THETwo ISOMERS OF B4C2He
Oct. 20, 1963
constant for eq. 6. The term [Q+ i-PrO-] can be replaced by an equilibrium constant times the product of the concentrations of free quaternary ammonium ion and isopropoxide ion, but again this would be a mechanism requiring participation by not one, but two, free ions and in CC1, a mechanism involving an associated species is far more plausible. Because Allen and Modena4 had attributed catalysis to chloride ion, an attempt was made to fit the data by replacing the last term of eq. 4 with kCA [Cl-1, and because their observed catalytic rate constants were in fact linear in total salt concentration rather than in ion concentration a term kCAS was also tried, but in neither case could an acceptable fit be obtained. Comparison of our findings with those of Allen and Modena4 relative to catalysis by HCl in the absence of added salt is impossible, because we find that the rate a t very short times is entirely dependent on the small amount of HCl initially present, while Allen and Modena determine rates a t zero time and take no account of the possibility of impurity HC1 being present. Our observations suggest that such rate constants must be meaningless without knowledge of the initial HCl concentrations. From our own studies we can conclude with reasonable certainty that there is little or no uncatalyzed reaction in CCl, between (C6H5)3SiC1and 2-propanol, t h a t the significant HCl catalysis is due to an associated species and not to free ions, and that the apparent catalysis by benzyltriethylammonium chloride results from uncatalyzed attack by an associated quaternary ammonium isopropoxide species on the (C6H5)3SiC1. These conclusions, initially somewhat surprising, are in fact quite reasonable. Failure to observe catalysis by H + or C1- may be attributed simply to the extremely low concentrations of the free ions, while catalysis by HC1, which must be similar in nature to t h a t which might be predicted for H + or C1-, is measurable because of the very much greater concentration of HC1. The direct reaction between (C,&)3SiCl and
[CONTRIBUTION FROM THE
3167
the quaternary ammonium isopropoxide, with no equivalent reaction for the alcohol, is explained by the fact that charge separation in (Q+ i-PrO-) is very much greater than in i-PrOH. The salt species, even though associated, is then a much stronger nucleophile than the alcohol. In a case where three species are involved in the rate equation, as in the HC1-catalyzed path described here, it is far more reasonable to accommodate the observation by postulating an intermediate comprising a t least two of the species than by requiring a three-body reaction. In the present case the most probable intermediate involves pentacovalent silicon, possibly as shown in structure 1. H5C6
\ ./ RO-&-Cl I
/ C6H5
The fact that no reaction is observed in the absence of HCl suggests that the Si-C1 bond is stronger than the Si-0 bond in the intermediate. The function of the HC1 is then to aid in strengthening the Si-0 bond relative to the Si-Cl bond. This may be done through hydrogen-bond formation with either the C1 or the hydroxyl H of I , although the latter possibility is not overly attractive when one considers that the solvent offers far more opportunities for similar interactions and c c l 4 is not a sufficient catalyst to effect reaction. The preliminary formation of an intermediate hydrogen-bonded species (C6H6)3SiC1. . . HCl does not seem nearly as probable as the type represented by I, since the great excess of solvent cc14 competing with (C6H6)3 Sic1 for the limited supply of HCI argues against any significant lifetime for such an intermediate and the mechanism would then not differ significantly from a three-body collision. In view of these arguments, we conclude t h a t our observations are best rationalized through an intermediate containing pentavalent silicon.
RESEARCH LABORATORY, OLIN MATHIESON CHEMICAL
The Carborane Series : B,C2H, + 2.
I
H
CeH5
CORP., PASADENA, CALIF.]
11. The Two Isomers of B,C2H,
BY I . SHAPIRO,' B. KEILIN,ROBERTE. WILLIAMS,AND
c. DAVIDGOOD
RECEIVED FEBRUARY 6, 1963 Two isomers of carborane-4, BaCzH6, have been isolated, and their structures deduced from the nuclear magnetic resonance, infrared, and mass spectral analyses of isotopic variants of the compounds. Some of the physical and chemical properties of the more stable carborane-4, syrn-%C1H6, are given.
The carborane series of compounds have been prepared from the reaction of acetylene with boron hydrides. The simplest member of the series, B3C2H5, has already been discussed. The isomers of carborane-4 were prepared from the reaction of pentaborane and acetylene in a silent discharge tube. When these same reactants are confined under pressure a t elevated temperatures, the product is another organoboron compound, B4C2Hs,3which differs structurally4 from the (1) Universal Chemical Systems, Inc.,Culver C i t y , Calif. (2) I. Shapiro C. D. Good, and R . E . Williams, J . A m . Chem. Soc., 8 4 , 3837 (1962). (3) I . Shapiro a n d H. G . Weiss, experimental work performed 1953-1957, and declassified Nov. 5, 1962; I. Shapiro, paper presented a t t h e XIX Meeting of t h e International Union of Pure and Applied Chemistry, London, July, 1963. (4) Attention is called t h a t t h e structure of B&Hs cited recently5 was taken f r o m t h e previous work8 without permission or acknowledgment of t h e senior investigator. ( 8 ) T. P. Onak, R. E. Williams, a n d H. G . Weiss, J . A m . Chem. SOL.,84, 2830 (1962).
carboranes. Data on other carboranes and their derivatives will be published later. Experimental Materials.-The isotopic variants of acetylene and pentaborane have been described previously.* Apparatus .-The apparatus used t o prepare and analyze the isomers of carborane-4 are the same a s described for carborane3.2 Procedure .-Although carborane-4 can be prepared by the high-temperature reaction of acetylene with diborane or pentaborane a t low pressures, i t was found more convenient t o prepare the carboranes by use of the silent discharge apparatus.2 An equimolar mixture of pentaborane-9 and acetylene (25 mmoles) was circulated through the silent discharge apparatus (15 k v . ) for 1 hr. at ambient temperature ( t h e temperature of the reactor increased noticeably during the course of the experiment). During this time, the total pressure of the system gradually decreased, and a light yellow-brown deposit formed on the walls of the reactor. After the high-voltage discharge was discontinued, the condensable products were transferred t o a fractionation train in the high vacuum apparatus. Inasmuch as the quantity of volatile product from a n y one experiment was low, this pro-
I. SHAPIRO, B. KEILIN,R. E WILLIAMS,AND C. D. GOOD
3168
Vol. 85
was observed; when a mixture of the two carborane-4 was heated, the sym-carborane-4 appeared to be the more stable form.
Results Spectral Analysis. Mass Spectra.-The parent grouping in the mass spectra of the various isotopic species of B4C2H6 is illustrated in Fig. 1 and the complete polyisotopic mass spectrum of sym-B4CzHe is given in Table I. The isotopic transformation6 of the B'O-enriched spectrum (Fig. l a ) to an isotopicallynormal spectrum (Fig. lb) reveals that the compound contains four boron atoms with a sharp cutoff corresponding to the B4CzHe+species. TABLE I POLYISOTOPIC MASS SPECTRUM OF ISOTOPICALLY-NORMAL symCARBORASE-4 A D J U S T E D FOR m / e 73 AT 100 U N I T S m/e
12
13 14 .. 21 22 23 24 25 26
27
,
28 29
65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80
..
b
'
.
.
.
.
.
.
MASS NUMBER
.
.
.
.
.
.
.
mIe
Fig. 1.-Comparison of parent groupings in mass spectra of isotopic variants of sym-carborane-4. cedure was repeated several times in order to accumulate sufficient material for separation and analysis. Infrared analysis of the reaction mixture indicated the most abundant carborane present was sym-carborane-4; the chief component of the mixture was unreacted pentaborane. hfter removal of B3C2H5 and other more volatile materials from the mixture by fractionation a t - 126°,2 the carboranes were separated from the unreacted pentaborane by complexing the latter with triethylamine and subsequently removing the excess amine with diborane. Since the volatility of pentaborane is similar t o those of the desired products, separation by straight fractionation is diflicult; however, complexing the pentaborane with amine (and destroying excess amine with the low boiling diborane) changes the volatility range of the compounds and permits separation of carboranes by low temperature fractionation. An example of this method of separation is given below. After removal of B3C2H5 a t -126O, the remaining material (consisting chiefly of carboranes and pentaborane-9) was condensed in a tube with a small quantity of triethylamine. On warming this mixture t o -80", a vigorous reaction occurred between the amine and pentaborane, forming a liquid of low volatility. Upon standing for a short time a t room temperature, yellow viscous material of uncharacterized nature appeared. The reaction tube was then immersed in liquid nitrogen and any noncondensable material was removed by evacuation. T h e products which were volatile a t room temperature (carboranes and triethylamine) were then transferred under vacuuni t o another tube and condensed a t - 196" with a n excess of diborane. On warming t u room temperature, reaction occurred, forming a small quantity of a mobile liquid (triethylarnineborane). .4fter fractionation a t -80' to remove the less volatile triethylamineborane, and a t -150' t o remove more volatile diborane, the resulting product was shown by infrared analysis t o consist chiefly of sym-carborane-4 with small quantities of unsynzcarborane-4, carborane-5, and traces of unknown materials. T h e addition of triethylamine (about one-fifth the quantity of carboranes) t o this mixture and then fractionation several times a t - 105' resulted in removal of the zLnsyni-carborane-4 and carborane-5 from sym-carborane-4. Traces of triethylamine present with sym-carborane-4 were removed by treatment with diborane and fractionation. In several experiments from the silent discharge apparatus a small quantity of unsym-carborane-4 was isolated in relatively pure form. No interconversion of the two forms of carborane-4
32 33 34 34.5 35
Peak h t
Peak ht
mje
Peak h t
m/e
4 8 4 1 0 5
35.5 36 36.5 37 38 39
2 15 0 17 0 0
56
1 5
57
58
3 6 5 6
59 60 61 62
3 6 2 3 0 6
0 2
1 1 1 0 2 0
5 6 1 5 1 5 3 3
0 9
0 6 2 3 0 4
7 0
8 7 5 1 4 3
. .
42 43 44 45 46 47 48 49 50 51
55
0 2 4 1 1 0 6 8 2 4
65 66 67 68 69 70 71 72 73
0 5
74 75
5 1 2 3 2 2 5 0
5 3
0 2 8 19 25 25 45 81
3 0 6 7
0 6 9 3
100
41 7 1 1
That the compound contains a t least six hydrogen atoms can be deduced from the shift in 6 mass units upon complete deuteration (Fig. le). From n.m.r. analysis i t is concluded that the compound contains only six hydrogen atoms. I n addition, one can learn the source of hydrogen atoms by partial deuteration. The product given by Fig. IC was prepared from deuterioacetylene and normal pentaborane , that given by Fig. Id, from normal acetylene and deuteriopentaborane. The shift of 2 to 4 mass units higher on carbon deuteration and boron deuteration, respectively, indicates that of the six hydrogen atoms, two of them came from the acetylene and four of them from pentaborane. Comparison of the monoisotopic spectra of the parent groupings in Fig. 1 indicates a difference in the relative fragmentation of B-H and C-H bonds. The monoisotopic distribution for the isotopically-normal compound is: B4C2He+,67.2; B4CzHs+, 100; B4CzH4+, 11.4; B4CzH3+, 24.1; BdC*H?+, 7.9; BqCzH+, 23.9; and B4Cz+, 4.8. In the case of the completely deuterated compound, the distribution of ion species follows in the same pattern; in neither case can one distinguish the loss of hydrogen or deuterium specifically from either a boron or carbon atom. However, in the partially deuterated species with the parent species as a basis, one finds that for the carbon-deuterated compound the relative concentration of B4C2H:4Dz+(hydrogen fragmentation from boron) is greater than that of B4C2H4D+ (deuteri'um fragmentation from carbon), and in the boron-deuterated compound the B4C?D3H?+ (deuterium fragmentation from boron) also is greater than B4C2D4H+ (hydrogen fragmentation from carbon). (fi) I Shapiro, C . 0. Wilson, J F D i t t e r , and W . J I.ehmann,Advancesin Chemistry Series, N o . 32, American Chemical Society, Washington, D C , 1961. p. 127.
THETwo ISOMERS OF B4C2H6
Oct. 20, 1963
An inspection of Table I reveals a low degree of destruction of the basic skeletal structure, with the occurrence of symmetrical cleavage into B2CH3+ fragments being greater than other B-C combination fragments. This low degree of cleavage of basic structure also had been noted in carborane-3.2 Infrared Spectra.-The infrared absorption spectra of the two isomers of B4C2H6are given in Table 11, and the salient features of isotopic variants of the symcarborane-4. in Table 111. The single sharp peak occurring in the 2600-2700 cm. region is easily recognized as the B-H asymmetric stretching frequency; and in the 2000 cm.-' region, as the B-D asymmetric stretching frequency. There are no bands for any of the compounds that can be attributed to B-H-B bridge groups. As in the case of carborane-32there are no frequencies which can be assigned t o a C-H (or C-D) stretching vibration, even a t a gas pressure of 560 mm. in the cell. TABLE I1 INFRARED .ABSORPTION SPECTRCM O F CARBORANE-4
s, strong, m, medium; w, weak; v, very. sh, shoulder; b, broad (cm sym-B4C?Hs 1144sh 1926rn 1332m 421 1SI1 1912111 1327m 1l40sh 4 181vw 1134vs 1859vvw 1318111 4097w 1784vvn 1301m 1128vs 4019sh 1757vvw 1290m 1121s 3798vw lll5sh 1742sh 1280m 3633w 1065vw 1732~ 1269m 3497~ I05Svw 1716u 1265sh 3374vw 934sh 1694w 1253111 3233vw 931m 1678sh 124'im 3107vw 927sh 1586~ 1237m 2962vw 923m 1569m 1192sh 2 8 6 6 ~ ~ 912sh 1518vwb 1179sh 2 667s 11 ,a,mb 1479sh 1176vvs 2657vvs 743m 1469~ 1165vvs 2310vwb 'i30shb I462sh 1157vvs 215lsh 726m 1395~ 1154~~s 2128~ 708n b 1330sh 1147sh 1970TV
"--
unsym-BaC2Hs
2618~s 1386~ 1309wb 12 56w 121iw 1188sh 1174sh 1149sh
1143s 1133s 1l27sh 1119sh 1073V\% 1071vw
1004n
1053w
990m 977w 943v\1 936vn 928m 900w
1 0 3 6 ~ ~
867~
TABLE I11 SALIENT FEATCRES OF ISFRARED .ABSORPTION SPECTRA OF ISOTOPIC VARIANTSOF S ~ V Z - C A R B O R A S E - ~ BICIH~IIL cm.-'
B , C ~ I h H cm ~.
-1
B I C ~ Dcm ~,
--I
2667~s 2632sh 1116s
202ovs 2008sh 2000sh
2008~s 2000sh 1976sh
1103s
1147s 1140s
1063, 1058sh
86%
730m
1055s
Nuclear Magnetic Resonance Spectra.-The HI and are B l 1 n.m.r. spectra of isotopically-normal sym-B4C2H~ given in Fig. 2. As in the case of carborane-3,2 the proton spectrum (Fig. 2a) reveals that the protons exist in two distinct environments-some protons connected to identical carbon atoms (single large peak) and others
3169
Field
I. SHAPIRO, B. KEILIN,R. E. WILLIAMS.AND C. D. GOOD
3170
Vol. 85
TABLE IV VAPOR PRESSURE O F SYm-CARBORANE-4
Temp "C
I
p , mm
---V
Dev , mm
Calcd
Obsd
Solid -82 -80 -64 -64 -57 -46 -42 -42 -40 -32
97 47 24 23 67 86 51 36 41 63
1 05 1 39 6 22 6 17 10 55 24 29 32 95 33 10 38 17 62 52
1 02 1 33 6 20 6 21 10 71 24 30 32 93 33 26 37 94 62 53 Av dev
-0 03 - 06 - 02 04 16 01 - 02 16 - 23 01
+ + + + +
0.074
Liquid -32 63 -31 66 -22 84 -17 12 0 05 5 01 11.13 14 49
+
62 52
68 00 108 146 316 391 499 564
4 3 6 4 3 1
63 6i 108 144 318 391 497 565
35
07
+0.8 - .9
- .1
3 9 3
-1.4 +1.7 -0.4 -1.5 +1.2
0 8 3 Av dev.
1 0
with those usually obtained for unassociated compounds. Chemical Properties. Stability.-Storage of pui e IC-H sym-caiborane-4 a t room temperature over a period of I 6 months resulted in no detectable decomposition. E'- H I :: " 1 , L 1 Reactivity.-sym-Carborane-4 has been in contact B-H with acetone, trimethylamine, triethylamine, ammonia, Fig. 3,-2:.111.r. spectra of unsym-carborane-4 : ( a ) H1 spectrum, air, and water a t room temperature without noticeable 40 Mc.; i b ) B*' spectrum, 12.6 Mc. reaction. The hydrogen atoms attached to the boron in symwhose spacings are displaced as shown in Fig. 3a. carborane-4 will undergo H-D exchange with deuterioSuch an interpretation would be in agreement with the diborane a t room temperature, but the rate of exchange interpretation of the Bl1 spectrum shown in Fig. 3b. is less rapid than in the case of carborane-3. Here the apparent triplet is considered to be the coincidental overlap of two doublets since there is a slight Discussion difference in spacings of the two end peaks from the Two carborane-4 compounds have been isolated. center peak. The high-field doublet has an apparent From the information derived from mass and n.m.r. value of J of 187 c.P.s.; the low-field doublet, an spectra, the stable compound contains two identical apparent J-value of 174 C.P.S. C-H units and four identical B-H units. The mass Physical Properties. Vapor Pressure.-The vapor fragmentation pattern of the compound also point to a pressure of sym-carborane-4 was measured over the symmetrical compound. The most probable structure temperature range of -83 to +l5' and is reported in of the molecule appears to be a tetragonal bipyramid, Table IV. The solid has an appreciable vapor pressure i . e . , four boron atoms and their hydrogens in one plane which can be expressed by the equation with one carbon and its hydrogen on each side of log P,, = 11.1324 - 1896.94/T - 0.006021T (solid) the boron plane such that the line joining the two The vapor pressure of the liquid can be expressed by carbon atoms will pass through the center of the square the equation formed by the four boron atoms. Such a structure log P,, = 9.7513 - 1676.40/T - 0.004068T (liquid) would indicate high symmetry and stability of the molecule and account for its volatility, which is greater Boiling Point.-Extrapola tion of the vapor pressure than comparable boranes or straight-chain hydrocardata to one atmosphere pressure gives a normal boiling bons Because of its symmetrical structure, this point of 22.7'. compound will be referred to as sym-carborane-4. Melting Point.--Attempts to determine the melting The exchange of one carbon atom with any one of the point by means of Stock's ring method were unsuccessfour boron atoms in the above structure should result ful. Visual observation of the melting point gave a in the formation of an isomer of sym-carborane-4. value of -32.63 f 0.05'. Simultaneous solution of The n.m.r. spectrum of the second carborane-4 fits the vapor pressure equations for the solid and liquid such a structure, Each of two boron atoms will be gives a value of -29.5'. This discrepancy of 3.1' is neighboring two carbon atoms and two boron atoms, attributed to the combination of deviations in measureand each of the other two boron atoms will be neighment and the small difference in slopes of the two vapor boring one carbon atom and three boron atoms, thus pressure equations. accounting for two identical C-H units and two pairs of Heat of Vaporization.-The molar heat of vaporizaB-H units, each pair differing from the other pair. tion a t the normal boiling point, calculated from the This compound is less stable than its isomer and is Clausius-Clapeyron equation, is 6042 cal./mole. Troutermed "unsym-carborane-4." ton's constant is 20.4 cal./deg. mole, a value consistent
J,
1 , I
PHOTOISOMERIZATION OF ISOCOLCHICINE
Oct. 20. 1963
In both compounds each carbon atom appears to have five ligands instead of the customary four ligands. T o circumvent this anomally, the concept of carbon participating in the so-called "three-centered bonds" with two boron atoms is accepted. In this connection the two carbon atoms can be visualized as substituting for two boron atoms, but with two additional electrons, thus forming the equivalent stable octahedral B6H6-? ion.Y I t already has been suggestedYthat carborane-3 (B3CYH5)may be isoelectronic with BsHs-2. The structure postulated for B3C2Hb2is indeed that pictured for B5H5-2.9 (8) \V H E b e r h a r d t , B Crawiord, a n d U'. N . Lipscomb, J . Chem P h y s , 2 2 , 989 (1954). 19) E B hloore, L . L Lohr, a n d W N Lipscomb, ibid , 3 5 , 1329 (1961).
[CONTRIBCTION F R O M THE
DEPARTMENT OF
3171
The apparent lack of hydrogen exchange between boron and carbon hydrogens permit the preparation of specifically labeled isotopic compounds. Although the hydrogens originally attached to the carbon and boron atoms in the parent compounds are retained in their same relative environment in the new compounds, the mechanism of formation of the new compounds must be complex since a t least the carbon atoms in acetylene must undergo drastic rearrangement in the SYWZ-B~C~H~. Acknowledgment.-The experimental work was supported by Navy Contract 52-1023 C and Air Force Contract 33(600)-33920 during the period 1953-1957, but was not released from restricted publication until 1961.
CHEMISTRY, IOWA STATE UNIVERSITY O F SCIENCE AND
TECHNOLOGY, AYES, IOWA]
Photoisomerization of Isocolchicinel BY 0. L. CHAPMAN, H . G . SMITH, AND P. A. BARK$ RECEIVED MARCH 4, 1963 Irradiation of isocolchicine in aqueous solution gives a photoisomer which is shown t o be 11. The mode of photochemical valence tautomerization appears t o be controlled by electronic factors. The controlling factor in this case appears t o be preservation of a trimethoxystyryl unit
The difference in mode of photochemical valence tautomerization of colchicine3-j and simple tropolone methyl ethers6 can be interpreted as a result of steric control in the colchicine photoisonierization and electronic control in the case of simple tropolone ethers.* Isocolchicine (I) poses an interesting problem in this light. There is little steric basis for preference for either gross structure I1 or I11 for the valence tautomer; consequently the photoisomerization of isocolchicine should be dominated by electronic factors.
m 1
II
Irradiation of isocolchicine in aqueous solution gives a photoisomer (3.85, 6.01 p ; 218 (4.42) and 260 mp (log E 4.34)) in 40-50% yield. The ultraviolet absorption of the photoisomer is similar to that of P-lumicolchic i ~ i e ~arid . ~ r-lumicolchicine,3 , 4 suggesting that the molecule contains a trimethoxystyryl chromophore. Structure I11 does not contain a styryl chromophore, and thus structure I1 is favored by the ultraviolet absorption. The nuclear magnetic resonance spec(1) P a r t IX of t h e Photochemical Transformation Series. ( 2 ) Petroleum Research F u n d Fellow, 1960-1962 D u P o n t Teaching F e l l o w , 1962-1963 (3) E. J Forbes. J . Chem Soc , 3864 (1555) (4) P D G a r d n e r , K 1- Brandon, a n d G K Haynes, J . A m . Chem S O L . , 79, 6334 (1957) (5) 0 1, C h a p m a n , H G S m i t h , a n d R . U'. King, ibid., 86, 803, 806 (1963) ( 6 ) 0 I. C h a p m a n and D J . P a s t o , %bid , 82, 3642 (1960) ( 7 ) 11' G D a u b e n , K . Koch, 0. I. C h a p m a n , and S . I, S m i t h , zbio , 83, 1768 (1961) (8) 0 I. C h a p m a n , C h a p t e r in "Advances in Photochemistry,'' Vol 1, edited by \t' A . S o y e s , Jr.. G S. H a m m o n d , and J S P i t t s , J r , John Wiley and Sons. Inc , New York, N Y . , in press
trum (Table I) of the photoisomer is also in accord with structure 11. The two olefinic protons overlap giving a two proton signal a t 3.65 T. It can be demonstrated that this two proton singlet arises from chemically nonequivalent protons by use of pyridine as solvent. The nuclear magnetic resonance spectrum of I1 in pyridine (Table I) shows three singlet absorptions due to the aromatic proton and the two olefinic protons now rendered magnetically nonequivalent by the specific shielding effect of pyridine. The highly specific shifts of the various proton resonance positions in pyridine offers additional support5 for Slomp's suggestiong that the shielding effect of pyridine is due to attachment of solvent molecules at specific sites on the solute rather than a bulk solvent effect. The remaining proton assignments are straightforward and are given in Table I. Reduction of I1 with sodium borohydride gives an enol ether alcohol (6.02 p , amide; 6.09 p , C=C-OCH3) which proved to be very sensitive. The nuclear magnetic resonance spectrum of IV (Table I) shows the aromatic proton and both olefinic protons clearly separated. The shift of one olefinic proton to high field (5.24 7 ) identifies it as the enol ether vinyl proton. The bridgehead proton (6.67 T) is coupled to both the proton on the carbon bearing the hydroxyl group (5.32 r , T. = 9.4 c.p.s.) and to the adjacent cyclobutene proton ( J = 0.86 c.P.s.). The identity of this latter coupling has been confirmed by a double resonance experiment. lo,ll Irradiation of the bridgehead proton 173 c.p.s. upfield from the cyclobutene proton collapsed the cyclobutene proton to a singlet. The small value for this coupling constant is normal for this system in which the coupling constant falls in the range 0-1 c.p.s.I2 Mild acid hydrolysis of IV gives the keto alcohol V. The infrared carbonyl absorption (5.71 p) of the keto alcohol confirms the presence of a fivemembered ring in the photoisomer 11. The keto alcohol shows the expected ultraviolet absorption for (9) G Slomp and F MacKellar, J . A m Chem S O L ,82, 989 (1560) (IO) K Kaiser, Rea. Scz I n s t v . , 31, 963 (1560) (11) K Freeman, Mol P h r s , 3, 433 (1960) (12) I. hl J a c k m a n , "Nuclear Magnetic Resonance Spectroscopy Pergamon Press New York, N Y , 1959, p . 86.
