William Adcock

Group IVb metalloidal substituent effects studied by carbon-13 nuclear magnetic resonance spectroscopy

Abstract The natural abundance carbon-13 NMR spectra of series of compounds ArX(CH 3 ) 3 (At = phenyl, 1- and 2-naphthyl, 4-biphenyl and 9-anthryl; X = C, Si, Ge, Sn, Pb), have been recorded and essentially completely assigned by specific deuteration, magnitudes of 13 CX couplings and other criteria. For these directly bonded metalloidal systems, the substituent chemical shifts at formally conjugated positions have been considered in terms of mesomeric electron withdrawal, associated with orbitals of π-symmetry on X, and opposing electron donation, resulting from the aryl CX bond polarisation. For silicon the former effect predominates, but for germanium, tin and lead, opposing electron-donating mechanisms assume importance. For 1-naphthyl systems, the effects of molecular distortion on carbon-13 chemical shifts are critical at certain positions. The substituent effects of M(CH 3 ) 3 in aromatic systems are very feeble in comparison with the conventional substituents of organic chemistry.

Metalloidalmethyl substituent effects by 13C and 19F NMR : electron release in the neutral ground state

Abstract 13C and 19F chemical shift studies of a series of CH2M(CH3)3 and CH2M(C6H5)3 (M  Si, Ge, Sn, Pb) - substituted aryl derivatives (phenyl; 1-naphthyl; 2-naphthyl) have established unambiguously that the order of hyperconjugative electron release in the neutral ground state is Pb>Sn>Ge>Si. This order is clearly at variance with the commonly accepted order(Pb>Sn>Ge>Si) based on studies of electron deficient substrates. The phenomenon is discussed in terms of current theories on σ-π interactions. In addition, substituent parameters (σI and σRo) for the PB(CH3)3 group have been derived utilizing new data from the fluorophenyl tag. These new constants are compared with those previously reported.

Arylthallium trifluoroacetates carbon-13 and fluorine-19 nuclear magnetic resonance: The substituent effect of the bis(trifluoroacetato)- thallium(III) group

13C NMR data for a series of arylthallium trifluoroacetates (ArTlX2, X = OCOCF3) are reported and assigned. The range of carbon—thallium couplings to be expected, the dependence on the disposition of coupled nuclei, and chemical shift effects are discussed. The Tl(OCOCF3)2 group is shown to be a powerful electron withdrawing group, from both the 13C data and 19F substituent chemical shifts of the p-fluorophenyl derivative.

DFT–GIAO and DFT–NBO studies of the origin of 19F NMR shielding effects in alkyl fluorides

Electron delocalization into the local environment of the fluorine nucleus in the anti (1) and syn (2) epimers of 7‐norbornenyl fluoride, 7‐fluoronorbornane (3), 7‐fluoronobornadiene (4) and the equatorial and axial epimers of cyclohexyl fluoride (5 and 6, respectively) were studied using the natural bond orbital (NBO) approach. The level of theory, B3LYP/6–311G**, was chosen in such a way that the GIAO‐calculated 19F magnetic shielding constants in all these compounds were in very good agreement with experimental values. The large 19F deshielding effects known experimentally in 1 and 4 with respect to 2 and 3 are rationalized in terms of differences in such electron delocalizations. The main contribution is that corresponding to the π→(C—F)* interaction, which is only efficient for an anti orientation of the C—F bond and the vinyl moiety. 19F and 13C NMR spectra of 5 and 6 were also measured because some ambiguity was detected in the literature. Despite the large calculated charge donation to the fluorine environment in 6, its 19F resonance appears upfield from that in 5. This trend is rationalized as originating in γ effects of the C—Hax bonds of the kind which have been invoked to explain 17O chemical shift trends in different cyclohexanols and their respective ethers. Electron delocalization effects on 1J(C,F) and 2J(C,F) couplings are also discussed. Copyright

Conformational consequences of metallo-methyl interactions: A 13C nuclear magnetic resonance study☆

Abstract A number of model benzyl-organometallic systems have been synthesized and characterised, and their proton-decoupled natural-abundance 13C NMR spectra measured and assigned. A comparison of the 13C chemical shifts of carbons formally para to the carbonmetal (CM) bond strongly supports the importance of hyperconjugative interactions in the neutral ground state for metallomethyl substituents. Further, an analysis of the carbonmetal coupling constants has defined unambiguously the generality and origin (hyperconjugation) of a new concept, namely, large five-bond carbonmetal coupling in benzylic systems. The utilization of 5J(13C117,119Sn) for structural and stereochemical assignments is demonstrated.

Carbon magnetic resonance of organometallic Compounds: Hyperconjugative effects in benzylsilanes

Summary A number of benzylsilanes have been synthesised and characterised, and the structural features of certain of these compounds are such that the C Si bond is constrained to varying degrees to the nodal plane of the aromatic system. The proton-decoupled, natural-abundance 13 C NMR spectra of these compounds have been obtained and essentially completely assigned on the basis of signal intensities, chemical shifts, selective deuteration and (in some cases) the magnitudes of 13 C- 19 F coupling constants. Comparison of 13 C chemical shifts of carbons formally para to the silylmethyl group (CH 2 Si) in the bicyclic silatetralins and -indanes with the corresponding carbon shifts in the carbon analogues, indicates suppression of the electron-donating effect of the CH 2 Si group, when the optimum alignment for interaction of the C Si σ-bond and the π-system is prevented. Effects related to strain and special influences operating on the chemical shifts of carbon bearing fluorine, render interpretation for some compounds less straight-forward.

The substituent effect of the trimethylstannylmethyl group, studied by carbon-13 nuclear magnetic resonance

Abstract The natural abundance carbon-13 NMR spectra of the compounds ArCH2Sn(CH3)3 (Ar = phenyl, 1- and 2-napthyl and 4-biphenyl) have been recorded and essentially completely assigned by specific deuteration, magnitudes of 13C117,119Sn couplings and other criteria. Comparisons have been made with the appropriate neopentyl compounds, and at conjugated positions, substantial shielding is experienced, and Δδ the substituent chemical shift correlates well with Δq (the SCF-πMO change in π-charge density for the change ArCH3 → ArCH2 ) as expected for a conjugative interaction. The present data indicate essentially no conjugative transmission to the 5-position in the 1-naphthyl system, in line with previous reports showing such mesomerism to be trifling. The conclusions that follow from these 13C studies agree with those previously based on 19F investigations. Some comments on 13C11, 119Sn coupling in ArCH2Sn(CH3)3 are recorded.

(Main-Group)-Metallomethyl substituent effects by 19F resonance: Effect of aryl ring size

Abstract The following series of metallomethyl substituted [(C 6 H 5 ) 3 M-CH 2 , where M=Si, Ge, Sn, Pb] fluoroaromatic compounds have been synthesised and characterised.

Transmission of polar substituent effects in the adamantane ring system as monitored by19F NMR

An extensive series of (E)/(Z)‐5‐substituted(X)adamant‐2‐yl fluorides (2 and 3, respectively) and (E)/(Z)‐4‐substituted(X)adamant‐1‐yl fluorides (4 and 5, respectively) were synthesized and characterized and their 19F chemical shifts measured in several solvents. Correlation of the 19F substituent chemical shifts (SCS) against polar field parameters (σF) together with comparisons against the 19F SCS of 4‐substituted(X)bicyclo[2.2.2]oct‐1‐yl fluorides (1) provide unequivocal evidence for the importance of electrostatic field and ‘through‐three‐bond’ electron delocalization (double hyperconjugation) effects as long‐range modes of transmission of polar effects in these saturated systems. The former effect is shown to be a function not only of spatial factors (angles and distance) but also the ‘stiffness’ of the C—F σ bond. The latter electronic mechanism is clearly the dominant factor regulating the 19F SCS of 2 and 4 but is switched ‘off’ in 3 and 5 as a result of specific stereoelectronic requirements. ©1998 John Wiley & Sons, Ltd.

Transmission of polar substituent effects across the bicyclo[1.1.1]pentane ring system as monitored by19F NMR shifts

A series of 3‐substituted(X)bicyclo[1.1.1]pent‐1‐yl fluorides (1) together with a more limited series of 3‐substituted(X)bicyclo[1.1.1]pent‐1‐yl trifluorides (4) were synthesized and their 19F and 13C NMR spectra were recorded. Correlation of the 19F substituent chemical shifts (SCS) of 1 against the 19F SCS of 4‐substituted(X)bicyclo[2.2.2]oct‐1‐yl fluorides (5), together with a linear multiple regression analysis of the SCS against polar substituent parameters, provide evidence for the importance of strong electronegativity effects (σχ) in 1. The shift pattern for 3 is also more consistent with a dominant substituent electronegativity influence than an electrostatic field effect (σF). Most importantly, the signs of the 19F SCS of 1 and 4 are diametrically opposed. The origin of the σχ effect in 1 is ascribed to hyperconjugative and homohyperconjugative interactions governing the degree of electron population of the σ* orbital of the C– F bond. By contrast, the perturbation of the fluorine π‐electron density appears to be the pertinent molecular parameter in 4. The 1J(C,F) values of 1 were not subject to any sensible empirical analysis. Copyright