2780
COMMUNICATIONS TO THE EDITOR
Vol. 83
K‘
I
11 --+ R--C=NOII
-1- K ” C H 0
+x
Haas and Bender.‘ who studied the reaction oi nine para-substituted benzyl halides with the sodium salt of 2-nitropropane, showed that only p-nitrobenzyl chloride gives carbon-alkylation ; all the other benzyl halides examined yield aldehydes, evidently arising from oxygen-alkylation, It now has been found that the uniqueness of the 9-nitrobenzyl system depends not only on the pnitro group but also, and in a dramatic way, on the leaving group. For example, whereas p-nitrobenzyl chloride gives 95% carbon-alkylation the use of p-nitrobenzyl iodide results in S l % oxygenalkylation; Table I summarizes our findings. Such dependence of the reaction course on the nature of the leaving group has riot been clenionTABLE I REACTION O F @ - O ~ N C S H ~ C HWITH ~ X THE LITHILJM SALT O F 2-NITROPROPANZ I N D M F AT -16’ A S A FUNCTION OF X“ X c, 5% 0,% + NMetb 93 0 NATURE
0
-I+-
OF
c1 Clh (-1
93
0
(‘I (’I CI OTos Br
95
1
32 65 I 7 81 All reactions were run to completion using 0.25 niolar P-OZNC~H~CH and ~ X0.5 molar lithium salt except for the pentachlorobenzoate ester reaction which was 0.035 molar in ester and 0.07 molar in lithium salt. AI1 yields refer to pure products isolated except for the yo0 in runs employing the tosylate, bromide and iodide; here the aldehyde yields are by gas chromatography and are ca. 10-15’% higher than the yields of pure p-nitrohenzaldehpde 2,4-dinitrophenylhydrazone isolated. Reaction times range from ca. 35 hr. 40 17
L
for X = k M e (slowest) to 0.6 hr. for X = I (fastest). Repetition of the p-nitrobenzyl iodide experiment using the X = C1 reaction time (20 hr.) did not alter the yield of carbon alkylate. * Run a t 25”.
strated before, either in the alkylation of nitroparaffin salts, or in the alkylation of any other ambident2anions. In contrast, the unsubstituted benzyl system shows no leaving group effect. The reactions of benzyl chloride, bromide, iodide or tosylate with the lithium salt of 2-nitropropane in DMF all give 8284% yields of benzaldehyde and ca. 1% yields of the carbon alkylate (2-benzyl-2-nitropropane). It is noteworthy that whenever carbon alkylation is observed in the p-nitrobenzyl system the reaction rate is much faster than for the corresponding benzyl compound whereas when the p-nitro group jq unable to effectuate carbon alkylation of the ?-nitropane anion i t simultaneously fails to produce a large increase in rate relative to the corrrsponding benzyl compound (Table 11). This suggests that oxygen alkylation, the usual mode of reaction of a nitroparaffin anion, derives (2) N. Kornblum, R. A. Smilep, R. K. Blackwood and D. C. Iffland, J . Am. Chcm Soc., 77,6269 (1955).
> 100
Nblet
>100
c1
>loo
OTos Br I
8 6 3
simply from nucleophilic displacement by the oxygen of the anion on the benzylic carbon; however, in the p-nitrobenzyl series, when X is a difficultly displaced group, a second mode of attack by the nitroparaffin anion has a chance to compete and it is this second process which is productive of carbon alkylation. The matter of mechanism is currently under investigation. Aside from its obvious significance as regards the alkylation of other ambident anions, the properties herein reported for the 9-nitrobenzyl system may very well be important in reactions involving non-anibident nucleophiles and this possibility is being explored. (3) Sponsored by the U. S. Army Research Office (Durham).
DEPARTMENT OF CHEMISTRY NATHAX KORNRLUM PAULPINK^ PURDUE UNIVERSITY LAFAYETTE, INDIANA KENNETHIJ. ’T-ORKA? RECEIVED APRIL 14, 1961
A MODEL FOR ANGLE STRAIN CALCULATIONS. OPTICALLY ACTIVE SYMMETRICALLY BRIDGED BIPHENYLS’**
Sir: The architecture of molecules of type I is uniquely suited for a comparison of theoretical and experimental evaluations of angle strain. A high degree
x
X
wQ+G / \
H2C CH,
C,H2 /CH2 Y
Ia,X=Y=O
BrCHz
CH2Br
CH300C COOCH:,
11
HOCH, CHzOII IIIa, X = 0 b,X=S
b,X=Y=S c,X=O,Y=S
of constitutional symmetry in cases where X = Y minimizes the assumptions ordinarily entailed in calculations for less symmetrical molecule^.^ We wish to report that calculated angle deformation energies in the transition state conformations for racemization of l a and Ib account for the experimentally observed energy barriers. (1) Acknowledgment is made t o t h e donors of T h e Petroleum Research Fund, administered by t h e American Chemical Society, for partial support of this research. We also thank t h e Alfred P.Sloan Foundation f o r partial support. (2) Satisfactory elemental analyses were obtained fur all substances reported. Chemical shifts a r e expressed for solulions in CDCIs, with reference t o external benzene. (3) E.g.,K. E. H o a l e t t , J . Chern. SOL.,1250 (1958); cf. F H. Westheimer, in M . S. Newman, “Steric Effects in Organic Chemistry.” Joliu IX’ile). and Sons, h’ew 170rk. N I,.,1956, Chapter 12.
June20, 1961
COMMUNICATIONS TO THE EDITOR
2781
I n the model for our calculations we assume that above the melting point (149.5-155.5', 4 min.). to a first approximation (a) over-all changes in The compound gives racemic I b with Na2S. The non-bonded interaction from ground to transition dramatic diference i n the chemistry of racemic state are negligible, (b) changes in the hexagonal and enantiomeric modijications of IIIb appears to geometry of the benzene rings are negligible, and be without precedent and is the subject of further (c) strain in the CCX angle is completely relieved investigations. by buckling, Le., by the folding of the CXC plane Preparation of IIIa, m.p. 139-140.5', [a]2% relative to the plane containing the biphenyl f32' (chf.), is effected by treatment of (+)-I1 framework (a mode of relief not considered by with AgzO/aq. acetone, followed by LiAIH4. Addition of (+)-IIIa to concd. Hi304 at -20', Howlett3). Use of bond bepding constants3v4 1.3, 1.0 and followed by a conventional workup in which the 0.7 X lo-" erg. rad.-2 molec.-l for COC, CSC temperature is kept as low as possible, gives (+)-Ia, and CCC, respectively, yields calculated values for initial [aIotuca. +230' (0-xylene); the absolute configuration, shown in the figure, follows from the E a c t of 22 and 42 kcal./mole for the racemization of Ia and Ib, respectively. I n harmony with these synthetic sequence.8 Racemic I a may be prepared estimates, we find that I a racemizes in o-xylene from IV and AgzO/aq. acetone, or from racemic according to the expression k1 = 1011.7e-z0.2/RTor active IIIa and TsCl/pyridine, or from IIIa and (e.g:, t ~ / , is ~ ~10.7 , ~ min.) whereas I b remains TsOH in refluxing benzene. It has m.p. 189-191' 255.5 mk (16,300), n.m.r. optically unchanged on heating in o-xylene a t after sublimation, 83.5' for a t least 3 hours. IC is intermediate: spectrum in the aliphatic region exhibits two spinit racemizes in o-xylene according to the expression coupled methylene protons, 6'~, 1.85, 6 ' ~2.23, kl = 1013.2e--30.0/RT (e.g., is 31.7 hr.). J'AB 0.192 p.p.m. The synthesis of I b revealed a number of highly Closure of (+)-IIIb (TsCl/pyridine) gives (R)unconventional features which we desire to record IC, m.p. 161-162', [CY]~~D +333' (0-xylene). a t this time. Bromination (NBS) of dimethyl Racemic IC, m.p. 161-162', A$? 253.5 m p (-) - (R) -6,6'-dimethyl-2,2'-diphenate5gives I1 (10,900), is similarly prepared from racemic (m.p. 105.5-106.5'), [a]2 4 4-21' ~ (C&), which is IIIb. The n.m.r. spectrum in the aliphatic region converted (NaZS, then LiAlH4) into IIIb, m.p. exhibits two pairs of spin-coupled methylene 123.5-124.5', [a]23D +58' (chf.). The reaction protons, B A 2.84, Bg 2.87, JAB0.30 (estd.), S'A 1.52, of solid (+)-IIIb with PBr3/C6Hs (trace of pyridine) 6'g 2.18, J'AB 0.292 p.p.m. at room temperature affords directly an equimolar DEPARTMENT OF CHEMISTRY NEWYORKUNIVERSITY KURTMISLOW mixture of I b and of 2,2',6,6'-tetrakis-(bromoM. A . W. GLASS methyl)-biphenyl (1V)'I; it seems likely that NEWYORK53, NEWYORK RECEIVED APRIL24, 1961 sulfonium salts intervene in this disproportionation.' I b has m.p. 213.3-215.5' (dec.), XXZt,"," SYNTHESIS OF SUBSTITUTED CYCLOBUTANE 242 mp (11,800), 291 mp (1040), [cY]*~D +415' HYDROCARBONS BY POTASSIUM CATALYZED (C~HO),n.m.r. spectrum in the aliphatic region DIMERIZATION OF 8-ALKYLSTYRENES' exhibits two spin-coupled methylene protons, Sir: SA 3.00, 6~ 3.10, JAB0.212 p.p.m. The absolute We wish to report a novel dimerization reaction configuration of (+)-Ib, shown in the figure, resulting in the formation of cyclobutanes. It was follows from the synthetic sequence.* A novel aspect of the disproportionation is the found that the dimerization of B-methylstyrene in fact that the reaction of solid racemic IIIb (m.p. the presence of catalytic amounts of potassium 154.5-155.5', prepared from racemic 11, m.p. produces high yields of an almost equimolal 131-134') with PBr3/C6Ha (trace of pyridine) a t mixture of stereoisomers of 1,3-dimethyl-2,4room temperature affords a single substance, pre- diphenylcyclobutane (11). The dimerization reaction was carried out in a sumably the expected CleH14SBr2 (found: C, flask provided with a high-speed stirrer. Two 48.47; HI 3.63; S, 7.89; Br, 40.16). This substance remains unchanged on recrystallization. grams of potassium metal was dispersed in 98 g. It is not formed from an equimolar mixture of of methylcyclohexane a t 100' and to this was added racemic Ib (m.p. 266-267' (dec.), prepared from 24 g. of B-methylstyrene consisting of 65% of IV and NazS) and IV in PBrs/C6HB, but it is the trans- and 35% of the cis-isomer. After 3 hours quantitatively (infrared) disproportionated into an of stirring a t 100' the product consisted of 50% equimolar mixture of racemic I b and IV by heating of recovered 0-methylstyrenes, 45% of 11, and of some higher boiling material. The yield of cyclo(4) H. Siebert, 2.anorg. u. allgem. Chem., 271, 65 (1952). butanes based on converted P-methylstyrene ( 5 ) For absolute configuration, cf. K. Mislow, Angew. Chcm., 70, amounted to about 90%. 683 (1958). The two stereoisomeric species of the substituted (6) E. Bergmanu and 2. Pelchowicz, Bull. Research Council Israel, 3 , No. 1/2, 91 (1953). cyclobutane (11) were detected by means of gas (7) C f . , e x . , F. E. Ray and I. Levine, J . Org. Chem., 2, 267 (1937); chromatography. The combined isomers distilled M. Kulka, Can. J . Chcm., 37, 325 (1959) and references cited. at 118-122' at 0.5 rnm., n20D 1.5455, and did not Symmetrical intermediates are excluded from any possible mechanism show any olefinic unsaturation. since the produced Ib is optically active and apparently optically pure. Ana,?. Calcd. for CI8Hm; C, 91.46; H, 8.54. (8) The system of configurational designation proposed by R. S. Found: C, 91.41; H, 8.62. Cahn, C. K. Ingold and V. Prelog, Experienfia, 19, 81 (1956), cannot The cyclobutanes had Xma, 2590 A., e = 1015. be applied to this case. However, elimination from this system of the class of optically active atropisomers is under active consideration (V.Prelog, private communication),
(1) Paper XXII of the series "Base Catalyzed Reactions" for paper XXI see J. Shabtai and H. Pines, J . Org. Chem.. in press.
