NMR ³J(C₁,H₃) couplings in 1‐X‐bicyclo[1.1.1]pentanes. FPT–DFT and NBO studies of hyperconjugative interactions and heavy atom substituent effects

Abstract

ABSTRACT:In this work a rationalization of the very large substituent effects on 3 J(C 1 ,H 3 ) couplings in 1-X-bicyclo[1.1.1]pentanes is presented. The Fermi contact contribution to such couplings was calculated in a series of 13 X-derivatives within the DFT-B3LYP framework using the finite perturbation theory. Core electrons for atoms beyond the Second Row were taken into account using effective core potentials. Calculated couplings are in very good agreement with experimental values. The role played by hyperconjugative interactions involving bonds or antibonds belonging to the coupling pathway are studied using the NBO approach. Heavy atom contribution to substituent effects on 3 J(C 1 ,H 3 ) couplings was estimated as small. This contrasts notably with trends observed in the corresponding 13 C substituent chemical shifts, SCSs. The latter were estimated comparing for X = Cl, Br, I, SnMe 3 , calculated SCSs with their experimental values. Such estimations are in line with explicit calculations of the spin-orbit contribution reported in the literature for smaller compounds.

Experimental and DFT studies on the transmission mechanisms of analogous NMR JCH and JCC couplings in 1‐X‐ and 1‐X‐3‐methylbicyclo[1.1.1]‐pentanes

“…It is observed that compounds of series (2) transmit these longrange 'trans-cage' SSCCs notably more efficiently than the analogous member of series (1) transmits the corresponding Table 1. Comparison between experimental 1 J C 3 H 3 in (1) and 1 H / 13 C 1 J C 3 C Me SSCCs in (2) (in Hz) and substituent effects 3 J C 1 H 3 X / 3 J C 1 H 3 H in (1) and 3 Table 1, substituent effects are smaller in (2) than in (1). This is equivalent to saying that SSCCs in (2) are basically more efficiently transmitted than in (1), but such transmissions in (2) are notably less affected by substituent effects than in ( 1).…”

“…This role can be identified with the role played by delocalization interactions in transmitting the spin information associated with the FC interaction corresponding to long-range SSCCs in saturated compounds. In this way, it is expected that any hyperconjugative interaction along the coupling pathway (Scheme 2) should increase the efficiency for transmitting, for instance, 3 J C 1 H 3 in (1) and 3 J C 1 C Me in ( 2). Of course, there are three similar coupling pathways for each compound.…”

“…This simple qualitative model 5 is based on the de convolution of the FC term of 1 J CH SSCCs as a sum of contributions from localized molecular orbitals, LMOs, based on the CLOPPA method: 17 -20 contributions from localized orbitals within the polarization propagator approach. For the special case of a 1 J CH SSCC, where C is assumed to be an sp 3 hybridized carbon atom, it is given by Eqn (1)…”

“…1 -6 For instance, dramatically large substituent effects on 3 J C 1 H 3 SSCCs were reported that approximately correlate with the electronegativity of X, although the halogen atoms depart notably from that trend. 2 Hereafter, substituent effects, SE-SSCCs, on 3 J C 1 H 3 SSCCs in (1) are defined as the ratio between the SSCC for a given X substituent and the respective coupling in the parent compound, i.e. X D H. Even though using this definition departs somewhat from the common usage, it presents the advantage of not requiring the use of reduced coupling constants when comparing SSCCs involving different isotopic species.…”

“…SE-SSCCs reported 1 for 3 J C 1 H 3 in series (1) range from 1 to 3.38. 1,3 It is worth mentioning that such unusual SSCCs are by far dominated by the Fermi contact (FC) term. On the other hand, the measured C 1 -SCSs are notably smaller than those observed for the same substituents attached to any other substrate.…”