Absolute rate constants for some reactions of the triethylamine-boryl radical and the borane radical anion

Laser flash photolysis (LFP) of di-tert-butyl peroxide or dicumyl peroxide at ambient temperatures in the presence of Et3N\u2192BH3 or BH4- generated the title radicals which were found to have broad, featureless absorptions in the visible region. Rate constants for H-atom abstraction from Et3N\u2192BH3 by cumyloxyl radicals show a small solvent dependence, e.g. 12 7 107 and 2.2 7 107 dm3 mol-1 s-1 in isooctane and acetonitrile, respectively. Rate constants for halogen atom abstraction by Et3N\u2192BH2\ub7 and BH3\ub7- from a number of chlorides and bromides were determined by LFP and by competitive kinetics, e.g., for Et3N\u2192BH2\ub7 + CCl4/PhCH2Cl/CH3 (CH2)2Cl, k = 4.4 7 109/1.1 7 107/5.1 7 105 dm3 mol-1 s-1 and for BH3\ub7- + CCl4/PhCH2Cl, k = 2.0 7 109/3.0 7 107 dm3 mol-1 s-1. Rates of addition of Et3N\u2192BH2\ub7 to 1- and 1,1-substituted olefins increase dramatically as the electron affinity of the olefin increases, confirming the nucleophilic character of amine-boryl radicals. A comparison of the present results with literature data for the addition of olefins of four nucleophilic carbon-centered radicals proves that Et3N\u2192BH2\ub7 is by far the most nucleophilic radical for which kinetic data are available. A few rate constants for abstraction of hydrogen from electron-deficient carbon by Et3N\u2192BH2\ub7 are also reported.Peer reviewed: YesNRC publication: Ye

Absolute rate constants of alkene addition reactions of a fluorinated radical in water

Absolute rate constants of \ub7RfSO3 -radical addition to a series of water-soluble alkenes containing ionic, carboxylate substituents were measured by laser flash photolysis experiments in water. The observed rate constants were all considerably larger than those of structurally similar analogues in a nonpolar organic solvent, with rate factors of 3-9-fold being observed. It is concluded that such rate enhancements derive at least in part from stabilization of the polar transition state for addition of the electrophilic fluorinated radical to alkenes by the polar solvent water.Peer reviewed: YesNRC publication: Ye

Thermal decomposition of <i>O</i>-benzyl ketoximes; role of reverse radical disproportionation

Thermolyses of seven dialkyl, two alkyl-aryl and two diaryl O-benzyl ketoxime ethers, (RRC)-R-1-C-2=NOCH2Ph, have been examined in three hydrogen donor solvents: tetralin, 9, 10-dihydrophenanthrene, and 9, 10-dihydroanthracene. All the oxime ethers gave the products expected from homolytic scission of both the O-C bond (viz., (RRC)-R-1-C-2=NOH and PhCH3) and N-O bond (viz., (RR2)-R-1 C=NH and PhCH2OH). The yields of these products depended on which solvent was used and the rates of decomposition of the O-benzyl oxime ethers were greater in 9, 10-dihydrophenanthrene and 9, 10-dihydroanthracene than in tetralin. These results indicated that a reverse radical disproportionation reaction in which a hydrogen atom was transferred from the solvent to the oxime ether, followed by beta-scission of the resultant aminoalkyl radical, must be important in the latter two solvents. Benzaldehyde was found to be an additional product from thermolyses conducted in tetralin. This, and other evidence, indicated that another induced decomposition mode involving abstraction of a benzylic hydrogen atom, followed by beta-scission of the resulting benzyl radical, became important for some substrates. Participation by minor amounts of enamine tautomers of the oxime ethers was shown to be negligible by comparison of thermolysis data for the O-benzyloxime of bicyclo[3.3.1]nonan-9-one, which cannot give an enamine tautomer, with that of the O-benzyloxime of cyclohexanone.</p

Radical deoxygenation of xanthates and related functional groups with new minimalist N-heterocyclic carbene boranes

Minimalist N-heterocyclic carbene boranes 1,3-dimethylimidazol-2- ylideneborane and 2,4-dimethyl-1,2,4-triazol-3-ylideneborane are readily available and have low molecular weights. They exhibit superior performance to first-generation NHC-boranes, providing improved yields in reductions of xanthates and related functional groups. © 2010 American Chemical Society

Estimated rate constants for hydrogen abstraction from n-heterocyclic carbene-borane complexes by an alkyl radical

Rate constants for hydrogen abstraction by a nonyl radical from 20 complexes of N-heterocyclic carbenes and boranes (NHC-boranes) have been determined by the pyridine-2-thioneoxycarbonyl (PTOC) competition kinetic method at a single concentration point. The rate constants range from <1 × 104 to 8 × 104 M-1 s-1. They show little dependence on the electronic properties of the carbene core, but there is a trend for increasing rate constants with decreasing size of the carbene N-substituents. Two promising new reagents with small N-substituents (R = Me) have been identified. © 2010 American Chemical Society

Polarity reversal catalysis in radical reductions of halides by N-heterocyclic carbene boranes

Otherwise sluggish or completely ineffective radical reductions of alkyl and aryl halides by N-heterocyclic carbene boranes (NHC-boranes) are catalyzed by thiols. Reductions and reductive cyclizations with readily available 1,3-dimethylimidazol-2-ylidene borane and a water-soluble triazole relative are catalyzed by thiophenol and tert-dodecanethiol [C 9H 19C(CH 3) 2SH]. Rate constants for reaction of the phenylthiyl (PhS•) radical with two NHC-boranes have been measured to be ∼10 8 M -1 s -1 by laser flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis. © 2012 American Chemical Society