COMMUNICATIONS TO THE EDITOR
April 5 , 19G3
TABLE I THEKEAClIONS OF Cis- AND trans-Coi S H d , ( SJ)?' and C O ( N H ~ ) & ~ ' VITH + Fe*? AT 25", B (Clod-) = 0.26 (Co(SHa)i-
kz,' .Lf-i min.-I
(s3)Z-l
Complex
X 108, .\I ( F e + * ) ,.lf
11.8 1.8 1.8 7.41 1.8 1.8 3.87 3.87
cis CIS
ClS
0.0186 .I1186 0186 ,0185 0093 ,0370 ,0186 0185 0185 . 0 185 009:3 .03;0 ,148
( H + ) , JI
0.072 131 189 2 19 183
A",', min:'
0.203 200
11.0 10.8 11.2 11.3 11.6 10.8 19Ud . 281d . 36gd
20;
210 108 CiS 144 40 1 trans 072 0 190,O 193 trans 0 280 131 3.87 380 trans 189 .114d trans 3.87 2 19 (1 116,O411 20id trans 3.8i 0 206 219 2.87 ~ ) trans-Co(NH3)4(N3)2+ ~+ >> C O ( N H ~ ) ~ N ~cannot ++, be explained invoking solely the effect of ligands trans to the bridging group2: according to the trans effect, one would expect cisCo("3)4(N3)2+ to react a t approximately the same rate as C O ( N H ~ ) ~ N ~ + Alternately, +.~ it could be assumed that replacement of one NH3 in Co(NH3)bN3++ by K3-, whether in the cis or trans positions, results in an increased rate of reaction with Fe++, perhaps by stabilization of the Co orbital which accepts the e l e c t r ~ n . ~However, ,~ in view of the efficiency of the acid-catalyzed path for the reaction of trans-Co(NH3)4(N3)2+with Fe++, it becomes difficult to understand why acid-catalysis is not observed for the cis isomer. We suggest, therefore, that the reaction of cis-Co(NH3)4(N3)2+with Fe++ proceeds zlia a double-bridged activated complex entirely analogous to the one recently demonstrated for the reaction between Cr(N&+ and Cr + + . j This interpretation receives further support when the ratio of the rate constants for the reactions of C ~ ~ - C O ( N H ~ ) ~and ( N Co(NH3)5N3++ ~)~+ with F e + + is compared with the corresponding ratio for the reactions of ~ i s - C r ( X ~ )and ~ + CrN3++ with Cr++. The observed ratio for the Co(II1) complexes is approximately 20, and, when corrected for the difference in ionic strengths,6this ratio will be close to the value of 50 observed5for the chromium system. The increased reactivity of trans-Co(YH3)4(N3)2+ as conipared to Co(hTH3)jhT3++ is consistent with a trans-effect in electron-transfer reactions, * and the observed acid-catalysis further supports this interpretation : the N3- trans to the bridging N3- has a site available for a proton attachment, and movement of the ( 2 ) I,. Orgel, Report of t h e Tenth Solvay Conference, Brussels, 195fi. p 289. (3) I t is n o t likely t h a t t h e increased reactivity of t h e cis complex as compared t o Co(SHa)sSz" i s a consequence of t h e decrease in charge. T h u s , a t a faster rate t h a n cis- or trans-CrFz (Y. T . Chia C r F - - reacts with Cr and E. L. King, Discu:sio?rs Faraday Soc., 29, 109 (1960)). Furthermore, an equilibrium mixture of cis- and Irans-Co(SHa)aOH? N 3 + + reacts with F e + +in 1.2 M HCIOI 70 times faster t h a n C O ( N H J ) ~ S( ~A .+ Hairn, ~ unpublished results). See also ref. 4 , ( 4 ) K . D. K o p p l e a n d R . R . Miller, P r o c . Chern. Soc., 306 (1962). (,j)R . Snellgroveand E. I,. King, J . A m . Chem. S o c . , 84, 41310(1962). (fi) 'The reactions of cis-Co(SHa)r(Na)z and Co(l\'Ha)aN\'a++were studied a t 2 (Clod-) values of 0.26 and 0.89. respectively + +
1017
trans N3- away from the Co center as electron transfer occurs' is facilitated by protonation.' Acid-catalyzed paths for electron-transfer reactions between metal ions, although not unprecedented, are not a common occurrence. Such paths have been observed previously in systems where addition of a proton improves the conjugation of two metal centers connected by an unsaturated bridging ligand,' and for the reactions of carboxylatotetraamminecobalt(111) complexes with CrA+.? (7) Facilitation of t h e removal of K3- b y protonation has been observed in o t h e r systems [A. Haim a n d W K . Wilmarth, litovg, Chein., 1 , 583 (19F2)) a n d is f u r t n e r supported by unpublished experiments showing t h a t t h e ) ; ( S ~ ) ~0.2 - a n d 1 . 2 41 H - are aquations of cis- a n d I ~ . Q ~ z ~ - C O ( N H ~between first order in complex and first order in H -, with undetectable contributions of H - independent paths.
DEPARTMEST OF CHEMISTRY THEPESNSYLVATIA STATE USIVERSITY UNIVERSITY PARK,PEXSSYLVASIA RECEIVED FEBRUARY 9, 1963
ALBERT HAIM
ON THE MECHANISM OF PHOTOCHEMICAL FORMATION OF CYCLOBUTANOLS
Sir: Aliphatic ketones containing r-hydrogen upon irradiation with ultraviolet light in solution undergoes two concurrent reactions, the type I1 cleavage1 and the formation of cyclobutanols.* Both reactions appear to be intramolecular with little or no detectable side react i o n ~ . In ~ the cases of 2-octanone and 20-ketosteroids, two isomeric cyclobutanols'are ~ b t a i n e d . ~Two mechanisms have been proposed for the photochemical formation of cyclobutanols, one a step-wise mechanism2 (eq. 2) and the other a concerted mechanism5 (eq. 3 ) . On the basis of available experimental data, these two mechanisms cannot be differentiated.
The irradiation of 6-hepten-2-one [I ] was undertaken in order to provide direct evidence as to the mechanism of the cyclobutanol formation. Should the reaction proceed by a concerted mechanism, the products would contain only cyclobutanols. However, if the intermediate is a free radical, an allyl radical [11] in this nlJ
.A
.-
+
(1) R. G. W . Xorrish, T r a n s . F a r a d a y SOC.,33, 1321 11937). (2) N. C . Yang a n d D . H . Yang, J . A m . C h e m . .Soc., 80, 2913 (1958). (3) W. Uavis, J r . , and W. A . S o y e s , J r . , ibid., 69, 2153 (1947); P. Ausloos and R. E. Rebbert, ibid., 83, 1897 (1961). (4) N. C . Yang and I) H . Yang. Telrahedioii Letters, 4, 10 (1960); 0 . Jeger, e l al., H e h . Chim. A d a , 43, 3 5 %(1960). ( 5 ) 0. Jeger, el al., ibicl., 42, 2122 (1939).
1018
COMMUMCATIONS TO THE EDITOR
case, it may cyclize in two possible manners to form methylvinylcyclobutanols [111] as well as methylcyclohexenol [IV] according to the following scheme: When a solution of 6-hepten-2-one in pentane was irradiated with a low pressure mercury resonance quartz lamp,6 acetone (32%), butadiene (23%) and an alcohol fraction (7.5%) isomeric with the starting ketone (found: C, 74.63; C, 10.48) were formed among some unreacted ketone (20%) and high molecular weight material. The major alcohol (6yG), separated by gas chromatography, was identified to be l-methyl2-vinylcyclobutanol [111] (found: C, 74.88; H, 10.77; 13% 1.4598; T m a x 3320, 3060, 1635, 990 and 910 cm.-I). The n.m.r. spectrum exhibits a singlet a t 8.85 r (- CHJ, a multiplet at 7.90-8.70 T (-CH2), a quartet a t 7.13 r (allylic), a singlet a t 6.36 7 (-OH), a triplet a t 5.52, 5.10 and 4.93 r (terminal vinyl) and a multiplet a t 3.504.45 T (non-terminal vinyl) and the relative intensities of these peaks are 3 : 4: 1: 1 : 2 : 1, respectively. Its double bond region in the n.m.r. spectrum was essentially identical with that of a known allyl system.' The other alcohol component ( I .4y0)had been identified tentatively to be 1-methyl-3-cyclohexenol [IV], Y~~~ 3320 and 3005 cm.-l; 8.80 T (singlet, -CHJ and 4.42 T (singlet, vinylene). Upon hydrogenation the alcohol fraction absorbed one mole equivalent of hydrogen, and the hydrogenated mixture was separated by gas chromatography into 1-methyl-2-ethylcyclobutano18and 1methylcyclohexanol in 4 : 1 ratio, identical in all respects with the corresponding authentic samples. The isolation of both 111 and IV in the present investigation conclusively substantiates the step-wise mechanism.' It also indicates that the reactive state responsible for this reaction may be a triplet since the intermediate radical was free to delocalize over an allylic system.g Acknowledgment.--A part of this work was carried out in the Department of Chemistry a t the University of Illinois where N.C.Y. was a visiting lecturer, during the summer of 1962. The hospitality and stimulation provided by various members of that Department, particularly by Professor Stanley Smith, are gratefully acknowledged. This work is supported by a grant from the Petroleum Research Fund, Grant No. 726.
Yol. 85
tr~meter.~ The detection of free radicals and other products is carried out by ionization of a portion of the product stream with electrons of low energy, such that only parent ions are formed. The thermal decomposition of 1,2-diiodobenzene in this reactor a t 960' (residence time sec., pressure -lo-? mm.) leads to the formation of the following products: iodine atoms, iodophenyl radicals, phenyl iodide, phenyl radicals, benzene, a product of parent mass 76 and a product of parent mass 152. The reactions giving rise to these products are thought to be
2
0
1
-
78 152
These reactions are similar to those reported in the photolysis of 1,2-diiodobenzene in ~ o l u t i o n . ~A comparison of the ionization efficiency curves for benzene, the product of mass 76 and an added xenon standard leads to the vertical ionization potentials : benzene, 9.50 v.; mass 76, 9.75 v . Identification of this species of mass 76 as benzyne by its parent mass alone is not sufficient, since three other structures for C6H4 hydrocarbons can be written: HC=CCH=CHC=CH (I), HsC=C=C=C=C=CH2 (11) and H,C=C=-C= CHCECH (111). Formation of any of these three species from 1,%diiodobenzene appears quite improbable. Formation of I would require one H-atom to migrate to a next-but-one carbon, I1 and I11 require the migration of two H-atoms. Moreover, the identity of the product of mass 76 with any of these can be ruled out on the basis of the expected ionization potentials. I1 and 111, being substituted butatrienes, will have ionization potentials less than that of butatriene itself, 9.28 v.j The ionization potential of I can be estimated sufficiently closely for the present purposes by comparing the changes in ionization potential along the two [ f i ) If.S . Kharasch and H S Friedlander, J . Oyg, C h e m . , 14, 21; (1949). series : ethylene, propylene, %butene ; and ethylene, ( 7 ) Varian S . m . r Spectra CataloR, spectrum S o 13ii. ( 8 ) Prepared by the photochemical reaction of 2-heptanane, I). H . Y a n g , vinylacetylene and I. Substitution of H in ethylene unpublished result. by -C=CH decreases the ionization potential slightly ( 9 ) G . S . H a m m o n d , Abstracts of Papers, 17th S a t i o n a i Organic Chemisless (0.72 v.) than substitution by -CH3 (0.78 v . ) . ~ t r y Symposium o f the American Chemical Society, 1901, p p , 48-58. Comparison with 2-butene, derived from ethylene by a (10) Fellow of the Alfred P. Sloan Foundation. (11) C. S. Public Hzalth Service Predoctoral Fellow. 1,2-disubstitution, gives the ionization potential of I DEPARTMEST OF CHEMISTRY S . C. 'I-AsG'O to be 9.40 v. The observed value for mass 76 is clearly THEUNIVERSITY OF CHICAGO .I.MORDUCHOWITZ"larger than expected values for I, I1 or 111. CHICAGO 3i, ILLISOIS DISG-DJUNG H . YANG The thermal decompositions of 1,4- and 1,3-diiodoRECEIVED FEBRUARY 7 , 1963 benzene under the same conditions were also examined. The former produced iodophenyl radicals, benzene, and IONIZATION POTENTIAL OF BENZYNE again a compound of mass f G , but no product of mass Sir : 132. The ionization potential of the species of mass 76 The chemistry of benzyne (1 ,z-dehydrobenzene) in in this case, however, was 9.46 v . , in good agreement solution has received considerable attention, and has with the estimated value for I. The course of this been reviewed by a number of authors.' Recently, the reaction is evidently transient spectrum of an intermediate has been observed in the flash photolysis of o-iodophenylmercuric iodide and other precursors, which is almost certainly attributable to benzyne. V-e wish to report the de1 1 tection of benzyne in the thermal decomposition of 1,2diiodobenzene in a reactor coupled to a mass spec(3) J . B Farmer aod F P Lossing, ( ' n i t J . C k e m , 33, 8ii1 (195.5); F. P. (1) G. Wittig, Angew. C h e w . , 6 9 , 213 ( 1 9 5 7 ) ; J. 11. Roberts, Chem. S O C . (1,tindon) Sper. Prtbi., 12, 11.5 (1'3,761, R. Huisgen, in "Organometallic Chemistry." H . H. Zeisi, E d . , Reinhold Publishing Co , S e w York, N.Y.. 1960, H. Heaney, Chem. R e a , 62, 81 (1962). ( 2 ) K . S. Berry, G. S . Spokes and R X I . Stiles, J . A m C h e m Soc., 82, ,5240 ( I H i i f l j , 84, :3;170 (1962).
Lossing, P. Kebarle and J. B. de SIusa in "Advances in hfass Spectrometry," Pergamon Press, I.undun, 1'3.59, p 4,31, t1) J . A . Kampmeier and IC. Hoffmeister, J A m C h c m . Sur , 84, 3767 (19tiZ).
( 5 ) A , Streitwieser, J r . , ibid . 82, 412d (19iiO). (6) R . E . Htinig, J Chenz. P h j s , 16, 10.5 (1946).
