John C. Cochran

Palladium(0) catalysis in hydrostannation of carbon-carbon triple bonds

Abstract Palladium catalyzed hydrostannation of acetylenes results in a high degree of regioselectivity and stereoselectivity, as well as good overall yields.

On the planarity of styrene and its derivatives: the molecular structures of styrene and (Z)-β-bromostyrene as determined by ab initio calculations and gas-phase electron diffraction

Abstract The molecular structures of styrene and (Z)-β-bromostyrene have been studied in the gas phase at nozzle temperatures of 303 and 338 K respectively. For both molecules the electron diffraction data were consistent with the results from ab initio calculations which described the vinyl torsional motion, near the planar configurations, in terms of a double minimum potential function with barriers of 243 cal mol−1 (styrene) and 430 cal mol−1 (bromostyrene) at the planar form, and with the minimum energy forms 27° (styrene) and 39° (bromostyrene) away. The perpendicular barriers were calculated to 2.73 kcal mol−1 (styrene) and 1.10 kcal mol−1 (bromostyrene). The important distances (ra) and angles (∠α) obtained from least squares refinements of the electron diffraction data are as follows: styrene, r(CH)Av = 1.102(7) A, r( CC ) = 1.355(16) A , r( CC ) Ph = 1.399(2) A , r( CC ) = 1.475(23) A , ∠CCC = 126.9(24)°; and bromostyrene, r(CH)Av = 1.082(13) A, r(CC) = 1.331(20) A, r( CC ) Ph = 1.400(2) A , r(CC) = 1.465(20) A, r( CBr ) = 1.893(8) A , ∠CCC = 132.8(23)°, ∠BrCC = 125.7(15)°, ∠C2C1C7 = 123.9(33).

The Reimer-Tiemann Reaction, Enhanced by Ultrasound

Abstract Significant improvement in yields for Reimer-Tiemann reactions are obtained when the reaction is carried out in the presence of ultrasound.

Alkyl- and acyl-substituted vinylstannanes: Synthesis and reactivity in electrophilic substitution reactions

ABSTRACT Six substituted vinylstannanes have been prepared. (E)- and (Z)-2-trimethylstannyl-2-butene, (1) and (2), respectively, 2-methyl-1-(trimethylstannyl) propene, (3), and 3-methyl-2-trimethylstannyl-2-butene, (4), were prepared by coupling the appropriate lithium or Grignard reagent with chlorotrimethylstannane. 3-Trimethylstannyl-3-butene-2-one, (5), and (Z)-3-trimethylstannyl-3-hexene-2-one, (6), were prepared by palladium(O) catalyzed hydrostannation of the appropriate ynone. This reaction was regiospecific such that the trimethylstannyl and carbonyl groups were bonded at the same vinyl carbon. The reaction was also stereospecific giving syn addition in each case. However, isomerization to a mixture of isomers was observed for the reaction of (5) with Me3SnD and complete isomerization of E-(6) to Z-(6). Each compound was characterized by 1H, 13C, and 119Sn NMR. The reactivity to protodestannylation was determined for each compound by spectrophotometric measurement of second order rate constants. The reactivity of the multimethyl-substituted vinylstannanes was consistent with the reactivity determined previously for monomethyl-substituted vinylstannanes. However, two methyl groups at the remote vinyl carbon exhibited a synergistic activating effect on the protodestannylation reactivity. The acyl group was found to be deactivating for protodestannylation. The stereochemistry of the reaction was found to take place with retention of configuration.

Synthesis of Bis-Stannylacetylenes from Calcium Carbide; Assisted by Ultrasound

Abstract Seven bis-stannylacetylenes have been prepared from the reaction of calcium carbide with a trisubstituted stannyl halide (R3SnX: R[dbnd]methyl, ethyl, n-propyl, n-butyl, phenyl, benzyl, cyclohexyl; X[dbnd]chloride, bromide) in dimethylformamide. The reactivity of the calcium carbide was enhanced using ultrasound generated from a cleaning bath. The products have been characterized by physical constants, elemental analysis, 1H NMR and 13C NMR. When allowed to react for similar time periods, stannyl chlorides and bromides, with identical organic substituents, gave similar yields, suggesting that the reaction does not proceed by an SN2 displacement of halide by carbide. Reactions with triethylgermyl chloride and triethylplumbyl chloride were also observed but the products were not isolated.

THE STRUCTURE OF THE INTERMEDIATE RADICAL IN THE HYDROSTANNATION OF PHENYLACETYLENE

Hydrostannation of phenylacetylene, both thermal and free radical catalyzed, results in a Z/E mixture of β-(trialkylstannyl)styrenes with the Z isomer the kinetic product and the E isomer the thermodynamic product. The stereoselectivity of the kinetic product is a result of the structure of the intermediate vinyl radical. Semi-empirical calculations using the PM3 Hamiltonian indicate the radical species formed in the addition of trisubstituted tin hydride across the triple bond of phenylacetylene leads to an intermediate with an sp-geometry at the radical carbon. The reaction proceeds to form the Z product for a number of reasons. First, there is a small barrier to tin group rotations hindering hydrogen abstraction syn to tin. There is a nearly unhindered reaction plane anti to tin. A trialkyltin hydride is quite sizable and does not readily fit in the reaction plane necessary to form the E product. Finally, the value of the Singly Occupied molecular Orbital (SOMO) is highest where radical attack occurs. The value of the SOMO is highest anti to the tin group in the sp-structures. Experimentally, the kinetic product isomerizes and the thermodynamically more stable E isomer is isolated.

CATALYSIS OF THE DIELS-ALDER REACTION BY ORGANOTIN HALIDES

Organotin halides catalyze the Diels-Alder reaction between isoprene and methyl vinyl ketone. The catalytic activity is a function of the number of proximate tin atoms in the molecule and the extent of halogen substitution on tin.

Kinetics of the Uncatalyzed Hydrostannation of Diethyl Acetylenedicarboxylate

Hydrostannation of carbon–carbon triple bonds is usually catalyzed by a free radical initiator or by metal complexes of palladium, molybdenum or rhodium. However, when the triple bond is substituted with an effective electron‐withdrawing group, the addition reaction will proceed in the absence of a catalyst. In this paper we report the kinetics of hydrostannation of diethyl acetylenedicarboxylate, (1), with trimethylstannane (2a), tri‐n‐propylstannane (2b), tri‐n‐butylstannane (2c), and triphenylstannane, (2d). Rate constants for these reactants were determined in acetonitrile at 25, 35, and 45 °C. Also, rate constants for trimethylstannane and tripropylstannane were determined in cyclohexane and trimethylstannane‐d1 (2e), in acetonitrile. Finally, the hydrostannation with tributylstannane was run at 25, 35, and 45 °C in 95% ethanol. Where appropriate, enthalpies and entropies of activation were determined.

Book Review: Organotin Chemistry

Abstract Alwyn G. Davies. VCH Publishers: Weinheim, Germany, and New York, 1997. 300pp. Figs and tables. ISBN 3-527-29049-4.

Organotin Chemistry (Davies, Alwyn G.)

180.