Article pubs.acs.org/joc
Computational Studies of Carbodiimide Rings Robert Damrauer,*,† Hai Lin,† and Niels H. Damrauer*,‡ †
Chemistry Department, University of Colorado−Denver Campus Box 137, P.O. Box 173364, Denver, Colorado 80217-3364, United States Department of Chemistry and Biochemistry, University of Colorado−Boulder, Boulder, Colorado 80309, United States
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S Supporting Information *
ABSTRACT: Computational studies of alicyclic carbodiimides (RNCNR) (rings five through twelve) at the MP2/6-31G(d,p)//MP2/6-31G(d,p) level of theory were conducted to locate the transition states between carbodiimides isomers. Transition states for rings six through twelve were found. The RNCNR dihedral angle is ∼0° for even-numbered rings, but deviates from 0° for rings seven, nine, eleven, and twelve. The even- and odd-numbered ring transition states have different symmetry point groups. Cs transition states (even rings) have an imaginary frequency mode that transforms as the asymmetric irreducible representation of the group. C2 transition states (odd rings) have a corresponding mode that transforms as the totally symmetric representation. Intrinsic reaction coordinate analyses followed by energy minimization along the antisymmetric pathways led to enantiomeric pairs. The symmetric pathways give diastereomeric isomers. The five-membered ring carbodiimide is a stable structure, possibly isolable. A twelve-membered ring transition state was found only without applying symmetry constraints (C1). Molecular mechanics and molecular dynamics studies of the seven-, eight-, and nine-membered rings gave additional structures, which were then minimized using ab initio methods. No structures beyond those found from the IRC analyses described were found. The potential for optical resolution of the seven-membered ring is discussed.
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INTRODUCTION On one extreme, carbodiimide (RNCNR) studies are exemplified by the importance of carbodiimides as synthetic reagents and on the other by questions concerning their structure and the nature of nitrogen configurational stability. Early reviews summarizing carbodiimide chemistry detailed their synthetic utility, notably as coupling reagents;1−3 more recent studies attest to the continuing importance of carbodiimide coupling.4,5 Questions concerning structure and nitrogen configurational stability in carbodiimides were considered early and have continued to be addressed,1−3,6−13 yet, in some ways, these issues are not well understood. These experimental and computational studies focused on carbodiimide structural features and the nature of the mechanism(s) transforming one enantiomer to another. The first mention of the possibility of optically active carbodiimides was made in 1932 by Roll and Adams,14 where attention was drawn to their obvious similarity to allenes and to the possibility of their resolution. Despite Roll and Adams’ failure to report optically active carbodiimides, their paper offers a remarkable analysis of various aspects of nitrogen configurational stability in both amines and oximes, all the while analyzing the nature of nitrogen in carbodiimides. A simple valence bond picture of a carbodiimide suggests a linear N−C−N bond and a perpendicular R−N···N−R dihedral angle (the R−N···N−R angle is called the dihedral angle throughout this article). The earliest systematic computational study of various diazacumulenes by Gordon and Fischer6 in 1968 using intermediate neglect of differential overlap (INDO) methodology investigated the R = H and R = F carbodiimides. Although no mention is given to the linearity of the N−C−N bond, the dihedral angles reported for the parent and the fluoro © 2014 American Chemical Society
derivatives are approximately 90°. For R = H, only a slight energy difference between stereoisomerization by inversion and rotation mechanisms (both ∼8 kcal/mol) was determined, with rotation being slightly favored. The barrier for R = F was much higher (∼22 kcal/mol), thus suggesting the possibility of isolating enantiomers. This compound has never been synthesized. It is generally recognized that isolation of interconvertible isomers requires a barrier near 20 kcal/mol at room temperature.15,16 The first ab initio computations of the R = H inversion barrier gave similar results.17 A somewhat later ab initio study by Nguyen and Ha in 1983 [Hartree−Fock 4-31G(d,p) and 6-31G(d,p)] reported the first indication of nonlinearity of the N−C−N bond angle,12 as do all subsequent computational studies when larger basis sets and electron correlation effects are included. The rotation and inversion barriers were reported as being nearly equal and quite small (∼4 kcal/mol). Related INDO studies of dimethylcarbodiimide in 19697 and 19718 by Williams and Damrauer addressed these issues in a more chemically relevant way, given the importance of dialkylcarbodiimides in synthesis and the fact that the parent R = H is a very minor10 contributor to the cyanamide (N CNH2) (>99%) ⇔ carbodiimide (HNCNH) (
