Conformational effects of through‐bond interaction in N‐aryl‐piperidine derivatives

Abstract

Through-bond interaction in an N-aryl-piperidine derivative carrying an electronegative substituent at C-4 is found to stabilize axial arrangement of the aryl group,which demonstrates the possibility of long-range stereoelectronic conformational control. INTRODUCTIONThe term through-bond interaction (TBI) was introduced by H o m n n e.a. -2 to designate interaction between functional groups a intervening sigma-bonds.Theoretical and spectroscopic have shown that TBI depends strongly not only on the number of intervening sigma-bonds but also in particular on the conformation of the compounds investigated. In the electronic spectra of compounds in which TBI occurs between groups with pronounced electron donor (D) and electron acceptor (A) properties the interaction may manifest68 itself by the presence of an additional absorption band due to an intramolecular transition with strong charge-transfer (CT) character. P-Aminoketones are amongst the earliest examples of systems where this phenomenon was recognized9-l ' and the confornational dependence of their electronic spectra has been instrumental in establishing the theory of TBI. RESULTS AND DISCUSSIONAn extremely strong intramolecular CT transition (hm,= 348 nm, see also Fig. 1) attributable to through-bond donor-acceptor interaction was now observed for compound 1 that contains an anilino electron-donor moiety and a 1,l-dicyano ethylene acceptor group.The strength of this absorption band could be easily mistaken to indicate n-conjugative interaction between the functional groups, whereas these are in fact separated by at least three sigma-bonds ! 6925 M-'.cm-' in n-hexane as a solvent at 2OoC, --Fiaure 1, Electronic absorption spectra (in methyltetrahydrofuran) of 1(-) and 2 (---) at 297 K and of 2 (--) at 175 K.From the theory of TBI it appears that such strong TBI can on!y occur if the phenyl substituent occupies an axial position thus leading to a situation in which the lone-pair orbital on the donor nitrogen is antiparallel to the central C-C bond coupling donor and acceptor. This is exactly the conformation found13 for 1 in the solid state by X-ray analysis (see Fig. 2). Conformational uniformity of 1 in solution is supported by the minor dependence of its UV/vIS absorption on temperature (range studied 298 -175 K). Thus it may safely be assumed that also in solution 1 adopts predominantly a conformation with an axially oriented phenyl group and that this allows for the strong TBI evident from its electronic absorption.data. While an axial orientation of the nitrogen substituent on a tropane skeleton like that of 1 is sterically acceptable14, such an axial orientation seems hardly accessible in a piperidine derivative like 2. Thus the equatorial orientation has been f o u n d ' to be more stable by 2.7 kcal/mol in phenylcyclohexane and this difference tends to be even larger in N-substituted piperidines14. Previous work8-12 has shown, that in such an equatorial conformation little if any TBI will occur. We were quite amazed therefore to find that th...