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Molecular conformation of cyclenes
Journal
of Molecular Structure
Elsevier Publishing
Company,
Amsterdam.
MOLECULAR
CONFORMATION
III. mm-CYCLOOCTENE,
Printed in the Netherlands
101
OF CYCLENES
tramCYCLONONENE,
cis- AND
tram-CYCLO-
DECENE
G. BUEMI, G. FAVINI AND F. ZUCCARELLO Institute of Physical Chemistry, Uniuersity of Catania (Italy)
(Received
June 7th, 1969)
ABSTRACT
The strain energy of some cyclenes with one double bond in the ring has been calculated as a function of various geometric parameters. The minimum energy forms for trans-cyclooctene and for trans-cyclononene are characterized by a twisting angle at the double bond of 17-18”. The most stable configuration for cis-cyclononene and cis-cyclodecene is like that found for ciscyclooctene in previous work. In trans-cyclodecene the minimum energy form with an axis of symmetry is angular strainless and remarkable for its flexibility.
INTRODUCTION
In a previous paper I, Hendrickson’s treatment’ was applied to the calculation of molecular conformations and energies of cyclenes with one double bond in the ring, ranging fromcyclohexene up to cyclononene. The strain energies in transcyclooctene and trans-cyclononene were found to be higher than those derived from the experimental thermochemical data, but the possibility of a twisted double bond was not considered. In the present paper this possibility was included in the c$culations and the n-energy dif%erencewith respect to ethylene was estimated as indicated in the second paper3. Calculations were also extended to cyclodecene, where the magnitude of the problem increases considerably in consequence of the greater flexibility of the ring. J. Mol. Structure, 5 (1970) 101-110
102
G. BUEMI,
G. FAWN&
F. ZUCCARELLO
CALCULATIONS
The energy terms were evaluated according to the formulae given in ref. 1. Only symmetrical forms were examined although it is no longer certain that all of the energy minima are represented by symmetrical forms. Calculations were performed on an IBM 1620 (20 K) computer and organized in different steps as before’.
RESULTS
AND
DISCUSSION
Cycloocfene
In the frans-derivative (form XIII of ref. 1 with C&3 symmetry), the minimum energy conformation was calculated by including as an independently variable parameter the torsional angle at the double bond. The energy lowering amounted to 6.77 kcal mole” as can be seen from the values collected in Table 1. 3
4
2 GEOMETRY (angles in degrees) TABLE 1 OFtT(IllS-(SYCLOOCENE
Form XIII
AND ENERGIES
(see ref. 1)
-
92.7
+ 107.9
15.86 12.37 1.56 0 29.79
5
1 I D
6 8
7
Twistedform
116.0 92.0 109.8 118.3 -180.0 + 67.5 + 29.7
(in kcal mole-‘)
117.5 98.0 108.8 114.5 -163.0 + 64.0 + -
32.7 96.4
-f-115.2 6.46 12.80 0.49 3.27 23.02
The strain energy in the frans-isomer with a double bond twisting angle of 17” is higher than that in the &-isomer by 13.86 kcal mole-‘, and the agreement with the difference between the heats of hydrogenation in acetic acid solution at 25OC (9.3 kcal mole -1)4 is greatly improved. J. Mol.
Structare, 5 (1970) 101-110
MOLECULAR
CONFORMATION
OF CYCLENES.
103
III..
In the previous paperr it was pointed out that the strain energy values obtained for cis-cyclononene were too high in all the forms with -C, symmetry. Consequently, we have here considered a further form characterized by a plane of symmetry in the CIC,C,C,C,C, fragment and by an axis of symmetry in the C,CSCsC,C8 fragment (form A); the ind~~~dentIy variable parameters selected were: S,( = S,), G3( = &), QS(= 9,). Q6 and the C,C, distance. The results of the calculations are reported in Table 2 and compared with TABLE 2 GEOMETRY ANI3 ENERGIES OF Cis-CYCLONONENE
Form XIV (see rejI I) undeforl?ted
h(==
&J w %) &i
023
-t_
WJ4
c
043
-
e.6 @se
-
~uw--E*b) z: w&-&3 =%it% &It
Minimum energy Undeformed
120.0 109.5 109.5 109.5 109.5
62)
h(== a* &x5
86.6 12.9 41.6 85.2 X2.9 0 10.62 8.02 18.64
Form A
-i+ -
Minimum energy
122.5 114.0 110.5 110.5 110.0
120.0 109.5 109.5 109.5 109.5
123.4 108.6 109.8 111.3 111.8
79.8 17.9 44.1 83.3 17.9
-b 97.8 -101.0 -+ 108.5 - 64.1 _t 25.6
-+ 96.6 - 96.2 +106.9 - 65.2 +- 22.6
1.32 10.21 4.44
15.97
0 10.25 6.17 16.42
1.04 10.15 2.02 13.21
those of the most energeticahy stable form found in ref. I. The strain energy di%erence between c&-cyclononene and cyclohexeae is reduced to Il.04 kcat mole-’ in fair agreement with the v&e of 9-4 kcal moleW1 obtained from the strain energy in cyeIononane’ and from the difference in the heats of hydrogenation in acetic acid solution at 25” C between cis-cyclononene and cyclohexene4. The dihedral angle of 3’?“, found’ from the NMRcoupling constant J2 J, is in very good agreement with the value obtained for form A (36.6”), and the uncertainty in the choice between the probable experimental angles (27” or 37”) therefore seems to be resoIt;ed. Two symmetrical forms were considered for the frarrJ-isomer: J. MM.Srructure, 5 11370) IOf-
104
G. BUEMI,
Form A (XIX in ref. I): chair-chair
180 + -
Form B: boat-chair
E30++-+
+ -
G. FAVINI,
F. ZUCCARELLO
c
f-f-
=
++-+ 2 c
The strain energy was first calculated by allowing no rotation around the double bond, and later by including also the torsional angle q2 as an independent variable. Unlike the c&-isomer, no angular strainless conformations are possible. The results of the calculations are reported in Table 3. Form A with the double bond twisted by 18.5” is the most stable conformation. The strain energy in trans-cyclononene was calculated to be higher than that in the &-isomer by 2.8 kcaf mole-l, in excellent agreement with the value of 2.9 kcal mole” experimentally observed for the enthaIpy of the c&-tram isomerization’. TABLE
3
GEOMETFtY AND ENEliGIES OF trOtIS-CYCLONONENE
Form A
-I+ -
Form B
Twisted B form
Twisred A form
120.0 99.5 121.1 119.5 114.5
118.0 94.5 109.6 117.5 112.0
118.4 103.7 109.6 113.1 115.1
118.4 101.4 107.7 113.5 114.7
180.0 76.3 30.6 93.4 70.6
180.0 -t 70.0 -t 45.3 -127.1 f 63.1
-161.5 -I- 89.6 - 56.7 +113.5 - 73.0
-165.1 -f- 68.6 -I- 44.8 -126.1 + 64.9
13.70 9.71 0 9.22 32.63
11.51 9.93 0 2.61
4.03 10.16 2.52 1.06 17.77
2.63 8.13 3.86 1.38 16.00
24.05
Seven symmetrical forms were considered for the &-isomer: Form A:
boat-boat
0
++----f-f
0
C,-B
Form B:
boat-chair
0
+ + - + -c-
0
C,-B
Form C:
chair-chair
0
+ -
+ -
0
C'-B
-+
-
I. Mol. Structure, 5 (1970) 101-l 10
-+
MOLECULAR
CONFORMATION
Form D:
chair-boat
0
Form E:
boat-chair
0
Form F:
chair-chair
0
Form G:
chair-boat
0
OF CYCLENES.
+-+
0
_+-+ --
+
+ --
+
+ -++--+--
+ --l--t
105
III.
C,-B
-
C,-B
+
C2-B
_
Cz--B
+-++
The highly strained boat-boat conformation with C, symmetry was not included in the calculations. Six independently variable parameters were selected for the plane-symmetrical forms: 8, (= a,), &(= %,), %a(= %), %(= %)? %(= 97) and dh9. The six parameters selected for the forms with an axis of symmetry were: a,(= a,), &(= W, Q4(= %), %(= &3), w2s and q (dihedral angle between CICz and C& bonds). The results of the calculations are reported in Table 4 for the forms without bond TABLE GEOMETRY
023 034 045 056 W67
W%+Es) JZE, +&‘I ZIEHH E *ot
4 AND
ENERGIES
OF
C;S-CYCLODECENE:
UNDEFORMED
FORMS
(d
=
120’
Form B
Form C
Form D
Form F
Form G
+
+ 82.6 - 138.8 +155.4 - 92.7 0
+ 81.8 -139.4 + 11.9 + 91.9 0
+ 37.5 -174.8 +138.8 - 74.4 + 80.0
f 80.5 -140.9 f 14.3 + 90.3 - 146.6
82.5 20.2 -155.5 + 92.6 0 +
0 14.90 51.21 66.11
0 15.09 51.54 66.63
0 18.03 129.42 147.45
0
AND
8
=
lo!ks”)
0
8.64 33.11 41.75
16.36 76.03 92.39
angle bending strain and in Table 5 for the minimum energy forms. Like cis-cyclooctene and cis-cyclononene, opening the tetrahedral angles appreciably reduces the non-bonded H - - - H interactions. Forms B and C among those with a plane of symmetry and forms F and G among those with an axis of symmetry were found to be the most stable. The dihedral angle of 15” between C,H and C,H bonds, obtained from the NMR coupling constant Jz3 for c&oyclodecene6, agrees more with values calculated J.
Mol. Sm~rrwe, 5 (1970) 101-110
106 TABLE GEOhiETRY
G. BUEMI,
G. FAVINI,
F. ZUCCARELLO
5 AND
ENERGIES
Form A
+ + -
OF CiS-CYCLODECENE:
Form B
MINIhiUM
Form C
122.0 118.0 118.5 117.5 119.5
124.0 110.0 110.0 110.5 118.5
122.0 108.5 110.5 110.5 118.0
78.6 26.0 22.9 77.7 0
+ 83.3 + 25.5 -143.4 + 85.7 0
+ 86.8 -139.2 + 142.4 - 86.1 0
12.96 14.21 8.74 35.91
4.39 14.84 3.80 23.03
ENERGY
Form D 122.0 108.0 115.7 117.7 120.0 + 87.1 - 140.9 + 22.1 + 77.9 0
3.28 15.52 4.90 23.70
FORMS
Form E 119.0 112.0 114.5 114.5 115.1 + 82.5 -115.4 - 31.5 +104.0 -101.4 3.57 17.69 6.97 28.23
8.92 14.23 8.05 31.20
Form F
Form G
126.0 112.0 112.0 109.5 111.8
121.7 111.8 112.2 117.5 118.2
+ 40.0 -158.5 +138.9 - 81.4 + 85.8
+ 70.3 -150.8 + 42.2 + 66.6 -121.7
3.16 11.58 7.23 21.97
6.27 10.45 7.35 24.07
for the forms with an axis of symmetry (10-20”) than with those calculated for the forms with a plane of symmetry (23-27”). Four symmetrical forms were considered for the trans-isomer, related to the schematic representations of Fig. 5 in ref. 8: 180
Form B: (15b in ref. 8)
180
Form C: (He
in ref. 8)
180
++--+ ++--
C,-B
Form D: (15f in ref. 8)
180
++-+ ++-+-
C,-B
The following
independently
+-+-+ +-+-+ +--+-
C,-B
Form A: (Ha in ref. 8)
+
C,-B
variable parameters were selected: S,( = S,), 9, (= %,), %(= %), %(= %), cp (d i hed ra1 angle between C,Cz and C,C, bonds), d (distance between the midpoints of the C,C, and C,C, bonds). The results of the calculations are reported in Table 6. It is evident that the forms A and B without angular deformations are also the minimum energy forms, while the most energetically stable forms of C and D are partially deformed. It may be noted that, in forms A and C, cp g 180” while, in forms B and D, q is equal to 120” and 70”, respectively, rather than 90” as indicated in the schematic J. Mol. Srrucrure,5 (1970) 101-110 3
MOLECULAR
CONFORMATION
OF CYCLENES.
107
III.
.BLE 6 wtx~Y.kND
EN~~RO~~F~~UPI.S-CYCLODEIZENE
Form A
Form B
Form C
Form D
Undeformed
=#,I
120.0 109.5 110.5 110.5 109.5
120.0 109.5 110.2 109.5 109.5
109.5 109.5 109.5 109.5
Q,
181.0
120.0
178.0
t116.4 - GO.7 + 73.5 - 123.6 +160.9
+ 123.4 - 23.0 - 50.4 f131.4 -157.9
0.08
0.02 10.13 1.51 11.66
= &1) := @lOI = w
=w
"23 "34 "45 "56
6.91 1.37 8.36
Minimum energy 120.0 109.5 109.5
120.0
Minimum energ
112.0
120.0 109.5 109.5 109.5 1oP.5
112.0 113.0 112.0 110.5
180.0
68.0
70.0
+ 42.6 + 63.4 - 82.6 + 106.3 -161.9
+ 47.4 + 57.0 - 77.3
114.5
+ 53.3 + 62.8 - 48.1 - 174.9
Undeformed
+ 56.3 + 62.8 - 55.8 - 62.4 f170.9 1.25 4.20 3.62 9.07
0
4.88 8.49 13.37
0 10.46 4.27 14.73
120.0
+104.9 -161.4 1.03 10.03 2.34 13.40
representation of Fig. 5 in ref. 8. Forms A and D are remarkable for their flexibility and, in the interconversion process, a barrier of about 15 kcal mole-l should be overcome (Fig. 1). E t kc*
mole
1 60
1 90
I l20
I 160
Fig. 1. Energy profile of irons-cyclodecene
I la0
Qp (*I
interconversion.
J_ Mol. Structure,
5 (1970) 101-110
108
G. BUEMI,
G. FAVINI,
F. ZUCCARELLO
From the strain energy in cyclodecane’ and from the difference in the heats of hydrogenation in acetic acid solution at 25” between trans-cyclodecene and cyclohexene4, the strain energy in trans-cyclodecene exceeds that of cyclohexene by 6.4 kcal mole- ‘, in good agreement with the calculated value for form A (6.2 kcal mole-‘). Also the dihedral angle between C,H and C3H bonds, obtained from the NMR coupling constant Jz3 for trans-cyclodecene6 (lo), agrees with the value calculated for form A (about 3”). The strain energy in the c&isomer is higher than that in the trans-isomer by about 13.5 kcal mole- ‘, whilst thermochemical data indicate that the &-derivative is more stable than the trans-derivative by 3.304-3.60’ kcal mole-‘. This discrepancy suggests that the conformation of lowest energy is not included among the symmetrical forms (C, or Cz) considered for cis-cyclodecene in our calculations. Consequently we considered, as for ci.s-cyclooctene and cis-cyclononene, two conformations characterized by a plane of symmetry in the C,C2C3C4C9C,, fragment and by an axis of symmetry in the C4C5C6C7CsC9 fragment: FormH:Of---
-_ --I-+
-
_;
formI:Ot---I-X -
--
+
The independently
variable parameters selected were: 6,(= S,), &(= 9,,), 9, and the distance between the midpoints of the C3C,,-, and (= %), %(= %), d49, C,C, segments_ The results are in Table 7_ The strain energy difference between the TABLE GEOMETRY
7 AND
ENERGIES
OF CiS-CYCLODECENE
Form H Undeformed
Form I Minimum energy Undeformed 124.0
120.0 109.5 109.5 109.5 109.5
113.5 109.1 109.5
f 130.5 - 49.0 - 53.6 f116.0 -156.1 + 84.4
+ 126.7 - 43.9 - 51.8 +115.2 -160.5 + 77.5
108.5
0 8.16
10.07 18.23 J. Mol. Structure, 5 (1970) lOI-
1.77 7.57 2.97 12.31
Minimum energy
120.0 109.5 109.5 109.5 109.5
125.0 109.0 113.0 112.0 111.0
t-116.9 - 22.0 - 50.7 - 39.0 + 169.0
+116.2 - 21.0 - 42.7 - 46.9 +172.7 + 99.0
t110.4
0 6.70 15.60 22.30
2.65 5.92 3.66 12.23
MOLECULAR
CONFORMATION
OF CYCLENES.
109
III.
cis- and trans-isomers is reduced to 3.9 kcal mole-‘, but the calculated energy for cis-cyclodecene is much too high. It seems likely that an asymmetrical conformation of the molecule should have the lowest energy. While the present work was in progress, a paper was published by Allinger et al.’ in which the basic method of Westheimer” (as subsequently modified by Hendrickson’) was applied to the calculation of the molecular structures of olefins. A substantial difference exists only in the torsional energy at the double bond which was evaluated by Allinger with a twofold sinusoidal function; his values are slightly lower than those obtained by us as indicated in Part I13. A comparison of the calculated heats of hydrogenation for the cycloalkenes (as a difference between the olefin and its reduction product, each in the lowest energy conformation) is given in Table 8, along with the experimental data. Our TABLE HEATS•
8 (in kcalmole-l)
FHYDROGENATION
Compound
Cyclohexene Cycloheptene cis-Cyclooctene
FORTHECYCLOALKENES
--d&,(expt.)
--dH,,(cufc.)
(4
(b)
Our ualues
Allinger’
28.59
27.10 25.85 22.98 32.24 23.62 26.49 20.67 24.01
28.59; 26.32; 23.68;
28.61 26.82 23.25 28.64 22.82 22.25
26.52 23.53
tram-Cyclooctene
cis-Cyclononene Pans-Cyclononene cis-Cyclodecene trans-Cyclodecene
27.10 24.83 22.19 36.05 25.24 28.03 27.56 23.69
(a) In the gas phase4. (b) In acetic acid solutionf
are calculated assuming a bond energy difference between the cycloalkene and the cycloalkane of - 26.42 kcal mole- ‘, which reproduces the heat of hydrogenation of cyclohexene in the gas phase and a constant difference of 1.5 kcal mole-’ for the heats of solution in acetic acid between the olefin and its reduction product. values
ACKNOWLEDGEMENT
This research was supported by the Italian Consiglio Nazionale delle Ricerthe. J. Mol.
Sfracrure,
5 (1970) 101-110
110
G. BUEMI,
G. FAVINI,
F. ZUCCARELLO
REFERENCES 1 G. FAVINI, G. BUEMI AND M. RAIMONDI, J. Mol. Structure, 2 (1968) 2 J. B. HENDRICKSON,J. Am. Chem. Sot., 83 (1961) 4537. 3 G. FAVINI, F. ZUCCARELLO AND G. BTJEMI,J. Mol. Structure, 3 (1969)
137. 385.
R. B. TURNER AND W. R. MJZADOR,J. Am. Chem. SOL, 79 (1957) 4133. J. B. HENDRICKSON,J. Am. Chem. Sot., 86 (1964) 4854. G. V. SMITH AND H. KRILOFF, J. Am. Chem. Sot., 85 (1963) 2016. A. C. COPE, P. T. MOORE AND W. R. MOORE, J. Am. Chem. Sot., 81 (1959) 3153. G. BINSCH AND J. D. ROBERTS, J. Am. Chem. Sot., 87 (1965) 5157. N. L. ALLINGER, J. A. HIRSCH, M. A. MILLER AND I.J. TYMINSKI, J. Am. Chem. Sot., 90 (1968) 5773. 10 F. H. WESTHEIMER, in M. S. NEWMAN (Editor), Srerhz effects in organic chemistry, Wiley, New 4 5 6 7 8 9
York,
1956, p. 523.
J. Mol. Strucrure, 5 (1970) 101-110
of Molecular Structure
Elsevier Publishing
Company,
Amsterdam.
MOLECULAR
CONFORMATION
III. mm-CYCLOOCTENE,
Printed in the Netherlands
101
OF CYCLENES
tramCYCLONONENE,
cis- AND
tram-CYCLO-
DECENE
G. BUEMI, G. FAVINI AND F. ZUCCARELLO Institute of Physical Chemistry, Uniuersity of Catania (Italy)
(Received
June 7th, 1969)
ABSTRACT
The strain energy of some cyclenes with one double bond in the ring has been calculated as a function of various geometric parameters. The minimum energy forms for trans-cyclooctene and for trans-cyclononene are characterized by a twisting angle at the double bond of 17-18”. The most stable configuration for cis-cyclononene and cis-cyclodecene is like that found for ciscyclooctene in previous work. In trans-cyclodecene the minimum energy form with an axis of symmetry is angular strainless and remarkable for its flexibility.
INTRODUCTION
In a previous paper I, Hendrickson’s treatment’ was applied to the calculation of molecular conformations and energies of cyclenes with one double bond in the ring, ranging fromcyclohexene up to cyclononene. The strain energies in transcyclooctene and trans-cyclononene were found to be higher than those derived from the experimental thermochemical data, but the possibility of a twisted double bond was not considered. In the present paper this possibility was included in the c$culations and the n-energy dif%erencewith respect to ethylene was estimated as indicated in the second paper3. Calculations were also extended to cyclodecene, where the magnitude of the problem increases considerably in consequence of the greater flexibility of the ring. J. Mol. Structure, 5 (1970) 101-110
102
G. BUEMI,
G. FAWN&
F. ZUCCARELLO
CALCULATIONS
The energy terms were evaluated according to the formulae given in ref. 1. Only symmetrical forms were examined although it is no longer certain that all of the energy minima are represented by symmetrical forms. Calculations were performed on an IBM 1620 (20 K) computer and organized in different steps as before’.
RESULTS
AND
DISCUSSION
Cycloocfene
In the frans-derivative (form XIII of ref. 1 with C&3 symmetry), the minimum energy conformation was calculated by including as an independently variable parameter the torsional angle at the double bond. The energy lowering amounted to 6.77 kcal mole” as can be seen from the values collected in Table 1. 3
4
2 GEOMETRY (angles in degrees) TABLE 1 OFtT(IllS-(SYCLOOCENE
Form XIII
AND ENERGIES
(see ref. 1)
-
92.7
+ 107.9
15.86 12.37 1.56 0 29.79
5
1 I D
6 8
7
Twistedform
116.0 92.0 109.8 118.3 -180.0 + 67.5 + 29.7
(in kcal mole-‘)
117.5 98.0 108.8 114.5 -163.0 + 64.0 + -
32.7 96.4
-f-115.2 6.46 12.80 0.49 3.27 23.02
The strain energy in the frans-isomer with a double bond twisting angle of 17” is higher than that in the &-isomer by 13.86 kcal mole-‘, and the agreement with the difference between the heats of hydrogenation in acetic acid solution at 25OC (9.3 kcal mole -1)4 is greatly improved. J. Mol.
Structare, 5 (1970) 101-110
MOLECULAR
CONFORMATION
OF CYCLENES.
103
III..
In the previous paperr it was pointed out that the strain energy values obtained for cis-cyclononene were too high in all the forms with -C, symmetry. Consequently, we have here considered a further form characterized by a plane of symmetry in the CIC,C,C,C,C, fragment and by an axis of symmetry in the C,CSCsC,C8 fragment (form A); the ind~~~dentIy variable parameters selected were: S,( = S,), G3( = &), QS(= 9,). Q6 and the C,C, distance. The results of the calculations are reported in Table 2 and compared with TABLE 2 GEOMETRY ANI3 ENERGIES OF Cis-CYCLONONENE
Form XIV (see rejI I) undeforl?ted
h(==
&J w %) &i
023
-t_
WJ4
c
043
-
e.6 @se
-
~uw--E*b) z: w&-&3 =%it% &It
Minimum energy Undeformed
120.0 109.5 109.5 109.5 109.5
62)
h(== a* &x5
86.6 12.9 41.6 85.2 X2.9 0 10.62 8.02 18.64
Form A
-i+ -
Minimum energy
122.5 114.0 110.5 110.5 110.0
120.0 109.5 109.5 109.5 109.5
123.4 108.6 109.8 111.3 111.8
79.8 17.9 44.1 83.3 17.9
-b 97.8 -101.0 -+ 108.5 - 64.1 _t 25.6
-+ 96.6 - 96.2 +106.9 - 65.2 +- 22.6
1.32 10.21 4.44
15.97
0 10.25 6.17 16.42
1.04 10.15 2.02 13.21
those of the most energeticahy stable form found in ref. I. The strain energy di%erence between c&-cyclononene and cyclohexeae is reduced to Il.04 kcat mole-’ in fair agreement with the v&e of 9-4 kcal moleW1 obtained from the strain energy in cyeIononane’ and from the difference in the heats of hydrogenation in acetic acid solution at 25” C between cis-cyclononene and cyclohexene4. The dihedral angle of 3’?“, found’ from the NMRcoupling constant J2 J, is in very good agreement with the value obtained for form A (36.6”), and the uncertainty in the choice between the probable experimental angles (27” or 37”) therefore seems to be resoIt;ed. Two symmetrical forms were considered for the frarrJ-isomer: J. MM.Srructure, 5 11370) IOf-
104
G. BUEMI,
Form A (XIX in ref. I): chair-chair
180 + -
Form B: boat-chair
E30++-+
+ -
G. FAVINI,
F. ZUCCARELLO
c
f-f-
=
++-+ 2 c
The strain energy was first calculated by allowing no rotation around the double bond, and later by including also the torsional angle q2 as an independent variable. Unlike the c&-isomer, no angular strainless conformations are possible. The results of the calculations are reported in Table 3. Form A with the double bond twisted by 18.5” is the most stable conformation. The strain energy in trans-cyclononene was calculated to be higher than that in the &-isomer by 2.8 kcaf mole-l, in excellent agreement with the value of 2.9 kcal mole” experimentally observed for the enthaIpy of the c&-tram isomerization’. TABLE
3
GEOMETFtY AND ENEliGIES OF trOtIS-CYCLONONENE
Form A
-I+ -
Form B
Twisted B form
Twisred A form
120.0 99.5 121.1 119.5 114.5
118.0 94.5 109.6 117.5 112.0
118.4 103.7 109.6 113.1 115.1
118.4 101.4 107.7 113.5 114.7
180.0 76.3 30.6 93.4 70.6
180.0 -t 70.0 -t 45.3 -127.1 f 63.1
-161.5 -I- 89.6 - 56.7 +113.5 - 73.0
-165.1 -f- 68.6 -I- 44.8 -126.1 + 64.9
13.70 9.71 0 9.22 32.63
11.51 9.93 0 2.61
4.03 10.16 2.52 1.06 17.77
2.63 8.13 3.86 1.38 16.00
24.05
Seven symmetrical forms were considered for the &-isomer: Form A:
boat-boat
0
++----f-f
0
C,-B
Form B:
boat-chair
0
+ + - + -c-
0
C,-B
Form C:
chair-chair
0
+ -
+ -
0
C'-B
-+
-
I. Mol. Structure, 5 (1970) 101-l 10
-+
MOLECULAR
CONFORMATION
Form D:
chair-boat
0
Form E:
boat-chair
0
Form F:
chair-chair
0
Form G:
chair-boat
0
OF CYCLENES.
+-+
0
_+-+ --
+
+ --
+
+ -++--+--
+ --l--t
105
III.
C,-B
-
C,-B
+
C2-B
_
Cz--B
+-++
The highly strained boat-boat conformation with C, symmetry was not included in the calculations. Six independently variable parameters were selected for the plane-symmetrical forms: 8, (= a,), &(= %,), %a(= %), %(= %)? %(= 97) and dh9. The six parameters selected for the forms with an axis of symmetry were: a,(= a,), &(= W, Q4(= %), %(= &3), w2s and q (dihedral angle between CICz and C& bonds). The results of the calculations are reported in Table 4 for the forms without bond TABLE GEOMETRY
023 034 045 056 W67
W%+Es) JZE, +&‘I ZIEHH E *ot
4 AND
ENERGIES
OF
C;S-CYCLODECENE:
UNDEFORMED
FORMS
(d
=
120’
Form B
Form C
Form D
Form F
Form G
+
+ 82.6 - 138.8 +155.4 - 92.7 0
+ 81.8 -139.4 + 11.9 + 91.9 0
+ 37.5 -174.8 +138.8 - 74.4 + 80.0
f 80.5 -140.9 f 14.3 + 90.3 - 146.6
82.5 20.2 -155.5 + 92.6 0 +
0 14.90 51.21 66.11
0 15.09 51.54 66.63
0 18.03 129.42 147.45
0
AND
8
=
lo!ks”)
0
8.64 33.11 41.75
16.36 76.03 92.39
angle bending strain and in Table 5 for the minimum energy forms. Like cis-cyclooctene and cis-cyclononene, opening the tetrahedral angles appreciably reduces the non-bonded H - - - H interactions. Forms B and C among those with a plane of symmetry and forms F and G among those with an axis of symmetry were found to be the most stable. The dihedral angle of 15” between C,H and C,H bonds, obtained from the NMR coupling constant Jz3 for c&oyclodecene6, agrees more with values calculated J.
Mol. Sm~rrwe, 5 (1970) 101-110
106 TABLE GEOhiETRY
G. BUEMI,
G. FAVINI,
F. ZUCCARELLO
5 AND
ENERGIES
Form A
+ + -
OF CiS-CYCLODECENE:
Form B
MINIhiUM
Form C
122.0 118.0 118.5 117.5 119.5
124.0 110.0 110.0 110.5 118.5
122.0 108.5 110.5 110.5 118.0
78.6 26.0 22.9 77.7 0
+ 83.3 + 25.5 -143.4 + 85.7 0
+ 86.8 -139.2 + 142.4 - 86.1 0
12.96 14.21 8.74 35.91
4.39 14.84 3.80 23.03
ENERGY
Form D 122.0 108.0 115.7 117.7 120.0 + 87.1 - 140.9 + 22.1 + 77.9 0
3.28 15.52 4.90 23.70
FORMS
Form E 119.0 112.0 114.5 114.5 115.1 + 82.5 -115.4 - 31.5 +104.0 -101.4 3.57 17.69 6.97 28.23
8.92 14.23 8.05 31.20
Form F
Form G
126.0 112.0 112.0 109.5 111.8
121.7 111.8 112.2 117.5 118.2
+ 40.0 -158.5 +138.9 - 81.4 + 85.8
+ 70.3 -150.8 + 42.2 + 66.6 -121.7
3.16 11.58 7.23 21.97
6.27 10.45 7.35 24.07
for the forms with an axis of symmetry (10-20”) than with those calculated for the forms with a plane of symmetry (23-27”). Four symmetrical forms were considered for the trans-isomer, related to the schematic representations of Fig. 5 in ref. 8: 180
Form B: (15b in ref. 8)
180
Form C: (He
in ref. 8)
180
++--+ ++--
C,-B
Form D: (15f in ref. 8)
180
++-+ ++-+-
C,-B
The following
independently
+-+-+ +-+-+ +--+-
C,-B
Form A: (Ha in ref. 8)
+
C,-B
variable parameters were selected: S,( = S,), 9, (= %,), %(= %), %(= %), cp (d i hed ra1 angle between C,Cz and C,C, bonds), d (distance between the midpoints of the C,C, and C,C, bonds). The results of the calculations are reported in Table 6. It is evident that the forms A and B without angular deformations are also the minimum energy forms, while the most energetically stable forms of C and D are partially deformed. It may be noted that, in forms A and C, cp g 180” while, in forms B and D, q is equal to 120” and 70”, respectively, rather than 90” as indicated in the schematic J. Mol. Srrucrure,5 (1970) 101-110 3
MOLECULAR
CONFORMATION
OF CYCLENES.
107
III.
.BLE 6 wtx~Y.kND
EN~~RO~~F~~UPI.S-CYCLODEIZENE
Form A
Form B
Form C
Form D
Undeformed
=#,I
120.0 109.5 110.5 110.5 109.5
120.0 109.5 110.2 109.5 109.5
109.5 109.5 109.5 109.5
Q,
181.0
120.0
178.0
t116.4 - GO.7 + 73.5 - 123.6 +160.9
+ 123.4 - 23.0 - 50.4 f131.4 -157.9
0.08
0.02 10.13 1.51 11.66
= &1) := @lOI = w
=w
"23 "34 "45 "56
6.91 1.37 8.36
Minimum energy 120.0 109.5 109.5
120.0
Minimum energ
112.0
120.0 109.5 109.5 109.5 1oP.5
112.0 113.0 112.0 110.5
180.0
68.0
70.0
+ 42.6 + 63.4 - 82.6 + 106.3 -161.9
+ 47.4 + 57.0 - 77.3
114.5
+ 53.3 + 62.8 - 48.1 - 174.9
Undeformed
+ 56.3 + 62.8 - 55.8 - 62.4 f170.9 1.25 4.20 3.62 9.07
0
4.88 8.49 13.37
0 10.46 4.27 14.73
120.0
+104.9 -161.4 1.03 10.03 2.34 13.40
representation of Fig. 5 in ref. 8. Forms A and D are remarkable for their flexibility and, in the interconversion process, a barrier of about 15 kcal mole-l should be overcome (Fig. 1). E t kc*
mole
1 60
1 90
I l20
I 160
Fig. 1. Energy profile of irons-cyclodecene
I la0
Qp (*I
interconversion.
J_ Mol. Structure,
5 (1970) 101-110
108
G. BUEMI,
G. FAVINI,
F. ZUCCARELLO
From the strain energy in cyclodecane’ and from the difference in the heats of hydrogenation in acetic acid solution at 25” between trans-cyclodecene and cyclohexene4, the strain energy in trans-cyclodecene exceeds that of cyclohexene by 6.4 kcal mole- ‘, in good agreement with the calculated value for form A (6.2 kcal mole-‘). Also the dihedral angle between C,H and C3H bonds, obtained from the NMR coupling constant Jz3 for trans-cyclodecene6 (lo), agrees with the value calculated for form A (about 3”). The strain energy in the c&isomer is higher than that in the trans-isomer by about 13.5 kcal mole- ‘, whilst thermochemical data indicate that the &-derivative is more stable than the trans-derivative by 3.304-3.60’ kcal mole-‘. This discrepancy suggests that the conformation of lowest energy is not included among the symmetrical forms (C, or Cz) considered for cis-cyclodecene in our calculations. Consequently we considered, as for ci.s-cyclooctene and cis-cyclononene, two conformations characterized by a plane of symmetry in the C,C2C3C4C9C,, fragment and by an axis of symmetry in the C4C5C6C7CsC9 fragment: FormH:Of---
-_ --I-+
-
_;
formI:Ot---I-X -
--
+
The independently
variable parameters selected were: 6,(= S,), &(= 9,,), 9, and the distance between the midpoints of the C3C,,-, and (= %), %(= %), d49, C,C, segments_ The results are in Table 7_ The strain energy difference between the TABLE GEOMETRY
7 AND
ENERGIES
OF CiS-CYCLODECENE
Form H Undeformed
Form I Minimum energy Undeformed 124.0
120.0 109.5 109.5 109.5 109.5
113.5 109.1 109.5
f 130.5 - 49.0 - 53.6 f116.0 -156.1 + 84.4
+ 126.7 - 43.9 - 51.8 +115.2 -160.5 + 77.5
108.5
0 8.16
10.07 18.23 J. Mol. Structure, 5 (1970) lOI-
1.77 7.57 2.97 12.31
Minimum energy
120.0 109.5 109.5 109.5 109.5
125.0 109.0 113.0 112.0 111.0
t-116.9 - 22.0 - 50.7 - 39.0 + 169.0
+116.2 - 21.0 - 42.7 - 46.9 +172.7 + 99.0
t110.4
0 6.70 15.60 22.30
2.65 5.92 3.66 12.23
MOLECULAR
CONFORMATION
OF CYCLENES.
109
III.
cis- and trans-isomers is reduced to 3.9 kcal mole-‘, but the calculated energy for cis-cyclodecene is much too high. It seems likely that an asymmetrical conformation of the molecule should have the lowest energy. While the present work was in progress, a paper was published by Allinger et al.’ in which the basic method of Westheimer” (as subsequently modified by Hendrickson’) was applied to the calculation of the molecular structures of olefins. A substantial difference exists only in the torsional energy at the double bond which was evaluated by Allinger with a twofold sinusoidal function; his values are slightly lower than those obtained by us as indicated in Part I13. A comparison of the calculated heats of hydrogenation for the cycloalkenes (as a difference between the olefin and its reduction product, each in the lowest energy conformation) is given in Table 8, along with the experimental data. Our TABLE HEATS•
8 (in kcalmole-l)
FHYDROGENATION
Compound
Cyclohexene Cycloheptene cis-Cyclooctene
FORTHECYCLOALKENES
--d&,(expt.)
--dH,,(cufc.)
(4
(b)
Our ualues
Allinger’
28.59
27.10 25.85 22.98 32.24 23.62 26.49 20.67 24.01
28.59; 26.32; 23.68;
28.61 26.82 23.25 28.64 22.82 22.25
26.52 23.53
tram-Cyclooctene
cis-Cyclononene Pans-Cyclononene cis-Cyclodecene trans-Cyclodecene
27.10 24.83 22.19 36.05 25.24 28.03 27.56 23.69
(a) In the gas phase4. (b) In acetic acid solutionf
are calculated assuming a bond energy difference between the cycloalkene and the cycloalkane of - 26.42 kcal mole- ‘, which reproduces the heat of hydrogenation of cyclohexene in the gas phase and a constant difference of 1.5 kcal mole-’ for the heats of solution in acetic acid between the olefin and its reduction product. values
ACKNOWLEDGEMENT
This research was supported by the Italian Consiglio Nazionale delle Ricerthe. J. Mol.
Sfracrure,
5 (1970) 101-110
110
G. BUEMI,
G. FAVINI,
F. ZUCCARELLO
REFERENCES 1 G. FAVINI, G. BUEMI AND M. RAIMONDI, J. Mol. Structure, 2 (1968) 2 J. B. HENDRICKSON,J. Am. Chem. Sot., 83 (1961) 4537. 3 G. FAVINI, F. ZUCCARELLO AND G. BTJEMI,J. Mol. Structure, 3 (1969)
137. 385.
R. B. TURNER AND W. R. MJZADOR,J. Am. Chem. SOL, 79 (1957) 4133. J. B. HENDRICKSON,J. Am. Chem. Sot., 86 (1964) 4854. G. V. SMITH AND H. KRILOFF, J. Am. Chem. Sot., 85 (1963) 2016. A. C. COPE, P. T. MOORE AND W. R. MOORE, J. Am. Chem. Sot., 81 (1959) 3153. G. BINSCH AND J. D. ROBERTS, J. Am. Chem. Sot., 87 (1965) 5157. N. L. ALLINGER, J. A. HIRSCH, M. A. MILLER AND I.J. TYMINSKI, J. Am. Chem. Sot., 90 (1968) 5773. 10 F. H. WESTHEIMER, in M. S. NEWMAN (Editor), Srerhz effects in organic chemistry, Wiley, New 4 5 6 7 8 9
York,
1956, p. 523.
J. Mol. Strucrure, 5 (1970) 101-110