Cct. 20, 1956
5363
REARRANGEMENT OF %BYDROXY-2’-NITRODIPHENYL SULFONE
[CONTRIBUTION FROM
THE
VENABLE CHEMICAL LABORATORY, UNIVERSITY OF NORTH CAROLINA]
Steric Acceleration through Regulation of Rotational Conformation : The Smiles Rearrangement 1*2 BY
J. F. BUNNETT AND TOSHIHIKO oKAMOT03 RECEIVEDMAY14, 1956
The rate of rearrangement of a 2-hydroxy-2‘-nitrodiphenylsulfone t o a 2-(o-nitrophenoxy)-benzenesulfinicacid is increased about 500,000-fold by the introduction of a methyl chloro or bromo substituent in the 6-position. Since both electron-releasing and electron-attracting 6-substituents cause similar tremendous increases in rate, the origin of the acceleration is steric and not electronic. The mechanism of the acceleration is discussed.
In 1937, McClement and Smiles4 reported that the Smiles rearrangement of 2-hydroxy-2’-nitrodiphenyl sulfones to 2-(o-nitrophenoxy)-benzenesulfinic acids is accelerated by methyl groups in the 6-position. The cause of the acceleration, they said, was electron-release from the methyl group to the sulfone bridge which thereby was rendered less
3
NO*
a 6-bromo or 6-chloro substituent caused a big increase in rearrangement rate, the effect would be shown to be steric, while a big decrease in rate would indicate that the effect was electronic. The former proved to be the case, gratifying our expectations, and i t then became desirable to have a means of estimating the normal electronic effect of 6-substituents on the reaction rate. For this purpose the kinetic effect of the same substituents in the 4-position was determined. The 4- and 6-positions are both meta to the phenolic hydroxy group while one is ortho and the other para to the sulfone bridge. As a first approximation, the electronic effect of a 6-substituent may be taken as equal to that of the same substituent in the 4-position.
able to draw electrons from the ionized 2-hydroxy group. I n consequence, they explained, the nucleophilic reactivity of the ionized hydroxy group was greater than it would otherwise have been, and the C H 3 - - ( ) - - S O * - p reaction went f a ~ t e r . ~ R-\ -OH NOz Bunnett and Zahler‘j considered this explanation unconvincing, and showed that the acceleration could be interpreted more agreeably as a steric IVa, R = H effect, that is, as a consequence of the bulk of the 6IVb, R = CH3 methyl groups. The element of proof was lacking IVc, R = C1 I V d , R = Br from their argument, however, and one of the aims of this work was to establish whether or not the acR celeration actually arises from the size rather than from the electron-releasing character of 6-methyl groups. Another purpose was to describe the acceleration more exactly than McClement and Smiles had doneG7 It was reasoned that if the acceleration is really Vb, R = CH3 due to the bulk rather than the electron-releasing Vc, R = C1 effect of 6-methyl groups, then groups of comparV d , R = Br able size but opposite electronic effect should also have an accelerating influence. On the other hand, if the acceleration were due to an unusual sensitivity of the reaction to the electronic influence of 6, substituents, the introduction of an electron-atC1 tracting group in that position should strongly deXIX celerate the reaction. Thus, if the introduction of (1) Described in part in a preliminary communication: T. Okamoto and J. F. Bunnett, J . Org. Chcm., 2 1 , 487 (1956). ( 2 ) Presented in part t o t h e Southeastern Regional Meeting, American Chemical Society, Columbia, S. C., November, 1955. (3) On leave from the Pharmaceutical Institute, University of Tokyo, 1954-1956; grateful recipient of a Fulbright travel grant. (4) C. S. McClement and S. Smiles, J. Chcm. SOL,1016 (1937). ( 5 ) In a series of researches on this and related rearrangements, summarized by Bunnett and Zahler,n Smiles and his co-workers demonstrated adequately t h a t the function of t h e promoting base is to convert t h e hydroxy group t o the corresponding ion which then effects a n intramolecular nucleophilic displacement a t the 1‘-carbon to form the anion of t h e sulfinic acid product. (6) J. F. Bunnett and R . E. Zahler, Chem. R e v s , 49, 369 (1951). (7) McClement and Smiles estimated rates rather roughly, and their measurements indicate only t h a t sulfones with &methyl groups rearrange, in general, more t h a n ten times as fast as sulfones rrith a vacant 6-position.
I
KO2 VIa, R VIb,R VIc, R VId,R
= = = =
H CH3 C1 Br
R
1
NO2
VIIb, R = CHI VIIc, R = Cl VIId, R = Br
c1
I
xx
NOx
CH3 I
C1
c1 XVb
NO, XVIb
The sulfones chosen for kinetics study comprised four pairs of isomers (’IVb and Vb, IVc and Vc, IVd and Vd, and X I X and XVb) which have a methyl, chlorine or bromine substituent first in the 4- and then in the 6-position. All of them possess a 5methyl group for reasons of convenience in synthesis. The parent sulfone (IVa) having both the 4-
J . F. BUNNETT AND 'TOSHIHIKUOKAMOTO
5364
and the 6-position unsubstituted was also prepared for study. The preparation of these sulfones and an examination of their rearrangement reactions are described in an accompanying paper.8 The several compounds are given the same roman numeral designations in both papers.
Experimental Rate Measurements by the Titration Technique.--A solution of about 0.001 mole of sulfone IVa or IVb in 507, aqueous dioxane ( b y weight) was combined with enough of a standard solution of sodium hydroxide in 50% dioxane to provide 1.2 moles of base per mole of sulfone, and the solution was made up to a volume of 250 cc., shaken thoroughly, and returned t o the thermostat. All components of the reaction mixture were brought t o thermostat temperature before being combined. Aliquots ( 2 5 cc.) were removed at recorded times, and discharged into beakers containing excess dilute sulfuric acid in standard amount. The quenched solutions were back-titrated potentiometrically with standard aqueous sodium hydroxide solution. The plot of pH us. volume of titrant showed two breaks: one with the steepest slope a t about PH 7, corresponding to neutralization of excess sulfuric acid plus the sulfiiiic acid which had been formed, and a second with steepest slope at about pH 11, corresponding t o neutralization of the phenolic hydroxyl of unreacted IVa or IVb. T h e second break occurred at the same volume for all samples within a given run. "Infinity" samples showed a single break at the volume characteristic of the second break. T h e logarithm of the difference in volume of titrant for the two breaks was plotted against time. T h e resulting plots were straight lines, the slopes of which were multiplied b y 2.303 to get the first-order rate coefficients. In a set of runs with sulfone IVa a t 46" ( T a h l c I ) , the effect of varying the sodium hydroxide concentration was studied. The rate is not dependent on sodium hydroxide concentration within the range studied. This means t h a t 1.2 moles of sodium hydroxide per mole of phenolic sulfone
TABLE I RATECOEFFICIENTS FOR SMILES REARRANCEMEXTS
Suifone IVa IVa
Moles NaOH per mnle Temp., sulOC. fonc 25.00 1.2 1.2 46.00 I.5 2.0 2 ,5
1.2 I ,2
IVb IVb IVc
28.00 46.00 40.00
1 2 1 2
I .2
1Vc
G7.2
1 2
IVd
46 00
I ,2
Vb vc
00 .O
1.2 1.2
Vd
0
1. 2
XVb
0
1. 9
XVb
14 4
1.2
p H 5 . 0 , -,hOv. Pol:rrocraphy OH 5 . 0 . -.,XJv. f'olarr,qr.iphy plI 4 C . -.Mlv. Phriinraphy
46 00
1.2
XIX
lii.2
1.2
\ ~ , l l t : l ~ easr e
Slean rate ciwiiicient, min - ' ' I 1 . 3 8 T 0 . 0 2 x 10-1 1 . 9 3 rt 0 . 0 8 x 10 2 3 . 0 2 X 10-2 1 . 9 7 X 10-2 1 . 8 3 X 10-2 1 .!I2 zt 0.08 X I O 2 2 , 33
I
0 I2 X 11)
2
I 26 I 0 04 x 10 - 3 1 . t i f j & . OG X I O 1 1.11 .01 X 1 0 - 3
+
.04 X
1.71' i-
10
-2
*
Pr,l:rrography 2 I 2.3 . 0 1 X 10 PI1 6 0 ; -.65v. '1'w Eu,t tu measure 2 9 . 2 =k 1 . 3 X 10-1 Pul;irocraphy
XIX
'1
Anslytirnl Si, methurl" runs Titration 3 Titration 3 Titration 1 Titration I Titration I Polarography 9 pi1 9.2 Polar I'olarography 2 ,bH 5 0 , - 00". P
