Charles A. Kingsbury University of Nebraska Lincoln, NB 68568
I
Conformations of Substituted Ethanes
Most oreanic chemistrv courses introduce conformational analysis near the beginning of the course and emphasize the ohenomena rather than the reasons behind the observed hehavior. Students find the subject interesting and frequently inquire concerning the reasons for the barrier to internal rotation in ethane, or question why the gauche conformational isomer of butane (I) is less stable than the trans (or anti') conformer (11). The former question forces the reluctant instructor to mumble something about steric hindrance and to hope for an early sound of the hell. The steric concept is easy for beginning students, but this explanation has long been regarded as inadeauate to exolain rotation barriers ( I ) . If a " steric explanation is used, students may later inquire why butane has onlv a slinhtlv . . larger .. harrier to internal rotation ( 4 . 5 kutethe barrier t o rotation in part to forces associated with the Pauli, exclusion principlc. The repulsion caused by penetration of rircLn,ns associated with one atom on a filled orbital associated with atoms could be considered a steric effert. other ., .'I hese I'aclms were mentioned hy various speakers in the S~mposiurn on No11-hndedlnteracticms. 1 i2nd National NIe~tin~afthe Arneriran Chemical Society. Ssn Francisco, CA. Sept. 1976, PHYS 79-81, ctr.
.
Volume 56. Number 7.July 1979 1 431
In other theoretical treatments, the interaction hetween electruns formally assigned to a given C-H bon(l in ethane with a 5,icinal C -H is considered to he most fuvorable when the two C-H honds are trans (8,1O, 12). The interaction may be viewed as delocalizingthe electrons in question into a trans C-H antibonding orbital (12). This represents a type of hyperconiugative interaction. Despite Dewar's intense criticism bf hyperconjugation in neutral molecules (131, the hyperconjugation concept is encountered frequently in recent paners 6-12),. , and i t is used to exulain the preferred staggered conformation of ethane. ~ v e in n early-work, interactions similar to VI were considered (14). In addition to these orbital interactions, electrostatic effects such as nuclear-electron attraction. and nuclear-nuclear or electron-electron repulsions play a role. Kinetic energy effects associated with thevelocity of motion of the electrons may be important also (7,9b).
.
interaction energy was considered to be 0.2 kcallmol. In IIa, various conformational adjustments have occurred in order to minimize the CH3-CH3 interaction; these hond angle changes, etc., cost another 0.2 kcal in energy. The remainder of the 0.8 kcal difference in energy between Ia and IIa results from a summation of the various group interaction energies shown in Figure 1. According to this work, the H-H interaction is particularly unfavorable. However, Fitzwater and Bartell disagree with Allinger's emphasis on H-H interactions (19). These workers consider the classical idea of a dominant CH3-CHa repulsive interaction in IIa as correct, and further state that the assignment of a specific energy value to a given class of interactions (say H-H) is not warranted, as the precise spatial relationships of groups will vary from case to case. Recently, a revised classical mechanics calculation program has been formulated (20).The inclusion of new torsional energy terms obviates the need for a "hard" hydrogen. A revised estimate of the group interaction energies in Ia and IIa is awaited with inte&t,even though these may be relevant only to butane. In the succeeding paragraphs, examples of the various factors that affect conformation will be considered in more detail. -~ ~ .with ~ - the ~ excention of auantum mechanical effects associaied with the "gkche efiect." Steric Effects-This t . w.e of interaction receives a meat - deal of attention in most undergraduate courses, for the concept is understood easilv. .. and firm examples are widespread. With regard tostericeifects on molerul;i; geometry, the rigid m u ecules \'I11 and 1X are illumative (1'1, 221. In these cyclic molecules, internal rotation is prohibited, and the effect of steric hindrance on hond angles is unambiguous. In VIII, the CH-C-C bond angle has spread from -119" to 122' in order to relieve the cis CH-H interaction. In IX, the much more ~ - severe ~ - ~ CH1-CH- " interaction results in a further widening of the CH3-C-C angle to 124". Similar effects occur in acyclic systems, e.g. hydrocarbons such as X in which the C C - C angles along the chain are -112", significantly larger bond angle has than tetrahedral (3.20). In X, the H-C-H undergone a compensating decrease to -106'. Present indications from X-ray crystallographic data (e.g. XIa) indeed strongly suggest that bond angle spreading frequently occurs between the larger groups, and a partial collapse of hond angle often involves honds to hydrogen (23). Thus, a formally tetrahedral carbon often is really a flattened tetrahedron (24). The hond angles in XII, though exaggerated, are illustrative of these angle adjustments, whose origin is most likely a simple steric effect. ~
Hyperconjugation has heen used to explain amforn~ntional pretcrcnces in other rnses, as well. I.owe (26) and \Volfe (1.51 initiallv drew attention to the ureference for gauche halo~ens in 1,~rdihaloethanes. i.e. thk "gauche efgct." ~ o w i v e r , Phillins and Wrav showed that this conformational preference occurs mainly f i r molecules having highly electionegative halogens (16). Many explanations have been advanced to account for these data (17), but Abraham feels that nospecial explanations are required, as the conformation of several fluoroethanes is reproducible hy classical mechanics calculations (18). Earlier, Radom, et al. interpreted the preference for gauche fluorines in VII in terms of a hyperconjugative interaction between fluorine and a trans hydrogen (8). The gauche fluorines, per se, were considered to interact in a repulsive manner (cf. dipole-dipole interactions discussed below).
~
~~
By means of classical mechanics calculations, Allinger and co-workers have provided an interesting though controversial dissection of the interactions in trans and gauche butane (Ia and IIa) (3).Allinger points out that the Ia-IIa interconversion involves much more than a simple rotation about the central C-C hond. Variations in other degrees of freedom, such as H-C-C bond angles, occur simultaneously with internal rotation so that an incipient severely repulsive interaction is relieved as much as possible. In the gauche form of butane (IIa), one hydrogen of each methyl group extends toward and impinges upon a similar hydrogen from the other methyl group. In this early work, the
Figure 1.
432 / Journal of Chemical Education
The picture that emerges from the crystallographic investi-
xv
XVI
I z9'\= i -i
d
Figure 2.
gations shows the interplay of changes in the state of internal rotation (cf. XIb) coupled with changes in hond angles as various molecules seek the minimum energy conformation. As noted some time ago, these variations are relatively cheap in energy requirements in contrast to changes in bond length ( l a ) . Molecules seldom undergo lengthening of C-C bonds to relieve steric interactions. The chief exceptions appear to be highly congested molecules such as pentaphenylethane, where the central C-C bond is 1.63 A in length (24,25). Steric congestion also affects the harriers to internal rotation in ways perhaps not expected. The barrier to rotation of the methyl gronp(s) in the cis oxirane IX is lower (1.6 kcal) than for the less congested trans isomer VIII (2.6 kcal) (22). A similar difference is found in the 3-halo~rooenesin which the harrier to mrthyl rotation is quite highin ihe unhindered trans isomer (-2.2 kcall and rounhls independent of?(. In the more hindered cis isomer ( 1 I I , the l~arrierdecreases assteric hindrancr increases in thennler: X = t' (harrier, 1.0 kcall; C1, (0.6kcal I, and HI (0.2 kcal~.The strrirnlly hindered molecules such as XIV n r v not really comfortable in any cmf~rrmntion. 'I'he enereies of the most stable conformation ithe .'valle\." of the enerp\. versus angle uf rotation plot) and the lrast stahle confimnatim lthe "~eak")arc h t h liieh hut no1 substdnriallv different. he difference in energy (the harrier to rotatin;) may therefore be quite small. H
\
7
3
FC\
X
H
XI,,
predicted to he the most stable for the erythro isomers, and this has been supported by a great deal of recent work. In the chloro-sulfide (XVII) shown in Figure 3, the fraction of all molecules occupying ET (i.e. the population of ET)regularly increases as the size of R increases up to R = tert-butyl(29). Thus. in E r . the stericallv demandine L mouos are trans. and furthkrmor;, the total number of gaGhehteiactions between sizable groups (L or M) is minimized compared to Em or Em. On the other hand, when R is smaller (e.g. CH3 or CzHs) there is no clear differentiation between the size of ArS and R, and the choice of conformation may no longer he dictated by steric factors alone. As expected, an increase in size of R a t a point distant from the ethanic skeleton has relatively little effect. Thus, R = C H ( C 2 H h induces about the same degree of preference for ET as R = CH(CH3)z. Also, R = CH2C(CH& induces a SUP prisingly small preference for ET. In contrast, neopentyl halides react verv slowlv in S Nreactions. ~ due to steric hindrance to the approach of ;he nucleophile ($1. However in this reaction, steric hindrance near the Derinherv . . .of the molecule is important. Compounds with R = tert-hutyl groups (seemingly, the epitome of a large group) frequently have anomalous conformations (29,31). As Figure 3 shows the preference for ET is less for R = tert-butyl than for R = CH(CH&. The clarification of the reasons for this behavior awaits moredefinitive evidence, such as X-ray crystallographic structures. However, present indications suggest that the molecule achieves the most comfortable fit of various mourn in a manner different from internal rotation. One p&ibkty is that hond angle changes take place, as in XVIId, thus reducing the steric iuterference of tert-butyl with vicinal groups. In any case, one cannot assume that tert-hutyl is strongly conformationally determinative in acyclic systems, in contrast to the usual assumptions in cyclic systems (I). ~
~
X
\
p
3
H/c=c\ H XIV
An interesting comparison of butane with 2-silahutane (XV) is possible through the work of Onellette (26). In XV, thegauche conformer is favored over the trans conformer hy 1.0 kcal. The Si-C bondlength (1.87 A) is substantially longer than the C-C distance (-1.53 A), which has the effect of senaratine" C1. from C in XV comnared to 11. The eauation relating potential energy to distance of separation of atoms has attractive terms as well as reodsive terms. The increased separation of C, from Cq has the-effect of placing the methyl groups nearer the minimum of the potential energy curve (cf. Figure 2) leading to a net attractive interaction. However, in 1-silabutane, (XVI) the trans conformer is the more stable (by 0.5 kcal). The longer C-Si bond lengtb is counterbalanced by the greater radius of the SiH3 group than CHz (27). In early work, Mateos and Cram predicted the conformatioual preferences of diastereomers on the basis of the space demands of the large (L), medium-sized (M) and small (S) groups (28). In particular, conformer E T (as ~ in Figure 3) was 4Theterm ET signifie~the conformer having trans vieinal hydrogens ofthe compound having the erythro configuration. The terminology is arbitrary, and no particular significance should he attached to these designations.
The R = cyclopropyl group induces a preference for conformer ET to a smaller extent than R = CHs, i.e., cyclopropyl acts as a very small group. Cyclohutyl is effectively less space-demanding than R = C2H5. In these small rings, the bond angles are compressed (cf. XVIIf); whereas, the bond angles between the cyclic group and the ethane skeleton are widened (cf. VIII-X). The latter effect reduces the steric interference of the ring carbons with vicinal substituents on the ethane skeleton.
d
t-C4Hg
-25
e
neo-c5Hlt
A3
1
0
77
Figure 3.
Volume 56, Number 7,July 19.79 1 433
In small ring compounds, such as XVIIf, Hb is preferentially trans to H.. The narrow separation of Cp and C3 due to the compressed ring bond angles requires that the smallest group possible (i.e., a hydrogen) be gauche to these two carbons; Hb must therefore be trans to H,. Similarly, in aziridines such as XVIII, the narrow C-N-C angle of the ring requires that the smallest group possible be oriented over the ring (32). The '3C chemical shift of the ring carbons is insensitive to the nature of R, as long as the N-alkyl group is primary or secondary. The R groups are directed away from the ring, as shown in structure XVIII.
Textbooks usually show Newman projections of various molecules in terms of dihedral anglesof 60'. However, it has been suspected since the early work of Mizushima (33) that considerable variation in dihedral angles occurs (as well as in other degrees of freedom) as the moleculeseeks the minimum energy conformation (25). Allinger, et al. commented on this fact in his discussion of the reasons for the surprising importance of the gauche conformer (XIXa) of 2,3-dimethylbutane relative to the trans conformer (XIXh) (34). The CH3C-CH3 bond angles have spread, probably to relieve repulsions of these groups. However, this diminishes the dihedral angles between vicinal methyl groups, as shown in XIXb. In the gauche form, XIXa, three gauche interactions exist between methyl groups, compared to two gauche interactions for XIXb. However, because of the unfavorable dihedral angles in XIXb, the gauche form XIXa is comparable in energy. Recently, Ingold and co-workers have shown that a conformation similar to XIXa is favored for tetra-tert-butylethaue. Interconuersion of conformers is slow in this molecule (35).
has indeed shown that XXa is dominant in some cases and XXb in others. When the L groups are extremely large, it seems clear that a gauche orientation of the L groups is prohibited. The avoided interaction of such large groups is apparent in-the sulfone XIb and in the sulfoxide XXI. The perturbation due to the additional oxygen in the sulfone results in a series of correlated changes that ultimately affect sites in the molecule distant from Sop. Thus, the additional oxygen in XIb results in an increase in the S-Ph, dihedral dihedral angle (74' in XXI, but 81" in XIb). The Ph,Phb angle is surprisingly small, but rather similar in the two The Phb-[(CH3)2(0H)C-] dihedral compounds, -50'. angle is also very large, -84', undoubtedly due to the great bulk of the latter group. Thus, although both L groups are formally trans, the dihedral angle varies from 153" in XXI to 146O in XIb (20).
L
M
xxb xxa The phenyl groups appear to be uncomfortably close in space. However, the interaction of these groups has been alleviated to a certain extent by a lateral movement of both phenyl groups away from one another in a manner not visible in the Newman projection (cf. XIa). The previous discussions have emphasized the interaction of gauche (1,2) groups, that is, groups situated on carbon 1and carbon 2 of the ethane skeleton. Polymer chemists have appreciated the importance of another type of interaction, i.e. 1,3 interactions, for some time (36).The interfering groups are located on carbons 1and 3 of a propane skeleton. In the preferred conformers XXIIa and XXIIb (similar to many vinyl polymers), a large group at Cp is opposed by a hydrogen a t C4 and vice versa. The preference for conformations similar to XXIIa in the repeating units of the polymer chain imparts a twist that ultimately leads to an extended helical structure of the polymer chain. Regrettably, textbooks continue to portray structures such as XXIII for isotactic vinyl polymers. Although eclipsed conformers similar to XXIIc are quite rare, a few cases are known in which similar conformers have substantial weight, e.g. XXIV. The lack of hydrogens in the interfering groups, Br and COpCH3, is currently believed to relax the exclusion of eclipsed conformers (37).
The previous discussion emphasized erythro diastereomers. For threo diastereomers, Mate& and Cram suggested that XXa should be somewhat more stable than XXh, although exceptions were considered quite probable. Subsequent work
_\xm
, Phb
.
CH3
Figure 4.
434 1 Journal of Chemical Education
XI,"
Hydrogen Bonding-Among uon-bonded interactions that are attractive in nature, hydrogen bonding bas been the most thoroughly studied. For example, Bodot and co-workers were able to approximate the weights of various conformers of certain chlorohydrios (Figure 5 ) , through a combination of nmr and ir methods of study (38). In the erythro isomers, the tendency for the alkyl groups to be trans favors El.,but the stabilization afforded by the OH-CI hydrogen bond favors one or both of the Ec isomers.
OH
OH.
.HO
sumably, a charge-dipole interaction is possible between the non-honded electrons on oxygen and the positively charged nitrogen. However, Beveridge and Radna postulated a hydrogen bond between N(CH3)3 and oxygen (42). In certain cyclic systems (XXIX) Eliel, et al. discuss the preferred axial conformation of such groups as X = SO, SOz, or NR3+ in terms of electrostatic interactions with the ring oxygens (176).
xxvm
Figure 5. Where R is comparatively small, Eel is highly populated, hut where R = i-C3H7, ET becomes dominant (-69%). In the case of the threo isomers, the repulsion of the alkyl groups is minimized in Tcz, and hydrogen honding also stabilizes this strongly dominant conformer. With regard to solvent effects, an intramolecnlarly hydrogen honded compound will respond in one of three ways (39). I f the conformation of the molecule is dominated by a n e x /;rnidy sinhlr hsdrofivn bond, a change to a more polar solvent will have little effect, since the intmmolecular bond is not easily disturbed. If the intramolecular hydrogen bond is of moderate stability, a change to a more polar solvent may disrupt the hydrogen hond, leading to an abrupt change in conformer populations. If a weak intramolecular hydrogen bond is present (say Ph-HO), the conformation is dominated by other factors, and disruption of the hydrogen hond by solvent change may have little effect on the choice of conformation. An example of the second response is compound XXVI. In CCla solution, intramolecular hydrogen honding , ET has a small weight stabilizes conformers such as E G ~and (-10%). In DMSO, the intramolecular hydrogen bond is replaced largely by intermolecular hydrogen honding, and the weight of ET increases to -45%. O-H
H,A,OH
The state of ionization of a hydrogen hond acceptor also dramatically affects conformation. Thus, the hydroxy acid XXVII heavily populates conformer ET in methanol solution (-80-90%) (40). The OH and COOH are trans and not hydrogen honded to one another. In basic methanol, COOH is converted to the more powerful hydrogen bond acceptor, COO-, and conformers such as Eel become populated at the expense of ET, whose weight drops to 40-50%. In the more solvating medium of water, (pH -8), ET is again preferred as water competes effectively with the intramolecular OH for the COO- groups.
XXlX
In some cases, electrostatic effects play a surprisingly small role. In XXX, the free acid (COOH groups un-ionized) prefers conformer TT in CDC13 solution (>SO%),and in aqueous solutions, the dianion of XXX also prefers TT to approximately the same extent (43). Solvation of the COO- groups in the highly polar medium probably reduces electrostatic repulsion of the COO- to a certain extent. In XXXI, a strong preference for conformer &is evident (-90%). Addition of dications such as Ba2+ does not enhance the population of conformers with gauche COO- groups (e.g. XXXIh) even though ioo-pairing is certain a t the concentrations used. However, a number of water molecules may separate the dication and anions in the ion-pair. In non-aaueous solvents. Porter and S i m ~ s o nrea striking change ported that 8-hydroxycarboxylates~ndergo in conformation nDon com~lexationwith lanthanide shift reagents (44).
Although the above results do not indicate a dominating influence of electrostatic effects, the work of Rahan and coworkers does show the effects of charge on molecular conformation (45). Thus, the ion-paired form of the enolate ion XXXII prefers the Z,Z conformation, whereas the free anion prefers the E,Z conformation. Presumably, the repulsion of the oxygens, which carry high charge density, favors the E,Z form. However, in the ion-paired form, the attraction of the negatively charged oxygens by the cation favors the Z,Z form.
XXXlla(Z,Z1
Electrostatic Effects-Eliel and co-workers have discussed the preferred (gauche) conformation of acetylcholine (XXVIII) in terms of electrostatic interactions (41). Pre-
XXXIL ( +Z
I
Dipole-Dipole, Dipole-Quadrupole, etc. Effects-In dl2,3-dihromohutane, (XXXIII), Bothner-By and Naar-Colin showed that conformer TGIis preferred (>60% in CCla solntions) (46). Methyl and bromine have rather similar sizes (van der Waals' radii -2 A). However, the C-Br hond is longer (1.95 A) than C-CH3 (1.50 A), and this greater length essentially removes bromine from the vicinity of interfering vicinal groups. Thus, on a steric basis, Tcz might he expected to be preferred over Tcl. However, the repulsion of the C-Br dipoles should destabilize T G ~leading , to a preference for Tcl, as is observed. Volume 56, Number 7,~uly1979 / 435
1
Br
H
xxx~rh( TGIl
xxxlllb TG2)
In certain diamides, Sandstrom and co-workers found that the population of the Z,Z form was much larger than expected on the basis of statistics (47). The preponderance of the Z,Z form was ascribed in part to an attraction of the opposed amide dipoles in the gauche form, i.e. XXXIVa. However, the Swedish workers also showed that steric effects destabilized the E,E form. C,"3
Biirgi has elegantly demonstrated the effects of the interaction of an amine with a transannular carhonyl group in a series of molecules, e.g. XXXVIII (51).In a sense, the interaction is a donor-acceptor interaction, although the Swiss scientists view the N-C=O interaction as a chemical reaction that is frozen at one stage of the reaction course by constraints imposed by the rings. In a series of molecules exhibiting the N - C 4 interaction, the carhonyl is shown to have developed progressive degrees of tetrahedral character with increasing proximity of nitrogen, culminating in a fully covalent bond to nitrogen XXXIX.
/
I
I
CH?
E,z (-30%)
2.2 I 60 *lo 1
Abraham and co-workers have carried out detailed studies of the effect of solvents on molecules such as l-hromo-2chloroethane (XXXV) (48). The gauche form, which has a high resultant dipole moment, becomes increasingly populated as the solvent oolarity increases. However, in contrast to predictions from theories-on the interaction of the resultant dipole with the polar medium, the gauche form never becomes dominant, no matter how polar the solvent. Quadrupolar interactions apparently oppose the dipole-solvent interaction mentioned above, and maintain the preference for the trans form. Regrettably, i t is difficult to portray quadrupolar interactions in structural formulae.
In conclusion, the complexity of the theoretical treatments of the harrier to internal rotation do not permit facile explanations to he oresented to colleee However, if - sovhomores. . conformational analysis were introduced later in the course, the instructor could oerhaos build an argument hased on hypenonjuga~ionfor ;he 11n;ricr to rotatio~;in ethane and for the vreferred confwmation of dihaloethanes. Although perhapH not easily assimilated by students, the argumen&w&dd partially satisfy today's skeptical brand of students that a reasonable basis exists for the material presented in class. Also by means of this article, it is hoped that the dependence on h in classroom nresentations will he lessened - r~i c~nhenomena - - ~ in view of the many factors affecting conformation. In one case (XIX) simnle first-order steric areuments do not in fact oredict the correct conformation. In future years, conformational analysis will most likely develop further toward theoretical analyses hased on quantum and classical calculations. Hopefully, the causes of the phenomena can he pinpointed with greater reliability. I t is hoped that these results will find their way into textbooks with due speed. L~~
~
~~
~
~
Literature Cited
xxxva xxxvb Donor-Acceptor Interactions-Kiefer and Gericke have shown that various types of groups X coordinate with mercury as shown in conformer XXXVIh (49) most stable interaction occurred with X=N(CH&, although evidence was found for complexation with other groups such as X=Br and CH30-
C&.
1'
m w a
xxxvm
A second case in which donor-acceptor interactions affect conformation is compound XXXVII, in which the bromine atom was considered to be bonded (weakly) to tin (the approach of Br to Sn was 0.4 A less than the sum of the van der Waals' radii) (50). Ph
\
/Ph
(1) (a) Eliel, E. L.,"StueochemistwofCarbonCompounds."Mffiraw-HillBookCo.,New York, N.Y.. 1962, p. 262. lbl Lehn. J. M.. in"ContormstionslAnalyais.)'Chiudoglu. E., Editor, Academic Press, New York, N.Y., 1971, p. 129: (d Millen, 0. J., in ."Progreaain Stereochemidry,"Vol. 3, de la Mare, P.B.D., and Klyne, W.,Edilors, Butterworthk, London, Eng., 1962, Ch. 4: (dl Eiiel, E. L.,Allinger. N. L.. Angysl, S.,and Mornson, G., "Conformstionai Adyais," Intemience, New Y0rk.N.Y.. 1965, pp. 13-17: (dl Bartel1.L. S.. J. Amer Chem Soc.. 99.2379(19771. (21 Lowe, J. P., Piopr Phya. O v . C h m , 6.1(1970). 131 la1 Allinper, N. L.. Tribble. M.. Miller, M.. and We*. D.. J Amsr Chem. Sor., 93.1637 (1971): (bi Allinger, N. L., Hirsch. J., Miller, M.,Tyrninski, I., and Van-Cstleda, F.,J. Amar. Chem. Sor. 90.1199 (1968); (cl Alater paper [Allingerand Wertz. Tetrahedron, 30,1579 (197411 pmientp slightly different dafa. (4) General references include: (a) Wiison, E. B., Chem Sac. Rau., 1, 293 (1972): (hl Scherags, H.,Aduan. Phy8 04.Chsm.. 6,103 (1968); Id Pethriek, R.,end WynJom8,E., Quart. Rsu. Chom. S o c , 23.301 (1969): (dl See. however, ZeSrou,N.S., Zh. Vm~,Khim.h a , 22.261 (19771. I51 Kallrnan, P.. J Amel. Chem. Soe., 99,4875 (1977): Aer. Chom. Rsa., LO, 365 (1977). end reE 151). (6) Ep~tein,I.R..and Lipsmmh, W. N., J. Am. Cham Soc.. 92,6094 (19701. 17) Eyring. H., J. CHEM.EDUC..35.550 119581. IS) Is) Radom. K.. Lathan, W. A,. Hehre, W. J.. and Pople. J . A., J. Am. Chsm. S o r . 95. 694 (19731; (b) Jeffrey, G., Paplo, J. A.. and Radom, L.,Corbohyd. R e g , 25, 117 (19721. (9) Is) Lows, J. P., J. Amsr Cham. Soe., 96,3759 (1974); J. Amzr Chem. Sor. 92,3799 (1970). (bl See,however, Allen, L. C., end Bssch, H., J. Amar Chem. Soc. 93,6373 (19711,sndrefs. 151 and (Ill. (101 (a1 England, W., Gordan, M. S.,andReudanbeq, K., Them. Chim. Act., 37,177 (1975): ibl England. W.. G0rdan.M. S.. J. Amsr Chsm. Soc, 93,4649 ((19711andrelafed "em-7~ v"yu.".
(11) Hoffman, R., Radom. L., Pople, J. A,, Sehleyer. P, von R.. Hehre, W., and Salem. L., J. Amer Chem. Soc. 94.6221 (19721. (121 Rojss.0.. J. Amm Cham. Soc 99.2902 (19771. (131 Dewar. M. J. S.. "Hypermnjugstion." The Ronald P ~ e s Co.. s NewYork. N.Y., 1962. (I41 Mullikon. R. S.. Rieke, C. A.,and Brom, W. G..J. Amer Chem S o c , 63,41 119411. see a h , G. Wheland, -Rrsonancein 0w"icChernlrtry." John Wileyand so"& h., New Y0rk.N.Y.. 1955,~.151. (151 Wolfe. S., Rauk,A., Tel, L. and Czismsdia. I., J Chem. Soc.B. 136 (1971); Acc Chem.
.
Ror 6 . 107,19791 . ...., . ..,.... ,.
(161 Phi1lips.L. and Wray, V., Chem. Cammun, 90l19731. 117) ( 8 ) Bingham. R. C.. J. Am,. Chem Soc.. 97.6742 (1974) and related papers: (b) Eliel, E. L.,Ane~iu.Ch~m.Infern.Ed.EngL.L1.739(1972);(clZeBrou,N.S.,Blsgovesh-
436 / Journal of Chemical Education
ensky, V..Kaznnirchik. I.,sndSurnvr, N., Totrohedron. 27,3111 119711and related papers; Id) Epiutis, N., J Amrr. Chrm Soc., 95,3067 11973) and related papers. (16) Ahrahem, R. L a n d Loftua,P.. Chem Commun.. I80 119741. (19) Fifzwater.S.,and Rarlel1.L. S.. J. Amer Chem S o e , 98.5107 11976): t h e e n e w d i f ference between ~ a u c h eand anti rotamem is rather paorly duplicated In this s p ~ proach, however. Sce, however, ref (251. (20) Allineer, N. L . r l Amer Chem. Soc, 99,8127(1977): (h) Ftef. Id. (211 In) Emptage,M. R., J. Chem. Phya., 47,1293 11967): IhJ Sage, M.L., J . Chem. Phy*. 2s. 142 11961): (c) Swatan. J., and Hernchbach, D., J. Amer Cham. Sor. 27,llo 119571. (22) weisrhurgpr, A. .'Technique of Organic Chemistry," Vol. IX. 2nd Edition, J . Wiley and Sons.Ine..New York. N.Y., 1970,~.478. Inn) h y , v., nay, R. and ~ i n g ~c.,b ~~; h~ l ~i ~. h eresdb. d (24) Borgi, H.-B.,snd Barte1l.L. S., J. Amer Chem Soc., 94.523611972). IZSl Hounrhell. W. D..Doushertv.D.A..Hummell.J.P..sndMislou.K..J.Amer. Chem.
(28) Mateoa,J.snd Cram.D. J., J.Amar Chem. Soc.. 81,2756 11959). 129) Underwood. G., Watts C.,Chan. A.,'Groon, T.. and Kingsbury, C., J 078. Cham., 38, 2786 119731. (30) Ingold, C. K., "Structure and Mechanism in Organic Chemistry," Carnell University Presl.1Ulaea.N.Y.. 1953.m405.
(351 Brawnstein.S..Dunogues, J.,Lindsay, D.andingold,K.U.,J.Amer Chem. Sot. 99, 2073 (1977).See a h : Lunazri, L., Macciantelli, D., Bernsnll. F., and Ineold. K. U., J . Amw Chrm. Soe 99,4573 11977). and Daugherty, D.. Mislow, K.,Biount, J., Waoten, L a n d Jacohus, J . , i l A m e r Chem. Sac. 99,6149 (1977). 136) la) McMahon. P.. end Tincher. W.. J . M a l . Soecfrosc.. 15.180 119651: lbl Bovev.. F. A..Tolyme~Configuration &d ~ o n h r m & %~~e.s.d e & i cPress. N;W York, N.Y., 1969, ~~73.80:le) Price,C.C.. J. CHEM. EDUC.50.744 119731. 137) K1ng~hury.C..Hutton, R a n d Durham,D., J. Ore. Chsm.. inpren. 138) B0dot.H.. Fediere, H.,Pouzsrd. G.,snd Puiol. L.,Bul!. Sor. Chim Fr., 326011968). (39) Kinaburv.C.. J. Ore. Cham.. 3L 1319 (19701.
.
pen.
142) Eweridge, D. L.,snd l3adna.R.. J. Am,. Chem Soc. 93,375911971). SeeslsoPort, G. N. J., and Pullmsn, A,, J . Amer. Cham. Sor. 97,4060 11975). (43) Wang, C.-H., and C. King8hury.J O m Chem., 40,3811 11975). 144) Porter,G..sndSimpson, J.,Angrw. Chrm 1ntrm.Ed. Engi., 17.49 11978). 145) Raban,M.. Noe,E.,endYamamBo,G., J . Amer. C h ~ mSor., . 99.6627 11977). 146) la1 Bothnor-By, A. A . and Nasr-Colin, C.. J . Amer Chem. Soc 84.743 (1962): (b) Hueblin. G . Kvhmstedt. R., Ksdura, P..and Dsweynski, H., Tetiohadron, 26.61 11970); (c) Hamer, G.. Reynolds, W. F., and Wmd, J.. Con. J. Chem., 49, 1755 ,,a7,, \.",.,.
147) Ksrlsion, S.. Liljefors. T.. and Sandstram, J.. Acfo Chem Seond.. SPT.B, B31, 399 (19711 ,... . ,.
148)
(a) Ahrahsm,R.J..
Cavelli, L.,and Psehler, K. G.R., Mol. Phvs., 11.471 119%): lb)
demandinggroup. 132) Tarhurton,P.,KingshuaC..Sopehik,A.,sndCromwdl,N. H., J. Or& Chsm., 43,1350 1197m
133) la) Mizushlma. S.,"The StrnetureofMoleculesand Internal Rotation,'AcademicPre%s New York, N.Y., 1954: lblSeeHounshell,etal.. reL 125h1,for agaphieexample. 1341 Ref (3b): see also, Chen, C.. and Bushweller, C. H., J. Amsr. Chrm Sor, 99, 313 119771.
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151) (a) BOrgi, H-B.,Aweu. C h ~ mIntern. . Ed. Eng!., 14.460 11975)sndrefereneeseited: lbi Bur@, H.~B.,Dunit?., J. D., and Shelter, E., J , Amar Chsm. Sor., 95, 5065 119731.
Volume 56. Number 7, July 1979 / 437
