Nov. 20, 1964
COMMUNICATIONS TO THE EDITOR
5047
singlet appears as two doublets of equal intensity, 7 7.95,8.15).
V.'" V I : m.p. 160-161';
Amax (acetonitrile) 243 mp 13,601)); infrared 3455 and 1675 cm.-l; CDC13 solution in ca. 50:50 equilibrium with its cyclic tautomer (determined by n.m.r. analysis) .5a VII: m.p. 50-52'; Amax 225 mp ( E 17,800), shoulder a t 232 mp ( e 16,400); infrared 3025 and 1020 cm.-'; n.m.r. multiplet T 2.5, quartet 7.0, multiplet 8.0, intensities 1 S : l : 2 . (e
2,5-isomer and underwent valence tautomerization6 to l-benzoyl-1,2,2-triphenylcyclopropane[IV (35yo) ; structure shown by spectral data, debenzoylation to 1,1,2-triphenylcyclopropane (YIII),3,i,Ra andsynthesisEb]. The 4.5-dihydrofuran (like the 2,5-isomer), and also the cyclopropyl ketone IV, underwent acid-catalyzed rearrangement without phenyl group migration to the acyclic b,y-unsaturated ketone V. The cyclopropyl ketone, when fused or subjected to boiling acetic acid without added acid catalyst, underwent rapid rearrangement back to the 4.5-dihydrofuran. Ir, proof of the sequence of reactions 117 + I1 + V under strong acid catalysis, n.m.r. tracking in CDC13-CF3COOH solution showed the rapid buildup of the dihydrofuran and relatively slow rearrangement of this to the unsaturated ketone. The conversion of the cyclopropane through the 4,5dihydrofuran to the P,y-unsaturated ketone IV + I1 + ITis of significance to small-ring chemistry and substituent effects on ring stabilities, and it is of particular importance to the mechanism of acid-catalyzed cleavages of cyclopropyl ketones where formation and intermediacy of the 4,s-dihydrofuran must now be considered as possibilities.g Investigations of scope, by-products, mechanism, and detail of these and related reactions will be discussed in the forthcoming paper. Analytical D a t a . I 0 11: m.p. 106-108°; Amax 245 and 303 r n p (e 10,300 and 9800); infrared 1640 cm.-' (enol ether)5d; n . m . r . (Varian -4-60) multiplet T 2.6, singlet 6.05, intensities 10 : 1. 111: m.p. 181.5-183'; Amax 248 mp ( e 21,000), shoulder a t 255 mp, (e l ( i , O O O ) ; infrared 1680 cm.-'; n.m.r. (110', dimethyl sulfoxide-d6)" multiplet T 2.9, two singlets 3.63 and 4.23. I V : m . p . 161-162'; Amax 22'7 mp (e 20,91)1)).shoulder a t 240-245 mp ( e 18,800-13,0i)i)) ; infrared 30503 0 0 0 , l(X0, 1018 cm.-'; n.m.r. (CDCl,) multiplet T 2.4, singlet i . 2 5 , intensities 1 0 : l (in benzene the (6) ( a ) A p p a r e n t l y novel, although t h e r m a l , interconversions o f t h i s t y p e are k n o w n , P . K . , cyclopr,,panecarboxaldehyde-2.3-dihydrofuran rquilibra, 3002 (19471, ( h ) S . I. tion a t 3 7 S C : C I.. U'ilson, J . A m C h e m S O L 69, Shuikin I . F . Rel'skii, a n d K A. K a r a k h a n o i , Doki A k a d . .Vauk S S S R , 126, 1051 (1959); (c) cf. 1x7. A. G i b b o n s , W.F . Allen. a n d H . E . G u n n i n g , ( ' a n . J . C h e w . , 40, 568 (1962). ( 7 ) B y potassium i - b u t o x i d e , f . f P. G . G a s s m a n a n d F . V. Zaler. 147th h'ational Meeting of t h e American Chemical Society, P h i l a d e l p h i a , P a , April, 1964, A b s t r a c t s , p . 7iV (8) Syntheses b y photocondensation of d i p h e n y l d i a z o m e t h a n e a n d (a) s t y r e n e or ( b ) a - p h e n y l a c r y l o p h e n o n e , c i . (c) H hf. Walborsky and I.. Plonsker. J A m . C h e v . S o c . , 83,2188 ( l g f i l ) , ( d ) J . E. H o d g k i n s a n d M P . H u g h e s , J . 0i.t ChPwz., 2 7 , 4187 ( 1 9 6 2 ) , ( e ) C G . Overberger a n d J P Anselme. i b i d . , 29, 1188 (1964). ($1) E E . . t h e acid-catalyzed cleavage of t w o highly s u b s t i t u t e d benzoylcycloproiianea. where optical a c t i v i t y a t C - l is lost a t a measurably hiaher r a t e t h a n prnduction of t h e acyclic 0,? - u n s a t u r a t e d k e t o n e , was explained in t e r m s o! rapid reversible formation of a homoallylic c a r h o n i u m i n t e r m e d i a t e 8r (10) All analyses for C a n d H were correct ( 1 1 ) By H H . F r e e m a n , I h w Chemical C o m p a n y .
COBBCHEMICAL LABORATORY UNIVERSITY OF 1-IRGINIA CHARLOTTESVILLE, VIRGINIA
DAVIDW. BOYKIS,J R . ' " ROBERT E. LCTZ
RECEIVED JULY 22, 1964
Photoreduction of Aminobenzophenones'
Sir. Photoreduction of benzophenone and other ketones and aldehyde^^,^ occurs readily via abstraction of hydrogen from alcohols and other hydrogen atom donors by the excited carbonyl compounds (eq. 1). Amino RK'C=O*
+ AH
+
KR'C-OH
+ A,
(1)
and hydroxy s u b s t i t ~ e n t ssuppress, ~'~ or greatly decrease, rates of photoreduction of benzophenones. I t has been suggested4m5that the lowest lying triplet in such compounds is T-T* rather than n--K* in nature, and is unreactive in abstraction of hydrogen. Recentlyfi p-aminobenzophenone and p-hydroxybenzophenone have been reported to be unreactive in 2propanol but to be photoreduced in cyclohexane. This unusual effect was attributed6 to the lowest lying triplet in the former solvent being of a "charge-transfer" type which is unreactive in abstraction of hydrogen, while in the hydrocarbon solvent i t is of the reactive n-n* type. We report now that aminobenzophenones undergo rapid photoreduction by 2-propanol when converted to their "onium" derivatives, either by protonation or methylation. p-Dimethy1arninobenzophenoneji DMABP, has important contributions in the ground state from forms I and 11. It shows intense absorption bands in 2 0
I
0-
I1
propanol a t 249 (log E 4.22) and 352 mp (log t 1.33), the former presumably arising from T--T* excitation of the aromatic ring and the latter from excitation to (1) We a r e pleased t o acknowledge generous s u p p o r t nf thiq work b y t h e U. S. Atomic E n e r g y Commission, A T ( 3 0 - 1 ) 2 4 9 9 . T h i s work was reported a t t h e I n t e r n a t i o n a l S y m p o s i u m on Organic P h o t o c h e m i s t r y , S t r a s b o u r g , F r a n c e , J u l y 2 1 , 1964. ( 2 ) G. Ciamician and P. Silber, B e y . , 33, 2911 ( 1 9 0 0 ) ; 34, 1.541 (1901). (3) C. W e i z m a n n , F B e r g m a n n , a n d Y . Hirshberg. J . A m . C h r m S o r , 60, 1530 (1938). F . Bergmann a n d Y Hirshberg. ibid.. 6 5 , 1429 (1943) (4) J. N . P i t t s , H . W. J o h n s o n , J r , a n d T . K u w a n a . J P h y s ( ' h e m . , 66, 2456 (1962). ( 5 ) A. B e c k e t t a n d G. P o r t e r , Traizs. F a r a d a y S O L ,69, 2051 (1963) (6) G. P o r t e r a n d P.S u p p a n , Proc Chem. .Soc., 191 (1961) (7) C . L). H u r d a n d C. h-. W e b b , in "Organic S y n t h e s e s , " Call Vol. I . A. H . B l a t t , Ed., J o h n Wiley a n d S o n s , I n c . , 9 e w Y o r k . N Y . . 1911, p. 2 1 7
COMXUNICATIONS TO T H E EDITOR
504s
a singlet of the type of form 11, referred to6 as the "charge-transfer" band. Addition of hydrogen chloride to the 2-propanol solution leads to no change in the 249-inp band, but t o marked decrease of the band a t 352 mp. The pk', of I)MAl13P.estimated from the decrease in this absorption with addition of hydrogen HC1 the long chloride, is 1 . 7 in 2-propanol. At 2 wave length band is a t 314 mp and of low intensity (log 6 2 . 2 8 ) , comparable t o the corresponding band of benzophenone a t 334 mp (log e 2.18); i t appears to arise from an n~-r*transition which leads to a chernically reactive state, presumably the triplet.8 Photoreduction of 0.1 -If DMXBP in 2-propanol in Pyrex by a C,. E. 851V h3 ultraviolet lanip proceeds very slowly, about n.00:3 the rate of benzophenone. The rate however rises regularly with addition of hydrogen hydrochloride, while the absorption a t long wave length decreases, leveling a t ca. 0.7 S HC1 a t a rate about 0.2 t h a t of benzophenone, indicating a quantum yield of about 0.4.The rate of photoreduction of benzophenone in 2-propanol is unaffected by 0.5 HCl. The rate of photoreduction of DMXBP in 2-propanol appears to rise more rapidly than the "charge-transfer" band is suppressed with addition of hydrogen chloride, indicating that the ground state may be a weaker base than is the n-r* excited triplet. In 1 : 1 2-propanol-water the rate of photoreduction of DMXBP is negligible and is again greatly increased by addition of hydrogen chloride. Higher concentrations of the acid are required to reach maximum rate of photoreduction than in the absence of water, reflecting the lower protonating power of the aqueous system. IYater reduces the maximum rate of photoreduction of DM;\BP and also that of benzophenone itself, effects which will be described in a subsequent report. The product of photoreduction of 0.1 M DMABP in 0.5 LV HC1 in %propanol, formed quantitatively, is 4,1'-bis(dimethylaminobenzpinacol) dihydrochloride, m.p. 150-151° dec. This was converted quantitatively to the free base, m.p. and m.m.p. 181-183', identical with an authentic sample prepared by reduction of I) \I.XHP by magnesium-magnesium iodide. :lnul. Calcd. for CJuHa??j202: C , 79.7; H , 7.OX; N , G.19. Found: C, SO.0; H , 7.10; N.6.21. The pinacol was readily distinguished from p-dimethylaminobenzhydrol, 1n.p (Xio, prepared by reduction of the ketone by sodium borohydride. .1 nul. Calcd. for ClsH,7r\;O: C. 7!).:3: H , 7.40; Ti, R . l f i , mol. wt. 227. Found: C.79.5; H , 7 . 2 4 : N, li.19; mol. wt. 230. Preliminary experiments have shown t h a t p - and o-aniinobenzophenorie are also reduced readily in 2propanol in the presence of acid. X 0.01 -21 solution o f the para compound was reduced less than 770 in the absence of hydrogen chloride after irradiation for 21 h r . . and was conipletely reduced in less than this time in the presence of 0.5 2' HC1. o-Aminobenzophenone, 0 . 0 1 .\I. showed no reduction after irradiation for 17 hr. in the absence of acid and was almost completely reduced after irradiation for 1 hr. in the presence of O . , j -VHC1. The o r t / compound ~~ is of particular interest since its reduction might normally be prevented C;, S
Iiammontl a t i d 11' 51
l l < , o r e , .I
C'hcw
.io(.. 81, 6 M 4
Vol X ( i
both because of an unreactive low triplet of structure related to that of form I1 and because of intramolecular hydrogen abstraction related to photoenolization.I f ) The observed photoreduction in acid medium indicates that the o-KHs- group. unlike -CH3, does not transfer hydrogen rapidly to the oxygen of the n-r* tripletcarbonyl group, and that the latter is relatively positive and its activity in hydrogen atom abstraction is electrophilic in character. Since this work is part of a study of photoreduction and its inhibition by sulfur compounds" in aqueous media, we have prepared the quaternary methochloride from DMABP, 4-benzoylphenyltrimethylammonium chloride, n1.p. 187-189' dec. ilnal. Calcd. for C160
H&OCl: C, 69.3; H , 6.53; N, 5.08; C1, 13.0. Found: C, 69.3; H , 6.42; N , 5 . 1 7 ; C1, 13.1. It showed Amax 340 mp (log E 2.20), presumably due to an n--K* transition, similar to that of benzophenone, and no high intensity long wave length absorption characteristic of the unquaternized and unprotonated aminoketones. This quaternary compound was also readily photoreduced in 1 : I 2-propanol-water, a t a rate about 0.35 that of benzophenone in this solvent pair, with estimated quantum yield ca. 0.25. (10) N . C . Y a n g and C. R i v a s , ibid., 85, 2213 (1961). (11) S. G. C o h e n , S. O r m a n , a n d D. 6. L a u f e r . ibid., 84, 3905 (1962)
DEPARTMEKT OF CHEMISTRY BRAXDEIS UNIVERSITY WALTHAM, MASSACHVSETTS02154
SAULG . COHEN M . NASIMSIDDIQUI
RECEIVED AUGUST21, 1964
Homolytic Aromatic Substitution. Phenylation of Phenanthrene
V.'
Sir : Electrophilic and homolytic aromatic substitutions of phenanthrene have been investigated to evaluate predictions, based on molecular orbital theory, that the reactivities of the five positions in this arene follow the sequence 9 > 1 > 4 > 3 > 2 . ? For example, nitration has been found to give the order 9 > l > 3 > 2 > 4,3 and homolytic phenylation with two sources (benzoyl peroxide and diazoaminobenzene) has been reported to correspond to the sequence 9 > 1 > 3 > 2 . 4 The latter studies failed to reveal 4-phenylphenanthrene among products from either source, and steric hindrance has been suggested by way of explanatian. We now report that the reactions of phenanthrene with three sources of phenyl radicals, including diazoaminobenzene, produce the 4-isomer in substantial amounts and that a t least one source yields 4-phenylphenanthrene in amounts t h a t are indicative of steric (1) P a p e r I V . S C. Ilickerrnan. M. Klein, a n d G B. V e r m o n t , J Orp C h ~ m, in press, paper 111: S C IXckerman, .4.5 4 . Felix, and I,. B. 1,evy. ibiii., 29, 26 (1i)fil) ( 2 ) 4 . Streitwieser. J r . , "Molecular Orbital T h e o r y for Organic Chemists," 1961. John R'iley and Sons, I n c . , New Y o r k , K.Y., (L3) AT J . S. I l e w a r , T. Mole, a n d I2 \I' T. Warford, J . C h r m . Sor., 3,511
I' 4) A . L. J . Beckwith and &I. J T h o m p s o n . ibtd , 7 3 (1961).
