The crystal and molecular structures of some highly strained and moderately ... Systems

Abstract

The thesis entitled “The Crystal and Molecular Structures of Some Highly Strained and Moderately Strained Organic Ring Systems” consists of two parts: Part One deals with the highly strained systems, and Part Two with the moderately strained systems.

Part One The highly strained system under consideration in this thesis is the cyclopropane ring. This system fascinates both the theoretical chemist and the experimentalist with its interesting properties — especially the high sensitivity of its geometry towards electronic factors. The present study was undertaken in order to assess the influence of substituents (both electron?withdrawing and electron?donating) on the cyclopropane ring geometry. Chapter I This chapter presents the crystal and molecular structures of two cyclopropane derivatives: cyclopropane?1,1?dicarboxamide (A) (Fig. 1) and 3?(2?methoxyphenyl)?1,1,2,2?cyclopropane tetracarbonitrile (B) (Fig. 2). Compound A crystallises in the monoclinic space group Cc or P2?/c with a = 10.236(2), b = 6.432(2), c = 9.444(2) Å, ? = 104.62(2)°, and Z = 4. The structure was solved by direct methods in space group P2?/c and refined to an R value of 5.2%. The C=O and C–N bonds of the amide groups show unusual bond lengths. A rotational disorder mechanism, which would account for both the bond?length anomaly and the space?group ambiguity, has been proposed. Compound B crystallises in the monoclinic space group P2? with a = 13.241(4), b = 7.446(2), c = 6.436(2) Å, ? = 93.23(2)°, and Z = 2. The structure, solved by direct methods, has been refined to an R index of 3.5%. In both structures (A and B), the substituents induce drastic changes in the geometry of the three?membered ring. The Walsh model for bonding in cyclopropane has been found to offer an adequate explanation for such changes.

A novel C?? carbocyclic system (Fig. 3), reported in Chapter II, bridges the gap between the two major parts of this thesis — the highly strained and moderately strained ring systems — as it contains both a cyclopropane ring and a bicyclo[3.2.1]octane system, which belong to the former and latter categories respectively. This compound crystallises in the space group P2?/a with a = 10.013(2), b = 9.620(2), c = 12.532(3) Å, ? = 95.87(2)°, and Z = 4. The structure, solved by direct methods and refined to an R value of 3.8%, was found to be totally different from the one proposed on the basis of spectroscopic results. A possible reaction mechanism for the formation of this novel ring system has been proposed. The conformational features of the cyclopropane ring and the bicyclic moiety are elaborately discussed. In addition, the geometries of the methanesulfonate and cyano groups are touched upon.

Part Two The X?ray structure analyses of moderately strained ring systems provide useful reference structures for the development of force fields applicable to molecular mechanics calculations. Chapter III deals with one such moderately strained ring system: a brendene derivative (Fig. 4). This compound crystallises in the monoclinic space group P2?/c with a = 12.193(4), b = 10.009(3), c = 14.699(5) Å, ? = 115.35(4)°, and Z = 4. Direct methods were used for the structure solution. The final R factor is 4.6%. The results are discussed in comparison with those derived by Schleyer (1977) for brendene, and with those obtained independently by Allinger (1978) and Schleyer (1977) for norbornane through molecular mechanics calculations. In addition, the geometries of the 3,5?dinitrobenzoate group and the cyano group are discussed.

Chapter IV This chapter describes the structures and conformations of two norbornenone derivatives:

Ethyl?1,4,5(exo)?trimethyl?2,3?diphenyl?bicyclo[2.2.1]hept?2?en?7?one?5(endo)?carboxylate (A) (Fig. 5a), and Ethyl?1,4?dimethyl?2,3?diphenyl?bicyclo[2.2.1]hept?2?ene?7?one?5(endo)?carboxylate (B) (Fig. 5b).

The structure analyses of these two compounds were undertaken (1) as part of studies on moderately strained ring systems — to examine the influence of bulky substituents on the geometry of the parent molecular framework, and (2) to verify a hypothesis by Prof. S. N. Balasubrahmanyam and co?workers regarding the anisochronism of the –OCH?– protons of the ester groups. Compound A crystallises in P1 with a = 9.197(2), b = 10.357(2), c = 11.086(3) Å, ? = 94.07(2)°, ? = 98.21(3)°, ? = 90.82(2)°, and Z = 2. Compound B crystallises in P2?/a with a = 11.584(2), b = 11.315(2), c = 15.900(4) Å, ? = 104.02(2)°, and Z = 4. Both structures were solved by direct methods and refined to R factors of 6.2% and 4.6% respectively. These structures, besides serving the above purposes, provide an opportunity to study the geometries of the cis?stilbene and ester moieties present in them.

Chapter V The last chapter presents the structure of yet another moderately strained system: azaisotwistanone (Fig. 6). This compound crystallises in the space group P2?/c with a = 10.680(2), b = 6.153(2), c = 13.432(2) Å, ? = 110.40(2)°, and Z = 4. The structure, solved by direct methods and refined to an R index of 5.3%, has a cis?amide group embedded in the isotwistanone skeleton. The geometry of the isotwistanone moiety is discussed in comparison with bicyclo[2.2.2]octane and with the twistane moiety present in azatwistanone reported by Ramakumar et al. (1977). The non?planar deformations of the cis?amide group are analysed. Prof. Blaha et al. (1978) have studied the chiroptical properties of this compound, and the X?ray structure analysis has been useful in interpreting their results. The details of a computer program written by the author to calculate the electron density in any general plane of the molecule have been recorded in the Appendix.