Time-resolved studies of the title reactions have been carried out over the pressure range 1−100 Torr (in SF6 bath gas) and at temperatures in the range 293−600 K, using laser flash photolysis techniques to generate and monitor the silylenes, SiH2 and SiMe2. All three reactions showed evidence of pressure dependence, consistent with third-body assisted association reactions to form silirane products. Extrapolation of the pressure-dependent rate constants gave the following Arrhenius parameters: SiH2 + C3H6, log(A/cm3 molecule-1 s-1) = −9.79 ± 0.03, Ea (kJ mol-1) = −1.9 ± 0.3; SiH2 + C4H8, log(A/cm3 molecule-1 s-1) = −9.91 ± 0.04, Ea (kJ mol-1) = −2.5 ± 0.3; SiMe2 + C4H8, log(A/cm3 molecule-1 s-1) = −12.12 ± 0.02, Ea(kJ mol-1) = −8.5 ± 0.2. These parameters are consistent with fast, nearly collision-controlled processes for SiH2 but a tighter transition state for SiMe2. Rice, Ramsperger, Kassel, Marcus theory (RRKM) modeling, based on consistent transition states for silirane decomposition, and employing a weak collisional deactivation model, gave good fits to the pressure-dependent curves for each system, provided an appropriate value of Eo (fitting parameter) was used for each reaction. The kinetic results are consistent with an electrophilically led addition mechanism, although methyl substitution in the alkene hardly affects the rate constants. The RRKMderived Eo values have been used to derive reaction enthalpies which are in reasonable agreement with values obtained by ab initio calculations at the G2 (MP2,SVP) level. The experimental ΔH° values yield strain energies of 190, 196, and 216 kJ mol-1 for 2-methyl-, 2,2-dimethyl-, and 1,1-dimethylsilirane, respectively. Compared to the strain enthalpy of 167 kJ mol-1 for silirane itself, this shows that methyl substituents in the silirane products substantially increase the strain energies. Theory supports this. Disciplines Chemistry CommentsReprinted (adapted) M. S. Gordon Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011-3111ReceiVed: April 22, 1998; In Final Form: June 16, 1998 Time-resolved studies of the title reactions have been carried out over the pressure range 1-100 Torr (in SF 6 bath gas) and at temperatures in the range 293-600 K, using laser flash photolysis techniques to generate and monitor the silylenes, SiH 2 and SiMe 2 . All three reactions showed evidence of pressure dependence, consistent with third-body assisted association reactions to form silirane products. Extrapolation of the pressuredependent rate constants gave the following Arrhenius parameters: SiH 2 + C 3 H 6 , log(A/cm 3 molecule -1 s -1 ) ) -9.79 ( 0.03, E a (kJ mol -1 ) ) -1.9 ( 0.3; SiH 2 + C 4 H 8 , log(A/cm 3 molecule -1 s -1 ) ) -9.91 ( 0.04, E a (kJ mol -1 ) ) -2.5 ( 0.3; SiMe 2 + C 4 H 8 , log(A/cm 3 molecule -1 s -1 ) ) -12.12 ( 0.02, E a (kJ mol -1 ) ) -8.5 ( 0.2. These parameters are consistent with fast, nearly collision-controlled processes for SiH 2 but a tighter transition state for SiMe 2 . Rice, Ramsperger, Kassel, Marcus theory (RRKM) m...

Time resolved studies of silylene, SiH2, generated by the 193 nm laser flash photolysis of phenylsilane, have been carried out to obtain rate coefficients for its bimolecular reactions with methyl-, dimethyl- and trimethyl-silanes in the gas phase. The reactions were studied over the pressure range 3-100 Torr with SF6 as bath gas and at five temperatures in the range 300-625 K. Only slight pressure dependences were found for SiH2+MeSiH3(485 and 602 K) and for SiH2+Me2SiH2(600 K). The high pressure rate constants gave the following Arrhenius parameters: [TABLE: SEE TEXT]. These are consistent with fast, near to collision-controlled, association processes. RRKM modelling calculations are consistent with the observed pressure dependences (and also the lack of them for SiH2+Me3SiH). Ab initio calculations at both second order perturbation theory (MP2) and coupled cluster (CCSD(T)) levels, showed the presence of weakly-bound complexes along the reaction pathways. In the case of SiH2+MeSiH3 two complexes, with different geometries, were obtained consistent with earlier studies of SiH2+SiH4. These complexes were stabilised by methyl substitution in the substrate silane, but all had exceedingly low barriers to rearrangement to product disilanes. Although methyl groups in the substrate silane enhance the intrinsic SiH2 insertion rates, it is doubtful whether the intermediate complexes have a significant effect on the kinetics. A further calculation on the reaction MeSiH+SiH4 shows that the methyl substitution in the silylene should have a much more significant kinetic effect (as observed in other studies).

Four 4-n-propyl-, one 4-n-pentyl-, and four 4-phenyl-coumarins have been isolated from the bark of Marnrnea africana G. Don and identified. All are derived from 5.7-dioxygenated coumarins, and eight of the structures contain an isoprene unit and an acyl side chain a t either C-6 or C-8. In four cases the isoprene unit exists as a 3-methylbut-2-enyl side chain but otherwise it forms part of a chromen or benzofuran system. The metabolites are 5,7-dihydroxy-6-(3-methylbut-2-enyl) -8-(2-methyl butyryl) -4-n-propylcoumarin (1 ) (mammea B/B B) 5.7-dihydroxy-8-(3-methyl but-2-enyl) -6-(3-methylbutyryl) -4-phenylcoumarin (5) (mammeisin, mammea A/AA) 5.7-dihydroxy-8-(3-methylbut-2-enyl) -6-(2-rnethylbutyryl) -4-phenylcoumarin (7) (mammea A/AB, MAB 1 ), 5,7-dihydroxy-8-(3-methylbut-2-enyl)-6-(2-rnethylbutyryl)-4-n-propylcoumarin (8) (MA5 2), 4',5'-dihydro-5-hydroxy-5'-(1 -hydroxy-1 -methylethyl) -6-(2-methylbutyryl) -4-phenylfuro[2',3' :7,8]coumarin (9) (MAB 3), 4',5'-dihydro-5-hydroxy-5'-(1hydroxy-I -methylethyl) -6-(2-methylbutyryl) -4-n-propylfuro [2',3' : 7.81 coumarin (1 0) (MAB 4). 5-hydroxy-6',6'-dimethyl-6-(2-methylbutyryl)-4-phenylpyrano[2',3':7,8]coumarin(1 1) (MAB 5), 5-hydroxy-6 ',6'-dimethyl-6-(2-methylbutyryl) -4-n-propylpyrano[2',3' : 7.81 coumarin (1 2) (MAB 6) and 5,7-dihydroxy-8-(2-methylbutyryl)-4-n-pentylcoumarin (1 3). The structures of the pyranocoumarins have been confirmed by partial synthesis. The possible biogenetic relationship between the coumarins and xanthones in Guttiferae is discussed. PREVIOUSinvestigations have shown the presence of six coumarins (1)-(6) in the seeds 3a of Mammea africnna G. Don and seven xanthones have been isolated from the h e a r t ~o o d . ~ An investigation of the bark of two M . africana G. Don trees grown in Nigeria has now led to the isolation of three known coumarins [(l), (5), and (7)] (previously isolated from the seeds of M . americalza L.395) together with six new coumarins to which structures (8)-(13) are assigned. Three of

Abstract:The synthesis of a range of phosphine-diamine, phosphine-amino-alcohol, and phosphineamino-amide ligands and their ruthenium(II) complexes are reported. Five of these were characterised by X-ray crystallography. The activity of this collection of catalysts was initially compared for the hydrogenation of two model ester hydrogenations. Turnover frequencies up to 2400 h -1 were observed at 85 o C. The catalysts turnover, albeit slowly at near ambient temperatures. Using a phosphine diamine Ru(II) complex that was identified as the most active catalyst, a range of aromatic esters were reduced in high yield. The hydrogenation of alkene-, diene-, and alkyne functionalised esters was also studied. Substrates with a remote, but reactive terminal alkene substitutent could be reduced chemoselectively in the presence of 4-dimethylaminopyridine (DMAP) co-catalyst. The chemoselective reduction of the ester function in conjugated dienoate ethyl sorbate could deliver (2E,4E)-hexa-2,4-dien-1-ol, a precursor to leaf alcohol. The mono-unsaturated alcohol (E)-hex-4-en-1-ol was produced with reasonable selectivity, but complete chemoselectivity of C=O over the diene is elusive. High chemoselectivity for the reduction of an ester over an alkyne group was observed in the hydrogenation of an alkynoate for the first time. The catalysts were also active in the depolymerisation reduction of samples of waste PET to produce benzene dimethanol. These depolymerisations were found to be poisoned by the ethylene glycol side product, although good yields could still be achieved.

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