7385-82-2

Alkene isomerization can be an atom-economical approach to generating a wide range of alkene intermediates for synthesis, but fully equilibrated mixtures of disubstituted internal alkenes typically contain significant amounts of the positional as well as geometric (E and Z) isomers. Most classical catalyst systems for alkene isomerization struggle to kinetically control either positional or E/Z isomerism. We report coordinatively unsaturated, formally 16-electron Cp Ru catalyst 5, which facilitates simultaneous regio- A nd stereoselective isomerization of linear 1-alkenes to their internal analogues, providing consistent yields of (E)-2-alkenes greater than 95%. Because nitrile-free catalyst 5 is more than 400 times faster than previously published nitrile-containing analogues 2 + 2a, very reasonable 0.1-0.5 mol % loadings of 5 complete ambient-temperature reactions within 15 min to 4 h. UV-vis, NMR, and computational studies depict the imidazolyl fragment on the phosphine as a hemilabile, four-electron donor in 2-P,N coordination. For the first time, we show direct experimental evidence that the PN ligand has accepted a proton from the substrate by characterizing the intermediate Cp Ru[??3-allyl][1-P)P-N+H], which highlights the essential role of the bifunctional ligand in promoting rapid and selective alkene isomerizations. Moreover, kinetic studies and computations reveal the role of alkene binding in selectivity of unsaturated catalyst 5.

Here we examine in detail two catalysts for their ability to selectively convert 1-alkenes to (E)-2-alkenes while limiting overisomerization to 3- or 4-alkenes. Catalysts 1 and 3 are composed of the cations CpRu(κ2-PN)(CH3CN)+ and Cp?Ru(κ2-PN)+, respectively (where PN is a bifunctional phosphine ligand), and the anion PF6-. Kinetic modeling of the reactions of six substrates with 1 and 3 generated first- and second-order rate constants k1 and k2 (and k3 when applicable) that represent the rates of reaction for conversion of 1-alkene to (E)-2-alkene (k1), (E)-2-alkene to (E)-3-alkene (k2), and so on. The k1:k2 ratios were calculated to produce a measure of selectivity for each catalyst toward monoisomerization with each substrate. The k1:k2 values for 1 with the six substrates range from 32 to 132. The k1:k2 values for 3 are significantly more substrate-dependent, ranging from 192 to 62 000 for all of the substrates except 5-hexen-2-one, for which the k1:k2 value was only 4.7. Comparison of the ratios for 1 and 3 for each substrate shows a 6-12-fold greater selectivity using 3 on the three linear substrates as well as a >230-fold increase for 5-methylhex-1-ene and a 44-fold increase for a silyl-protected 4-penten-1-ol substrate, which are branched three and five atoms away from the alkene, respectively. The substrate 5-hexen-2-one is unique in that 1 was more selective than 3; NMR analysis suggested that chelation of the carbonyl oxygen can facilitate overisomerization. This work highlights the need for catalyst developers to report results for catalyzed reactions at different time points and shows that one needs to consider not only the catalyst rate but also the duration over which a desired product (here the (E)-2-alkene) remains intact, where 3 is generally superior to 1 for the title reaction.

The azoisopropane-initiated thermal reaction of (E)-CH3CH=CHCH3 has been studied in the temperature range 489.5-542.0 K.For the reactions (CH3)2CHN=NCH(CH3)2 --> 2 2-C3H7(.) + N2 (1) 2-C3H7(.) + (E)-CH3CH=CHCH3 --> C3H8 + (E)-C4H7(.) (4) --> (CH3)2CHCH(CH3)CH(CH3)(.) (5) 2 2-C3H7(.) --> (CH3)2CHCH(CH3)2 (2) the following Arrhenius parameters were determined: log(k1/s-1) = (16.42 +/- 0.30) - (201.9 +/- 3.0) kJ mol-1/θ log4/k21/2)/dm3/2 mol-1/2 s-1/2> = (3.64 +/- 0.40) - (46.9 +/- 2.1) kJ mol-1/θ log5/k21/2)/dm3/2 mol-1/2 s-1/2> = (2.53 +/- 0.60) - (39.5 +/- 2.4) kJ mol-1/θ where θ = RT in 10.For the croos-combination ratios of the radicals 2-C3H7(.) and (Z)-C4H7(.), Φt = 2.12 +/- 0.10 was obtained, where the subscript t refers to the terminal combination.Formation of certain characteristic products was observed in various addition/isomerization/dissociation processes. 2-C3H7(.) addition to (E)-CH3CH=CHCH3 is suggested as the rate-determining step, followed by 1,4- and 1,5-H-atom shifts.

The characteristic NMR and vibrational spectroscopic properties of 1-methylene-2-vinylcyclopropane 1 and some of its derivatives are described and discussed.The exocyclic C=C stretching vibration is consistently found at 1745 cm-1.The 1H NMR spectra show coupling patterns among the methylene and ring protons that are essentially independent of the substituents.The coupling constant between H-2 and the α-proton of the substituent depends on the conformation. 13C chemical shifts and coupling constants are readily interpreted using common substituent effect rules.MNDO calculations provide proton-proton distances and both orders which are used to interpret proton relaxation rates and proton-proton coupling constants.