7642-04-8Relevant articles and documents
Photochemistry of Alkenes. 7. E/Z Isomerization of Alkenes Sensitized with Benzene and Derivatives
Snyder, John J.,Tise, Frank P.,Davis, Robert D.,Kropp, Paul J.
, p. 3609 - 3611 (1981)
Contrary to numerous previous reports placing the (E/Z)pss ratios from benzene-sensitized isomerization of various alkenes at unity, the (E/Z)pss ratios for alkenes 1-4 (Table I) have been found to (a) differ from unity, with the less highly strained isomer predominating, (b) vary depending on the structure of the alkene, and (c) bear an apparently linear relationship with the triplet excitation energy of the sensitizer.The results are tentatively interpreted in terms of "nonvertical" energy transfer, with transfer occuring more efficiently to the thermodynamically less stable isomer.Possible difficulties with the quantitative aspects of this interpretation are dicussed.Of several sensitizers evaluated for efficiency in effecting E/Z isomerization of 3,4-dimethyl-2-pentene (2) (Table III), p-xylene and phenol were found to be superior.The latter has the added advantage of being easily separated from the alkene by extraction with base.
Steric and Electronic Effects of Phosphane Additives on the Catalytic Performance of Colloidal Palladium Nanoparticles in the Semi-Hydrogenation of Alkynes
Staiger, Lena,Kratky, Tim,Günther, Sebastian,Tomanek, Ondrej,Zbo?il, Radek,Fischer, Richard W.,Fischer, Roland A.,Cokoja, Mirza
, p. 227 - 234 (2020/10/14)
We report on the influence of phosphanes on the catalytic activity and selectivity of colloidal, tetraoctylammonium bromide (TOAB) stabilised palladium nanoparticles (NPs) in the semi-hydrogenation of alkynes to olefins. Full characterisation of the catalytic system (HRTEM, EDX, XPS, IR, NMR) confirmed the formation of spherical particles with a narrow size distribution (1.9±0.5 nm). The catalytic performance of the Pd NPs in the semi-hydrogenation of 1-octyne, 2-octyne and phenylacetylene to the respective olefins and the influences on the selectivity was investigated. The system shows high activities and selectivities at mild conditions (0 °C and 1.0 bar H2 pressure). It was shown that generally, phosphanes lead to an increase of both the reaction rate and selectivity towards the olefin where both steric and electronic effects of the ligand play a crucial role for the catalyst performance. A moderate steric demand of the ligand with a rather weak σ-donating ability turned out to give the highest catalytic performance.
Highly Z-Selective Double Bond Transposition in Simple Alkenes and Allylarenes through a Spin-Accelerated Allyl Mechanism
Kim, Daniel,Pillon, Guy,Diprimio, Daniel J.,Holland, Patrick L.
supporting information, p. 3070 - 3074 (2021/03/08)
Double-bond transposition in alkenes (isomerization) offers opportunities for the synthesis of bioactive molecules, but requires high selectivity to avoid mixtures of products. Generation of Z-alkenes, which are present in many natural products and pharmaceuticals, is particularly challenging because it is usually less thermodynamically favorable than generation of the E isomers. We report a β-dialdiminate-supported, high-spin cobalt(I) complex that can convert terminal alkenes, including previously recalcitrant allylbenzenes, to Z-2-alkenes with unprecedentedly high regioselectivity and stereoselectivity. Deuterium labeling studies indicate that the catalyst operates through a π-allyl mechanism, which is different from the alkyl mechanism that is followed by other Z-selective catalysts. Computations indicate that the triplet cobalt(I) alkene complex undergoes a spin state change from the resting-state triplet to a singlet in the lowest-energy C-H activation transition state, which leads to the Z product. This suggests that this change in spin state enables the catalyst to differentiate the stereodefining barriers in this system, and more generally that spin-state changes may offer a route toward novel stereocontrol methods for first-row transition metals.
