C. W. Jefford,' R. McCreadie, P. Muller, and J. Pfyffer Universitv of Geneva 121 I Geneva 4, Switzerland
I
Spectroscopy and Structure Elucidation
I
Coordinated experimental exercises in advanced organic chemistry
1
Spectroscopy is an invaluable aid towards structural identification in modern organic chemistw. Conseauentlv. every student of c h e m i s t i a t some stage in his training should be made aware of the range of information potentially availahle from spectroscopic techniques, and he given a practical introduction to the basic spectroscopic methods. It has been our experience that a student more readily develops a meaningful interest in spectroscopy when it is presented not as a collection of unrelated facts in class, hut rather when it is integrated with a sequence of simple experiments in the laboratory. Although isolated experiments in spectroscopy have been described for the beginner (I), no experiments have been devised for the more advanced student who would like to learn how different techniques can he brought to bear on a set of interconnected structural ~rohlems.With this aim in mind. we have developed appiopriate experiments which demonstrate the versatilitv of modem s ~ e c t r o s c o ~methods ic and the variety of structural information which can he obtained. Our experimental sequence uses readily availahle starting materials and consists of five easy-to-perform, highyield reactions in which the structures of the products are determined from infrared (ir), ultraviolet (uv) and nuclear magnetic resonance (nmr) ~ p e c t r a . ~ The reaction sequence (Fig. 1) entails the treatment of commercially availahle isophorone (I) with methyl magnesium iodide. When the reaction is carried out in the presence of cupric acetate, 3,3,5,5-tetramethylcyclohexanone (11) is ohtained. Bromination of ketone (11) with pyridinium hydrobromide perhromide gives 2-hromo-3,3,5,5-tetramethylcyclohexanone (111) (2). On the other hand the reaction of iso~horone(I) with methvl maanesium iodide alone furnishks, presumably via the tertiary alcohol IV, a mixture of the endocyclic and semicyclic dienes V and VI from which, on treatment with maleic anhydride, the DielsAlder adduct VII is ohtained. Protonation of the diene mixture V and VI generates the common cation VIII. As semicyclic diene VI cannot undergo the Diels-Alder reaction the formation of more adduct W from the mother liquors on adding a trace of concentrated sulfuric acid confirms the interconversion of dienes V and VI via VIII. These manipulations can he accomplished in four to five sessions of 5-hr duration; two sessions are needed for the preparation of 11, one session each for 111 and the mixture of V and VI, and VII. However, much time can he saved if II is p u r c h a ~ e d . ~
'To whom enquiries may be directed at the Dkpartement de Chimie Organique, 30 quai de I'Ecole-de-MBdecine, 1211 Genhe 4.
2These experiments have been satisfactorily carried out by students in their fifth semester of a three-year program leading to the "licence" in chemistry at the Univenity of Geneva. 33,3,5,5-Tetramethyl~y~I~hex~n~ne is obtainable from the Aldrich Chemical Ca., Ine. or from Fluka AG, Buchs, Switzerland.
Figure 1. Flow sheet of the preparation of 3.3.5.5-tetramethylcycIohexanone 11, its monobromo derivative i l l . 1.3.5.5-tetramethylcy~Iohexa-l.3diene (V), l-methylene-3.5.5-trimethylcyclohexa-2-en VI, the DielsAlder adduct VII and the 1.3.5.5-tetramethyicyclohexa-1.6dienyi cation VIII.
The foregoing compounds all give clean-cut spectra which variously reveal the conformational behavior of the a bromocyclohexanones 111, the isomerization between the endocyclic and semicyclic dienes V and VI, the symmetrical structure of the cyclohexenyl cation VIII and the geometry of the rigid adduct VII. The spectral determinations (three for the ir, four for the uv, and six for the nmr) are best performed by groups of three to four students. If a simple, rugged 60 MHz nmr spectrometer is not available, the instructor may run one set of spectra for all the students on a research instrument. The structure of 3,3,5,5-tetramethylcyclohexanone(11) is conclusively established by spectroscopy. The carbonyl stretching frequency of 1715 cm-1 (ir in carbon tetrachloride) and the position and intensity of the n-a*,ahsorption band in the uv hmexMeOH286 mp; r 20) are typical for cyclohexanones. The nmr spectrum of I1 a t 60 MHz in carbon tetrachloride shows three sharp singlets a t 1.02, 1.45, and 2.10 ppm in a ratio of 12:2:4 which may be immediately assigned to the methyl, y-methylene and a-methylene protons. The signals are sharp because the chair-chair interconversion at room temperature is so rapid on the nmr time-scale that axial and equatorial protons are indistinguishable. The spectral properties of 2-hromo-3,3,5,5-tetramethylcyclohexanone (111) are in marked contrast to those of the parent ketone. The differences stem mainly from the fact that the two interconverting chair conformations, IIIa and IIIe, are not identical, hut are related as diastereomers or, more precisely since I11 is a racemic mixture, as enantiomeric pairs of diastereomers. Volume 50, Number 3.March 1973 /
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For example, conformer IIIa, in which the bulky halogen is axial, experiences unfavorable, syn-axial, non-honded interactions, whereas the electrostatic repulsion hetween the nearly orthogonal carbon-oxygen and carhonhromine dipoles is slight. In conformation IIIe, where the a-bromine suhstituent is equatorial, the opposite is the case; the steric interactions are now trivial, hut the electrostatic repulsion between the almost parallel carhonyl and carbon-halogen bond dipoles is severe. As a result, this dipolar field effect decreases the polarity of the carhonyl bond in equatorial a-bromoketones and thereby increases its vibrational stretching frequency (usually by shifts of 15 to 25 cm-') relative to that of the parent ketone. This effect is virtually non-existent for axial a haloketones. Colisequently the value of the carbonyl frequency constitutes the hasis of the ir spectral method, used by Corey, Allinger and others (2-4) for determining the configuration of the halogen suhstituent of a halocyclohexanones. In a similar way the uv spectra of a-hromoketones are affected by the relative orientations of the carbon-bromine and carbon-oxygen bonds (2-5). Thus the geometry of axial a-bromoketones is such that the antihonding s* orbital of the carhonyl group is appreciably stabilized by overlap with the unfilled orhitals on the adjacent halogen atom. Consequently, the n-a* uv absorption maximum for such hromoketones occurs a t longer wavelength and is more intense than that of the parent ketone, for example a t 300 mfi ( e of 120) compared to 280 mfi ( t of 20-30). On the other hand for equatorial a-bromoketones, where the dipoles are almost parallel, it is the non-bonding orbitals on oxygen which back-bond to hromine. These non-bonding orbitals are, however, only slightly stabilized since they are already of low energy. Hence the intensity and position of the n-r* maximum of equatorial a-hromoketones are similar to those of the parent ketone. Naturally the position of the equilihrium between these limiting conformations will he affected by solvent polarity (2-6). In general, nonpolar solvents favor the less polar conformers in which the halogen is axial, since intramolecular electrostatic repulsion is the key factor determining the conformation. In polar solvents the situation is reversed; the more polar, but less crowded, conformation in which the halogen is equatorial predominates since its inherent dipolar instability is compensated by favorable intermolecular electrostatic interactions with the solvent. These conformational preferences are neatly demonstrated by the markedly different ir spectra of JII in carhon tetrachloride (Fig. 2a) and acetonitrile (Fig. 2h). The relative intensities of the carhonyl absorptions a t 1715 and 1730 cm-' due to IIIa and IIIe are roughly reversed in the two solvents (2, 7). The nmr spectra of I11 (Fig. 4) in carhon tetrachloride and in benzonitrile (Fig. 5) provide an impressive corrohoration of this conformational behavior. Since nmr has a time-scale much slower than the rate of interconversion (at room temperature) of the two conformers, what is ohserved is the "average" spectrum of the two conformers IIIa and IIIe. A rigorous analysis is not therefore possible. However, since the conformational equilihrium is displaced largely towards either IIIa or me, these "average" spectra are very similar to those expected for the pure conformers IIIa and IIIe. In both spectra the most distinctive resonance is due to the bromomethine proton A. This proton shows well downfield as it is geminal to a bromine atom. However the magnetically anisotropic carhonyl group differentially affects the ring protons. Axial protons are deshielded; equatorial protons are shielded (8, 9). In carhon tetrachloride, I11 spends most of its time in the axial conformation 182
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Journal of Chemical Education
l730
' ' 1111
4730
cm-'
' ' 1715
cm'
Figure 2. Position of the carbonyl absorption of I l l in carbon tetrachloride (a) and acetonitrile (b).
Figure 3. Ultra-violet absorption spectra of I i i in carbon te and acetonitrile (bl
chloride (a)
L
+' i
1
mm
Figure4. Nmr spectrum of I I i in carbon tetrachlorideat 60 MHz.
. .. . 4.5
1
i
Figure 5. Nmr spectrum of i l l in benzonilrila a t 6 0 MHz.
1
w-
IIIa, as attested by the shift of 4.03 ppm for HA,a value typical for an equatorial proton. In henzonitrile the equatorial conformation IIIe makes a considerable contribution to the time averaged species and the lower value of 4.53 ppm., is an indication that H A has a lot of axial character (10). Apart from the deshielding-shielding properties of the carhonyl group, the a-bromine atom, when i t is axial, exerts a supplementary deshielding effect, hut only on contieuous nrotons. namelv the svn-axial ones (8, 11). This egect is'immediate~~ obvious i n comparing Figures 4 and 5. The difference in chemical shifts between the axial and equatorial geminal protons is greater in IIIa than IIIe. In IIIa, this difference is marked for protons B and C, and somewhat less for D and E as they are farther away from the carhonyl, (Fig. 4). In IIIe the difference is modest for protons B and C and so small for protons D and E that their shifts start to converge (Fig. 5). A further corroboration of the predominance of the axial conformation IIIa in carhon tetrachloride is the ohservation that the equatorial protons B and D exhibit fine structure due to mutual long range coupling over the four intervening sigma bonds. Such long-range coupling is the rule only for protons which find themselves on the ends of four sigma bonds arranged in a W shape; in conformation IIIa protons B and D satisfy this rule (6, 8, 11). A completely different structural problem is posed by the reaction of isophorone (I) with methylmagnesium iodide alone. The endocyclic and semicyclic dienes (V and VI) are the logical products of the dehydration of the initially formed alcohol IV. Kharasch (12) managed to isolate IV and assumed the endocyclic diene V to be the compound ohtained on dehydration. Later Wheeler concluded from chemical and uv spectral evidence that the diene was the semicyclic isomer VI (13). On repeating these experiments, we found, as did Deno (14), that dehydration of the intermediate alcohol (IV) gives a mixture of both dienes V and VI in high yield. Under our conditions the dienes are formed in almost equivalent proportions, as is clearly revealed by the nmr and uv spectra. Separation of the dienes is not necessary since their structures can he assigned directly from the spectra of the mixture. Thus the uv spectrum in cyclohexane shows two hands at 268 mp ( 6 4,000) and 237 mp ( 6 12,000). The position and relative intensities of these hands are in good agreement with the values of 273 and 234 mp for endocyclic (V) and semi-cvclic (VI) dienes ~ r e d i c t e dfrom the WoodwardFieser k l e s (15). The ratio of V to VI is readily obtainable from the nmr spectrum of the mixture at 6 0 . ~ ~in 2hexadeuteriobenzene (Fie. 6). Four different vinvl resonances are clearly visible a i d their assignment to the various protons, A-D, in V and VI is based on em~iricalconsiderations (11). Integration of the intensities of the B, C and A, D signals gives the ratio of V to VI, which is about 1:l. A further check on the ratio can he ohtained from the relative intensities of the signals due to the geminal dimethyl groupings K and
six gem-dimethyl protons (D)gamma to the allylic system are scarcely influenced by the positive charge in that their signal a t 1.10 ppm is only slightly displaced downfield from those observed for methyl groups in saturated hydrocarbons (11). Further proof that structure VIII correctly represents the cation is furnished by the uv spectrum of the diene mixture in concentrated sulfuric acid (14); a single ahsorption hand, X max. 314 mp (e 9,000) is seen. The position of this maximum agrees with the value predicted from an empirical comparison with the uv spectra of other alkenyl cations (16) and is in accord with the electronic absorption energy calculated for VIII from simple L.C.A.O. theory (14, 17). A supplementary detail, which follows from the symmetry of the spectra, is that cation VIII effectively has Cz, symmetry; which means that the conformations VIIIa and VIIIh undergo rapid interconversion. In recent years, many empirical and theoretical studies have been made for correlating chemical shifts and coupling constants with the geometric arrangements of protons in molecules (11). In selecting suitable model compounds to test the theory, certain criteria need to he respected. Firstly, molecules containing small numbers of protons with widely separated chemical shifts are desirable since these usually exhibit simple first-order spectra (11). Secondly the model compound should possess a well-
Figure 6. Nmr spectrum of dienes V and VI in hexadeuteriobenzene at 60 MHz.
L.
When the diene mixture is dissolved in concentrated sulfuric acid the nmr spectrum simplifies to just four singlets (Fig. 7). The integrated relative intensities of the signals (1:4:6:6) and their chemical shifts can only he satisfactorily accommodated by the 1,3,5,5-tetramethylcyclohexenyl cation VIII (14). The signal at 7.68 ppm represents the vinyl proton (A) attached to a positively charged system (11). The peaks a t 3.10 and 2.83 ppm are due to the four methylene (B) and six methyl hydrogens (C) adjacent to the cationic system. Owing to the proximity of the positive charge, these peaks occur a t lower fields than those of allylic protons (11). On the other hand the
8
,
S
S
4
3
%
~
P
F
Figure 7. Nmr spectra of cation Vlil obtained by dissolving dienes V and VI in concentrated sulfuric acid.
Volume 50, Number 3, March 1973
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183
defined geometry (8,11). These desiderata are most conveniently found in rigid bridged hicyclic compounds. T h e present example, t h e bicyclic adduct VII, gives a particularly satisfying demonstration of the dependence of chemical shifts a n d coupling constants on the disposition and type of protons. T h e nmr spectrum of W (Fig. 8), at 60 M H z in deuteriochloroform, is fully interpretable a s i t is first-order. All t h e different kinds of protons exhibit well-separated and distinguishable resonances. T h e assignment-of t h e chemical shifts t o their appropriate protons follows from a consideration of t h e vaiuks kxpected, reinforced by t h e coupling constants. T h e solitary vinyl proton a shows a t 5.6 p p m a s a multiplet owing t o long range coupling with t h e allylic protons at t h e bridgehead, Hd, and t h e methyl group e on the double bond. T h e chemical shifts for the tertiary protons, b, c, a n d d are typical a n d each signal is easily assigned by virtue of their characteristic splittines. Proton d is instantly recognized by i t s doublet of doublets a t 2.55 p p m springing from vicinal coupling with Hb a n d the lone - ranee - coupline with t h e vinyl proton a. Proton c, on t h e other hand, merely c o u p l e s - 4 t h its neighbor b and shows a s a doublet a t 2.8 ppm. Proton b appears at 3.5 p p m as t h e expected doublet of doublets. T h e methyl groups give instantly recognizable signals. T h e hridghead methyl, f, signal appears a t 1.4 as a sharp singlet. However, t h e singlets at 1.1 a n d 0.9, d u e t o t h e geminal methyls i and j are broadened owing t o mutual W-plan coupling. These singlets also overlap with the signals of t h e methylene protons g, h, which have very similar chemical shifts. At first sight, these protons tend t o be missed, however integration reveals their presence. T h e last remaining signal, t h e fine doublet at 1.8 p p m is characteristic of t h e vinyl methyl group e. Experimental
Infrared svectra are recorded on a Perkin-Elmer 257 Grating Infrared Spectrophotometer; uv spectra on a Perkin-Elmer 402 Ultraviolet-Visible Soectronhotometer a n d nmr spectra on a Perkin-Elmer R12 Spectrometer operating a t 60 MHz, with tetramethylsilane as a n internal standard.
To magnesium turnings (9 g, 0.39 mol) in dry ether (20 ml) in a 1-1 flask is added methyl iodide (1 ml). When the effervescence has started dry ether (150 ml) is added in one portion. This is followed by the dropwise addition of methyl iodide (60 g) in dry ether (60 ml) with stirring during 20-30 mi". The mixture is stirred for a further 30 min a t room temperature and cooled to 5°C in an ice bath, and finely powdered eupric acetate manohydrate (45 g, 0.23 mol) is added. To the resultant black suspension (due to traces of colloidal copper) is added, dropwise, isophorone (I; 45 g; 0.23 mal) in ether (300 ml). After addition, the reaction mixture is allowed to warm to roam temperature and then heated under reflux for 2 hr. The cooled reaction mixture is decomposed by the dropwise addition of a saturated ammonium chloride solution, followed by dilute hydrochloric acid. The insoluble inorganic salts are removed by filtration and the two layers of the filtrate are separated. The ethereal extract is washed successively with a saturated solution of sodium thiosulfate (until colorless), water, saturated sodium bicarbonate and sodium chloride solutions and then dried and evaporated. Distillation of the residual oil in vacua yields 16.5 g (50%) of 3.3.5.5-tetramethylcyclohexnnone (If), b.p. 79"/12 mm; ir (CCU 2960, 2910, 2870, 1715, 1416, 1310, 1280 and 1230 cm-I; nmr (CCln)three singlets in ratio 4:2:12 at 2.10.1.45 and 1.02 ppm, uv AmaXMeoH 286 mp ( r of 20).
4This product is commercially available. Alternatively it may be prepared as in "Reagents for Organic Synthesis,'' L. F. Fieser and M. Fieser, John Wiley and Sons, New York, 1967. 184
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I
4
3
2
I
0
w m
Figure 8. Nmr spectrum of adduct VII in carbon tetrachloride at 60 MHz.
2-Bromo-3,3,5,5,-tetramethylcyclohexanone (111) (3) To 3,3,5,5-tetramethylcyeIohexanone(11) 4.62 g; 0.03 mole) in glacial acetic acid (30 ml) is added in small portions pyridinium hydrobromide perhromide4 (10 g; 0.31 mole). After stirring at room temperature for a further 30 min, the reaction mixture is dissolved in water and then extracted several times with ether. The combined ethereal extracts are concentrated, and the residue is dissolved in pentane. On cooling, small quantities of dibrominated tetramethylcyelohexanones separate. The mother liquors, after filtration, are concentrated and distilled to yield 6.5 g (90%) (III), b.p. 114of 2-bromo-3,3,5,5-tetrame~hyleyc10he~nnone 116" C/9 mm. Pure 111, m.p. 50-51T, is obtained by sublimation. The ir, uv and nmr spectra are shown in Figures 2, 3, 4, and 5. 1,3,5,5-Tetramethylcyclohexadiene ( V ) and 1,5,5-Trlmethyl-3-methylenecyclohexene (V1) Magnesium (6.0 g; 0.25 mole) is put in a 250-ml flask and is covered with a little dry ether. Methyl iodide (1 ml) is next added; after effervescence has started, more dry ether (150 ml) is added. Additional methyl iodide (36 g) in dry ether (35 ml) is added dropwise, with stirring, over 15 min and the mixture is stirred at room temperature for 30 min and then cooled to 5'C. Isophorone (I; 31 g; 0.23 mole) in ether (30 ml) is added, with stirring, at such a rate that the temperature of the solution remains about 15". The mixture is heated under reflux for 1 hr, allowed to cool, then decomposed with ice and 15% hydrochloric acid. The ethereal layer is separated and the aqueous layer is further extracted with ether (2 X 25 ml). The combined extracts are washed suecessively with water, saturated sodium thiosulfate, sodium bicarbonate, sodium chloride solution, dried over magnesium sulfate and concentrated to yield a yellow oil. Distillation through a fractionating column yields 26 g (85%) of a roughly 1:l mixture. b.p. 152-154". nDZ01.4700 of 1.3.5.5-tetromethyleycloheradiene ( V ) and 1.5.5-trimethyl-3-methylenecyclohexene (VI) as shown by ir, (film) 3080, 3010, 1650, 1610, 900, 870 and 810 em-'; uv, ,A,, (cyclohexane) 234 and 273 mp (G of appmx. 12,000 and 4,000, respectively, assuming a 1:l mixture of two products); for nmr in CaDs see Figure 6. In several experiments the ratio of V and VI remained approximately 1:l. The ratio varied slightly with the heating times on distillation. Partial separation was effected by careful distillation in uocuo. For example, the fraction collected
a t b.p. 39-4ZSC/10 mm was richer (as shown by nmr) in V, whereas the fraction, b.p. 42-43'/10 mm., was richer in VI. Dissolving the dime mixture in 96% sulfuric acid (13) gives a solution of cation (Vm), sufficiently stable for uv (A max, 314 m r ; r 9,000) and nmr spectral purposes (Fig. 7).
Maleic Anhydride Adduct (VII)
of
1,3,5,5-Tetrarnethylcyclohexadiene (V) Maleic anhydride (4.9 g; 0.05 mol) and 6.8 g (0.05 mol) of the mixture of diems V and VI are dissolved in 10 ml of dry benzene and heated under reflux for 2 hr. On cooling, colorless crystals (5.2 g) of VII separate. Addition of 1 drop of concentrated sulfuric acid to the mother liquars, followed by 30 min of reflux yields, on cooling, a further 3.5 g of crystals. Both batches of crude crystals are combined and recrystallized from benzene/petroleum ether 40-60" (1:3) to yield 8.1 g (69%) of adduct VII, m.p. 89-99" ir (nujol) v(C=O) a t 1855 and IT55 cm-', (C-H, olefinic) a t 3.060 em-'; nmr (Fig. 8).
Acknowledgment
We are grateful to the Fonds National Suisse pour la Recherche Scientifique for the award of a grant (No. 5202.2) for the purchase of the spectrometers, which we used for testing these experiments. Literature Cited (11 Glaros, G.. and Cromwell, N. H.. J. CHEM. EDUC.. 46. 854 (1969). Glscos. G.. and cmmwe11. N. H., J C H E M EDUC.. 48, 202. 204 (19711 Harhiron. K. 6..
J. CHEM. EDUC.. 47, 837 (1970). Taber, R. L., and Grsnfham. G. D.. J. CHEM. EDUC., 47. R45 119701. Fairleu. B, d.. Dunn, H. E.. and Fmter. D. 0.. J. C H E M EDUC.. 48. 827 119711. Suydan, F. H.. and Yodor. C. H.. J . CHEM. EDUC..48,849(197LI.McCannoll,J.F., J. C H E M EDUC, 48.552l1971l. (21 Waegell. B.. and Ouriuon. G., Bull Soc. Chim. Fmnce. 496 (19631 end references cited therein. (31 la) Jones. R. N.. Rsrnaay, D. A,. Herling, F.. and Dobriner. K.. J . A m m Chem. S o r . 71, 2828 11952). (bl Joner. R. N . , J Amer Chpm Soc., 75.4839 (1953). lci Dickson. D. H. W.. and Pago. J. E., J. Chsm Soc. 447 119551. (41 (a) Corey, E. J.. J . Amer Chem. Soc.. 75, 2301 11953). (bl Corey. E. J.. Topie. T. H..and Wmniak. W. A,. J. Amer Chem Soe., 77. 5415 (19651. (cl Corey. E.J.. and Burke. H. J.. J Amer. Chsm. S o c , 77. 5418 (19561. Id) Allinper. J.. and Allinger. N . L., Tetrohedmn, 2.64 11958). 151 Kosowor, E. M.. Wu. G.. and Sorensen. T. S.. J . Amar C h m . Soc, 83, 3147 119611. ( 6 ) Waegell, B..Bull. Sor. Chim. Fmnce. 855 119641. 17) Bellamy. L. J.. "The Infrared SrxcLra of Comdex Molecules." 2nd Ed., John Wile~sndSons.N e w York. 19581 (8) Jcflord, C. W.. and Waegell. B., Bull Soc Chim. Baig 79. 427 119701 and references cited therein. (91 Aprimon. J. W.. Dernsreo. P. Y.,Mathbron, D. W.. Craig, W. G.. Karim, A,. Saundors. L.. and Whalley. W. 8.. Teirahedmn. 26.119(19701. 110) Bsretls. A.. Zshra, d. P., Wasgall. B., and Jefford. C. W.. T e r m h e d m 26, 15 (19701. 111) Jackson. L. M.. and Sternhell. S.. "Applieafions of N.M.R. Spectroscopy in OrganicChemistry,"2ndEd..Pe~gamon.NewYork. 1969. (12) Karasch. M. S.. andTawney P.O..J.Amrr. Chem. Sac.. 63.2308.ll9411. 113) Wheeler. 0. W.. J . Org. Chem., 20. I672 11955). oen0.N . c.. R ~ c ~ JI.. ~ Y H, G.. ~ ~ J. D., d and ~wisotsky, ~ M. J.. J. ~ m e r . Chem. Soc., 84. ,498 119621. (15) Smft. A. I.. "Interp~etstionof the Ultra Violet Spectra of Natural Products." Pergammon. London, 1961. 1161 Deno. N . C . . h Phva 018. Chem.. 2,129119641. (17) Streifwieser Jr.. A,. ''Molecular Olbital Theory for Organic Chemists." John Wiley and Sons. New York. 1961. 1181 A modification of the cupnc acetate procedure used by Marnhall. J. A,, and Roebke. H.. J. 0rg. Chem.. 33. 840(19681.
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