Abstract: A study was carried out, with use of the Cambridge Structural Database, to examine the role of CH/~ interaction in the conformation of organic ...
TETRAHEDRON Pergamon
Tetrahedron 55 (I 999) 10047-10056
CH/~ Interaction in the Conformation of Organic Compounds. A Database Study Yoji Umezawa, Sei Tsuboyama,t Hiroki Takahashi,t Jun Uzawat" and Motohiro Nishiot* Instituteof MicrobialChemistry(BIKAKF.N),3-14-23,Kamiosaki,Shinagawa-ku,Tokyo 141, Japan l'The Instituteof Physicaland ChemicalResearch(RIKEN),Wako, Saitama351-01, Japan Received 6 May 1999; accepted 14 June 1999 Abstract: A study was carried out, with use of the CambridgeStructuralDatabase, to examine the role of CH/~
interactionin the conformationof organiccompounds.A numberof short intramolecularCH/~ distanceshave been disclosed in the crystal structure of these compounds.The structure was inspectedto know whether the crystal conformationis a consequenceof the so-calledpackingforces, or the CI-I/~interactionplays a role. The resulthas demonstrated that the CH/g interaction plays an appreciable role in controlling the conformationof organic cumpounds. © 1999ElsevierScienceLtd. All rights reserved.
INTRODUCTION It has been suggested that the CI-I/~ interaction1, 2 plays important roles in supramolecular chemistry,3,4 molecular recognition,5 chiral discrimination,6 self-assembly of complex molecules,7 coordination chemistry8 and protein biochemistry.9 In a previous paper, we reported that the CH/x interaction plays a crucial part as a driving force of the crystal packing.t0 Here, a database study was carded out to understand the significance of the CHh¢ interaction in terms of the conformation of organic compounds.
METHOD The Cambridge Structural Database II (CSD version 515, 181309 entries) was used. The method of exploring XH/n interactions in the crystal structure was reported earlier, t0 Thus, a computer program was written, by use of the CSD software, to find contacts between XH groups and ~ systems. Several kinds of distances (Dpln, Datm, Dlin) and angle (0, o~) parameters were defined to cover every possibility (Fig. 1). Nonbonded atomic contact was sought between a XH and an sp 2 carbon, in compounds beating sixmembered carbon aromatic ring as a part of the structure, with appropriate cut-off values for the distance (/)max) and angle (0, ¢0) parameters. This paper is dedicatedto the memoryof the late Pmfessc¢Sir Derek H. R. Baxton,the fotmdexof the conceptof conformationin organicchemistry.The authors sincerelyregret the immeasurablelossof an outstandingfigurein the worldof science. *
e-mail:dionisio@tim~i-ho.ne.jp
0040-4020/9915 - see front matter © 1999 Elsevier Science Ltd. A l l rights reserved. P/l: S0040-4020(99)00539-6
Y. Umezawa et al. / Tetrahedron 55 (1999) 10047-10056
10048
(a)
~
(b)
:.,...
i,,
~f~--"-~:~NO.l 2 D p ~,f"-~ "~ an
region~3'!~
DmH,
Fig. I. Method for exploring X I ~ contacts. (a) O: centre of the plane. C 1 and C2: nearest and second nearest sp2-carbons, respectively, to H. co: dihedral angle defined by C1OC 2 and HC1C2 planes. & ZHXC 1. Dpln: FF~-plane distance (H/I). Daun: interatomic distance (I-FC1).Dlin: distance between H and line C1C2 (H/J). (b) I: region where H is above the aromatic ring. 2 and 3" regions where H is out of region 1 but may interact with ~-orbitals. The program was run to search for I ~ distance shorter than a cut-off value Dmax in every region: Dpln < Dmax, 0 < 60 °. IO~< 90o for region I;/)fin < Dmax, 0 < 60°, 90*< IcoI< 130" for region 2, and Daun
