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The conformation of pivalophenone
Journal of Molecular Structure, 75 (1981) 265-270 Elsevier Scientific Publishing Company, Amsterdam
S. D. BARKER, School-of
D. MIRARCHI,
Chemistry,
R. K. NORRIS,
The University of Sydney,
-Printed
in The Netherlands
L. PHILLIPS and G. L. D. RITCHIE* N.S. W. 2006
(Ausiralia)
(Received 25 March 1981)
ABSTRACT Experimental dipole moments and infmitezlilution molar Kerr constants are reportei for pivalophenoae and five psubstituted pivalophenones (t-BuCO. C,H,* X, where X = I Me, t-Bu, NO,, Cl, Br) as soIutes in carbon tetrachloride. Analysis of results yields the dihedral angle, 9, between the planes of the carbonyl and aryl groups in the effective conformation of each molecule (X = H, G!J = 49 + 5”; Me, 39 + 8”; t-Bu, 33 * 9”; NO,, 49 + 20”; Cl, 45 + 7”; Br, 41 5 10”). INTRODUCTION
Although much effort has been directed towards establishing the conformational preferences of many benzaldehydes and acetophenones, the corresponding pivalophenones, in which there is significant steric hindrance [l] have received relatively little attention. For pivalophenone itself, different physical methods have yielded a range of unreliable estimates of the dihedral angle, 0, between the planes of the carbonyl and phenyl groups (UV spectra, 34” [l],33-38” [2] ,50” [3] ; “C-NMR spectra, 25O [4] ,
33” [3] ; dipole moments, 63” [5] ; IR spectra, 33-38” [2] ; molar reli-actions, 37” [6] ). In an attempt to clarify this matter we have applied considerations of molecular polarity and polarizability to pivalophenone and five p-substituted pivalophenones (t-BuCO. C6H4=X; X = Me, t-Bu, NO*, Cl, Br). Experimental dipole moments and molar Kerr constants for these molecules as solutes in carbon tetrachloride at 298 K are here recorded and analysed. The conformational results are of interest in relation to recent references to the occurrence and consequences of steric inhibition of resonance in molecules containing the pivaloyl group [ 7-91. EXPERlMENTAL
The procedures described by Pearson [lo] were used to obtain pure samples of pivalophenone, p-methyl-, p-t-butyl-, and p-bromo- pivalophenone; p-nitropivalophenone was prepared by direct nitration of pivalo*Author
for correspondence.
0022-2860/81/0000-0000/$02.50
0 1981 Elsevier Scientific Publishing Company
266
phenone [7] and had physical constants in agreement with literature values [ll] . To prepare p-chloropivalophenone, p-nitropivalophenone (5.9 g) was catalytically reduced in ethanol (200 ml) using PtO,; the unpurified amine (5.0 g) was diazotised and chlorinated using CuCl 1121 to give the required product (3.2 g, 47% overall yield), b.p. Sl-85”Cf0.5 mmHg (lit. [13] 84-86”C/O.7 mmHg). The solvent, analytic&reagent grade carbon tetrachloride (min. purity 99.5%), was stored over anhydrous calcium chloride. Relative permittivities [ 143 and Kerr effects [ 151 at 633 nm were determined with apparatus as previously described. Details of procedures, symbols, solvent constants, etc. have been given elsewhere [ 15,163 . In this paper dipole moments, p, molar Kerr constants , &it, and polarizabilities, cx,are express;edin SI units. Conversion factors are: 1 C m = 0.2998 X 1030D (dipole moment); 1 ms V-* mol” = 0.8988 X 1Oi5 e.s.u. mol-’ (Kerr constant); 1 C m2 V-l = 0.8988 X 1016cm3 (polarizability). Except for two earlier me~~ernen~ of the dipole moment of pivalophenone [5,17], the results summarized in Table 1 are the first determinations for these molecules. DISCUSSION
Our procedure [16,18,19] is to use known bond and group dipolemoment and polarizability components to calculate expected Kerr constants for the possible stereostructures of each molecule. Comparison of the TABLE
1
Molar polarizations and refractions, dipole moments, and molar Kerr constants of solutes t-BuCO= C,H, . X at 298 K and 633 nm from observations of incremental relative permittivities, densities, refractive indices and electric birefringences of solutions in carbon tetrachloride= X
&@I
B
v’nla
,p2
RD (cm’)
(cm3 1 H Me t-&i NC, Ci Br
8.77 9.41 7.36 13.44 5.92 4.95
-0.653 --o-651 -9.699 4.435 4.434 --0.215
0.27 0.33 0.32 0.43 0.42 0.42
199.9 229.0 238.2 350 t 175.0 180.0
+. 1.1 * 1.5 k 1.9 6 f: 1.1 r 1.0
50.7 55.9 70.7 60.0 56.5 60.3
i 0.7 z!z0.4 f 0.6 r 1.2 + 1.2 + 1.3
103Op (C m)
7
6
8.94 + 0.04 9.63 z 0.04 9.45 c 0.06 12.5 -t 0.1 7.94 +, 0.06 7.97 ir 0.06
0.062 O.-O70 0.069 0.091 0.089 0.089
40.9 112.5 103.0 288 -38.2 -36.3
lo=_(mK:) (ms V-* ma1 61* 190.5 218 f 579 F -77 f -89 +
aSee ref. 16 for details of apparatus, symbols, solvent constants, etc. For each solute and property (relative permittivity, density, refractive index and electric birefringence) at least six solutions having weight-fraction concentrations less than 2% were examined; linear concentration dependenees were observed over such ranges. Quoted uncertainties are 95% confidence limits derived by standard statistical treatment of experimental data; the absolute accuracy of the Kerr constants ia limited by systematic errors estimated as 25%. Dipole moments were calculated assuming # = 1.05 R,.
3 r 2.7 4 7 7 3
267
experimental result with the range of predicted values indicates the actual molecular conformation. We assumed regular valence angles of 120” for the &--CC-C group; anisotropic polarizabilities are given in Table 2, and exaltations of refraction and polarizability in Table 3. Since the correct apportionment of the exaltation, Aar, is uncertain, we performed the calculations of Kerr constants using four arbitrarily chosen distributions, the subscripts X, Y, Z denoting the reference axes shown in Fig. 1: (A) 2Aarxx = A(uYY = 2aa!/3, Aazz = 0; (B) Aarxx = Aqy = A42, Aazz = 0; (C) Aaxx = Aaryy = Acuzz= A&/3; and (D) in the direction of the C&-X bond. TABLE
2
Polarizability
components
(expressed as 10%/C
0.72 0.29 1.56 11.69 13.07 19.17 13.85 13.24 13.75
0.72 2.56 1.08
C-Hb gzb b C,H,-= MeC,H,-c t-BuC,H,-d NO,C,H,--e ClC,H,-f BrC,H,-f
11.69 14.90 21.11 16.57 15.72 18.00
m2
V-') for bonds and grrupsa
0.72 0.29 0.51 7.56 9*40 15.05 8.29 8.41 9.20
aNote that for the substituted phenyl groups QL is collinear with the 1,4-axis. bRef. 16. =R. 3. W. Le FBvre and L. Radom, J. Chem. Sot. E3,(1967) 1295. dM. J. Aroney, K. E. Calderbank, R. J. W. Le FBvre and R. K. Pierens, J. Chem. Sot. B, (1970) 1120. ‘K. E. Calderbank, R. J. W. Le FIvre and G. L. D. Ritchie, J. Chem. Sot. B, (1963) 503. fR. J. W. LeFGvre and B. P. Rao, J. Chem. Sot., (1958) 1465.
TABLE
3
Observed and calculated molar refractions, sums of constituent bond and group polarizabilities, and polarizability exaltations Substituent
H
Me
t-Bu
NO2
ED (obs.) (cnl’)a
50.7 49.6 63.3 0.9
55.9 54.5 69.7 1.2
70.7 68.3 87.7 2.1
60.0 56.1 71.1 4.6
RD (Cal&) (Cm3)b 10” Zar (bonds) (C m2 V”) 104’ Aor (C m2 V-‘)c
-cl 56.5 54.5 69.7 1.9
Br 60.3 57.4 73.3 3.1
‘From Table 1. bEva!uated from bond data in ref. 18 together with the following molar refractions (&,/cm’): tokene 31.3, nitrobenzene 32.7, chlorobenzene 31.1, and bromobenzene 34.0 (A. I. Vogel, J. Chem. Sot., (1948) 607,1833,654); t-butylbenzene 44.9 (Dictionary of Organic Compounds, 4th edn., Eyre and Spottiswoode, E. and F. N. Span,
London, 1965 1. =See refs. 16,18 and 19 for discussion of exaltation; values shown are means of estimates from the equations (i) A Q = 0.95(9~,/NA)RD(obs) - zP(bonds) and (ii)Aa = 0.95(9E~/~A)A~D.
(CH,),C
Y t
\
Fig. 1_ Dipole moments and conformations
of pivalophenones.
It is necessary also to establish the directions of action of the dipole moments of these molecules. The moment, p, of a p-substituted pivalophenone can be considered to arise from the moment, pl, of the appropriate monosubstituted benzene, assumed coincident with the C,,-X bond, and a component, pz , having the same magnitude and direction as the moment of pivalophenone (Fig. 1). Analysis of the dipole moments of the p-substituted pivalophenones (Table 1) in this way yields, in each case, the angle, OL,which the line of action of pz makes with the axis of the C=C group, and the inclination, x, of the resultant moment to this direction (Table 4). Such a procedure is not reliably applicable to p-methyl- and p-t-butyl- pivalophenone, since for these p1 is very much smaller than p2 so that the derived values of orand x are uncertain. From the data for p-nitro-, p-chloro- and p-bromoTABLE
4
Analysis of dipole moments Substituent
H
10'"p(C rn)= 10"Op,(Cm) = (") x ("P
8.94 (4) -3
of p-substituted
pivalophenones
Me
t-Bu
NO,
9.63 (1.36) (-3) +4
9.45 (1.03) (-3) t2
12.5 13.2b -5 -7s
Cl 7.94 5.30b -2 -38
Br 7.97 5.04= -2 -36
aFrom Table 1. bC. G. Le F&e and R. J. W. Le FQvre,J. Chem. Soea (1953) 4041. =C. G. Le FIvre and R. J. W. Le FBme, Aust. J. Chem., 7 (1954) 33. A plus sign indicates that x represents an anticlockwise rotation, a minus sign a clockwise rotation from the X-axis.
269
pivalophenone a emerges as -3 + 2”, and this result was assumed to apply also to g-methyl- and p-t-butyl- pivalophenone. Our interpretation of the dipole moments makes no allowance for possible electronic interaction between the pivaloyl group and the p-substituent. However, interactions of this kind are known to be very small even in the corresponding benzaldehydes and acetophenones [20,21] and, because of reduced conjugation, can be expected to be negligible in the pivalophenones. The conclusion that the line of action of the moment of pivalophenone is close to the axis of the C=O group is consistent with the view of Lumbroso and co-workers 1221. In the calculations of Kerr constants we have allowed a variation of *3” in the values of x given in Table 4, so that possible uncertainties from this source in the derived conformational angles are adequately taken into account. Representative examples of the ranges spanned by the theoretical Kerr constants as the dihedral angle, 9, is varied from O” to 90” are shown in Table 5. The full set of calculations, incorporating the four different apportionments of exaltation together with the range of dipole moment directions, is not reproduced here, but the results of the complete analysis are included in Table 5. It can be seen from Table 5 that the six pivalophenones here examined have rather similar effective conformational angles, the range from minimum (X = Mu, $J= 33”) to maximum (X = H, NO1, $J= 49”) being only 16” and the mean value 43 + 6”. The exceptionally large error in the result for p-nitropivalophenone originates in two factors: the large polarizability exaltation for this molecule, and the sensitivity of the calculated Kerr constants to variation of the dipole-moment direction. Only the unsubstituted molecule has previously been studied; OLWresult (4 = 49 A 5”) may be compared with the range of values, summarized above, obtained by other TABLE
5
Calculated molar Kerr constants, 10z7’,K(~)/mS piva.lophenonesa X
H Me t-Bu NO, Cl Br
V-l mol-‘,
for conformations,
9, of
Result (” )
a (“1 0
25
30
35
40
45
50
55
90
310 351 378 879 183 156
238 286 305 776 100 77
209 261 277 735 67 45
177 232 244 689 31 22
143 202 210 640 -8 -27
108 171 175 590 -48 -66
73 140 140 540 -88 -105
40 109 106 491 -127 -142
-93 -9 -28 301 -279 -288
49 + 39* 33 t 49 * 45 + 41 f
5 8 9 20 7 10
a8ample calculations only, with exaltations of polarizability, Aa, assigned as (B) AQXX = AQYY = AIY!/~, A&z2 = 0, and dipole-moment directions as in Table 4. The conformational angles shown in the final column were derived from the complete analysis (see text).
270
methods. Although the dihedral angle in p-t-butylacetophenone appears to be significantly less than that in pivalophenone itself, the uncertainties are, in general, of such magnitude as to obscure effects of particular substituents. In fact, it seems that the conformational preferences of these molecules are determined largely by steric factors, with the additional interactions provided by the p-substituents playing only a minor role, as might be expected. Finally, it may be noted that recent qualitative inferences concerning the effective conformations of molecules containing the pivaloyl group are entirely consistent with our results. For example, the nitration of pivalophenone has been found [ 71 to yield an unexpectedly low proportion (44%) of the M-isomer, an observation which is readily explicable in terms of substantial steric hindrance to coplanarity of the carbonyl and phenyl groups. Similar conclusions have been drawn from kinetic and mechanistic studies of ring-opening in 5-azido- [S] and nucleophilic substitution in 5-nitro- [9] 2-fury1 alkyl ketones. REFERENCES 1 E. A. Braude and F. Sondheimer, J. Chem. Sot., (1955) 3754. 2 A. R. Katritzky, R. F. Pinzelli and R. D. Topsom, Tetrahedron, 28 (1972) 3449. 3 C. G. Andrieu and D. Debruyne, personal communication, D. Debruyne, Thesis, University of Caen, 1976. 4 K. S. Dhami and J. B. Stothers, Can. J. Chem., 43 (1965) 498. 5 A. G. Pinkusand H. C. Custard, J. Phys. Chem., 74 (1970) 1042. 6 B. A. Zaitsev, Izvest. Akad. Nauk S.S.S.R., Ser. Khim., (1974) 780. 7 S. D. Barker, R. K. Norris and D. Randles, Aust. J. Chem., 34 (1981) in press. 8 P. J. Newcombe and R. K. Norris, Tetrahedron Lett., (1981) 699. 9 P. J. Newcombe and R. K. Norris, unpublished results. 10 D. E. Pearson, J. Am. Chem. Sot., 72 (1950) 4169. 11 R. K. Norris and D. Randles, Aust. J. Chem., 29 (1976) 2621. 12 A. I. Vogel, Practical Organic Chemistry, Longmans, London, 3rd edn., 1962, p. 601. 13 E. J. Skerrett and D. Woodcock, J. Chem. Sot., (1950) 2718. 14 R. J. W. Le F&re and D. J. Millar, Chem. Ind., (1971) 399. 15 M. R. Battaglia and G. L. D. Ritchie, J. Chem. Sot. Faraday Trans. 2,73 (1977) 209. 16 C. G. Le Fevre and R. J. W. Le Fevre, The Kerr Effect, in A. Weissberger (Ed.), Techniques of Chemistry, Vol. 1, Part IIIC, Wiley-Interscience, New York, 1972, p. 399. 17 C. Cherrier, C. R. Acad. Sci., 225 (1947) 1306. 18 R. J. W. Le F&re, in V. Gold (Ed.), Advances in Physical Organic Chemistry, Vol. 3, Academic Press, London, 1965. p. 1. 19 R. J. W. Le F&-re, Rev. Pure Appl. Chem., 20 (1970) 67. 20 R. A. Y. Jones, A. R. Katritzky and A. V. Ochkin, J. Chem. Sot. B, (1971) 1795. 21 E. A. W. Bruce, G. L. D. Ritchie and A. J. Williams, Aust. J. Chem., 27 (1974) 1809. 22 C. C. Andrieu, P. Metzner, D. Debruyne, D. M. Bertin and H. Lumbroso, J. Mol. StNCt., 39 (1977 ) 263.
S. D. BARKER, School-of
D. MIRARCHI,
Chemistry,
R. K. NORRIS,
The University of Sydney,
-Printed
in The Netherlands
L. PHILLIPS and G. L. D. RITCHIE* N.S. W. 2006
(Ausiralia)
(Received 25 March 1981)
ABSTRACT Experimental dipole moments and infmitezlilution molar Kerr constants are reportei for pivalophenoae and five psubstituted pivalophenones (t-BuCO. C,H,* X, where X = I Me, t-Bu, NO,, Cl, Br) as soIutes in carbon tetrachloride. Analysis of results yields the dihedral angle, 9, between the planes of the carbonyl and aryl groups in the effective conformation of each molecule (X = H, G!J = 49 + 5”; Me, 39 + 8”; t-Bu, 33 * 9”; NO,, 49 + 20”; Cl, 45 + 7”; Br, 41 5 10”). INTRODUCTION
Although much effort has been directed towards establishing the conformational preferences of many benzaldehydes and acetophenones, the corresponding pivalophenones, in which there is significant steric hindrance [l] have received relatively little attention. For pivalophenone itself, different physical methods have yielded a range of unreliable estimates of the dihedral angle, 0, between the planes of the carbonyl and phenyl groups (UV spectra, 34” [l],33-38” [2] ,50” [3] ; “C-NMR spectra, 25O [4] ,
33” [3] ; dipole moments, 63” [5] ; IR spectra, 33-38” [2] ; molar reli-actions, 37” [6] ). In an attempt to clarify this matter we have applied considerations of molecular polarity and polarizability to pivalophenone and five p-substituted pivalophenones (t-BuCO. C6H4=X; X = Me, t-Bu, NO*, Cl, Br). Experimental dipole moments and molar Kerr constants for these molecules as solutes in carbon tetrachloride at 298 K are here recorded and analysed. The conformational results are of interest in relation to recent references to the occurrence and consequences of steric inhibition of resonance in molecules containing the pivaloyl group [ 7-91. EXPERlMENTAL
The procedures described by Pearson [lo] were used to obtain pure samples of pivalophenone, p-methyl-, p-t-butyl-, and p-bromo- pivalophenone; p-nitropivalophenone was prepared by direct nitration of pivalo*Author
for correspondence.
0022-2860/81/0000-0000/$02.50
0 1981 Elsevier Scientific Publishing Company
266
phenone [7] and had physical constants in agreement with literature values [ll] . To prepare p-chloropivalophenone, p-nitropivalophenone (5.9 g) was catalytically reduced in ethanol (200 ml) using PtO,; the unpurified amine (5.0 g) was diazotised and chlorinated using CuCl 1121 to give the required product (3.2 g, 47% overall yield), b.p. Sl-85”Cf0.5 mmHg (lit. [13] 84-86”C/O.7 mmHg). The solvent, analytic&reagent grade carbon tetrachloride (min. purity 99.5%), was stored over anhydrous calcium chloride. Relative permittivities [ 143 and Kerr effects [ 151 at 633 nm were determined with apparatus as previously described. Details of procedures, symbols, solvent constants, etc. have been given elsewhere [ 15,163 . In this paper dipole moments, p, molar Kerr constants , &it, and polarizabilities, cx,are express;edin SI units. Conversion factors are: 1 C m = 0.2998 X 1030D (dipole moment); 1 ms V-* mol” = 0.8988 X 1Oi5 e.s.u. mol-’ (Kerr constant); 1 C m2 V-l = 0.8988 X 1016cm3 (polarizability). Except for two earlier me~~ernen~ of the dipole moment of pivalophenone [5,17], the results summarized in Table 1 are the first determinations for these molecules. DISCUSSION
Our procedure [16,18,19] is to use known bond and group dipolemoment and polarizability components to calculate expected Kerr constants for the possible stereostructures of each molecule. Comparison of the TABLE
1
Molar polarizations and refractions, dipole moments, and molar Kerr constants of solutes t-BuCO= C,H, . X at 298 K and 633 nm from observations of incremental relative permittivities, densities, refractive indices and electric birefringences of solutions in carbon tetrachloride= X
&@I
B
v’nla
,p2
RD (cm’)
(cm3 1 H Me t-&i NC, Ci Br
8.77 9.41 7.36 13.44 5.92 4.95
-0.653 --o-651 -9.699 4.435 4.434 --0.215
0.27 0.33 0.32 0.43 0.42 0.42
199.9 229.0 238.2 350 t 175.0 180.0
+. 1.1 * 1.5 k 1.9 6 f: 1.1 r 1.0
50.7 55.9 70.7 60.0 56.5 60.3
i 0.7 z!z0.4 f 0.6 r 1.2 + 1.2 + 1.3
103Op (C m)
7
6
8.94 + 0.04 9.63 z 0.04 9.45 c 0.06 12.5 -t 0.1 7.94 +, 0.06 7.97 ir 0.06
0.062 O.-O70 0.069 0.091 0.089 0.089
40.9 112.5 103.0 288 -38.2 -36.3
lo=_(mK:) (ms V-* ma1 61* 190.5 218 f 579 F -77 f -89 +
aSee ref. 16 for details of apparatus, symbols, solvent constants, etc. For each solute and property (relative permittivity, density, refractive index and electric birefringence) at least six solutions having weight-fraction concentrations less than 2% were examined; linear concentration dependenees were observed over such ranges. Quoted uncertainties are 95% confidence limits derived by standard statistical treatment of experimental data; the absolute accuracy of the Kerr constants ia limited by systematic errors estimated as 25%. Dipole moments were calculated assuming # = 1.05 R,.
3 r 2.7 4 7 7 3
267
experimental result with the range of predicted values indicates the actual molecular conformation. We assumed regular valence angles of 120” for the &--CC-C group; anisotropic polarizabilities are given in Table 2, and exaltations of refraction and polarizability in Table 3. Since the correct apportionment of the exaltation, Aar, is uncertain, we performed the calculations of Kerr constants using four arbitrarily chosen distributions, the subscripts X, Y, Z denoting the reference axes shown in Fig. 1: (A) 2Aarxx = A(uYY = 2aa!/3, Aazz = 0; (B) Aarxx = Aqy = A42, Aazz = 0; (C) Aaxx = Aaryy = Acuzz= A&/3; and (D) in the direction of the C&-X bond. TABLE
2
Polarizability
components
(expressed as 10%/C
0.72 0.29 1.56 11.69 13.07 19.17 13.85 13.24 13.75
0.72 2.56 1.08
C-Hb gzb b C,H,-= MeC,H,-c t-BuC,H,-d NO,C,H,--e ClC,H,-f BrC,H,-f
11.69 14.90 21.11 16.57 15.72 18.00
m2
V-') for bonds and grrupsa
0.72 0.29 0.51 7.56 9*40 15.05 8.29 8.41 9.20
aNote that for the substituted phenyl groups QL is collinear with the 1,4-axis. bRef. 16. =R. 3. W. Le FBvre and L. Radom, J. Chem. Sot. E3,(1967) 1295. dM. J. Aroney, K. E. Calderbank, R. J. W. Le FBvre and R. K. Pierens, J. Chem. Sot. B, (1970) 1120. ‘K. E. Calderbank, R. J. W. Le FIvre and G. L. D. Ritchie, J. Chem. Sot. B, (1963) 503. fR. J. W. LeFGvre and B. P. Rao, J. Chem. Sot., (1958) 1465.
TABLE
3
Observed and calculated molar refractions, sums of constituent bond and group polarizabilities, and polarizability exaltations Substituent
H
Me
t-Bu
NO2
ED (obs.) (cnl’)a
50.7 49.6 63.3 0.9
55.9 54.5 69.7 1.2
70.7 68.3 87.7 2.1
60.0 56.1 71.1 4.6
RD (Cal&) (Cm3)b 10” Zar (bonds) (C m2 V”) 104’ Aor (C m2 V-‘)c
-cl 56.5 54.5 69.7 1.9
Br 60.3 57.4 73.3 3.1
‘From Table 1. bEva!uated from bond data in ref. 18 together with the following molar refractions (&,/cm’): tokene 31.3, nitrobenzene 32.7, chlorobenzene 31.1, and bromobenzene 34.0 (A. I. Vogel, J. Chem. Sot., (1948) 607,1833,654); t-butylbenzene 44.9 (Dictionary of Organic Compounds, 4th edn., Eyre and Spottiswoode, E. and F. N. Span,
London, 1965 1. =See refs. 16,18 and 19 for discussion of exaltation; values shown are means of estimates from the equations (i) A Q = 0.95(9~,/NA)RD(obs) - zP(bonds) and (ii)Aa = 0.95(9E~/~A)A~D.
(CH,),C
Y t
\
Fig. 1_ Dipole moments and conformations
of pivalophenones.
It is necessary also to establish the directions of action of the dipole moments of these molecules. The moment, p, of a p-substituted pivalophenone can be considered to arise from the moment, pl, of the appropriate monosubstituted benzene, assumed coincident with the C,,-X bond, and a component, pz , having the same magnitude and direction as the moment of pivalophenone (Fig. 1). Analysis of the dipole moments of the p-substituted pivalophenones (Table 1) in this way yields, in each case, the angle, OL,which the line of action of pz makes with the axis of the C=C group, and the inclination, x, of the resultant moment to this direction (Table 4). Such a procedure is not reliably applicable to p-methyl- and p-t-butyl- pivalophenone, since for these p1 is very much smaller than p2 so that the derived values of orand x are uncertain. From the data for p-nitro-, p-chloro- and p-bromoTABLE
4
Analysis of dipole moments Substituent
H
10'"p(C rn)= 10"Op,(Cm) = (") x ("P
8.94 (4) -3
of p-substituted
pivalophenones
Me
t-Bu
NO,
9.63 (1.36) (-3) +4
9.45 (1.03) (-3) t2
12.5 13.2b -5 -7s
Cl 7.94 5.30b -2 -38
Br 7.97 5.04= -2 -36
aFrom Table 1. bC. G. Le F&e and R. J. W. Le FQvre,J. Chem. Soea (1953) 4041. =C. G. Le FIvre and R. J. W. Le FBme, Aust. J. Chem., 7 (1954) 33. A plus sign indicates that x represents an anticlockwise rotation, a minus sign a clockwise rotation from the X-axis.
269
pivalophenone a emerges as -3 + 2”, and this result was assumed to apply also to g-methyl- and p-t-butyl- pivalophenone. Our interpretation of the dipole moments makes no allowance for possible electronic interaction between the pivaloyl group and the p-substituent. However, interactions of this kind are known to be very small even in the corresponding benzaldehydes and acetophenones [20,21] and, because of reduced conjugation, can be expected to be negligible in the pivalophenones. The conclusion that the line of action of the moment of pivalophenone is close to the axis of the C=O group is consistent with the view of Lumbroso and co-workers 1221. In the calculations of Kerr constants we have allowed a variation of *3” in the values of x given in Table 4, so that possible uncertainties from this source in the derived conformational angles are adequately taken into account. Representative examples of the ranges spanned by the theoretical Kerr constants as the dihedral angle, 9, is varied from O” to 90” are shown in Table 5. The full set of calculations, incorporating the four different apportionments of exaltation together with the range of dipole moment directions, is not reproduced here, but the results of the complete analysis are included in Table 5. It can be seen from Table 5 that the six pivalophenones here examined have rather similar effective conformational angles, the range from minimum (X = Mu, $J= 33”) to maximum (X = H, NO1, $J= 49”) being only 16” and the mean value 43 + 6”. The exceptionally large error in the result for p-nitropivalophenone originates in two factors: the large polarizability exaltation for this molecule, and the sensitivity of the calculated Kerr constants to variation of the dipole-moment direction. Only the unsubstituted molecule has previously been studied; OLWresult (4 = 49 A 5”) may be compared with the range of values, summarized above, obtained by other TABLE
5
Calculated molar Kerr constants, 10z7’,K(~)/mS piva.lophenonesa X
H Me t-Bu NO, Cl Br
V-l mol-‘,
for conformations,
9, of
Result (” )
a (“1 0
25
30
35
40
45
50
55
90
310 351 378 879 183 156
238 286 305 776 100 77
209 261 277 735 67 45
177 232 244 689 31 22
143 202 210 640 -8 -27
108 171 175 590 -48 -66
73 140 140 540 -88 -105
40 109 106 491 -127 -142
-93 -9 -28 301 -279 -288
49 + 39* 33 t 49 * 45 + 41 f
5 8 9 20 7 10
a8ample calculations only, with exaltations of polarizability, Aa, assigned as (B) AQXX = AQYY = AIY!/~, A&z2 = 0, and dipole-moment directions as in Table 4. The conformational angles shown in the final column were derived from the complete analysis (see text).
270
methods. Although the dihedral angle in p-t-butylacetophenone appears to be significantly less than that in pivalophenone itself, the uncertainties are, in general, of such magnitude as to obscure effects of particular substituents. In fact, it seems that the conformational preferences of these molecules are determined largely by steric factors, with the additional interactions provided by the p-substituents playing only a minor role, as might be expected. Finally, it may be noted that recent qualitative inferences concerning the effective conformations of molecules containing the pivaloyl group are entirely consistent with our results. For example, the nitration of pivalophenone has been found [ 71 to yield an unexpectedly low proportion (44%) of the M-isomer, an observation which is readily explicable in terms of substantial steric hindrance to coplanarity of the carbonyl and phenyl groups. Similar conclusions have been drawn from kinetic and mechanistic studies of ring-opening in 5-azido- [S] and nucleophilic substitution in 5-nitro- [9] 2-fury1 alkyl ketones. REFERENCES 1 E. A. Braude and F. Sondheimer, J. Chem. Sot., (1955) 3754. 2 A. R. Katritzky, R. F. Pinzelli and R. D. Topsom, Tetrahedron, 28 (1972) 3449. 3 C. G. Andrieu and D. Debruyne, personal communication, D. Debruyne, Thesis, University of Caen, 1976. 4 K. S. Dhami and J. B. Stothers, Can. J. Chem., 43 (1965) 498. 5 A. G. Pinkusand H. C. Custard, J. Phys. Chem., 74 (1970) 1042. 6 B. A. Zaitsev, Izvest. Akad. Nauk S.S.S.R., Ser. Khim., (1974) 780. 7 S. D. Barker, R. K. Norris and D. Randles, Aust. J. Chem., 34 (1981) in press. 8 P. J. Newcombe and R. K. Norris, Tetrahedron Lett., (1981) 699. 9 P. J. Newcombe and R. K. Norris, unpublished results. 10 D. E. Pearson, J. Am. Chem. Sot., 72 (1950) 4169. 11 R. K. Norris and D. Randles, Aust. J. Chem., 29 (1976) 2621. 12 A. I. Vogel, Practical Organic Chemistry, Longmans, London, 3rd edn., 1962, p. 601. 13 E. J. Skerrett and D. Woodcock, J. Chem. Sot., (1950) 2718. 14 R. J. W. Le F&re and D. J. Millar, Chem. Ind., (1971) 399. 15 M. R. Battaglia and G. L. D. Ritchie, J. Chem. Sot. Faraday Trans. 2,73 (1977) 209. 16 C. G. Le Fevre and R. J. W. Le Fevre, The Kerr Effect, in A. Weissberger (Ed.), Techniques of Chemistry, Vol. 1, Part IIIC, Wiley-Interscience, New York, 1972, p. 399. 17 C. Cherrier, C. R. Acad. Sci., 225 (1947) 1306. 18 R. J. W. Le F&re, in V. Gold (Ed.), Advances in Physical Organic Chemistry, Vol. 3, Academic Press, London, 1965. p. 1. 19 R. J. W. Le F&-re, Rev. Pure Appl. Chem., 20 (1970) 67. 20 R. A. Y. Jones, A. R. Katritzky and A. V. Ochkin, J. Chem. Sot. B, (1971) 1795. 21 E. A. W. Bruce, G. L. D. Ritchie and A. J. Williams, Aust. J. Chem., 27 (1974) 1809. 22 C. C. Andrieu, P. Metzner, D. Debruyne, D. M. Bertin and H. Lumbroso, J. Mol. StNCt., 39 (1977 ) 263.