3778
COMMUNICATIONS TO THE
EDITOR
Vol. 84
TABLE I Yield,. Mercaptan
1 2 3 4
Ether
Thiophenol Thiophenol Thiophenol Thioplienol Thiophenol
Product (s)
%
Diethylene glycol dimethyl ether Diethylene glycol diethyl ether Ethylene glycol dimethyl ether Tetrahydrofuran 2-Methyltetrahydrofuran
Thioanisole 89 Thiophenetole 26 Tnioanisole 59 4-Thiophenoxybutanol 36 5 5-Thiophenoxypentanol-2 82 4-Thiophenoxypentanol-1 2 6 n-.2myl Diethylene glycol dimethyl ether SGb n-Amyl methyl sulfide 7 Benzyl‘ Diethylene glycol dimethyl ether Benzyl methyl sulfide 56 8 Thiophenol Diethylene glycol diethyl ether and diethyleiie glycol dimethyl Thioaniso!e 95 ether (1 : 1 mole ratio) Thiophenetole 0 9 Thioplienol ilnisole and diethylene glycol diethyl ether ( 1: 5 mole ratio) 6 Thioanisole 15 Thiophenetole 10 Tiiiophenol 4-1~rt-Butylcyclohexyl methyl ether and diethylene glycol di11 Thioanisole ethyl ether 7 Thiophenet ole a The reaction of the mercaptan, for example thiophenol, with sodium borohydride gives trithiophenoxyborane and sodium thiophenoxide, the latter being inert in the transfer reaction. The additional diborane generated in situ converts the trithiophenoxyborane into the active monothiophenoxyborane. The yields of product are calculated on the basis of the available thiophenoxy groups for transfer. * This yield is based on n-amyl methyl sulfone isolated after permanganate oxiReaction time of 18 hours, the rest being 21 hours. dation of the crude sulfide.
tion of substituted boranes6 and certain other intramolecular transfer reactions currently under study.’ The mechanism proposed for the above ether cleavage is distinctly different from that proposed for the cleavage of ethers as their boron trichloride complex which proceeds by a carbonium ion mechanism.8 Experiments designed to investigate intramolecular alkyl exchange via four-centered transition states resembling bicyclo[2.1.1], [3.1.1] and [4.1.1] systems are being carried out. (6) R. Roster and B. Glinther, A n n . , 629, 89 (1960). (7) D. J. Pasto and J. L. Miesel, manuscript in preparation ( 8 ) W. Gerrard and M. F. Lappert, J. Chem. Soc., 1486 (1952)
perchloric acid solution^.^^^ Values of [BH+]/ [B1 were obtained by direct solution of equation 1 a t five or six wave lengths in each of several acid percentages. Arbitrary shifting of spectral curves before application of equation l (the Hammett “isosbestic method”)’ was unnecessary because medium effects on the spectral bands are relatively small.
-
-
[BH+l/[Bl = ( e EB)/(~BH+ e) (1) log ([BH+]/[B]) PKBE+ = Ho (2) p K ’ ~ s += HK’ log ([BH+]/[B]), where HR‘ = H R - l o g ~ u ~ (3) o (4) [BH+l/[Bl = ( A - AB)/(ABH+ - A ) k‘ €BE?+ ( h / ( €- e n ) ) - h e / ( € - €B) 0 (5)
+
+
+
The relative constancies with changing perchloric of values of ~ K B H(equation + 2) and ~ K ‘ B H + acid DEPARTMENT OF CHEMISTRY OF NOTRE DAME UNIVERSITY D. J. PASTO (equation 3) obtained by the direct method can be compared in Table I. The protonation of phloroNOTREDAME,INDIANA glucinol shows somewhat less than an Ho acidity deRECEIVED JULY 18, 1962 pendence, -d log ( [BH+]/ [B])/dHabeing 0.85, and correlates poorly with HR’. The protonation of 1ACIDITY DEPENDENCE OF T H E CARBONmethoxy-3,5-dihydroxybenzene correlates very well PROTONATION OF PHLOROGLUCINOL AND ITS withHo,-dlog ( [BH+]/[B])/dRa=0.98,andpoorly METHYL ETHERS with HR’. An acidity dependence between HOand Sir: HR’ is shown for the protonation of 1,3,5-trimethIn a recent paper, Kresge and Chiangl reported oxybenzene, with - d log ([BH+]/[B])/Wo = 1.26 that equilibrium carbon-protonation of 1,3,5-tri- and -d log ([BH+]/[B])/dHp.’ = 0.78. Our methoxybenzene in aqueous perchloric acid is more results on the protonation of 1,3,5-trimethoxybenclosely dependent on the $1~‘acidity function2 than zene differ in detail from those reported by Kresge on Ho. Earlier, Deno, Groves and Sairies had con- and C1iiang.l However, if one excludes from concluded that the protonation of diarylolefiris was sideration their calculated [BH +]/[B] values cortlependent on HR’ rather than Ha.* responding to greater than 97yo and less that 2% Results bearing on the question of whether pro- protonation,* the disagreement is relatively minor. tonation on carbon is in general dependent on a dif( 3 ) This work was begun in 1968 as a necesqary adjunct to a study ferent acidity function than protonation on nitrogen of the kinetics of the hydrolysis of the ethers. or oxygen have been obtained in an ultraviolet ( 6 ) Ultraviolet spectral evidence that protonation of 1,3,5-triinethspectrophotometric examination of the protonation oxgbenzene occurs on carbon rather than on oxygen is given in reference 1. The closely similar spectral changes undergone in strong perchloric of phloroglucinol and its methyl ethers in aqueous acid by phloroglucinol and its mono- and dimethyl ethers leave no (1) A. J. Kresge and Y.Chiang, Proc. Chem. Soc., 81 (1961). (2) HR’ = H R log a ~ * owhere , H R is the acidity function for the
-
+
+
complex ionization ROH H+ e R+ HzO.* The function H R log a ~ , owas first defined by Deno, Groves and Saines‘ and conveniently labeled HR’ by Kresge.1 (3) N. C. Deno, J. J. Jaruzelski and A. Schriesheim, J . A m . Chem. SOC.,77, 3044 (1955); N. C. Deno, H. E. Berkheimer, W.L. Evans and H. J. Peterson, ibid., 81, 2344 (1059). (4) N. C. Deno, P.T. Groves and G. Saines, ibid., 84,6700 (1959).
-
doubt that they protonate in the same manner as 1,3,5-trimethoxybenzene. The n.m.r. spectrum of 1,3,5-trimethoxybenzene in 66% perchloric acid (unpublished work, these laboratories) is unequivocally interpretable as being t h a t of the carbon conjugate acid. (7) L. P. Hammett, C. A. Flexser and A. Dingwall, J. A m . Chem. Soc., 57, 2103 (1935). (8) We feel that measurements of such extremes of protonation have little reliability, even when medium effects on spectral bands are small; see. e.&, footnote p. Table I.
3779
COMLIMUNICATIONS TO THE EDITOR
Oct. 5, 1962
TABLE I VALUESOF [BH+]/[B], ~ K B HAND + PK'BH+' "2104,
%
1.3.5-CeHa(OH)sb [BH+]/ [B]/s' - ~ K B E + -PK'BH+
..
1,3,5-CHsOCsHa(OH)rC [BH+]/[BIh - ~ K B E + -PK'BEC
1,3,5-C"s(OCHs)sd" [BH +l/lBli PKBH+ -PK'BH+
-
.. .......... .. 0.136 f 0.005 3.60 6.89 43.8 ........ .. 7.26 0.085 f 0.001 3.91 3.58 6.94 7.10 0.192 f .006 44.8 0.135 f 0.008 2.73 7.37 3.83 .289 f .002 3.60 7.14 7.26 0.486 f .018 47.8 , 3 i l f .013 3.72 7.44 .349 f ,003 3.78 3.61 7.17 7.53 0.516 f .019 48.1 3.81 .673 f ,005 7.48 3.73 ,847 f .008 .. 1 . 0 5 f .04 3.63 7.40 50.1 .... .. 7.64 3.65 2 . 1 8 f .02 3.59 7.56 7.86 2.26 f .04 52.2 1.30 f .03 3.S4 7.80 3.62 4 . 4 1 f .07 3.62 7.80 8.07 4.29 =!= .25 3.91 54.1 2.09 f .04 7.75 3.43 11.81 f 1.23 3.64 7.99 8.29 7.13 f .33 3.96 55.7 3.48 f .21 The decline in a Spectra of the ethers in the higher acids were taken within 40 sec. of mixing, due to slow ether cleavage, EBH+ is experimentally negligible in that period of time. The "Area Method" (equation 4 ) gave: -pPKs=+ = 3.71-3.90; - ~ K ' B H += 7.10-8.24. The least squares method? gave: - ~ K B E=+ 3.56-3.62; - ~ K ' B E += 6.82-7.46. The least = squares method? gave: - ~ K R H +3.81-3.96; -PK'BE+ = 7.22-7.65. The area method using least squares gave - ~ K B H + 3.85-3.98. e For 1,3,5-HOCeH3(OCH3)2,the least squares method7 gave -PPKBH+ = 3.61-3.65; - ~ K ' B H += 7.32-8.29. Average value, for the wave lengths 242,335,340,345 and 350 ml. 0 In 63.5% HC104, the calculated [BH+]/[B]ranged from 14.9 to 31.1, illustrating the inaccuracy in determining very large indicator ratios. Average value, for the wave Average value, for the wave lengths 252, 335, 340, 345, 350 and 355 mp. lengths 244, 335, 340, 345 and 350 m l . f
.
.
.
*
The least squares method of Hammett also was used to calculate ~ K B Hand + PK'BH+of phloroglucinol and its methyl ethers. For all four compounds, the ~ K B Hvalues + obtained in this manner are fairly constant with changing perchloric acid percentage, whereas ~ K ' B Hdrifts + badly. Of course, the linear equation that is a ~ p l i e d5, , ~assumes that the particular acidity function used therein is applicable. A new method, in which the areas of the spectra in the region 290-380 mM were employed, also was used t o determine [BH+]/[B] (equation 4) for phloroglucinol (direct method) and 1,3,5-trimethoxybenzene (least squares method). The Area Method, which requires that the area of the spectral bands change little with medium, is independent of lateral shifts of the bands, but offers no special advantage here. Values of [BH+]/ [B] determined by this method correlate well with €10 and poorly with HR'. The results herein constitute a n exception to any belief that carbon-protonation should, in general, be dependent on the HK' function. The difference in behavior between the carbon-protonation of phloroglucinol and its methyl ethers and that of diarylolefins4 may lie in the fact that in the former instance both the free bases and conjugate acids have structures not unlike the free bases and conjugate acids of the indicator bases used to define Ho in the media used; ;.e., solvation of free base and conjugate acid is such as to cause fB/fBH+ to change with medium in approximately the same way as for the Hainmett indicator bases. I t is to be noted that the greatest departure froin Ho behavior (toward 13~'behavior) found in this work is for 1,3,5trimethoxybeiizene, the free base or conjugate acid of which has no positive hydrogens bonded to a hetero-atom, and that the value of -d log ( [ B H f ] / [B])/d& declines as methoxyl substituents are successively replaced by hydroxyl substituents. Hydrogen-bonding solvation of the positive OH hydrogens of the conjugate acid may be a major factor here. Such solvent stabilization of BH+ relative to B would be expected to decrease with increasing mineral acid percentage (decreasing U E ~ O and ) hence act to decrease -d log [BH+]/ [B]/ CWO.
The results reported herein may indicate that
neither the Ho nor the HR' function is unique in describing protonation equilibria. This would not be surprising, since variations of fB/fBH+ with medium, while presumably primarily dependent on charge type and on whether protonation is on carbon or a h e t e r ~ a t o m ,should ~ also depend ou the specific structure and charge distribution in base and conjugate acid (cf. refs. 4,9, lo). Support of this work by the National Science Foundation is gratefully acknowledged. (9) L. P. Hammett, Chem. Rev., 16, 67 (1935). (10) M. A. Paul and F. A. Long, ibid.. I T , 1 (1957), particularly p. 10.
W. M. SCHUBERT DEPARTMENT OF CHEMISTRY UNIVERSITYOF WASHINGTON RODNEY H. QUACCHIA SEATTLE 5, WASIIINGTON RECEIVED MAY10, 1962 FORMATION OF BICYCLO-OCTANE SYSTEMS BY AN ACID-CATALYZED CONDENSATION OF ENOL ETHERS OF a,fi-UNSATURATED KETONES
Sir: When a methanol solution of 16-dehydropregnenolone acetate1 is treated with trimethyl orthoformate in the presence of an acid catalyst, there precipitates after three minutes a t room temperature the corresponding 20-dimethylketd (I). This substance soon redissolves in the reaction mixture, and after fifteen minutes there precipitates a dimeric enol ether (1n.p. 301-304') which was shown to possess the decacyclic structure (IV). The same dimer is obtained by treatment of the monomeric enol ether (11, m.p. 115-120', Amax 238, e = 11,000, obtained by heating I in xylene) with boron trifluoride in benzene, and appears to arise ain an intermediate Diels-Alder type adduct (IIIa or IIIb) which undergoes further cyclization to produce the bicyclo [2.2.2]octane structure2 (IV). Assignment of structure IV is based on the evi(1) Similar treatment of 3@-acetoxy-58-pregna-l6-en-ZO-onegives rise to a decacyclic enol ether (m.p. 213-217') and ketone (m.p. 2522539 which are the tetrahydro (58) analogs of IV and V, respectively. (2) In order to illustrate its derivation, the numbering shown in I V is that of the component steroids. The product, IV. may be precisely designated as l,~dimethoxy-3'8-acetoxyandrost-6'-eno[l7',16':2,313"~-acetoxyandrost-5"-eno[l7",16":4,5]-bicyclo~2.2.2loct-7-ene, while V is l-methoxy-3'~-acetoxyandrost-5'-eno[17',16':2,3]-3"~-acetoxyandrost-6'-cno[ 17',16": 4,5]-hicycIo [2.2.2]octan-&one.
