May 5, 105s
ELECTRICAL EFFECTS IN BIPHENYL AND NAPHTHALENE SYSTEMS [COSTRIBUTION FROM
THE
2283
CHEMICAL LABORATORIES O F PURDUE UNIVERSITY]
Electrical Effects in the Biphenyl and Naphthalene Systems. The Influence of Alkyl Groups Attached to Silicon on Desilylation Reactions BY ROBERTA. BEXKESER,WILLIAMSCHROEDER AND OWENH. THOMAS RECEIVEDOCTOBER7 , 1957 Quantitative evidence is presented for the operation of a conjugative effect between the rings of biphenyl during electrophilic substitution. Evidence for the operation of a n important inductive effect in this system also is presented. The rates of desilylation of 0- and p-trimethylsilylnaphthaleneindicate the 0-position of naphthalene to be a t least four times as reactive as the beta toward electrophilic attack. These data correlate well with those obtained from carbinyl chloride solvolysis. The rates of desilylation of phenyltrimethylsilane, dimethylethylphenylsilane, triethylphenylsilane and triisopropylphenylsilane were found to decrease in the order Measi-> MenEtSi- > E t a s & >> (z-Pr)&i-. A steric effect is thus clearly indicated in such cleavage reactions.
Biphenyls.-There has been a great deal of interest for a number of years in the electrical effects which are operative in the biphenyl sysIn 1928, Le Fevre and Turners concluded t h a t the two biphenyl rings function independently of one another. This conclusion was based primarily on the observation t h a t electrophilic substitution occurs almost invariably ortho-para in the unsubstituted ring of a biphenyl containing a mdirecting group. Recent work by Berli~~er‘j-~ and co-workers has done much to clear up these early misconceptions. It is now definitely established t h a t the two biphenyl rings are not isolated electrically one from the other; however transmission of electrical effects between the rings is considerably damped. From data on the hydrolysis of certain biphenyl esters, it was est.mated t h a t electrical effects are transmitted through this system a t approximately onequarter the efficiency of transmission through a single benzene ring.7 Because of this damping effect i t is difficult at times* to decide whether electrical effects are transmitted via a n inductive or a conjugative (resonance) type process. Apparently in the ground state there is little, if any, significant conjugation between the two rings.$ It was thought t h a t a study of the rate of removal of the trimethylsilyl group (desilylation) from various positions in the biphenyl ring would be a convenient method for determining directly and quantitatively the electrical effects operative a t these positions.I0 I n this same connection we have studied the rates of desilylation of 4-trimethylsilylbiphenyl, 2’- and 4’-methyl-4-triniethylsilylbiphenyl and 2’- and 4’-chloro-4-trimethylsilylbiphenyl. The results are presented in Table I. (1) D. Vorlander. Be?., 58, 1803 fl92:). (2) F . Bell, J . Kenyon a n d P. H. Robinson, J . Chem. SOL.,12-12 (1926). (3) H. A . Scarborough and \A? A \T‘aters, ihid , 557 (l92G). (4) W.S. RI. Grieve and D. H. Hey, i b i d . , 22-16 ( 1 9 3 2 ) . ( 5 ) R . J. iT. Le Fevre and E. E . T u r n e r , ibid, 245 (1028). (GI E. Berliner a n d E A . Blommers, Tms J O U R N A L , 7 3 , 2-170 (1051). ( 7 ) E . Berliner and I.. H . T,iu, i b i d , 75, 2417 (1053). ( 8 ) E. Berliner, B . S e w m a n and T. RI. 1 iaboff, ibid , 77, 478 (1955).
(9) V. P. Kreiter, \V. A . Bonner and I t . H Rastman, ibid , 76, 5770 (1954). (10) We have since learned from Professor Eaborn (private communication) t h a t he has undertaken a similar s t u d y . As a conseciuence we are curtailing t h i s portion of our investigation.
The reactions employed to synthesize the isomeric methyl- and chlorotrimethylsilylbiphenyls are Sequence A (4-chloro-4’-triinetliylsilylbiphenyl) 4-nitrobiphenyl SnClz V
Brz ----+
HOAC
4-nitro-4’-bromobiphenyl
4-dmino-4’-broinobiphenyl
HC1 4,4‘-bromochlorobipheiiyl
.-+
1, XaS02-HC1 2 , CUzC12 1, n-BuLi _____)
2 , MeaSiCl
product
--
Sequcnce B (4-methyl-4’-trirnethylsilylbiplienyl) p-toluidine
1, KaNOt-HCl
2, CeHe-RTaOH
4-methylbipheiiyl
4-bromo-4’-methylhiphenyl
-
1, n-BuLi
2, hlC3SlCl
Br2
-+ product
Sequence C (2-chloro-4’-trimethylsilylbiplienyl) 2-nitrobiphenyl
Brz; FeCh
2-nitro-i’-brotnwbiplieti~l
KHIC1 ~liazoniuni-xnercuricchloride complex 2-cliloro-4-’-broniwhiplieiiyl
A
1, n-BuLi ____f
2, MejSiCl
rlroduct
Sequence D (2-methy1-4’-trimetliylsilylbiphenyl) 0
IIp-&C6H,MgBr --f
2, A, vacuum 1, Brz
X a ( E t z 0 ) prod+4-bromo-2-methylbiphenyl 1, -
2, M e 3 S z
uct
In sequence ?., an alternate procedure for the synthesis of 4,4’-bromochlorobiphenyl was found satisfactory also. This involved the direct chlorination of 4-bromobiphenyl with sulfuryl chloride in the presence of sulfur chloride and aluminum chloride. A mixture of nitrobenzene and tetrachloroethane was employed as solvent. 4-Bromobiphenyl
sozc12; _____)
SoClz; AlCla
4,4’-brwmoclilorobipheny1
I,--1
, / ~
f 11' Y . Oklimotv and
H
C . Bruivn, TIIIS Jonxxii., 7 9 , I',JO:3
(1!!.5i!. 1 I 2 1 s c c ' I l i , , I:. 13crl,ner >,,,
