L w i s . Curtin, Paul
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Dehydration of p-Hydroxytriarylmethanols
5717
Thermal, Photochemical, and Photonucleated Thermal Dehydration of p-Hydroxytriarylmethanols in the Solid State. (3,5-Dimeth yl-4- hydroxypheny1)diphenylmet hanol and (3,5-Dibromo-4-hydroxyphenyl)diphenylmethanol. X-ray Crystal Structures of (4-Hydroxypheny1)diphenylmethanol and Its 3,SDimethyl Terry W. Lewis, David Y. Curtin,* and rain C. Paul* Contribution from the Department of Chemistry and the Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801. Received February 28, I979
Abstract: (4-Hydroxyphenyl)diphenylmethanol ( l a ) , (3,5-dibromo-4-hydroxyphenyl)diphenylmethanol (lb), and (3,5-dimethyl-4-hydroxyphenyl)diphenylmethanol (IC) have been found to constitute a class of compounds which crystallizes with the phenolic hydroxyl prealigned by hydrogen bonding to an alcoholic hydroxyl group of an adjacent molecule in a fashion suitable for a self-catalyzed dehydration reaction to form the corresponding yellow or orange fuchsones (2a, Zb, and 2c). The solidstate thermal reactions of these compounds previously observed by Gomberg and his associates have been extended to observations of single crystals and some preference for reaction in the direction of the hydrogen-bonded chains noted. The photochemical reaction which had been observed by Gomberg has been extended to single crystals and, most interesting, it has been found possible to photonucleate the thermal reactions of l b and lc. The crystal structures of l a and IC have been determined by X-ray methods. Colorlesscrystalsof l a are monoclinic, a = 8.816 (3) A, b = 15.379 (4) A, c = 10.863 (4) A, fl = 101.43 (3)'; the space group is P21/c and the structure has been refined to an R factor of 0.061 on 2265 nonzero reflections. The hydroxylic protons were found to be disordered. Colorless crystals of ICare monoclinic, a = 22.394 (5) A, b = 8.580 (3) A, c = 17.556 (7) A, fl = 99.20 (2)'; the space group is C2/c. IC is isostructural with l b whose structure was previously determined. Molecules of IC are linked by hydrogen bonds between the phenolic and alcoholic hydroxyl groups but there are in addition hydrogen bond cross-links. The time for 90% dehydration of the crystalline dibromophenol l b at 110 "C was approximately one-tenth the value for the isostructural dimethyl compound IC.
Introduction A major focus of interest in solid-state organic chemistry) is on the utilization of the crystalline state to lock molecules into orientations favoring a desired reaction. Examples have included conformational locking of substrate molecules to favor a stereospecific elimination produced by a gaseous reagent.4 A second approach involves the juxtaposition of two or more reactant molecules with reactive positions correctly aligned for the intermolecular reaction desired. This second class of examples includes photochemical [2 21 cycloadditions5 and nucleophilic displacement reactions.6 A reaction from this latter class and which has been investigated in our laboratory is an acid-base-catalyzed dehydration of a carbinolhydrazine to a h y d r a ~ o n eIn . ~ this case, the acid and base are approximately prealigned by hydrogen bonding in the crystal before reaction. In a search for further examples of the latter sort we were attracted to the thermal dehydration of certain p-hydroxytriarylmethanols ( l ) * which ~ ~ seemed to possess a combination of functional groups which should lead to a general molecular packing characterized by hydrogen bonding of the relatively good hydrogen-donating phenolic hydroxyl group directed toward the relatively good hydrogen-accepting alcoholic hydroxyl group of an adjacent molecule.'O In fact, the crystal structure of the dibromo derivative l b had been r e p ~ r t e d . ~ Although the determination of structure l b was not of sufficient precision to locate the hydroxylic protons, it suggested that the hydrogen-bonding pattern was that desired for promotion of the dehydration reaction. In this paper we describe the synthesis of the dimethyl analogue IC,the determination of the crystal structures of l a and IC,and the thermal, photochemical, and photolytically induced thermal reactions of l b and IC.
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0002-7863/79/1501-5717$01 .OO/O
HO
la, X = H b, X = Br c , X.= CH,
2a, X = H b, X = Br c, X = CH,
Experimental Section Spectra and other supplementary experimental data are available in ref la. Synthesis and Characterization of Compounds. (3,5-Dimethyl-4hydroxypheny1)diphenyImethanol(IC).Stirring the fuchsone 2c in a M c ~ S O - H ~ O - N ~ Osolution H under N2 overnight, followed by filtration, dilution with H20, and precipitation of the carbinol with C02 gas,' I gave yellowish product (90% yield). Recrystallization from toluene gave IC as a white powder: mp 188-1 89 OC, turning yellow - I 50 O C ; 1 R ( KBr pellet) 3400, 1495, 1485, 1450, 1205, 1 180, 1 100, 700 cm-I; N M R (Me2SO-d6) 8.00 (1 H, s), 7.18 ( I O H, s), 6.69 ( 2 H, s), 6.02 (1 H, s), 2.07 ppm (6 H, s), the signals at 8.00 and 6.02 ppm diminished on dilution with DzO;UV A,, nm (e X 277 (l.59), 284 ( 1 S 9 ) ; DSC, sharp endotherm, extrapolated onset 200 O C , broad cndotherm centered at 270 OC. Anal. Calcd for C21H2002:C, 82.85; H, 6.63. Found: C, 83.00; H, 6.86. Crystals from ether-benzene (habit I) were rectangular tablets elongated along [OOI] with (100) as the most prominent faces and dispiayed 2/m symmetry: interfacial angles (deg), & l o (calcd) (loo), (001) 100 (99); (IOO), (110) 69 (69); (loo), (001) 82 (81); (OOI), (110)94(93);(001),(110) 87 (87);(1TO),(IlO) 138 (138). Crystals from ether-hexane (habit 11) were six-sided tablets with { O O l ) as the prominent faces and displayed 2/m symmetry: interfacial
0 1979 American Chemical Society
5718
Journal of the American Chemical Society
angles (deg), f1" (calcd) (OOI), (100) 81 (81); (OOI), (110) 87 (87); ( I IO), (100) 68 (69); (iio), ( I IO) 137 (138); (iio),( i l o ) 179 (180); ( I I O ) , ( i i o ) 42 (42); ( i i o ) , (Too) 112 (111); (loo), ( i i o ) 1 1 1
(111). 2,6-Dimethyl-4-(a,cu-diphenylmethylene)1,4-benzoquinone (2c). The method of Gomberg and Jicklingi2 was adapted to 2c. From 20 g (0.16 mol) of 2,6-dimethylphenol and 17.4 g (0.07 mol) of dichlorodiphenylmethane was obtained I5 g (75% yield) of the fuchsone 2c. Recrystallization from hexane-benzene gave 2c as a yellow-orange powder: mp 200-202 "C (lit.I3 mp 201-202 "C); 1R (KBr pellet) 1630, 1600, 1510, I030,700cm-'; N M R (CD2C12) 7.13-7.47 ( I 2 nm (t X 262 (1.53), 365 H, m), 1.97 ppm (6 H, s); UV A,, (3.07). (3,5-Dibromo-4hydroxyphenyl)diphenylmethanol(Ib).The reaction of methyl 3.5-dibrom0-4-hydroxybenzoate~~ with phenylmagnesium bromide gave, after recrystallization from ether-hexane, a white powder: mp 137-139 "C (turning yellow--120-125 "C) (lit.I5 mp 138 "C); IR (KBr pellet) 3450, 1475, 1450, 1160,70O~m-~; N M R (acetone-d6)8.43(1 H,s),7.39(2H,s),7.30(10H,s),5.41ppm(l H, s), the signals at 8.43 and 5.41 ppm diminished on dilution with D20; nm ( E X IO-)) 285 (2.20), 293 (2.68); DSC, sharp endoUV A,, therm, extrapolated onset 140 "C, broad ill-defined endotherm centered -210 "C, sharp endotherm, extrapolated onset -238 "C. Crystals from ether-hexane were six-sided tablets with {OOl)as the prominent faces and displayed 2/m symmetry: interfacial angles (deg), f l (calcd)_(OOI), (100) 81 (80); (OOI), 94 (94); ( 1 IO), ( 100) 68 (68); ( 1 1O ) , ( 1 IO) 1 36 ( 137); ( 1TO), 44 (43); (710), (Too) 70 (68); (loo), (ioo) 180 (180); ( i i o ) , (Too) 112 (112). Crystals corresponding to habit I of IC were not observed. 2,6Dibromo-4-(cu,cu-diphenylmethylene~1,4benzoquinone(2b) was prepared in virtually quantitative yield by the thermal dehydration of a powdered sample of lb at 1 I O "C for 30 h. Recrystallization from hexane-benzene gave reddish-orange crystals: mp 236-238 "C (lit.I5 mp 238 "C); IR (KBr pellet) 1640, 1590, 1505, 1310, 700 cm-'; N M R (CDCI)) 7.80 (2 H, s), 7.20-7.75 ppm (10 H, m); UV A,, nm ( E X 276 (1.53), 396 (2.84). (p-Hydroxypheny1)diphenylmethanol (la) was prepared by the method of Gomberg and JicklingI2 and separated from undesired side products by column chromatography on silica gel (ether-hexane eluant) or by the reaction of ethyl p-hydroxybenzoate with phenylmagnesium bromide in somewhat better yields. Repeated recrystallization from EtOH-HzO plus 1 drop of N H 4 0 H followed by repeated recrystallization from toluene-heptane at -80 "C gave la as a white powder: mp 157-159 "C, turning yellow -I I,O-120 "C ( M I 6 mp 157-1 59 "C); UV A,, nm ( E X 280 (1.75). 286 (1.39); DSC, broad endotherm centered at 1 17 "C, sharp endotherm, extrapolated onset I70 "C. When la was recrystallized from dilute EtOH-AcOH or from 50% AcOH, there was obtained a yellow powder, mp 139-140 "C (lit.'' mp 139- 140 "C), determined from the UV spectra of pure la (white) and 2a to be composed of -99% la and -I% 2a in agreement with results reported earlier.8 The IR, N M R , and elemental analyses of both the white and yellow powders of l a were effectively identical; IR ( K B r pellet) 3500, 1600, 1375,1285, 1 1 8 0 , 1 1 5 7 , 7 0 0 ~ r n - ~ ; N M R (acetone-ds) 8.18 (1 H, s), 7.22 (10 H, broad s), distorted AB quartet, A 7.04, B 6.70, J A B = 9 Hz (4 H), 4.93 ppm ( I H, s), the signals at 8.18 and 4.93 ppm diminished on dilution with D20. Crystals from ether-benzene were cube shaped with (1201and (001) as the prominent'faces and showed 2/m symmetrxinterfacial angles (deg), 1 1 " (calcd) (009, (120) 83 (82); (OOI), (120) 97 (98); (120). (120) 98 (97); (120), (120) 83 (83); (120), (-0) 180(180); (i20), (720) 179 (180). Although crystals of la suitable for comprehensive solid-state work were not obtained, the thermal reactions of imperfect chips were observed under the microscope at temperatures of 70-100 "C; the yellow coloration due to the formation of 2a was observed. However, no detailed solid-state work on the rearrangement of crystals of la was carried out. Several attempts (cocrystallization of la and 2a and heating of a microcrystalline sample of la at 90 "C for varying lengths of time) to obtain the 1:l complex of la:2a reported by Baeyer and VilligerI* were unsuccessful. a,a-Diphenylmethylene-l,4-benzoquinone (2a). Thermal dehydration of solid la at 110 "C for 24 h followed by recrystallization of the glassy, orange-red residue from cyclohexane and purification by sublimation (100 " C , 0.01 Torr) gave 2a: mp 166-169 " C ( M I 8 mp 168-170 "C); IR (KBr pellet) 3060, 1620, 1600, 760, 705 cm-I;
(no) (no)
/ 101:19 1 September 12,1979
N M R (CDCI3) 7.08-7.52 (12 H, m), 6.4 ppm (2 H, d, J = 10 Hz), nm ( E X 260 (1.35), half of a distorted AB quartet; UV A,, 365 (2.76). X-ray Structure Determinations. (3,5-Dimethyl-4-hydroxypheny1)diphenylmethanol (IC).A crystal of IC (dimensions 0.4 X 0.4 X 0.2 mm) suitable for X-ray work was obtained by slow evaporation of an ether-benzene solution. Crystal Data: CzrH2002; mol wt 304.4; monoclinic; a = 22.394 (5) A, b = 8.580 (3) A, c = 17.556 (4) A, p 99.20 (2)"; V = 3330A3; Z = 8; d, = I .21 g/cm3; F(000) = 1296; systematic absences, hkl when h + k = 2n + I , hOl, when I = 2n I ; space group C2/c or Cc, the former was confirmed by the results of the analysis. Unit cell parameters were determined by a least-squares fit to the settings for I4 accurately centered high-order reflections (Cu K a , = 1.541 78 A). Intensity data were collected on a computer-controlled four-circle Syntex P21 diffractometer. Procedures for data collection have been described p r e v i o ~ s l yA . ~ total ~ of 2202 reflections was considered nonzero at the 1.960 significance level out of 2837 possible reflections in the 28 range 2-130". Absorption or extinction corrections were not applied and there was no evidence for crystal decomposition. The weights were taken as I / [ o ( F , ) ~ (0.02F0)2],where o(F,) is the standard deviation based on counting statistics. The atomic scattering factors were taken from the analytical expressions given in the International Tables for X-ray Crystallography.20 The structure was solved using the MULTAN program.2' Leastsquares refinement incorporating positional and anisotropic temperature parameters for the nonhydrogen atoms gave an R factor of 0.091. The positions of the aromatic hydrogen atoms were obtained by applying standard geometric criteria; those for the hydrogen atoms of the methyl groups and for the hydroxyl hydrogen atoms were obtained from difference maps. The final model was obtained by fullmatrix least-squares refinement varying positional and anisotropic temperature parameters for all nonhydrogen atoms and positional and isotropic temperature parameters for all hydrogen atoms to convergence with a final R factor of 0.056 and an R2 of 0.063.22 The "goodness of fit" between the calculated and observed models was 2.06.22A final difference map did not reveal any significant electron density above 0.19 e/A3. The final atomic coordinates for IC are listed in Table I. The final values of the thermal parameters and of the structure factors have been dep~sited.~) (p-Hydroxypheny1)diphenylmethanol (la). A crystal of la (dimensions 0.3 X 0.3 X 0.1 mm) suitable for X-ray analyses was obtained by slow evaporation of an ether-hexane solution. Crystal Data: C19H160; mol wt 276.1; monoclinic; a = 8.816 (3) A, b = 15.379 (4) A, c = 10.863 (4) A, = 101.43 (3)"; V = 1444 A3;Z = 4; d , = 1.27 g/cm3; F(000) = 584; systematic absences, hOl when I = 2n I , OkO when k = 2n I ; spacegroup P21/c. Unit cell parameters were obtained by a least-squares fit to the settings for ten hand-centered reflections (Cu Ka,A = 1.541 78 A). Intensity data were collected on a Picker FACS-I diffractometer. Procedures have been described previously.24A total of 2265 reflections out of a possible 2622 with 28 5 130" was considered nonzero at the 20 level. Absorption or extinction corrections were not applied; there was no evidence for crystal decomposition. Least-squares The structure was solved by the MULTAN refinement, varying positional and anisotropic thermal parameters for the nonhydrogen atoms, converged to an R factor of 0. I02 and an R2 of 0.126. A difference map allowed the positions for all the hydrogen atoms to be assigned except for the hydroxylic protons. A diffuse volume of electron density appeared in the region corresponding to the hydroxylic hydrogen atoms, and a relatively large electron density (0.8 e/A3) appeared near the region of C ( 1 I ) , corresponding to H( 11). Eleven low-order reflections2* were removed from the data, and after further refinement a second difference map was obtained containing three peaks of 0.20 (H( I)), 0.1 3 (H( I')), and 0.21 (H(2)) e / A 3 which corresponded to chemically reasonable positions for hydrogen atoms. These hydrogen atoms were included in the calculations with half-occupancy and their thermal parameters were held constant at 4.0 After two final cycles of full-matrix least-squares refinement varying positional and anisotropic thermal parameters for all nonhydrogen atoms, positional and isotropic thermal parameters for the nonhydroxylic hydrogen atoms, and positional parameters for the three half-hydrogens, the final values of R and R2 were 0.061 and 0.067. The "goodness of fit" between the observed and calculated models was 3.62. Both final R-factor calcu-
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Lwis. Curtin, Paul
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Dehydration of p-Hydroxytriarylmethanols
Table 1. Final Atomic Coordinates for
atom
H(21)C
X
0.3775( 1) 0.3182(1) 0.2770( I ) 0.2234( 1 ) 0.2102(2) 0.2502(2) 0.3038(2) 0.3725( 1) 0.3200(2) 0.3 I61 (2) 0.3641(2) 0.41 56(2) 0.4210(2) 0.3960( 1) 0.4 I46( I ) 0.4329( I ) 0.4331(1) 0.4146( 1 ) 0.3960( 1 ) 0.4530(2) 0.41 76(3) 0.4232( 1) 0.4550( I ) 0.457(2) 0.434(2) 0.285( 1 ) O.I93(2) 0.169( 2) 0.244( 2) 0.329( I ) 0.284( I ) 0.275(2) 0.361(2) 0.453(2) 0.459(2) 0.414( 1) 0.381( 1) 0.496( 2) 0.442( 2) 0.437( I ) 0.387 (2) 0.412(2) 0.448(2)
IC
5719
Table 11. Final Atomic Coordinates for l a
v 0.581 7(3) 0.5024( 3) 0.4469( 3) 0.3768(4) 0.3588(5) 0.4130(5) 0.4869(4) 0.7579( 3) 0.8374(3) 0.9979(4) 1.0776(4) 1.005(4) 0.8405(3) 0.5440( 2) 0.3932(3) 0.3494( 3) 0.4630(3) 0.6 I49( 3) 0.6522(3) 0. I845(3) 0.7395(4) 0.5154(2) 0.41 82(2) 0.536(4) 0.454(5) 0.462(4) 0.336(5) 0.3 l4(4) 0.405(5 ) 0.53 l(4) 0.779(4) 1.044(4) I . 190(4) I.058(5) 0.788(4) 0.316(3) 0.753(3) 0. I86(4) 0.125(5) 0.145(4) 0.7 l8(5) 0.845( 5) 0.727(6)
z
0.621 2( 1) 0.6321 ( I ) 0.5706(2) 0.5821(2) 0.6542( 3) 0.7 I59(3) 0.7054( 2) 0.6345( 1) 0.6065(2) 0.6 186(2) 0.658 2( 2) 0.6855(2) 0.6739(2) 0.5428( I ) 0.5294( I ) 0.46 IO( 1) 0.4041 ( I ) 0.41 46( I ) 0.4845( 1) 0.4488(2) 0.3544(2) 0.6805( I ) 0.3374( 1) 0.67 2 (2) 0.292(2) 0.522(2) 0.532( 2) 0.666(2) 0.768(2) 0.749(2) 0.580( 2) 0.599( 2) 0.669( 2) 0.715(2) 0.696(2) 0.571 ( I ) 0.490( 1 ) 0.445(2) 0.494( 2) 0.401 (2) 0.309( 3) 0.372(2) 0.327(3)
Hydrogen atoms are given the numbers of the atoms to which they are bonded.
X
0.9286( 3) 0.8285(3) 0.7418(3) 0.6550(4) 0.65 1 l ( 4 ) 0.7394(4) 0.8259( 3) 1.0990(3) 1.1508(3) 1.3067(4) I .4143(3) I .3664(4) I .2096(3) 0.8743(2) 0.7371 (3) 0.6837( 3) 0.7664( 3) 0.8990(3) 0.9522(3) 0.91 17(2) 0.7 I 15(2) 0.830(6) 0.933(5) 0.809( 5) 0.591 (0) 0.752(3) 0.583(4) 0.584(3) 0.748 (4) 0.884(3) 1.069(3) 1.337(4) 1.559(2) 1.445(4) 1 . I 72(3) 0.680(3) 0.586(3) 0.959( 3) 1.042(3)
Y
0.9653( 1 ) 1.0425( I ) 1.0432(2) 1 . 1 I53(2) 1.1 872(2) 1 . I 882(2) 1.1 171(2) 0.9824( 1) 1.0474(2) 1.0607(2) 1.0118(2) 0.9464( 2) 0.9314(2) 0.88 l6( 1) 0.8422(2) 0.7673(2) 0.7302( 1) 0.7700(2) 0.8447(2) 0.9502( 1) 0.6542( I ) 0.958(4) 1 .OOO( 3) 0.626(3) 0.634(0) 0.99 I(2) 1 . 1 18(2) 1.239(2) 1.240(2) 1.115(2) 1.084(2) 1 .l08(2) 1.021 ( I ) 0.904( 2) 0.879(2) 0.870(2) 0.74 l(2) 0.748( 2) 0.868( I )
z 0.7702(2) 0.7 170( 2) 0.5959( 2) 0.5489(3) 0.62 14(4) 0.7440( 4) 0.7890(3) 0.7748( 2) 0.7039(2) 0.7096(3) 0.78 5 8 (4) 0.8573(4) 0.8514(3) 0.6968(2) 0.71 23(2) 0.649 I (2) 0.5677(2) 0.5465(2) 0.61 17(2) 0.8985(2) 0.5093(2) 0.890( 5 ) 0.9 36 (4) 0.449(4) 0.545(0) 0.545( 2) 0.472( 3) 0.595(3) 0.805( 3) 0.875(3) 0.642( 3) 0.662(3) 0.8 18(2) 0.911(3) 0.893(3) 0.769(2) 0.662(2) 0.496(2) 0.603(2)
Hydrogen atoms are given the numbers of the atoms to which they are attached. These atoms were included with an occupancy of 0.5. This atom was not included in the structure factor calculations, but is a possible site for the disordered H ( 0 2 ) . See supplementary text. (I
Quantitative Studies of the Dehydration of the Dimethyl Alcohol
lations and also the "goodness-of-fit" calculations included all the nonzero reflections of the data et.^^.^^ The atomic scattering factors of Cromer and MannZ9and that of Stewart, Davidson, and Simps0n3~ were used for all nonhydrogkn atoms and hydrogen atoms, respectively, in the structure factor calculations. The final coordinates for l a are given in Table 11. The final values of the thermal parameters and of the structure factors have been deposited.23 X-ray Powder Photography. Debye-Scherrer powder photographs were taken with a camera manufactured by Charles Supper Co., using llford Type G film and nickel-filtered Cu Ka radiation (A = 1.541 78 A). A freshly ground sample of the hydroxyphenol IC (recrystallized from ether-hexane) gave a photograph showing spacings in agreement with those calculated3~from the crystal data of IC. The powder photograph of fuchsone 2c obtained by heating a microcrystalline sample of IC for 48 h a t 1 I O "C was identical with a photograph of a pseudomorph obtained by heating a single crystal of IC for 7 days a t 110 "C (without grinding) except that the latter photograph showed streaking indicating some residual order. The d spacings obtained from both photographs agreed with those calculated from crystal data for the p(P212121) form of 2c.31,32 Single Crystal Experiments. All thermal and photochemical experiments were conducted with samples with a satisfactory elemental analysis ( f 0 . 3 ) . Single crystals of light-sensitive compounds were grown by slow evaporation of a solution protected from light - by. aluminum foil.
[IC) and Dibromo Alcohol (lb). The extent of the dehydration of single crystals of l b and IC was determined by UVANL, a computer program based on the Dewar-Urch method of UV a n a l y ~ i s . 3The ~ measurements at the 28 wavelengths utilized in U V A N L were made every 5 nm from 255 to 390 nm in the spectra of the mixtures of IC and 2c and a t 39 wavelengths (every 5 nm) over the range 260-450 nm for mixtures of l b and 2b. Agreement to f 3 % was found using four samples of known composition, the ratios of 1:2 varying from 30:70 to 80: 20. The rate data presented in Figure 1 were obtained using single crystals of both habits of IC; crystals were chosen which appeared to bc without major defects and of approximately the same size. Twenty to thirty single crystals were placed on a microscope slide and the slide placed on a calibrated Kofler hot stage (preheated to 110 f 2 "C). As the crystals were removed a t the times indicated, they were analyzed individually (without further purification) by dissolution in diethyl ether in a UV cell. The UV spectra thus obtained were analyzed for percent reaction by U V A N L . Each point in Figure l represents the analysis of one crystal. The data presented for both l b and IC are a composite of two such experiments. Similar experiments conducted with single crystals of both l b and IC which were heated at 1 I O "C for various times and then analyzed for percent reaction by elemental analysis gave results in good agreement with those above. Single crystals (-50 mg) of IC were heated at I I O "C for IOdays and then analyzed without further purification by N M R . The spec-
Journal of the American Chemical Society
5720 100
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i=
2n
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-A
/ September 12, 1979
0
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0
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trum thus obtained was identical with the N M R spectrum of an authcntic sample of 2c. Similarly Ib, after being held at I I O "C for 24 h, gave an N M R spectrum and mp 236" C (lit. 236-238 "C), identical with that of 2b. The irradiation of single crystals of IC was conducted using either a Mineralight UVSL-25 T L C indicator lamp (X 254 nm)34 a t a distance of 2.5 cm or a Rayonet photochemical reactor (high intensity,35 X 254 nm). The UV spectrum of a very concentrated solution in diethyl ether of a single crystal of ICwhich had been irradiated for 48 h with a Mineralight lamp showed a slight absorption at -360 nm, which corresponds to the A,, of 2c. The concentration of 2c was too small (
