1118-10-1Relevant articles and documents
Electon-Transfer, Halogen-Metal Exchange and Direct Processes in Formal Nucleophilic Substitutions on Alkyl Halides by Trimethyltinsodium
Smith, Gary F.,Kuivila, Henry G.,Simon, Reyna,Sultan, Leslie
, p. 833 - 839 (2007/10/02)
Formal nucleophilic substitutions have been studied by simple trapping techniques designed to separate and estimate contributions of reactions proceeding by way of free radicals, by way of anions, and by way of geminate or synchronous processes.Reactions of trimethyltinsodium with organic halides in tetrahydrofuran at 0 deg C were examined, using dicyclohexylphosphine for trapping free radicals and tert-butylamine for free anionoids.Among 22 halides included in this study nine have been shown to involve two or all three of the mechanistic pathways.Primary chlorides reacted predominantly by a direct mechanism (SN2, geminate reaction of intermediates, or multicenter process).Branching, as in isobutyl and neopentyl, led to contributions from electron-transfer (free radical) and halogen-metal exchange (anionoid) mechanisms.Secondary bromides reacted predominantly by the ET process (major) and HME (minor) while the relative contributions from these were reversed in importance with the iodides.Triethylcarbinyl chloride reacted exclusively by elimination while the bromide reacted by ET alone or in competition with elimination. 1- and 2-bromoadamantanes reacted by ET, and 1-chloroadamantane showed no reaction after 10 days at -4 deg C.
Donor-Acceptor Complexes of Organometals and Iodine. Alkyl Ligands as Probes for Steric Effects in Charge Transfer
Fukuzumi, S.,Kochi, J. K.
, p. 608 - 616 (2007/10/02)
Charge-transfer (CT) absorptions are observed between iodine and a variety of homoleptic alkylmetals including dialkylmercury (R2Hg) and tetraalkylmetals (R4M) of the group 4A elements (where M = lead, tin, germanium, and silicon) in carbon tetrachloride solutions.These alkylmetal-iodine complexes are all classified as weak, the formation constants, K, being generally less than 5 M-1 for dialkylmercury and less than 3 M-1 for the methylethyllead compounds.The formation constants of tetraalkyltin, -germanium, and -silicon are too small to measure (K -1).The frequency of the charge-transfer bands (hνCT) varies lineary with the vertical ionization potential (ID) of the alkylmetal, determined independently from the photoelectron spectra.However, two separate correlations are required for these alkylmetals-one for the series of two-coordinate, linear dialkylmercury compound and another for the series of four-coordinate, tetrahedral tetraalkylmetals corresponding to a sterically open and a quasi-spherical configuration of electron donors, respectively.Steric effects in these alkylmetal-iodine complexes may be evaluated in two ways.By the direct method, the role of steric effects in determining the charge transfer transition energy is associated with the Coulombic term (e2/rDA) in the first-order treatment of weak complexes according to the Mulliken theory.The mean separation rDAin the CT complexes of R2Hg and R4M, calculated from the measured values of βCT, ID, and the vertical electron affinity of iodine, shows two paralell trends, both increasing with decreasing values of the ionization potentials.This behavior is the same as that evaluated for the CT complexes of tetracyanoethylene (TCNE) with the same series of alkylmetals, in which K is larger and can be measured for as well as complexes.By the indirect method, steric effects are evaluated relative to a reference alkylmetal (Me2Hg for R2Hg and Me4Sn for R4M).The difference ΔE, taken as the relative change in steric effects, is shown to be essentially the same in TCNE and iodine complexes.The later bears on the general question as to whether small (intermediant) values of K (-1) or -ΔH (-1) can be used as adequate criteria for contact charge transfer.
