116 Bulletin of Magnetic Resonance

H-1 and C-13 NMR Spectra of the CarbanionsProduced from Phenylpropene Derivatives

Akihiro Yoshino, Kensuke Aoki, Masahiro Ushio,and Kensuke Takahashi

Department of Applied Chemistry, Nagoya Institute of Technology,Gokiso-cho, Showa-ku, Nagoya 466, Japan

Abstract l,l-Diphenyl-2-methyl-l-propene produces an equimolar mix-ture of two carbanions in contact with excess potassium-sodium alloy intetrahydrofuran. The carbanions can be interpreted as the products of dispro-portionation reaction between two radical anions produced by one-electronreduction of the phenylpropene with alkali metal.

1 IntroductionVarious carbanions can be pre-

pared from substituted ethyleniccompounds (1) in contact with al-kali metal in tetrahydrofuran(THF).In these reactions a radical anion(2) is formed as shown in Scheme 1

Scheme 1and its reactivity or stability willdepend on the nature of the sub-stituents. The three routes of re-actions can be considered as fol-lows. (1) If the ethylene has bulkysubstituents on both Ci and C2, thecorresponding ethylene dianion isproduced (Scheme l).[l-4] (2) Ifthe ethylene has no or stericallysmall substituent such as only onemethyl group on either Ci or C2, thecorresponding dimer dianion is pro-duced (Scheme 2).[5-7] (3) In thepresent paper, three title phenyl

R, © • _R2 X >C,-C2<_ THF

Scheme 2propenes (la-lc, where Ri and R3are fixed to C6H5 and CH3,respectively) are used as startingmaterials (Table 1). The proposedreaction scheme for the carbanionscan be represented as given inScheme 3. A characteristic point isthat the reduction products are anequimolar mixture of two

© • .R, R, . 6 R, R. e R,C,-Cj< 3 > '>C,-CH< 3 + >C,-C2< '

^ R4THF R / V R 4 R / 2 K R 42 X

2 5 6Scheme 3

carbanions, 5 and 6 respectively,whose structures are confirmed by!H and 13C NMR spectra. One ofthese anions(5) is a phenylalkylcarbanion and the other is a phenyl-allyl carbanion (6). Thus the resultmay be interpreted as a dispro-

Vol. 14, No. 1-4 117

portionation reaction between twoanion radicals generated first byone-electron reduction of thestarting phenylpropene with alkalimetal. As far as we know, this typeof simultaneous formation of twodifferent carbanions has not yetbeen reported. The conditions ofthese reactions will be discussed interms of steric bulkiness of thesubstituents.2 Experimental

The starting materials were pre-pared from dehydration of the cor-responding phenylpropanols whichwere prepared by Grignard reactionand followed by dehydration withanhydrous acetic acid. The startingmaterials dissolved in THF or THF-d8 were kept in contact withpotassium-sodium alloy in vacuumat room temperature for about 24 h.The resulting dark red solutionswere filtered, concentrated if nec-essary, and then sealed into a 5-mmNMR sample tube. Their concentra-tions were about 1 M. The *H and

13C NMR measurements were car-ried out at 22°C using Varian XL-200 or Unity-400 spectrometer.The chemical shifts were evaluatedfrom the upfield peak of THF orTHF-dg, used as an internal refer-ence. This peak was taken as 1.79or 1.75 for lH and 26.4 or 26.0 ppmfor 13C resonances, respectively,from TMS.3 Results and Discussion1H NMR Spectra of the Carbanions.

A typical lH NMR spectrum andchemical shifts are shown in Fig. 1and Table 1, respectively. In Fig. 1,there are three signals in the regionfrom 0 to 3 ppm, which are as-signed to one methyl and one iso-propyl groups. There are twoethylenic signals in the region from3 to 4 ppm. One problem in thespectrum is the origin of hydrogenin the isopropyl group. To clarifythe origin, the experiments usingtwo different solvents, THF andTHF-d8 were carried out. Sincethese two spectra are similar each

Table 1. *H Chemical Shifts of Related Carbanions and Their Precursors.

No.

11155

5

666

abca

•b

c

abc

R ?

PhPhCH3PhPh

CH3

PhPhCH3

R 4

CH3C2H5CH3CH3C2H5

CH3

CH3C2H5CH3

22

2

H 2

.904

.591

.540

t

1

443

H 3rans c i

1.8621.107

. 5 8 0 a ) 1.1.1151.114

0.875

. 0 3 5 b > 4 .

. 3 7 5 b ) 4 .

. 1 4 1 b > 3 .

s

780

695811343

a )

b )

b )

b )

H o( H o )

1

11

6.7416.756

5.0845.3687.1197 .0916.584

H m( H » - )

7 .247 .257.14

6.5936.563

6.0766.0446.7116.6616.656

H p

1

1

5.7605.726

4.364

6.1215.9665.735

H o t h eCH3

1.8421.950

0.872

1.287

1.8701.0681.8581.971

r s

CH2

2.802

1.4671.628

2.252

a) Trans(R4) and cis(Ra) configurations are defined forb) They are defined for Ci.

118 Bulletin of Magnetic Resonance

l6Hm 6HMe 5H3

5Ho

6Ho

P h C H 3 P h C H s\ - / \ - /

C-C H + C-C/ 1 2 \ / 1 2 ^

P h 3 C H i P h 3 C-H- /H

5a 6a

8 46 I ppm

Fig.l !H NMR spectrum of an equimolar mixtureof 5a and 6a in THF-d8 at 200MHz.

6Cm

5Cm

6C2

6Ci 5Ci

6 C o P h e n , P h C H ,

\ - / \ - /C-CH + C-C

5CO / 1 2 \ / 1 2 VP h 3 C H j P h 3 C - HH

5a 6a6Cp

5Cp 6CMe5C3

6C35C1

Jt5C2

1 f160

I0

l l l 1 1 i i

80l i i i i i i

61 ppmFig .2. 13C NMR spectrum of a mixture of 5a

and 6a in THF-d8 at 50.3 MHz.

0

Vol. 14, No. 1-4 119

other, it must be concluded that theCH hydrogen in the isopropyl groupof 5a is coming from another radi-cal anion (2), but not from the sol-vent. This is also supported by anexperimental fact that the productratio of 5 and 6 is 1 to 1. Anotherpoint of interest is that twomethylene hydrogens of 5b givedifferent chemical shifts, 1.467and 1.628 ppm, respectively. Thisis ascribed to the presence of anasymmetric center on C2.13C NMR Spectra of the Carbanions.A typical 13C NMR spectrum andchemical shifts are shown in Fig. 2and Table 2, respectively. In Fig. 2,there are 15 signals for a mixtureof 5a and 6a except for two solventpeaks. One methyl signal is over-lapped with a more shielded solventpeak. The carbon species can bedifferentiated by the DEPT tech-nique. The assignment of each peakis shown in Fig. 2.IH and 13C NMR Chemical Shifts.All the *H and 13C except for C2 andCj of the carbanions (5 and 6) showupfield shifts as compared withthose of the starting materials.(1)These upfield shifts are mainlyTable 2. 13C Chemical Shifts of Re

ascribed to the extra negativecharges on carbons of the carban-ions.

Since the carbanions have theirstability due to partial delocaliza-tion of the extra charge into thephenyl rings, phenyl rings arenecessary for formation of a stablecarbanion. In fact, tetramethyl-ethylene does not react with apotassium-sodium alloy in asimilar condition used for thepresent study.

The extent of charge delocaliza-tion can be thus estimated by com-parison of the *H or 13C NMR chemi-cal shifts. The *H and 13C chemicalshifts of Hp and Cp of 6a are lessshielded than those of 5a, by about0.361 and 5.49 ppm, respectively.These shift differences dividedby 10.7 and 160 ppm, respectively,gave about 0.034 unit of extracharge difference on Cp. [8]Therefore, the chemical shiftdifference between Hp or Cp of 6aand 5a is explained in terms oftheir localized excess charges. Thesame is true for 6b and 6c incomparison with 5b and 5c, re-spectively. The especially large

lated Carbanions and Their Precursors.

No. R 2 R.4 C(C

C) (c

V-» o t h e r s

CH3 CH2

l a Ph CH3 1 3 8 . 4 8 1 3 1 . 2 41 b Ph C2H5 138 . 47 1 3 6 . 7 4

1 c CH3 CH3 1 4 6 . 1 1 1 2 7 . 4 7

2 2 . 7 1 1 4 4 . 2 2 1 3 0 . 5 8 1 2 8 . 6 1 1 2 6 . 8 5 - - -1 9 . 2 6 1 4 4 . 2 5 1 3 0 . 1 7 1 2 8 . 6 4 1 2 6 . 8 3 1 3 . 6 4

1 3 0 . 4 0 1 2 8 . 7 1 1 2 6 . 8 91 3 1 . 1 4 1 2 9 . 0 8 1 2 8 . 7 1 1 2 6 . 5 5 2 2 . 3 3

29.20

5 a5 b5 c

Ph CH3Ph C2H5CH3 CH3

90 .2 689 .5678 .19

30 .3 038 .2429 .10

2021221921

6407114676

6 a Ph6 b Ph6 c CH3

CH3 89.61 147.54C2H5 88.65 156.93CH3 80.24 144.30

144.28 118.75 128.90 107.91 —145.02 118.77 128.81 107.80 14.19 29.45135.54 105.30 130.75 88.50 12.09

105.47 130.2593.16 148.38 124.86 128.61 113.40 24.7697.66 148.25 123.21 128.61 112.11 15.87 29.9476.36 146.24 118.64 128.59 107.46 19.39

27.07

120 Bulletin of Magnetic Resonance

difference between 6c and 5c, isascribed to the delocalized capacityfor excess charge consisting of onlyone phenyl ring. Therefore, Ci of 5cis more shielded by about 10 ppmthan that of 5a or 5b. The same istrue for Ci and C3 in 6c; they aremore shielded than those in 6a or6b. Thus the p-orbital of Ci canconjugate with those in both phenyland allyl groups in 6., while that of5 can only conjugate with phenylrings.

For the allyl anions the averageshift of the outer carbons (Ci andC3) is shielded by about 60 ppmthan that of the center carbon (C2),as shown in Table 2. This is a char-acteristic nature of the allyl an-ions.[12]

Dunkelbium and Brenner reportedon the same carbanion (6a) withlithium as a counterion.[13]Although their chemical shifts of6a were deshielded by about 0.1ppm, their *H NMR data are consis-tent with ours in consideration ofdifferent counterion.Internal Rotations. Three internalrotations can be considered aroundthe bonds of Ci to Ci, Ci to C2, andC2 to C3 for 5 and 6 (these willhereafter be referred to as rota-tions A, B, and C). In the *H and 13CNMR spectra of 5c, two signals forHo, Co, Cm, Hm were observed andtheir chemical shifts are given inTables 1 and 2, respectively. Thisis interpreted as a restricted rota-tion A for the phenyl ring of 5ceven at room temperature. A simi-lar observation was presented ear-lier in a methylbenzyl anion.[14] Achemical shift difference of 0.284ppm for Ho of 5c is smaller thanthat of 0.43 ppm for Ho of methyl-

benzyl anion. The cause of theshifts cannot be clarified yet; theshift differences for Ho, Co, Cm,and Hm changed to 0.284, 0.17,0.50, and 0.032 ppm. These foursites are adjacent to each other.But the values change alternatively.Therefore the origin of the shiftmay not be explained simply. On theother hand, the internal rotations Aof the phenyl groups of 5a and 5bwere not restricted at room tem-perature. At lower temperature,however, the rotation B was inhib-ited at -60°C for 5b. For 5a thearomatic carbon signals were sig-nificantly broadened at -80°C butwere not split. In comparison withthis behavior, however, those aro-matic carbon signals of 6a and 6bwere observed sharply even at-80°C. Further study is necessary.

Two Ho signals of 5c at 5.084 and5.368 ppm were correlated withtwo Co signals at 105.30 and105.47 ppm, respectively, in a 2DCH COSY spectrum for their assign-ment purpose. [15] Similarly, twoHm signals (6.044 and 6.076 ppm)are correlated with two Cm signals(130.25 and 130.75 ppm, respec-tively). Which one of the two Ho'slocates near to methyl or isopropylgroup? NOE of the methyl signal(1.287 ppm) was observed on themore shielded Ho signal (ca. 20 %).Therefore the methyl group on Ci isnearer to Ho than Ho'. NOE of an-other methyl signal (0.875 ppm)was observed on the less shieldedHo" signal (ca. 10 %).Condition of DisproportionationReaction. The condition can bediscussed in terms of bulkiness ofthe substituents. Four groups are

Vol. 14, No. 1-4 121

used as stubstituents on Ci and C2;namely H, CH3, C2H5, and CeHs. Theyare differentiated to have approxi-mately 1, 2, 3, and 5 A in their di-ameters as spherical models. Forsimplified discussion, we considerthat 2 has bulkier substituents onCi than on C2. Thus the C2 site willbe more reactive than the Ci sitefor dimerization. In this occasiondiscussion will be rather limited tothe bulkiness of substituents on C2.In cases where substituents aretwo C6H5 or one CgHs and one H,monomer dianions are formed(Scheme 1). Dimerization of theradical anion occurs in cases wherethe substituents are two hydrogensor one H and one CH3 (Scheme 2). Incases where the substituents aretwo CH3, disproportionate occurs(Scheme 3). Therefore bulkiness ofthe substituents may control theprogress of the reaction. Two alkylgroups are large in size for dimer-ization, and small for dianion for-mation, and may be suitable fordisproportionation. For dispropor-tionation reaction, the substituentmust have a hydrogen to be ab-stracted. One CH3 on C2 of la issubstituted by C2H5 in order to in-vestigate which hydrogen in twosubstituents, either CH3 or C2H5, iseasily abstracted. From analyses ofa mixture of the products, it isconcluded that the hydrogen-leavingpower is about ten times strongerin CH3 than in C2H5.Acknowledgement

The authors wish to thank Mr. K.Kushida and A. Ono (VarianInstruments, Ltd.) for their kind-ness in measuring several 2D CHCOSY spectra at 100.6 MHz.

4 References1) K.Takahashi,Y.Inoue, and R.Asami,Org. Magn. Reson., 3, 349 (1971).2) M.Morikawa, H.Matsui,A.Yoshino,and K.Takahashi,£«//. Chem.Soc.Jpn.,57,3327(1984).3) H.Fujiwara,A.Yoshino,Y.Yokoyama, and K.Takahashi,Bull.Chem.Soc.Jpn., 65, No.8, inpress (1992).4) Y. Yokoyama, T.Koizumi, andO.Kikuchi, Chem. Lett., 1991, 2205.5) K.Takahashi and R.Asami,Bull.Chem.Soc.Jpn., 41, 231 (1968).6) K.Takahashi,M.Takaki, andR.Asami, J.Phys.Chem.,75,1062(1971).7) Y.Yokoyama, et.,Bull.Chem.Soc.Jpn., 61, 1557(1988).8) The charge calculation was fol-lowed by using an empirical equa-tion proposed by Fraenkel et al.[9]and proportional factors presentedby Schaefer et al.[10,ll]9) G.Fraenkel, et al.,J.Am.Chem.Soc.,82, 5846(1960).10) T.Schaefer and W.G.Schneider,Can. J. Chem., 41, 966 (1963).11) H.Spiesecke and W.G.Schneider,Tetrahedron Letters, 1961, 468.12) D.H.O'Brien/'ComprehensiveCarbanion Chemistry," ed byE.Buncel and T.Durst, Elsvier, NewYork (1980), Vol 5A, p.273.13) E.Dunkelblum and S.Brenner,Tetrahedron Letters, 1973, 669.14) K.Takahashi, et al.,Org.Magn.Reson., 3, 539 (1971).15) The 2D CH COSY spectra weremeasured at 75.4 MHz with aHitachi R-3000 spectrometer. Theauthors wish to thank Mr. M. Tamura(Instrument Division Hitachi, Ltd.)for his kindness in measuring thespectra.