c) G. Modena and U. Tonellato. Adcan. Phys. Org. Chem., 9, 185 (1971); (d) P. J . Stang, Progr. P h i s . Org. Chem.. 10,276 (1973). (4) Z. Rappoport and Y. Houminer, J . Chem. Soc., Perkin Trans. 2, 1506 (1973). ( 5 ) 'Z. Rappoport, T. Bassler, and M. Hanack, J . Amer. Chem. Soc., 92, 4985 (1970). (6) P. J. Stang and T. E. Dueber, J . Amer. Chem. Soc., 95, 2683 (1973). . . (7) K. Yates, G. H. Schmid, T. W. Regulski, D. G . Garratt, H.-W. Leung, and R . McDonald, J . Amer. Chem. SOC.,95,160 (1973). (8) C. U. Pittman, Jr., and G . A. Olah, J . Amer. Chem. Soc., 87, 5632 (1965). (9) (a) H. G. Richey, Jr., L. E. Rennick, A. S. Kushner, J. M. Richey, and J. C. Phillips, J. Amer. Chem. Soc., 87, 4017 (1965); (b) H. G. Richey, Jr., J. C. Phillips, and L. E. Rennick, ibid., 87, 1381 (1965).
Y..
B
c,=c,,=c.,-z
' 0
X
IV I, C, sp2 hybridized
*
v
However, proton chemical shifts alone are not, in general, sufficiently informative to give a firm indication of the charge distribution in carbocations. This is particularly apparent in the case of alkynylcarbenium ion systems where only the tertiary ions (X, Y, or Z # H) were stable and thus it was not possible to observe any direct electronic changes at C,, C, or C,. The related alkynoyl cations have not previously been prepared or studied.
Results and Discussion Because of our continuing interest in vinyl cations, we have studied the cmr spectra of the carbocations VIa-e and VIIIa-c. Cmr studies were well suited to overcome the limitations, mentioned above of the previous pmr studies because the carbon atoms C,, C, and C, can be observed directly. Carbon-13 chemical shifts cannot be directly equated with, but obviously do reflect, the trend of charge densities at carbon atoms of similar hybridization and substitution la and can thus be successfully applied to the study of charge distribution in carbocations.llb The cmr data for the ions VIa-e and VIIIa-c allowed estimates of the relative (10) (a) M. D . Schiavelli, S. C. Hixon, and H. W. Moran, J . Amer. Chem. Soc., 92, 1083 (1970); (b) M. D. Schiavelli, R . P. Gilbert, W. A. Boynton, and C. J. Boswell, ibid., 93, 5061 (1971); (c) M. D. Schiavelll, S. C. Hixon, H . W. Moran, and C. J. Boswell, ibid., 93, 6989 (1971); (d) M. D. Schiavelli and D. E. Ellis, ibid., 95,7917 (1973). (11) (a) H. Spiesecke and W. G. Schneider, Tetrahedron Lett., 468 (1961): (b) G. A. Olah and P. W. Westerman, J . Amer. Chem. SOC.,95, 7530 (1973).
Olah, et al.
13CNmr Studies of Alkynylcarbenium Ions
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Journal of the American Chemical Society
96:18
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contributions of the mesomeric forms IV and V to be made, and indicate significant contributions from the allenyl form V in the former (VIa-e) but not the latter (VIIIa-c) examples. The preparation of the alkynylcarbenium ions VIa-e from their alcohol precursors, VIIa-e, respectively, has been reported previously8 (the present work utilized these preparations with some improvement on the methods used previously and is described in the Experimental Section). The novel alkynoyl cations VIIIa-c were generated from their corresponding acyl fluorides IXa-c * using antimony pentafluoride in sulfur dioxide solution (the preparation of XIIIa-c and IXa-c is fully described in the Experimental Section and the relevant ‘H and 19Fparameters are summarized in Table 111). The carbon-13 nmr spectra were obtained by the fast Fourier transform method l 3 on a Varian Associates XL-100 spectrometer. The cmr data for the ions VIa-e and their precurser alcohols VIIa-e are given in Table I. The assignment of C,, C,, and Cy did represent some difficulty but it was possible to identify Cy in VIa and VIb by the large longrange coupling to the C,-CH, protons in the spectra obtained without proton decoupling. Dreeskamp, et al. l 4 have determined the geminal and vicina1l3C-lH coupling constants in a number of methylacetylenes (Xa-c) shown below. The analogous coupling constants were CH3-CkC3-X
Xa (X = H),“* JcH,,c, = 10.6 Hz; J c H ~ . c=~ 4.8 HZ b (X = CH2CI),’4bJ~~,.~, = 8.2; JcH,.c~= 4.3 c (X = CH2Br),14b JcH~,c, = 7.9; JCE~.C, = 4.7
determined in VIa, VIb, VIIa, and VIIb (Table I) and they clearly differentiate C,, C,, and C,. There was no observable coupling between the C,-CH3 and C, in these four examples while the inability to observe a coupling between the Cy-CH3 and C, in VIa and VIIa is probably due to the broadening of the C, signal by smaller long-range coupling to the C,-CHI protons. The aromatic and methyl resonances were identified by “off-resonance’’ decoupling experiments. The marked deshielding of C, and C, in the ions VIa-e relative to their precursor alcohols VIIa-e (Table I, shown in brackets) clearly indicate substantial contribution from the mesomeric vinylic cation form V in all cases; positive charge at C, should result in some deshielding of C, (compare XIa and b, below) while Cy should not be significantly affected. Aromatic substitution at C, and Cy results in these carbons becoming less deshielded in the carbocations relative to their precursor alcohols and this is consistent with the expected Iessening of positive charge at C, and C, by delocalization into the phenyl rings. There is clearly more delocalization of charge into the C,-phenyl group since the para carbon in the C,-phenyl group in VId and e (Table I) is significantly more deshielded, relative to the precursor alcohols VIId and e, than is the para carbon of the Cy-phenyl group. T o estimate the relative contributions of the mesomeric forms IVa and Va in VIa, we have chosen, as a (12) VIIIc can also be generated from the more readily accessible phenylpropynoyl chloride, using the same conditions. (13) (a) R. Emst, Adtian. Mugn. Resonance, 2 , 108 (1966); (b) R. Ernst and W. A. Anderson, Reti. Sci. Instrum., 37,93 (1966). (14) (a) H. Dreeskamp, E. Sackman, and G . Stegmeier, Ber. Bunsenges. P b s . Chem., 67, 860 (1963); (b) H. Dreeskamp and E. Sackman, Z . Phys. Chem., 34,273 (1962).
model for the C , carbon in the mesomeric form IVa, the tert-butyl cation XIa, l5 where the central carbon is deshielded by 257.5 ppm relative to its precursor 2-methyl2-propanol (XIb). 3-Methyl- 1,2-butadiene (XII)16 was considered as a model for C, in Va, but the deshielded central carbon of the allene is not reflected in the chemical shift of CBin the ions VIa-e; this is presumably due to the sp hybridization of Cy in Va. Therefore, 2methyl-2-propene (XIII) was used as a model-it is expected that the positive charge at Cy will not have a significant effect on the chemical shift of C, in Va; thus C, should be deshielded by 79.5 ppm ( = 144.5 - 65.0) relative to the precursor VIIa (Table I). Experimentally C, is deshielded by 204.0 ppm in VJa relative to VIIa and substituting the values from the model compounds (above) reveals that the relative contributions of IVa and Va are in the ratio of about 2:l. This conclusion is supported by the fact that the deshielding of C para in the C,-phenyl group of VId is approximately twice that of C para in the C,-phenyl group, consistent with the amount of positive charge at C, being approximately twice that at Cy. For the ion Vlc, XIa and b serve as suitable models for mesomeric form IVc (AC, = 257.5 ppm) while XIII serves as a model for Vc (AC, = 144.5 - 66.7 = 77.8 ppm). C, is deshielded by 170.4 ppm in VIc relative to VIIc and this represents equal contributions of IVc and Vc in VIc. Schiavelli concluded, on the basis of his solvolytic studies of haloallenes,lOa-d, that the intermediate allenyl cations (including VIe from 1chloro-l,3,3-triphenylallene)were extensively delocalized with more charge residing at C, than Cy, consistent with our conclusions. It should be noted, however, that the charge distributions are not reflected in the subsequent reactions of these ions since attack by solvent occurs exclusively at C, to give propargyl alcohols unless C , is sterically hindered ( i e . , VI, R1 = R2 = tBu, R3 = t-Bu or Ph), where 10-15% of the solvolyzed products originated from attack at C,.loa~c The results for VIa indicate that where there is competitive charge localization in a carbocation, a tertiary carbenium center (Le., the positive charge residing at C,) localizes approximately twice the charge as does a secondary vinylic cation center l8 (Le., the charge residing at Cy). When C y is a secondary benzylic vinyl cation18 (Vc, VIc), the charge localization at Cy is approximately equal to that of a tertiary carbenium center. The alkynoyl cations VIIIa-c, prepared from their parent alkynoyl fluorides IXa-c (see Experimental Section), can exist in the mesomeric forms XIVa-c, XVa-c, and XVIa-c, of which the latter are vinyl cations. The cmr, pmr, and fmr data for VIlIa-c and IXa-c are given in Tables I1 and 111, respectively. O=&,-C@=Cy-R XIVa, R = H b, R = CHI C, R = Ph
&C,-Cp=sC,-R XVa, R = H b, R = CH; C, R = Ph
+
O=C,=Cp=C,-R XVla, R = H b, R = CHa C, R = Ph
The C,, C,, and Cy resonances were readily differen(15) G . A. Olah and A. M. White, J . Amer. Chem. SOC., 91, 5801 (1969). (16) R. Steur, J. Van Dongen, M. DeBie, and W. Drenth, Tetrahedron Lett., 3307 (1971). (17) In SO2 solution at -40”. (18) The term “primary” has been used previously3d for the C= C+-H ion.
Olah, et al.
13C Nmr Studies of Alkynylcarbenium Ions
5858 Table 11. Cmr Data of the Alkynoyl Cations VIIIa-ca-c and Their Precursor Acyl Fluorides IXa-ca+
c,
Compd VIIIad
c,
124.1, Jc,,H= 7 . 6 ( - 17.6) 124.2, Jc,.H= 2 . 2 (-18.3) 124.21 (-25.5) 141.7, Jc,,H 5.7, Jc,.F = 318.4 142.5, J c ~ , H< 1.5, JCa,F 315.7 149.7, JC,F = 313.9
VIIIbd VIIIcd IXae IXbc IXce
CY 110.7, J c ~ , H = 54.4 J c ~ ,= H294.9 (- 20.2) (28.3) 45.9 130.8, Jc,,H= 3 . 5 Jc-,,H= 11.0 (-21.5) (36.4) 56.9 134.21 (-26.9) (39.3) 69.1 82.4, J c y , ~= 265.3, Jc,.H = 51.7 JCD,F = 105.8 Jc-,.F= 14.5 67.4, 94.4, Jc,,H= 3.5, Jc?,H= 10.5, Jc,.F = 103.5 Jc-,,F = 14.1 83.8, 94.9 JCB,F = 103.1 JC./,F = 14.0
CH3
Cipso
C.3
CUI
CP
48.9,
8.6, JCH = 138.8 (5.6)
107.1 (-9.8)
141.8 (7.6)
131.2 (1.8)
143.0 (9.9)
117.3
134.2
129.4
1-13.1
3.0, JCH = 133.3, JCF< 1 . 5 ~
~~~
The coupling constants (in Hz) were obtained directly from the spectra recorded without proton decoupling. * The numbers in parentheses refer to the difference in chemical shift between the ion and precursor. Chemical shifts are in ppm from TRIS. In SbF5-S02a t -40". e In SO2at -40". 1 The alternate assignment is equally possible. a
Table 111. Pmr and Fmr Data for the Alkynoyl Cations VIIIa-ca3*and Their Precursor Acyl Fluorides IXa-ca,c Compd VIIIad VIIIbd VIIIcd IXae
HC=C
CHa
HDl
H O
HP
F
5.21 (1.85) 2.94(1.15) 3.36, JH,F
8.22(-1.02)
C H r k C - >> H-C-C-
(22) G . A. Olah and P. W. Westerman, J. Amer. Chem. Soc., 95, 3706 (1973). (23) Converted to 6 l3C from external TMS from the original data given from external CS:, using @;?S 193.7.
The precursors of the alkynylcarbenium ions VIa-e used in this study were commercially available or were prepared by standard methods.* Propynoyl Fluoride (IXa). A mixture of propynoic acid (7 g) and benzoyl fluoride (37 g) was heated to 150". The distillate boiling in the range 24-40' was collected and redistilled to give IXa (4.6 g) as a colorless liquid, bp 23-24" (lit. 2 4 bp 22-23 "). 2-Butynoyl Fluoride (IXb). A mixture of 2-butynoic acid (5 g) and benzoyl fluoride (35 g) was heated to 150". The distillate boiling in the range 67-80" was collected and redistilled to give IXb (3.6 g) as a colorless liquid, bp 77-78': vmax (neat liquid) 2440, 1810, 1225, 1008, and 746 cm-l; mass spectrum (70 eV, 50') m/e 86 (P+), 67 (P - 19+), 58, 57,43, 39. Cmr data are given in Table 11, and pmr and fmr data are given in Table 111. Phenylpropynoyl Fluoride (IXc). Selenium tetrafluoride25 (4.5 g) was added dropwise, with stirring, to a slurry of sodium phenylpropynoate (from phenylpropynoic acid (3.65 9)) in benzene (30 ml) at 0". The mixture was stirred overnight at room temperature and worked up by the addition of water. IXc was obtained as a colorlessliquid(0.8 g), bp76-78" (10mm) (lit.zabp 53" (2mm)). Preparation of Ions. The preparation of the alkynylcarbenium ions VIa-e has been described previously! It was found, in this study, that if the carbocations VIa and b were generated by the addition of solutions of VIIa and b, respectively, in SOzClF at - 130" to a rapidly stirred (shaken) solution of 1 : 1 SbFs-FSOaH in S0,ClF at - 130°, by-product formation was completely avoided. The pmr spectrum of VIa in SOzClF at -80" was slightly different to that reported previously in SOz solution at -60°:s C,-CHs, 6 3.70 (9, JCH*,CH* = 2 Hz); C,-CH3,6 3.50 (septet). The alkynoyl cations VIIIax were prepared by adding a solution of the corresponding fluorides, IXa-c, respectively, in SOPat -80" to a slurry of SbF5 in SO, at -80". The resulting mixtures were allowed to warm up to -40" to afford complete dissolution and ionization. Proton and Fluorine Magnetic Resonance Spectroscopy. Pmr and fmr spectra were obtained using a Varian Associates Model A56/ 60A nmr spectrometer equipped with a variable-temperature probe. External TMS (capillary) and CFC13(capillary) were used as a reference for pmr and fmr spectra, respectively. Carbon-13 Nuclear Magnetic Resonance Spectroscopy. Cmr spectra were obtained using a Varian Associates Model XL-100 nmr spectrometer equipped with a broad-band decoupler, Fourier transform accessory, and a variable-temperature probe. The spectrometer was operated at 25.16 MHz and was interfaced with a Varian 620-L computer. Chemical shifts were measured from the 13C signal of 5 % 'IC-enriched TMS in a 1.75-mm capillary held concentrically inside the standard 12-mm sample tube. Coupling constants were obtained directly from the cmr spectra measured without proton decoupling.
Acknowledgment. Support of our work by the National Institutes of Health and the National Science Foundation is gratefully acknowledged. (24) G . A. Olah, M. Nojima, and I. Kerekes, Synthesis, 487 (1973). (25) G . A. Olah, M. Nojima, and I. Kerekes, J. Amer. Chem. SOC.. 96, 925 (1974).
Olah, et al. 1 13C Nmr Studies of Alkynylcarbenium Ions
