Maxwell Bullpitt

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.

Nuclear magnetic resonance and 203Hg exchange studies of allylmercury systems: A comment on allylmercuric perchlorate

Abstract Nuclear magnetic resonance data for allyl-, β-methallyl-, crotyl- and cinnamylmercuric halides and acetates are presented and the dramatic effect of added mercuric salts on some of these spectra is discussed. The exchange of mercury between allyl- and β-methallylmercuric halides (chloride or bromide) and the appropriate mercury-(II) halide, utilising 203Hg as a tracer, is very rapid (statistical exchange in less than a minute at 20°) and appears to be one of the fastest organic-inorganic HgII exchanges reported. Mechanisms of the SEi′ variety are suggested, and the significance of the exchange in relation to the effect of HgII halides on the PMR spectra of allylmercury systems is pointed out. The preparation and some properties of a compound argued to be allylmercuric perchlorate are presented.

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.

Thiocyanogen cleavage of carbon-tin bonds

A number of tetraorganotin compounds of the types (CH3)3SnR and, to a lesser extent, (C6H5)3SnR, where R is a group normally readily cleaved by electrophilic agents, have been demonstrated to react readily with thiocyanogen (SCN)2, in benzene or carbon tetrachloride solution. When R cis- or trans-2-butenyl(crotyl), 2-propynyl or 1,2-propadienyl (allenyl), the rearranged α-methallyl, allenyl or 2-propynyl thiocyanates, respectively, are produced. The reactions are quantitative as judged by direct PMR observations. When R possesses only moderate reactivity towards electrophiles, e.g., CH3 as in (CH3)4Sn, polymerisation of (SCN)2 is more rapid than cleavage. The stereochemical path in the cases of cis- and trans-styryltrimethylstannanes is shown to be retention at the vinylic carbon.

Metal hydride reduction of some σ- and π-allyl and benzyl derivatives of mercury and palladium

The reductionof 2-butenyl-mercuric (crotyl-mercuric) systems, RHgX where X = Cl, Br, OCOCH3, by sodium borohydride and trimethyltin hydride has been investigated and under all conditions examined, 1-butene is the sole organic product. Crotylmercuric hydride seems likely to form and then to undergo either intramolecular transfer of hydrogen from mercury to carbon in an SNi′ manner, or SN2′ displacement of mercury by some hydride species leading to 1-butene, or reactions producing crotyl-Hg1 or crotyl-Hg- which are cleaved by water or methanol to again yield 1-butene. π-Crotylpalladium chloride on reduction yields only trans-2-butene, but in the presence of increasing concentratins of triphenylphosphine, which promotes the π ⇄ σ equilibrium of allylpalladium chloride systems, substantial amounts of 1-butene are formed. In aqueous tetrahydrofuran, sodium borohydride converts venzylmercuric chloride cleanly to divenzylmercury. Reduction by tetramethylammonium borohydride in benzene, yields toluene (70%) and dibenzylmercury(30%) whereas toluene was the only product from trimethyltin hydride reduction.

Trifluoroacetolysis of benzyltrimethylstannanes

Abstract The reactions of a series of m - and p -substituted benzyltrimethylstannanes with excess trifluoroacetic acid in benzene solvent have been studied. While the rate of disappearance of stannane is strictly first-order, both benzyl—tin and methyl—tin cleavages were occurring for the parent compound, as well as for substituted compounds, m - and p -Cl, m - and p -CF 3 , p -CH 3 and p -OCH 3 . From product analyses, the appropriate k -values could be factored from the overall pseudo first-order rate constant. For m -CH 3 and m -OCH 3 , the rates were abnormally fast (for m -OCH 3 essentially instantaneous) and CH 3 Sn cleavage was not detected, indicating a change in mechanism. Evidence is presented that in these two cases, the strong concerted activating effects of  OCH 3 (and CH 3 ) and  CH 2 Sn(CH 3 ) 3 , when m -disposed, promotes attack at the ring. For the other substituents, relative rates are correlated satisfactorily with σ n parameters, and there seems to be little resonance interaction with the ring, in the transition state.