K. TAKAHASHI, M. TAKAKI, AND R. ASAMI
1062
Nuclear Magnetic Resonance Spectra of Carbanions.
11. Carbanions
Produced from &-Methylstyreneand Cumyl Methyl Ether' by Kensuke Ta.kahashi,*Mikio Takaki, and Ryuzo Asami Department of Synthetic Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Japan (Received January 30,1970) Publication costs borne completely by The Journal of Physical Chemiatry
The pmr spectra of the title carbanions have been observed in a well separated shape suitable for first-order analysis. The excess charge of the carbanions, localized in the phenyl rings, is estimated from their chemical shifts and compared with that of 1,l-diphenylethylene dimer dianion. It was found that the spectrum of a-methylstyrene dimer dianion shows signals attributable to two different ortho protons at room temperature.
It was reported before that the aromatic proton spectrum of the 1,l-diphenylethylene dimer dianion can be observed in a well separated form suitable for first-order analysis.2 Similar experiments have been extended to the carbanions produced from a-methylstyrene and cumyl methyl ether by following reactions 1and 2 .
-K-Xa
~CH~-C=CHB in THF I CsH6 CHs-&-CH~-CH~-&-CH3
I
+ 2K+
(1)
+ K + + KOCH3
(2)
I
CJI6
C6Hs CH3 CH3-~-OCH3
K-Na ___f
in THF
CH3--C-CH3
b6H6
I
C6H5
Experimental Section a-Methylstyrene or cumyl methyl ether dissolved in T H F mas placed in contact with excess potassiumsodium alloy in a vacuum at room temperature for about 24 hr. The dark red solutions obtained were filtered, concentrated as far as possible, and then sealed into a 5-mm nmr sample tube. The concentration of the sealed solution was about 0.3 mol/. or more. Pmr spectra were observed at 60 MHz with a Varian A-60A or a Hitachi H-60 spectrometer at room temperature. The chemical shifts were evaluated from the higher field peak of THF, used as an internal reference. This peak of T H F was taken as 1.79 ppm from TIIS.
Results and Discussion A . Spectra of the Carbanions. Typical spectra and their chemical shifts are shown in Figure 1 and Table I, respectively. The well-separated signals allowed easy assignment. For example, the spectrum of the aromatic proton region of a-methylstyrene dimer dianion shown in Figure l a consists of four parts. A triplet signal at the highest is easily assigned to the para protons, similar to that observed in 1,l-diphenylethylene The Journal of Physical Chemistry, Vol. 76, N o . 8, 1971
dimer dianion.2 The multiplet structure and intensity indicate that the two broad doublets at 4.80 and 5.38 ppm are due to the ortho protons, which are therefore nonequivalent in a-methylstyrene dimer dianion. The shape of the doublet at 4.80 ppm is a little different from that at 5.38 ppm because of overlapping of the carbon13 satellite signal of the solvent at 4.83 ppm, as is shown in Figure IC. The meta proton signals of a-methylstyrene dimer dianion seem to have two slightly different chemical shifts. An impurity signal near 7.15 ppm in Figure l a is attributable to diphenylhexane, which is produced from the carbanion in contact with moisture. a-Methylstyrene dimer dianion treated with water gave 2,5-diphenylhexane, which shows pmr signals at 1.16 (doublet, J = 6.5 Hz), 1.44 (tripletlike multiplet), 2.56 (broad, probably multiplet), and 7.07 ppm in a carbon tet'rachloride solution. The carbanion gives a singlet signal at 1.32 ppm, attributable to the methyl group, but 2,5-diphenylhexane gives a doublet ( J = 6.5 Hz) at 1.21 ppm. The signal of the methyl protons in the carbanion therefore appears a t lower field than those in the corresponding hydrocarbon. A similar observation has been made previously with methylene protons in 1,l-diphenylethylene dimer dianion.2 This tendency may be ascribed to the change of hybridization in the carbon atom adjacent to the phenyl ring from sp3in hydrocarbons to near sp2 in the carbanions. Cumyl carbanion produced from cumyl methyl ether shows a rather simple spectrum in the aromatic proton region, as shown in Figure lb, except for two impurity peaks at 7.12 and 7.20 ppm. The methyl proton signal of the cumyl carbanion appears a t 1.47 ppm as a singlet which changes to a doublet ( J = 6.5 Hz) at 1.29 ppm in T H F on treatment with a small amount of HzO. The cumyl carbanion treated with (1) .Presented partly before the 20th Annual Meeting of the Chemical Society of Japan, Tokyo, April 1967, Abstract Val. 1, p 160. (2) K. Takahashi and R. Asami, Bull. Chem. Soc. Jap., 41, 231 (1968).
NMRSPECTRA OF CARBANIONS
1063
~~~
Table I : The Proton Chemical Shifts of the Carbanions Produced from 1,l-Diphenylethylene, Cumyl Methyl Ether, and a-Methylstyrene in Contact with Excess Alkali Metals in THF, in Ppm a t 60 MHz A -gsinme-n-t Starting material
Metal used
1,l-Diphenylethylene
Potassium Sodium Lithium Potassium
a-Methylstyrene Cumyl methyl ether
Potassium
7.0
6.5
6.0
5.5
5.0
4.5
pprn
7.0
6.5
6.0
5.5
5,O
4.5
pprn
--.
--.
Phenyl-
c
Ortho
Meta
Para
7.01 7.18 7.03 4.80 5.38 5.16
6.55 6.61 6.49 5.89 6.01 6.115
5.67 5.75 5.66 4.20
2.48 2.45 2.51 1.32
4.39
1.47 (CHs)
Figure 1. The spectra in the region of aromatic protons of the carbanions produced from a-methylstyrene (a) and cumyl methyl ether (b) in contact with excess potassium sodium alloy in THF. (c) An expanded spectrum of a part of (a). Applied radiofrequency increases from right to left near 60 MHz at a constant magnetic field.
D20 also shows the methyl signal at 1.29 ppm (triplet, J = 1 Hz). The methyl signal of cumyl carbanion appears at a lower field than that of the corresponding hydrocarbon, similar to the case of a-methylstyrene. The extracts obtained from the solution of the cumyl carbanion treated with H,O give impurity signals a t 1.63 and 7.04 ppm in addition to the signals of cumene, in a carbon tetrachloride solution. These peaks are assigned to the methyl and the phenyl protons. The singlet structure of the methyl signal shows the absence of a proton at the adjacent carbon atom. The
Others
(CHz) (CHz) (CHz) (CHs)
substance giving the signals at 1.63 and 7.04 ppm is therefore suggested to be 2,3-dimethyl-2,3-diphenylbutane. This compound may be obtained by the dimerization of cumyl radical, which is estimated to be present as an intermediate in reaction 2. These experimental results are summarized as follows. (1) The aromatic proton shifts of the carbanions produced from a-methylstyrene and cumyl methyl ether are in the order of meta, ortho, and para positions from low to high field; these are different from those of the carbanions produced from 1,l-diphenylethylene, which are in the order of ortho, meta, and para positions. (2) a-Methylstyrene dimer dianion shows two nonequivalent ortho phenyl proton chemical shifts but 1,l-diphenylethylene dimer dianion does not a t room temperature, even though the phenyl group seems to be bulkier than the methyl group. These two points mill be discussed in the two following sections. B. Comparison of the Extra Charges in the Carbanions. Since the carbanions studied here have extra charges coming from alkali metals, the charge distribution in the molecules is interesting from a theoretical point of view. If we assume that the chemical shift of an aromatic proton of the carbanions corresponds to the charge density of the adjacent carbon atom, we can
Table 11: The Proton Chemical Shifts of the Hydrocarbons Produced from the Corresponding Carbanions in Contact with Water in THF, in Ppm a t 60 MI3z Compd
Solvent
,------Assignment--CeHs CH
1,1,4,4-Tetraphenylbutane 2,5-Diphenylhexane
THF CCL THF CCl4 THF CCla THF CCla
7.20 7.14 7.15 7.07 7.20 7.14 7.12 7.04
Cumene 2,3-Dimethyl-2,3-diphenylbutane (estimated)
CHz
CHa
a
3.88 2.56 2.865
2.00 1.50 1.44
1.21 1.16 1.29 1.24
1.63
The chemical shift in the blank space is not available because of overlapping of the large solvent peaks. Q
The Journal of Physical Chemistry, Vol. 76,No. 8,1971
K. TAKAHASHI, M. TAKAKI, AND R. ASAMI
1064
Table 111: Charge Distribution in the Carbanions Produced from 1,l-Diphenylet,hylene, Cumyl Methyl Ether, and a-Methylstyrene in Contact with Excess Alkali Metals in THF, in Units of the Absolute Value of the Charge of an Electron 7
Starting material
1,I-Diphenylethylene" a-Methylstyrene Cumyl methyl ether
Metal used
Ortho
Potassium Sodium Lithium Potassium Potassium
-0.02 -0.00 -0.015 -0.205* -0.205
Position--------Meta
-0.065 -0.06 -0.07 -0.12b -0.105
a Corrected for ring current effect using the mean values obtained for model I in Table IV. ortho and meta positions in each ring are not equivalent.
estimate the charge density on the carbon atom from the observed shifts. On treatment with HzO, the carbanions form corresponding hydrocarbons, whose chemical shifts are given in Table 11. When we want to estimate the charge distribution in the carbanions, a suitable reference compound must be chosen, for example the starting materials, the corresponding hydrocarbons produced from the carbanions, or another such as benzene. I n this article we prefer to take the hydrocarbons produced from the carbanions as reference compounds. The chemical shifts of the hydrocarbons produced from the corresponding carbanions in contact with water are given in Table 11. The aromatic proton signals of the carbanions shift to higher field than those of the reference compounds, due to the delocalization of the excess charge, as estimated in Table 111. This estimation of the charge delocalization is not exact because effects other than those due to charge and ring current are neglected. At the early stage of this study we neglected the ring current effect in estimating the charge distribution in 1,ldiphenylethylene dimer dianion, because phenyl protons show almost the same chemical shifts in cumene, 2,5-diphenylhexane, and 1,1,4,4-tetraphenylbutane,the products of the reaction between the carbanions and water (see Table 11). However, the configuration of the carbon atom adjacent to the phenyl ring in these hydrocarbons is sp3 hybridized. A problem lies in the configuration of the carbon atom adjacent to the phenyl ring in the carbanions. A referee of this journal suggested that the ring current effect cannot be neglected in the estimation of the charge density distribution in these carbanions because the ortho protons give an abnormal lower-field shift in 1,1-diphenylethylene dimer dianions than those of the carbanions prepared from a-methylstyrene and cumyl methyl ether. We agreed with his comment and estimated this ring current effect, which is dependent upon the angles of twist 81 and t12 in Figure 2. Three typical models, I, 11, and 111, are taken for the estimation of the ring current effect. The angles of twist, el and e2, are measured from a standard position in the molecular plane. Both el and O2 in I are taken as the same magnitude, 30", and The J O U Tof ~Physical Chemistry, Vol. 76,No. 8,1971
Para
-0.155
-0.145 -0.155 -0.295 -0.28
Sum for a phenyl ring
-0.325 -0.265 -0.325 -0.945 -0.90
* These are average values since the two
Model
Figure 2. Models for estimating the ring current effects in 1,l-diphenylethylene dimer dianion.
in 11, go", but O1 and O2 in 111 are 0" and go", respectively. Geometric arrangement of the atoms in 1,ldiphenylethylene dimer dianion is estimated with the assumption that the carbon atom adjacent to the phenyl ring is sp2 hybridized. The C-C and C-H bond lengths are taken as 1.39 and 1.08 A, respectively, in the phenyl ring, and the C-C bond length betwoeen the phenyl ring and the adjacent carbon is 1.51 A. The twist angles of 30" in I are determined by taking the distance of two nearest ortho protons in two different twisted phenyl rings as 2.4 A, which is equal to twice the van der Waals radius of hydrogen. This twist angle in I is approximately coincident with the equilibrium angle estimated from the extended Huckel c a l ~ u l a t i o n . ~With these assumptions, the ring current effects are estimated for the three models, based upon Johnson-Bovey's calculation^.^ The results in Table IV show that model I11 gives a shielding effect at the ortho and meta positions and therefore this model cannot explain the experimental results shown in Table I. The ring current effects a t the ortho protons in model I are negligibly small as a mean, consistent with Sandel and Freedman's statement in the case of diphenylmethylcarbanion.6 If we assume a shorter distance of 2 between the two nearest ortho protons in the two phenyl rings and a smaller twist angle of 24", the ring current effect at the ortho protons amounts to 0.13 ppm as a (3) R. Hoffman, R. Bissell, and D. G. Farnum, J. Phys. Chem., 73, 1789 (1969). (4) C.E. Johnson and F. A. Bovey, J. Chem. Phys., 29, 1012 (1958). (5) V. R. Sandel and H. H. Freedman, J. Amer. Chem. Soc., 85, 2328 (1963).
1065
NMRSPECTRA OF CARBANIONS
Table IV : An Evaluation of the Ring Current Effects on Chemical Shift for the Aromatic Protons in the 1,l-Diphenylethylene Dimer Dianion in Ppm. Positive or Negative Sign Shows the Shift to Lower or Higher Field, Respectively --III-, Position
I
Ortho (near) Ortho (far) Meta (near) Meta (far) Para
-0.158 0.160
I1
dl
