Unusual behavior of isopropyl substituients in 9,10‐dihydroanthracenes: An NMR and molecular mechanics study

Abstract

A collection of cis-and trans-9-isopropyl-lO-R-9,lO-dihydroanthracenes were investigated. The cis compounds show large H-9-isopropyl methine coupling constants in the series R = Me, Et, i-Pr, t-Bu (8.8-9.9 Hz) due to a preferred orientation of the isopropyl group(s), caused by a transannular steric effect, wherein the H-!l-isopropyl methine dihedral angle is close to 180". These preferred conformations are predicted by molecular mechanics calculations (MM2 and MMP2) and data for the optimized geometries are presented. The trans compounds show considerably smaller coupling constants ( J = 2.6, 5.0 and 7.8 Hz for R = t-Bu, i-Pr and Et, respectively) and these numbers are easily rationalized from the calculated geometries. However, the compound with R = Me shows an unusually high value (9.6 Hz), and calculations suggested the presence of a structure with unusually large puckering of the dihydroanthracene central ring. This was confirmed by nuclear Overhauser enhancements and by proton and carbon relaxation measurements. KEY WORDS Hindered rotation 'H and 3C relaxation measurements Molecular mechanics calculations NOE experimentsA considerable amount of effort including x-ray crystallograph y,'-' ' NMR'Z-'6 and theoretical calculations'7-z0 have been directed towards an understanding of the conformational properties of 9,lO-dihydroanthracene (DHA) (1) and its derivatives. In the solid state, DHA adopts a boat conformation with a folding angle of 145" (i.e. the angle between the two planes containing the benzene rings).'.'' In solution, however, separate signals for the pseudo-axial and pseudo-equatorial protons at C-9 and C-10 were not observed by NMR, even at low temperatures, and so rapid boat-to-boat interconversion was suggested (i.e. 1 * 1 y 1 R 9 1 I I'This concept was extended to include derivatives with the expectation that the equilibrium should be shifted to allow for the pseudo-axial preference of substituent groups." More recently, molecular mechanics calculations have suggested a fairly wide range of conformations for DHAs, including a boat-shaped central ring for monosubstituted and cis-disubstituted derivatives, and an essentially planar geometry for (symmetrical) trans-disubstituted cases.' 8-20 However, in almost every case, calculations suggest a parabolic potential well * Author to whom correspondence should be addressed. (which may be displaced from 180") and do not include double minima expected for a boat-to-boat interconversion.The unusual behavior of isopropyl substituents in 9,lO-dihydroanthracenes was first noted in 1969 by Zieger et a1.,z2 who investigated the proton NMR spectrum of cis-9,lO-diisopropyl-DHA (2). They observed a large coupling constant (9.5 Hz) between isopropyl methine hydrogen(s) and H-9 (H-lo), and attributed this to restricted movement of the isopropyl groups owing to the transannular steric interaction. Hence, the preferred geometry has the isopropyl methyls bisected by H-9 (H-10) (as illustrated for 2-4). A subsequent investigationz3 of the related 1,Cdimethyl and 1,4-...