20748-24-7Relevant academic research and scientific papers
Palladium catalysed alkyne hydrogenation and oligomerisation: A parahydrogen based NMR investigation
Lopez-Serrano, Joaquin,Duckett, Simon B.,Dunne, John P.,Godard, Cyril,Whitwood, Adrian C.
body text, p. 4270 - 4281 (2009/02/03)
The role phosphine ligands play in the palladium(ii)-bis-phosphine-hydride cation catalysed hydrogenation of diphenylacetylene is explored through a PHIP (parahydrogen induced polarization) NMR study. The precursors Pd(LL′)(OTf)2 (1a-e) [LL′ = dcpe (PCy2CH 2CH2PCy2), dppe, dppm, dppp, cppe (PCy 2CH2CH2PPh2)] are used. Alkyl palladium intermediates of the type [Pd(LL′)(CHPhCH2Ph)](OTf) (2 and 3) are detected and demonstrated to play an active role in hydrogenation catalysis. Magnetization transfer experiments reveal chemical exchange from the α-H of the alkyl ligand of 2a (LL′ = dcpe) and linkage isomer 2e′ (LL′ = cppe) into trans-stilbene on the NMR timescale. Activation parameters (ΔH≠ and ΔS≠) for this transformation have been determined. These experiments, coupled with GC/MS data, indicate that the catalytic activity is greater in methanol, where it follows the order: dcpe > cppe > dppp > dppe > dppm, than in CD 2Cl2. All five of the phosphine systems described are less active than those based on bcope [where bcope is (C8H 14)PCH2-CH2P(C8H14)] and tbucope [where tbucope is (C8H 14)PC6H4CH2P(tBu) 2]. cis, cis-1,2,3,4-Tetraphenyl-buta-1,3-diene is detected as a minor reaction product with Pd(LL′)(PhCH-CHPh-CPh=CHPh)+ (4) also being shown to play a role in the alkyne dimerisation step.
Palladium-catalyzed hydrogenation: Detection of palladium hydrides. A joint study using para-hydrogen-enhanced NMR spectroscopy and density functional theory
Lopez-Serrano, Joaquin,Duckett, Simon B.,Lledos, Agusti
, p. 9596 - 9597 (2007/10/03)
Pd(PEt3)2(OTf)2, acting as an in situ source of Pd(PEt3)2, reacts with an alkyne and hydrogen via phosphine loss to form the detectable hydride-containing species Pd(PEt3)2(H)(CHPhCH2Ph), cis- and trans-Pd(PEt3)2(H)(CPh=CHPh), and Pd2(PEt3)3(H)(CHPhCH2Ph)2+, which map onto the reaction scheme predicted by density functional theory. Copyright
Unimolecular reactions in isolated and collisional systems: Deuterium isotope effect in the photoisomerization of stilbene
Courtney, Scott H.,Balk, Michael W.,Philips, Laura A.,Webb, Steven P.,Yang, Ding,et al.
, p. 6697 - 6707 (2007/10/02)
The isomerization of t-stilbene (stilbene h12 ) and three deuterated derivatives has been studied in a supersonic expansion, the thermal gas phase, and solution.In the jet we find that almost all effect of full deuteration (stilbene d12 ) is produced by deuteration of the two ethylinic hydrogens only (stilbene d2 ).Complete deuteration of the phenyl rings (stilbene dlo ) has rather little influence on the decay of the jet-cooled molecule.Nonexponential decays are found at intermediate excess energies in the jet-cooled system, with the degree of nonexponentiality decreasing with increasing excess energy.The ordering of the decay rates observed in the jet is not consistent with previous RRKM calculations of the isomerization rates of stilbene h12 and d2.Using similar parameters the calculations consistently place the stilbene d2 and stilbene dlo curves in the wrong order.Our results suggest extensive but not complete vibrational relaxation in the isolated molecule.Vibrational redistribution rapidly becomes complete in the presence of buffer gas.In thermal samples the isomerization rates of stilbene h12 and stilbene d10 are identical over a wide range of solvents and temperatures.By contrast the isomerization rates in stilbene d2 and stilbene d12 are 1.4 and 1.5 times slower than in stilbene h12 Again, these ratios appear constant over a wide range of experimental conditions.
Conformations and Rotational Barriers of 1,3-Diphenylallyllithium Compounds
Boche, Gernot,Buckl, Klaus,Martens, Diether,Schneider, Dieter R.
, p. 1135 - 1171 (2007/10/02)
The phenyl substituents of the 1,3-diphenylallyl anions 10 (gegenion lithium, solvent tetrahydrofuran) can exist in the exo,exo-, endo,exo- and/or endo,endo-conformations.We have investigated the influence of substituents R at C2 on the equilibria of these solvent separated ion pairs.While 10a (R = H) is the only one to exist predominantly in the exo,exo-conformation, and in 10b and c (R = CH3 and CN, respectively) the endo,exo-conformers predominate, in 10d, e and f (R = C2H5, C6H5 and iPr, respectively) there is increasing preference for the endo,endo-conformation, which in 10g (R = tBu) is the dominant (>/= 95percent) conformation.A vast congestion in the endo,endo-conformation is avoided by a rotation of the phenyl rings out of the plane of the allyl carbon atoms, and an expansion of the sp2 angles in the allyl moiety.The rotational barriers around the allyl anion bonds decrease from 19.1 kcal*mol-1 (10a) to 12.5 kcal*mol-1 (10f).Since this trend parallels to the above mentioned shift of the equilibria, it is due to ground state effects.The rotational barriers are only slightly (10a,b) if at all influenced by gegenion effects, which is in sharp contrast to the parent allyl "anion".Therefore, the rotational barriers of the allyl anions 10 are qualified for a comparison with the corresponding radicals and cations.Furthermore, with ΔG(excit)273 deg C = 19.1 kcal*mol-1 as a lower limit value for the rotational barrier of the parent allyl anion, one can estimate that the true value of this species must be close to barriers calculated with STO-3G and 4-3l-G programs (ca. 26 kcal*mol-1).
