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Relevant academic research and scientific papers

HOMOLYTIC SUBSTITUTION IN TRIALKYLTIN IODIDES BY PHOTOCHEMICALLY GENERATED IODINE ATOMS. V. Mixed Dimethylalkyltin Iodides, Me2R'SnI.

Mixed trialkyltin iodides form EDA complexes with iodine.The λCT data suggest an electron transfer from the iodide lone pairs.Rate constants for the reaction under photochemical conditions are determined by kinetic measurements.The selectivity parameter derived from the rate constants and from mass spectrometric measurements prove the presence of a fragmentation step in the reaction process.

Charge-Transfer Mechanism for Electrophilic Reactions. SE2 Cleavage of Alkyl Metals with Iodine

The absorption spectra of transient charge transfer (CT) complexes are observed immideately upon mixing iodine and various organometals RM, where M = tin, lead, and mercury.The formation constants KCT and transition energies hνCT of these CT complexes vary with the ionization potentials and the steric properties of the alkylmetals.The subsequent disappearance of the CT absorption band is accompanied by the cleavage of the alkylmetal by iodine (iodinolysis).The kinetics of the iodine disappearance are consistent with a preequilibrium formation of the CT complex followed by the rate-limiting iodinolysis of the alkylmetal.The selectivity in the iodinolysis of unsymmetrical tetraalkyltin compounds is determined by product analysis and shown to be strongly dependent on the solvent polarity.The solvent effect is also shown to affect the formation constant of the CT complex and the rate constant for iodinolysis in a parallel manner.A charge-transfer mechanism is proposed for iodinolysis in which the rate-limiting step involves the unimolecular decomposition of the CT complex by electron transfer from the alkylmetal donor to the iodine moiety to form the ion pair + I2->.This activation process is akin to the charge-transfer interaction, as formulated in the Milliken theory.Accordingly, the difference ΔE in the CT transition energy hνCT of a complex relative to that of a reference alkylmetal (either Me4Sn or Me2Hg) is used to evaluate the interaction energy of the ion pair.The change in the overall driving force ΔGr for electron transfer in the CT complex is determined from ΔE and the ionization potential of the alkylmetal.The activation free energy Δgr for the electron transfer is developed from the rate data by a similar comparative procedure, and shown to respond directly to the free-energy change, i.e., ΔGr=ΔGr.This linear free energy relationship, together with a pronounced macroscopic solvent effect on ΔGr based on Kirkwood's equation, supports a highly polar transition state for iodinolysis in accord with Scheme II.The same CT formulation can be quantitatively applied to the solvent effect on the relationship between the selectivity and the rate constants for iodinolysis in Figure 8, as well as the relationship between the selectivity and the formation constant of the CT complexes in Figure 9.It correctly predicts the inverse relationship often observed between selectivity and rate.Importantly, the charge-transfer formulation provides a quantitative foundation for the description of electrophilic processes, heretofore provided only in qualitative forms.

Electron Donor-Acceptor Complexes. 1. Linear Free Energy Correletion of the Charge-Transfer Transition Energy with the Kinetics of Halogenolysis of Alkylmetals

Mulliken charge-transfer theory is used to relate the properties of transient donor-acceptor complexes between alkylmetals and halogens with the kinetics of the accompanying cleavage reaction (halogenolysis).The formulation derives from the charge-transfer transition energy hνCT which is proportional to the second-order rate constant for halogenolysis of a variety of tetraalkyltin compounds in hexane or carbon tetrachloride solutions.The description of the activation process for halogenolysis as an electron transfer in the CT complex, e.g., --> , leads to a linear free energy relationship in which the activation free energy is equal to the driving force for ion pair formation.The latter is equated to the charge-transfer transition energy plus a contribution from the solvation energy, by employing a comparative procedure for the evaluation of alkylmetals.An independent measure of the solvation energy obtained from the gas-phase ionization potentials of alkylmetals and their free energy changes in solution supports the electron-transfer formulation of the activation process.The charge-transfer mechanism is generalized for the halogenolysis of alkylmetals.

Mechanisms of the rupture of the carbon-tin bond by halogens I. Electrophilic substitution at a saturated carbon atom

The reaction between halogens and tetraalkyltins is strongly influeced by the dielectric constant, the polarisability and the nucleophilicity of the solvent. These three aspects of solvent action are relatd to the ability of tin to make use of its empty 5d orbitals: the Sn polarisation, enhanced by the pentaco-ordination of the metal, governs the reactivity of the alkyl groups attached to it, appropriate attention being given to the incursion of steric effects. In polar media, the solvent itself acts as the nucleophilic catalyst and the reaction is best described by the following scheme {A figure is presented}. In less active solvents, the halogen molecule plays the role of nucleophile in a predetermining step the most likely mechanism may be written as follows: {A figure is presented}.

Mechanisms of the rupture of the carbon-tin bond by halogens II. Free-radical substitution in solution

The experiments describes in this paper show that the light-induced bromodemetallation of tetraalkyltins in chlorobenzene is a free radical substitution on tin, followed by a propagation step: {A figure is presented}. The stabilisation of R? by hyperconjugation seems to be an important factor for the reaction, but there is evidence for the influence of the other three substituents of tin and of the nature of the attacking radical on the reaction mechanism. The relation between the strucutre and the reactivity of tetraalkyltins suggests that the carbon-tin bond is only slightly loosened in the transition state; this agrees with the great reactivity of the Br atom.

Mechanisms of the rupture of the carbon-tin bond by halogens I. Electrophilic substitution at a saturated carbon atom

In chlorobenzene, some tetraalkyltins are able to alter the reactivity of bromine towards their homologues and this is no more true in solvents of high nucleophilicity. This perturbating power, which is linked to the nature of the substituents bound to the metal, is best explained by the formation of a complex R4Sn(-)-X(+)2 which acts as a modified halogen towards another alkyltin molecule. This mixing-effect changes sometimes considerably the kinetics and the mechanism of the reactions and suggests that the accessibility of empty 5d orbitals of the metal plays an important part in the reactivity of tetraalkyltins, as well in SE2 as in radical substitutions.