This is a general revision of the kinetics of iodine addition, w i t h a note on the equilibria in The rate of iodine addition over the concentration range that can be measured these reactions. is given by the expression the first term being chiefly operative in solvents such a s chlorobenzene, carbon tetrachloride, and carbon disulphide, and the second term in isobutyl ether, acetic acid, and nitrobenzene solutions. With certain aromatic compounds such as styrene and its derivatives iodine addition is accompanied by polymensation.AN earlier investigation on the kinetics of iodine addition (Bythell and Robertson, I., 1938, 173) established that these reactions, like the corresponding bromine and iodine chloride additions, showed third-order kinetics, with a small temperature coefficient in acetic acid and nitrobenzene solutions. Erroneous theoretical conclusions, however, were made from the measurements in carbon tetrachloride solution, chiefly as a result of confining the observations to a single compound, ally1 alcohol, the behaviour of which is now found to be anomalous. In the meantime several communications on the kinetics of iodine addition have appeared. Bhattacharyya and Rao ( J . Indian Chem. Soc., 1941, 18, 253), using 2-pentene and phenylacetylene as reactants, found third-order kinetics for iodine addition in acetic acid and ethyl alcohol solutions. Ghosh et al. (ibid., p. 245) state that iodine addition in carbon tetrachloride, carbon disulphide, and benzene solutions proceeds by a fourth-order mechanism. These observers, following Groh and Szelestey (2. anmg. Chem., 1927, 162, 333), who had previously investigated erucic acid in carbon tetrachloride and carbon disulphide and found f ourth-order kinetics, employed an elaborate formula, derived from the integration of the expression, -d[IJ/dt = k,[A][IJ3 -h,[AII][I2]2, and obtaining constant K, coefficients for p-amylene and pinene, concluded that the reaction proceeds, A + I,We, on the other hand, have followed the simpler procedure of adjusting the concentrations with one reactant A in excess, to reduce the effect of the reverse reaction, and have evaluated the reaction order by measurements a t different initial concentrations. To obtain the relative contributions of A and I, to the total reaction order, use was made of the formula : A11 +-21,. t,/t, = (c,/c,)"-* (G'"")":where t , and t, are the times of an equal fractional change of iodine addition, with concentrations cI and c2 for I,, and c' and c" for A, n being the order for I, and n' for A. The proposed formula holds only for the initial stages of the reaction, to -10% iodine addition, over which range, as is found experimentally, the yo addition-time curves are approximately straight lines.Employing this formula, we have established that in polar solvents, e.g., acetic acid, isobutyl ether, chloroform, chlorobenzene, and nitrobenzene, the reaction order for the organic compound (A) is unity ; in the non-polar solvents carbon tetrachloride and carbon disulphide, on the other hand, the v...
The synthesis of 2-substituted furans via palladium-and iron-catalyzed coupling utilizing 2-bromofuran is described. Whereas palladium-catalyzed Suzuki coupling effectively provided the corresponding aryl furans, little or no product was obtained by palladium-catalyzed coupling with various alkyl nucleophiles. Ironcatalyzed coupling proved effective for the synthesis of primary and secondary alkyl furans in modest yields and aryl furans in low yields.Oxabicyclic alkenes are valuable synthetic intermediates because they can serve as a general template to create highly substituted ring systems. 1 For instance, asymmetric ring opening of these alkenes allows the formation of several stereocenters in a single step. 2 We have recently investigated different modes of transition-metal-catalyzed reactions of oxabenzonorbornadiene (1), and found that, depending on the reaction conditions, several products 2-7 could be obtained (Scheme 1).For example, when 7-oxabenzonorbornadiene (1) is treated with an alkyne in the presence of the ruthenium catalyst, [Cp*Ru(cod)Cl], a [2+2] cycloaddition is observed and cyclobutene cycloadduct 2 is formed. 3 When 1 is treated with the secondary propargylic alcohol 8 in the presence of the neutral Ru catalyst, [Cp*Ru(cod)Cl], in methanol or using a cationic Ru catalyst (e.g., [CpRu(MeCN) 3 ]PF 6 ), isochromene 3 is formed. 4 On the other hand, if the same reaction between 1 and the secondary propargylic alcohol 8 is carried out with [Cp*Ru(cod)Cl] in tetrahydrofuran (THF), cyclopropane 4 is produced. 5 More recently, we have observed that in the absence of an alkyne, [Cp*Ru(cod)Cl] catalyzes the isomerization of 1 to the corresponding naphthalene oxide 5 or naphthol 6. 6 We have also reported that asymmetric cationic rhodium(I)-catalyzed cyclodimerization of 1 produces dimers 7 in excellent enantioselectivity (up to 99% ee). 7Because oxabenzonorbornadiene (1) is symmetrical, no regiochemical information could be gained from the above studies. To explore the regioselectivity of the above Scheme 1 Transition-metal-catalyzed reactions of oxabenzonorbornadiene (1)
Reactions. -Aryl 2-furans (III) are prepared by palladium-catalyzed Suzuki coupling reaction, whereas alkyl 2-furans (V) are more efficiently prepared by iron-catalyzed coupling reaction. No coupling product can be obtained with tertiary alkyl or alkenyl Grignard reagents. The iron-catalyzed coupling reaction affords in all cases homocoupled by-products. -(HANER, J.; JACK, K.; NAGIREDDY, J.; ABDUL RAHEEM, M.; DURHAM, R.; TAM*, W.; Synthesis 2011, 5, 731-738, http://dx.doi.org/10.1055/s-0030-1259484 ; Dep. Chem., Univ. Guelph, Guelph, Ont. N1G 2W1, Can.; Eng.) -C. Gebhardt 26-098
Efficient Procedure for the Preparation of 2-Bromofuran and Its Application in the Synthesis of 2-Arylfurans. -A simple, straightforward, and scalable procedure for the preparation of 2-bromofuran using NBS in DMF is reported. The described preparation is conducted on a 20 to 50 g scale and does not require extractive work-up procedures or chromatographic purifications. To illustrate the synthetic applications, Suzuki coupling reactions of the prepared 2-bromofuran with various aryl boronic acids are investigated. -(RAHEEM, M.-A.; NAGIREDDY, J. R.; DURHAM, R.; TAM*, W.; Synth.
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