• • • • H H H H H H H H 1 • • • • Me Me Me Me Me Me Me Me 2 376 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 376–377† Configurations of Cyclododeca-1,2,4,5,7,8,10,11-octaene and Its Octamethyl Derivative† Issa Yavari,*a Davood Nori-Shargh,b Robabeh Baharfara, Rahim Hekmat-Shoara and Hassan Norouzi-Arasib aDepartment of Chemistry, University of Tarbiat Modarres, P.O. Box 14155-4838, Tehran, Iran bDepartment of Chemistry, Islamic Azad University, Arak, Iran MNDO, AM1 and PM3 semi-empirical SCF MO calculations are used to calculate the structure optimization and configurational properties of cyclododeca-1,2,4,5,7,8,10,11-octaene (1) and its octamethyl derivative (2); the combination of four allenic units of the same chirality yields an enantiomeric pair of D4 symmetry, which is the most stable configuration of 1 and 2.Cyclododeca-1,2,4,5,7,8,10,11-octaene (1), with four allenic chromophores, could experience eight-electron cyclic interactions of both the in-plane and out-of-plane p bonds of the four allenic moieties.1–3 This ‘expanded cyclooctatetraene’ is expected to manifest special configurational properties.Conceptually, 1 may be regarded to be constructed by inserting a carbon atom in the carbon–carbon double bonds of cycloocta-1,3,5,7-tetraene. This structural feature suggests that combination of four units of the same chirality yields an enantiomeric pair (RRRR and SSSS) of D4 symmetry, while combination of three units of the same chirality and a unit of opposite chirality produces an enantiomeric pair (RRRR or SSSR) of C2 symmetry (see Scheme 1).Combination of two pairs of units of opposite chirality yields two meso compounds RRSS and RSRS of C2h and D2d symmetries, respectively. Intrigued by the fascinating molecular structure of the cyclic tetra-allene 1, we carried out semi-empirical SCF MO calculations4,5 on eight possible configurations of 1 and 2. Although 1 and 2 are presently unknown, their configurations are of interest.Calculations Initial estimates of the geometry of structures 1 and 2 were obtained by a molecular-mechanics program PCMODEL (88.0)6 followed by full minimization using semiempirical MNDO,7 AM18 and PM39 methods in the MOPAC 6.0 computer program,5,10 implemented on a VAX 4000-300 computer. Optimal geometries were located by minimizing energy, with respect to all geometrical coordinates, and without imposing any symmetry constraints. All geometries were characterized as stationary points, and true local energy minima on the potential energy surface were found using Keyword FORCE.All geometries obtained in this work are calculated to have 3Nµ6 real vibrational frequencies.11 Results and Discussion Heats of formation (DH°f ) for the eight configurational diastereoisomers of cyclododeca-1,2,4,5,7,8,10,11-octaene (1) and its octamethyl derivative (2), as calculated by MNDO, AM1 and PM3 methods, are shown in Table 1.The highly symmetrical crown configuration, with D4 point group, is calculated by all three methods to be the most stable geometry of 1 and 2. This configuration is constructed by a combination of four allenic moieties of the same chirality. While the axial symmetrical twist configuration of 1 is calculated by MNDO and PM3 methods to be the next most stable geometry, the best (RSRS, D2d) configuration of 1 is predicted by the AM1 method as the second most stable diastereoisomer.All three methods predict the twist (C2) configuration to be the second most stable geometry of the octamethyl derivative 2. *To receive any correspondence. †This is a Short Paper as defined in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is therefore no corresponding material in J. Chem. Research (M). Scheme 1 Table 1 Calculated energies (kJ molµ1) in various configurations of cyclododeca-1,2,4,5,7,8,10,11-octaene (1) and its octamethyl derivative (2) 1 2 MNDO AM1 PM3 MNDO AM1 PM3 Geometry DH°f DDH°f a DH°f DDH°f a DH°f DDH°f a DH°f DDH°f a DH°f DDH°f a DH°f DDH°f a RRRR, D4 731.6 00.0 801.0 00.0 819.4 00.0 475.5 00.0 581.2 00.0 537.8 00.0 RRRS, C2 748.7 17.1 809.4 8.4 829.2 9.8 511.8 36.3 596.3 15.1 558.6 20.8 RSRS, D2d 750.0 18.4 804.5 3.5 829.3 9.9 545.3 69.8 547.6 16.4 566.8 29.0 RRSS, C2h 752.6 21.0 809.7 8.7 829.3 9.9 522.2 46.7 598.4 17.2 560.7 22.9 aRelative to the best configuration of the same compound.(–78)–76 (86)84 (–71)–72 77(83) –15 –75 87 (–18) (89) (–78) 119 174 124 126 (178)180 (123) (122) (174) (118) 172(171) 120 • • • • • • • • • • • • • • • • (118) 122(121) 177(178) Crown ( RRRR, D4) Twist ( RRRS, C2) (118)116 (–73)–72 (0)0 (72)69 (0)0 123(121) 174(175) Boat ( RSRS, D2d) Chair ( RRSS, C2h) 120(119) 174(175) 123(121) J.CHEM. RESEARCH (S), 1997 377 The relevant structural parameters for various con- figurational diastereoisomers of 1 and 2, as calculated by the AM1 method, are given in Fig. 1. The C�C�C moieties are bent in various configurations of 1 and 2 and they are 1–9° compressed from the normal value of 180°. In general, the internal angle deformations increase from chiral geometries, such as crown and twist, to meso configurations. The C�C·C bond angles in all geometries of the octamethyl derivative 2 are 1–2° more compressed in comparison to those in 1, as a result of the methyl substituents.The C(sp2)·C(sp2)·C(sp2)·C(sp2) arrangements in the allenic moieties of the twist and meso configurations (boat and chair) of 1 and 2 are fairly twisted (7–21°) from their energy minimum at 90°, as a result of ring strain. However, the extent of this torsional deformation in the crown geometry is much smaller. The boat configurations of 1 and 2 have favoured torsional angles of single bonds flanked by two double bonds. The stabilization (negative strain energy) resulting from the conjugative effects of double bonds in the twist and meso geometries are more than offset by the unfavourable torsions and bond angles around the allenic moieties in these configurations.In conclusion, semi-empirical calculations provide a picture of the configurations of cyclododecaoctaene 1 and its octamethyl derivative (2) from both structural and energetic points of view. All three methods employed in this work predict that combination of four allenic chromophores of the same chirality yields an enantiomeric pair of crown (D4) configuration, which is the most stable diastereoisomer of 1 and 2.It would be valuable, of course, to have direct structural data on 1 and 2 for comparison with the results of the semi-empirical SCF MO calculations. We gratefully acknowledge financial support from the Research Council of the Islamic Azad University, Arak. Received, 2nd May 1997; Accepted, 1st July 1997 Paper E/7/03617E References 1 A. Greenberg and J. F. Liebman, Strained Organic Molecules, Academic Press, New York, 1978. 2 R P. Johnson, Chem. Rev., 1989, 89, 1111. 3 P. G. Garrat, K. C. Nicolaou and F. Sondheimer, J. Am. Chem. Soc., 19734, 95, 4582. 4 D. M. Hirst, A Computational Approach to Chemistry, Blackwell Scientific Publications, Oxford, 1990. 5 J. J. P. Stewart, J. Comput.-Aided Mol. Des., 1990, 4, 1. 6 Serena Software, Box 3076, Bloomington, IN, USA. 7 M. J. S. Dewar and W. Thiel, J. Am. Chem. Soc., 1977, 99, 4899, 4907. 8 M. J. S. Dewar, E. G. Zeobish, E. F. Healy and J. J. P. Stewart, J. Am. Chem. Soc., 1985, 107, 3907. 9 J. J. P. Stewart, J. Comput. Chem., 1989, 10, 221. 10 J. J. P. Stewart, QCPE 581, Department of Chemistry, Indiana University, Bloomington, IN, USA. 11 J. W. McIver, Jr., Acc. Chem. Res., 1974, 7, 72. Fig. 1 Calculated AM1 structural parameters (bond angles and dihedral angles in degrees) in various configurations of 1: the parameters shown in parentheses belong to the octamethyl derivati