Investigations of Substituent Effects by Nuclear Magnetic Resonance Spectroscopy and All-valence Electron Molecular Orbital Calculations. II. 4-Substituted α-Methylstyrenes and α-t-Butylstyrenes

Abstract

Substituent-induced 'H chemical shifts (S.C.S.) for 19 4-substituted a-methyl-and a-t-butylstyrenes have been determined a t infinite dilution in C6Hl2 and 13C S.C.S. have been determined for 0.4 M solutions in CCI,. S.C.S. are correlated with field and resonance substituent parameters and compared with charge densities determined by C N D 0 / 2 MO calculations. The variation of S.C.S. with the dihedral angle, p, between phenyl and vinyl groups and the overall pattern of S.C.S. can be largely accounted for by a model of substituent effects based on field, resonance, and n polarization effects, with conjugative interactions varying as cos2p. Both I3C chemical shifts and charge densities indicate that the n polarization effect consists of two components: (I) a through-space polarization of the vinyl system by the polar C-X bond and (2) polarization of the entire conjugated styrene n electron system. However, significant deviations are noted for some of the 'H S.C.S. correlations. The CND012 calculations indicate that these deviations are primarily due to electronic effects not predicted by the nlodel outlined above. CNDO/2 calculations for related compounds provide a partial explanation by indicating that the magnitude of the field effect depends upon the nature of the molecular framework. Les deplacelnents chimiques en 'H induits par des substituants (S.C.S.) pour 19 a-niethyl et a-tbutylstyrenes substitues en position 4 out Cte determines a dilution infinie dans le C,HIz; ceux-ci en 13C ont Cte determines en solution 0.4 M dans le CC14. Les diplacements chimiques induits par des substituants sont relies aux parametres de champ et de resonance caracteristiques des substituants et compares avec les densites de charge obtenues par des calculs CNDO/2 MO. La variation dans les deplacements chimiques avec I'angle diedre, p, entre les groupes phenyle et vinyl ainsi que le schema general de ces dkplacements chimiques peuvent tres bien s'interpreter a parlir d'un modele des effets de substituants base sur le champ, la risonance et des effets de polarisation n avec des interactions de conjugaison variant selon le cosZp. Les diplacements chimiques en I3C et les densites de charge indiquent tous les deux, que I'effet de polarisation n possede deux composantes: (I) une polarisation a travers l'espace d u systeme vinylique par la liaison polaire C-X et (2) une polarisation du spsteme d'klectrons n du styrene conjugue en entier. Cependant, des Ccarts notables ont Cte observes pour quelques-unes des correlations dans les dtplacements chimiques en 'H. Les calculs CNDO/2 montrent que ces tcarts sont imputables principalement a des effets electroniques imprevisibles selon le n~odele illustri: precedemment. Les calculs CND012 pour des composCs voisins permettent Line explication partielle en montrant que I'amplitude des effets de champ depend de la nature du squelette molCculaire.[Traduit par le journal]Can. J. Chem.. 51. 915 (1973)

Substituent-induced chemical shifts in 3- and 4-substituted styrenes: factor analysis of the styrene data matrix

“…Most new 4B derivatives were synthesized by reaction of the corresponding 4-substituted acetophenone with tert-butylmagncsiun~ chloride, followed by aqueous acidic work-up to yield the appropriate 2-(4-X-phcnyl)-3,3-dimethyl-2-butanol. Catalytic dehydration over activated alumina (2) then yielded the appropriate 4-substituted a-tert-butylstyrcnc (X = SCH,, COCH,, CF,, CN). The NH, derivative was synthesized from thc NO, derivative ( I ) by reduction with Sn/HCI (31).…”

“…The NH, derivative was synthesized from thc NO, derivative ( I ) by reduction with Sn/HCI (31). The Si(CH,), and C0,CH3 derivatives were synthesized from the corrcsponding C1 derivative via Grignard routes (2). All 4B derivatives were produced in low yield at 80-90% purity (typically contaminated with starting acetophenonc) and were used without furthcr purification.…”

“…'H chemical shifts for other derivatives were omitted since these are very sensitive to variations in ring current effects originating in the phenyI group. These variations are due to minor substituentinduced changes in vinyl group conformation (2). The 15 common substituents listed in the Introduction were used, yielding an 18 X 15 data matrix.…”

“…For AS, the break occurs at two factors, again corresponding to two substituent parameters (see eq. [2]). The only difference is that RSD is slightly larger (ca.…”

“…Our two main conclusions were: (a) that a dual substituent parameter (dsp) treatment was necessary to adequately account for substituent-~nduced long-range chemical shifts in styrenes, and (b) that the actual chemical shifts reflected a combination of resonance, field, and .rr inductive (polarization) effects (1, 2). The 4H, 4M, and 4B derivatives were deal for this purpose since the increasing size of the alkyl groups progressively decreased pheny I-vinyl conjugation, thus altering the ratio of resonance to fieldlinductive effects in a qualitatively predictable manner (2).…”