Search results for "Reactivity"

1976

tert-Butoxy radicals were generated by decomposing di-tert-butyl hyponitrite. The relative reactivities of substrates were measured from the ratio of tert-butanol and acetone formed. Polystyrene was found to be less reactive than model compounds. Activation parameters for the hydrogen abstraction reaction for cumene, 3-phenylpentane, and 2,4-diphenylpentane are reported and show small differences for the enthalpy of activation; large negative values for the entropy of activation demonstrate the importance of steric hindrance in the hydrogen abstracting step. 2,3-Diphenylbutane as model compound for head to head links in polystyrene exhibits low reactivity, whereas 2,5-diphenylhexane, a mode…

Unveiling the Ionic Diels-Alder Reactions within the Molecular Electron Density Theory

2021

The ionic Diels–Alder (I-DA) reactions of a series of six iminium cations with cyclopentadiene have been studied within the Molecular Electron Density Theory (MEDT). The superelectrophilic character of iminium cations, ω &gt

Uncommon coordination behaviour of P(S) and P(Se) units when bonded to carboranyl clusters: experimental and computational studies on the oxidation o…

2010

Oxidation of closo-carboranyl diphosphines 1,2-(PR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr) and closo-carboranyl monophosphines 1-PR(2)-2-R'-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr, Cy; R'=Me, Ph) with hydrogen peroxide, sulfur and elemental black selenium evidences the unique capacity of the closo-carborane cluster to produce uncommon or unprecedented P/P(E) (E=S, Se) and P=O/P=S chelating ligands. When H(2)O(2) reacts with 1,2-(PR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr), they are oxidized to 1,2-(OPR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr). However, when S and Se are used, different reactivity is found for 1,2-(PPh(2))(2)-1,2-closo-C(2)B(10)H(10) and 1,2-(PiPr(2))(2)-1,2-closo-C(2)…

Steric interactions controlling the syn diastereofacial selectivity in the [3 + 2] cycloaddition reaction between acetonitrile oxide and 7-oxanorborn…

2017

A Molecular Electron Density Theory study of the zw-type 32CA reactions of acetonitrile oxide (NO) with two 7-oxanorborn-5-en-2-ones (ONBs) has been performed at the DFT B3LYP/6-31G(d) computational level. These cycloadditions proceed through one-step mechanisms with high activation energies and present low para regio and complete syn diastereofacial selectivities. While the non-polar character of these zw-type 32CA reactions, which is the consequence of the insufficient electrophilic activation of ONBs, according to the analysis of the conceptual DFT reactivity indices, accounts for the high activation energies, and low para regioselectivity, NCI topological analyses at the anti/syn pairs …

Reactivity of di-tert-butyldimethoxystannane with carbon dioxide and methanol: X-ray structure of the resulting complex

2006

Abstract The synthesis of dimethyl carbonate from carbon dioxide and methanol was studied with ditert-butyldimethoxystannane under pressure at temperatures ⩽423 K. The formation of dimethyl carbonate is accompanied by transformation of the stannane into a trinuclear complex, the structure of which has been determined by single-crystal X-ray diffraction technique. The relevance of this specie in the catalytic cycle is demonstrated by conducting recycling runs. A preliminary kinetic study underlines the steric influence of the tert-butyl ancillary ligands in the stabilisation of intermediates, by comparison with the n-butyl homologue.

Synthesis of Sterically Hindered Chiral 1,4-Diols from Different Lignan-Based Backbones

2013

Methods for synthetic modifications of the natural dibenzylbutyrolactone lignan hydroxymatairesinol into chiral 1,4-diols with different lignan-derived backbones have been developed. A stepwise procedure, involving alkylation and oxidation, was shown to be successful and several highly substituted 1,4-diols were prepared. Some substituted butyrolactones resisted alkylation and led to the formation unusually stable hemiketals (butyrolactols). The formation of stable hemiketals was investigated in detail, showing that different backbone structures influence the formation and reactivity of the hemiketals.

The role of fluorine in the stereoselective tandem Aza-Michael addition to acrylamide acceptors: An experimental and theoretical mechanistic study

2007

Aza-Michael additions of alpha-amino esters to fluorinated acceptors take place in a highly stereoselective manner, to give partially modified Psi-[NHCH2]retropeptides incorporating a hydrolytically stable trifluoroalanine mimic. The reaction mechanism has been investigated experimentally and theoretically, in order to explain the effect of the trifluoromethyl group on the reactivity and the origins of the experimentally observed stereocontrol. The reaction is a two-step process, involving a tandem aza-Michael addition followed by a stereoselective hydrogen transfer. Both steps are base-catalyzed. The high level of stereocontrol is the result of a combination of electrostatic interactions a…

Can Green Dimethyl Carbonate Synthesis be More Effective? A Catalyst Recycling Study Benefiting from Experimental Kinetics and DFT Modeling

2014

Dibutyldimethoxystannanes are known to catalyze the reaction between carbon dioxide and methanol leading to dimethyl carbonate. Despite similarities between din-butyl- and ditert-butyldimethoxystannane, the recycled complexes have different structural features. In the din-butyl series, a decatin(IV) complex has been characterized and is less active than the stannane precursor. Kinetic experiments likely indicate that all the tin centers are not active, which is confirmed in comparing with the related dinuclear 1,3-dimethoxytetran-butyldistannoxane complex. In the ditert-butyl series, the tritin(IV) complex isolated upon recycling features the steric effect of bulky tBu ancillary ligands. In…

Trapping a Highly Reactive Nonheme Iron Intermediate That Oxygenates Strong C—H Bonds with Stereoretention

2015

An unprecedentedly reactive iron species (2) has been generated by reaction of excess peracetic acid with a mononuclear iron complex [FeII(CF3SO3)2(PyNMe3)] (1) at cryogenic temperatures, and characterized spectroscopically. Compound 2 is kinetically competent for breaking strong C-H bonds of alkanes (BDE ≈ 100 kcal·mol-1) through a hydrogen-atom transfer mechanism, and the transformations proceed with stereoretention and regioselectively, responding to bond strength, as well as to steric and polar effects. Bimolecular reaction rates are at least an order of magnitude faster than those of the most reactive synthetic high-valent nonheme oxoiron species described to date. EPR studies in tande…

Tin( II ) and Lead( II ) 4‐Acyl‐5‐pyrazolonates: Synthesis, Spectroscopic and X‐ray Structural Characterization

2004

Novel tin(II) β-diketonate Sn(Q)2 complexes [HQ = 1-R1-3-R3-4-R4(C=O)-pyrazol-5-one; HQC: R1 = Ph, R3 = Me, R4 = Cy; HQS: R1 = Ph, R3 = Me, R4 = CHPh2; HQL: R1 = Ph, R3 = Me, R4 = CH2Ph; HQT: R1 = Ph, R3 = Me, R4 = CH2tBu; HQE: R1 = Ph, R3 = Me, R4 = Et; HQB: R1 = Ph, R3 = Me, R4 = tBu; HQW: R1 = Ph, R3 = Me, R4 = p-(tBu)Ph; HQR: R1 = Ph, R3 = Me, R4 = p-[(CH2)5CH3]Ph; HQN: R1 = p-NO2Ph, R3 = Me, R4 = Ph; HQM: R1 = Me, R3 = Me, R4 = Ph; HQD: R1 = Me, R3 = Me, R4 = Me; HQP: R1 = Ph, R3 = Ph, R4 = Ph; HQG: R1 = Ph, R3 = Ph, R4 = Me; HQF: R1 = p-CF3Ph, R3 = Me, R4 = Ph; HQH: R1 = p-CF3Ph, R3 = Me, R4 = Me] have been isolated and characterized by single-crystal X-ray diffraction analyses, IR, 1…