Proton deshielding by spatial proximity: An experimental and theoretical study for ortho‐substituted anisoles

Abstract

A quantitative estimation of the proton deshielding effect produced by a steridy crowded environment was obtained experimentally, based on departures from the chemical shift additivity rule of protons ortho to OCH, and NOz groups in benzene derivatives, where a conformational change from these side-chain groups is expected. The intluences of these conformational changes were also analysed from a theoretical point of view. Calculations of the magnetic shielding tensor were carried out at the INDO approximation with gauge invariant atomic orbitals, together with the uncoupled Hartree-Fock approach. Theoretical results for these proton deshielding effects are in very good agreement with those determined experimentally.

Tuning the Reactivity of Peroxo Anhydrides for Aromatic C–H Bond Oxidation

“…Preferential formation of 3-methoxy- o -cresol ( 6l2 ) over 2-methoxy- m -cresol ( 6l3 ) could be explained by the steric hindrance between the methoxy moiety and adjacent methyl group, which defines the methoxy group conformation to be cis relative to H-6 (Scheme , 5l - cis ). This conformation suffers from steric crowding around the position 6 of the ring and substantially reduces the approach of P4 to this position. , …”

“…This conformation suffers from steric crowding around the position 6 of the ring and substantially reduces the approach of P4 to this position. 45,46 Hydroxylation of methoxybiphenyls 5m and 5n occurred only at the activated rings and left the unsubstituted phenyl moiety untouched, further implying that donor substituents are essential to promote the reaction. Acetamides generally yield the para-substituted products in EAS reactions.…”

The use of inner projections of the polarization propagator to study different contributions to the magnetic shielding tensor. I. Proton deshielding by steric compression

“…On the other hand, there may be some electronic contributions which are not so difficult to calculate within the molecular orbital theory. In fact, we have recently reported this for the stereospecific substituent-induced chemical shift of the ortho proton to the methoxy group in substituted anisoles [ 5 ] .…”

“…The properties of the HF ground state guarantee that i l depends only on the virtual orbitals. The second-order energy obtained variationally from (26) is identical to that of the CHF scheme [5,18]. Written in matrix form it is The plus or minus sign corresponds to HI being real or pure imaginary, respectively.…”

“…The second-order energy obtained variationally from (26) is identical to that of the CHF scheme [5,18]. Written in matrix form it is where 10) is the HF ground state, ll is the identity matrix, and C is a column vector with the c,, coefficients and is the solution of the first-order equation The plus or minus sign corresponds to HI being real or pure imaginary, respectively.…”