Unexpectedly high barriers to M–P rotation in tertiary phobane complexes : PhobPR behavior that is commensurate with tBu2PR

The four isomers of 9-butylphosphabicyclo[3.3.1]nonane, s-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl, have been prepared. Seven isomers of 9-butylphosphabicyclo[4.2.1]nonane (a5-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl; a7-PhobPBu, where Bu = n-butyl, isobutyl, tert-butyl) have been identified in solution; isomerically pure a5-PhobPBu and a7-PhobPBu, where Bu = n-butyl, isobutyl, have been isolated. The σ-donor properties of the PhobPBu ligands have been compared using the JPSe values for the PhobP(═Se)Bu derivatives. The following complexes have been prepared: trans-[PtCl2(s-PhobPR)2] (R = nBu (1a), iBu (1b), sBu (1c), tBu (1d)); trans-[PtCl2(a5-PhobPR)2] (R = nBu (2a), iBu (2b)); trans-[PtCl2(a7-PhobPR)2] (R = nBu (3a), iBu (3b)); trans-[PdCl2(s-PhobPR)2] (R = nBu (4a), iBu (4b)); trans-[PdCl2(a5-PhobPR)2] (R = nBu (5a), iBu (5b)); trans-[PdCl2(a7-PhobPR)2] (R = nBu (6a), iBu (6b)). The crystal structures of 1a–4a and 1b–6b have been determined, and of the ten structures, eight show an anti conformation with respect to the position of the ligand R groups and two show a syn conformation. Solution variable-temperature 31P NMR studies reveal that all of the Pt and Pd complexes are fluxional on the NMR time scale. In each case, two species are present (assigned to be the syn and anti conformers) which interconvert with kinetic barriers in the range 9 to >19 kcal mol–1. The observed trend is that, the greater the bulk, the higher the barrier. The magnitudes of the barriers to M–P bond rotation for the PhobPR complexes are of the same order as those previously reported for tBu2PR complexes. Rotational profiles have been calculated for the model anionic complexes [PhobPR-PdCl3]− using DFT, and these faithfully reproduce the trends seen in the NMR studies of trans-[MCl2(PhobPR)2]. Rotational profiles have also been calculated for [tBu2PR-PdCl3]−, and these show that the greater the bulk of the R group, the lower the rotational barrier: i.e., the opposite of the trend for [PhobPR-PdCl3]−. Calculated structures for the species at the maxima and minima in the M–P rotation energy curves indicate the origin of the restricted rotation. In the case of the PhobPR complexes, it is the rigidity of the bicycle that enforces unfavorable H···Cl clashes involving the Pd–Cl groups with H atoms on the α- or β-carbon in the R substituent and H atoms in 1,3-axial sites within the phosphabicycle

Development of homogeneous catalysts for the selective conversion of levulinic acid to caprolactam

The aim of this thesis is to introduce the readers to the importance of bio-based products and their potential applications in chemical industries. Alternative routes to the widely used nylon precursor caprolactam are discussed and compared with the current fossil-based synthesis. Furthermore, the aim of the research - the development of a novel synthesis route to caprolactam based on biomass - is reported; the envisaged reaction sequence includes the development of new catalytic chemistry, namely reductive amidation and hydroamidomethylation.</p

Palladium(0)​/NHC-​catalyzed reductive Heck reaction of enones: a detailed mechanistic study

We have studied the mechanism of the palladium-​catalyzed reductive Heck reaction of para-​substituted enones with 4-​iodoanisole by using N,​N-​diisopropylethylamine (DIPEA) as the reductant. Kinetic studies and in situ spectroscopic anal. have provided a detailed insight into the reaction. Progress kinetic anal. demonstrated that neither catalyst decompn. nor product inhibition occurred during the catalysis. The reaction is first order in the palladium and aryl iodide, and zero order in the activated alkene, N-​heterocyclic carbene (NHC) ligand, and DIPEA. The expts. with deuterated solvent ([D7]​DMF) and deuterated base ([D15]​Et3N) supported the role of the amine as a reductant in the reaction. The palladium complex [Pd0(NHC)​(1)​] has been identified as the resting state. The kinetic expts. by stopped-​flow UV​/Vis also revealed that the presence of the second substrate, benzylideneacetone 1, slows down the oxidative addn. of 4-​iodoanisole through its competing coordination to the palladium center. The kinetic and mechanistic studies indicated that the oxidative addn. of the aryl iodide is the rate-​detg. step. Various scenarios for the oxidative addn. step have been analyzed by using DFT calcns. (bp86​/def2-​TZVP) that supported the inhibiting effect of substrate 1 by formation of resting state [Pd0(NHC)​(1)​] species at the cost of further increase in the energy barrier of the oxidative addn. step

Enantioselective Hydroformylation by a Rh-Catalyst Entrapped in a Supramolecular Metallocage

International audienceRegio-and enantioselective hydroformylation of styrenes is attained upon embedding a chiral Rh complex in a nonchiral supramolecular cage formed from coordination-driven self-assembly of macrocyclic dipalladium complexes and tetracarboxylate zinc porphyrins. The resulting supramolecular catalyst converts styrene derivatives into aldehyde products with much higher chiral induction in comparison to the nonencapsulated Rh catalyst. Spectroscopic analysis shows that encapsulation does not change the electronic properties of the catalyst nor its first coordination sphere. Instead, enhanced enantioselectivity is rationalized by the modification of the second coordination sphere occurring upon catalyst inclusion inside the cage, being one of the few examples in achieving an enantioselective outcome via indirect through-space control of the chirality around the catalyst center. This effect resembles those taking place in enzymatic sites, where structural constraints imposed by the enzyme cavity can impart stereoselectivities that cannot be attained in bulk. These results are a showcase for the future development of asymmetric catalysis by using size-tunable supramolecular capsules

Monitoring the disassembly of virus-like particles by (19)F-NMR

Virus-like particles (VLPs) are stable protein cages derived from virus coats. They have been used extensively as biomolecular platforms, e.g., nanocarriers or vaccines, but a convenient in situ technique is lacking for tracking functional status. Here, we present a simple way to monitor disassembly of (19)F-labeled VLPs derived from bacteriophage Qβ by (19)F NMR. Analysis of resonances, under a range of conditions, allowed determination not only of the particle as fully assembled but also as disassembled, as well as detection of a degraded state upon digestion by cells. This in turn allowed mutational redesign of disassembly and testing in both bacterial and mammalian systems as a strategy for the creation of putative, targeted-VLP delivery systems