Chiral Auxiliary-Bearing Isocyanides as Synthons: Synthesis of Strongly Fluorescent (+)-5-(3,4-Dimethoxyphenyl)-4-[[N- [(4S)-2-oxo-4-(phenylmethyl)-2-oxazolidinyl]]carbonyl]oxazole and Its Enantiomer

Both (4S-(+)-3-(isocyanoacetyl)-4-(phenylmethyl)-2-oxazolidinone (R)-1 and its enantiomer (S)-1 have been synthesized as potentially useful synthons in asymmetric synthesis. Optically active (+)-5-(3,4-dimethoxyphenyl)-4-[[N-[(4S)-2-oxo-4-(phenylmethyl)-2-oxazolidinyl]]carbonyl] oxazole (S)-2 and its enantiomer (R)-2 obtained by treating 3,4-dimethoxybenzoyl chloride with (S)-1 and (R)-1, respectively, in the presence of the nonionic superbase P(MeNCH(2)CH(2))(3)N, have high fluorescence quantum yields. The molecular structure of (S)-2 obtained by X-ray means is also presented.     

P(MeNCH2CH2)3N: a highly selective reagent for synthesizing trans-epoxides from aryl aldehydes

In contrast to its acyclic analogue P(NMe2)3 (1), which in benzene at room temperature reacts with two aryl aldehyde molecules bearing electron-withdrawing groups to give the corresponding diaryl epoxide as an isomeric mixture (trans/cis ratios: 72/28-51/49), P(MeNCH2CH2)3N (2a) under the same reaction conditions is found to be a highly selective reagent that provides epoxides with trans/cis ratios as high as 99/1. These reactions are faster with 2a, because its phosphorus atom is apparently more nucleophilic than that in 1. Thus, it is found that 2a more easily forms 1:1 and 1:2 adducts with one or two molecules of aldehyde, respectively. These adducts apparently are intermediates in the formation of the product epoxide and the corresponding phosphine oxides of 1 and 2a.     

Scope and limitations of Pd2(dba)3/P(i-BuNCH2CH2)3N-catalyzed Buchwald-Hartwig amination reactions of aryl chlorides

Proazaphosphatrane ligands in combination with Pd(2)(dba)(3) generate highly active catalysts for Buchwald-Hartwig amination of aryl chlorides. In particular, commercially available P(i-BuNCH(2)CH(2))(3)N is a highly general and efficient ligand, allowing the coupling of an electronically diverse set of aryl chlorides, including chloropyridines, with a wide variety of amines using 1 mol % of Pd at 100 degrees C. Either a 1:1 or 2:1 ratio of ligand to Pd was found to be effective. This catalyst system performs exceptionally well for sterically hindered substrates, even with only 0.25 mol % of Pd. It is shown that NaOH can also be used as the base (instead of NaO-t-Bu) allowing functionalized substrates to participate in these reactions.     

Application of a new bicyclic triaminophosphine ligand in Pd-catalyzed Buchwald-Hartwig amination reactions of aryl chlorides, bromides, and iodides

The new bicyclic triaminophosphine ligand P(i-BuNCH2)3CMe (3) has been synthesized in three steps from commercially available materials and its efficacy in palladium-catalyzed reactions of aryl halides with an array of amines has been demonstrated. Electron-poor, electron-neutral, and electron-rich aryl bromides, chlorides, and iodides participated in the process. The reactions encompassed aromatic amines (primary or secondary) and secondary amines (cyclic or acyclic). It has also been shown that the weak base Cs2CO3 can be employed with ligand 3, allowing a variety of functionalized substrates (e.g., those containing esters and nitro groups) to be utilized in our amination protocols. This ligand provides a remarkably general, efficient, and mild palladium catalyst for aryl iodide amination. Although 3 is slightly air and moisture sensitive, easy procedures can be adopted that avoid the need of a glovebox. Comparisons of the efficacy of 3 in these reactions with that of the proazaphosphatrane P(i-BuNCH2CH2)3N (2) reveal that in addition to the opportunity for transannulation in 2 (but not in 3), other significant stereoelectronic contrasts exist between these two ligands which help account for differences in the activities of the Pd/2 and Pd/3 catalytic systems.     

Calorimetrie

Ohne Zusammenfassung     

3/2-order chain kinetics involving a postulated dicationic intermediate in the isomerization of a p-isocyano to a p-cyano azaphosphatrane monocation

Kinetic evidence suggests the possibility of a dicationic intermediate in the title reaction. Thus the linkage isomerization reaction, PNC+ = PCN+, is described by the rate law, nu = 3/2k[PNC+]3/2, which can be interpreted by a chain mechanism with the propagation reaction PNC+ + P2+ --> P2+ + PCN+. Such propagation is unusual in that the intermediate regenerates itself in this single step rather than forming a different intermediate for a second propagation step. Cyanide ions inhibit the rate because they participate in the termination step, P2+ + CN- --> PCN+. The rate constant in CD3CN at 100 degrees C is 3/2k = 7.2 +/- 0.6 x 10-5 L1/2 mol-1/2 s-1; 3/2k represents the composite (kinit/kterm)1/2 kprop. When the reaction is carried out in the presence of PBr+, however, the reaction becomes much faster and is described by the rate law, nu = kBr[PBr+][PNC+]; because [PBr+] remains at constant concentration, the time-course experiments follow first-order kinetics.