Search PubMed⌕ Search

Biomedical subjects

Guy Bertrand

Publications and source records attributed to Guy Bertrand.

At least 37 records · Page 2Linked to original sources
PubMed2004

Stable optically pure phosphino(silyl)carbenes: reagents for highly enantioselective cyclopropanation reactions.

The stability of phosphino(trimethylsilyl)carbenes bearing cyclic diamino substituents on phosphorus is strongly dependent on the steric hindrance of the nitrogen substituents. Phosphinocarbenes 3 and 7, derived from the trans-N,N'-diisopropylcyclohexane-1,2-diamine and N,N'-diisopropyl-1,2-ethanediamine, are not observed; instead the 1,3-diphosphete 4 and a novel six-membered heterocycle 8, which results from the dimerization of 3 and the reaction of 7 with its diazo precursor 6, respectively, have been isolated. In contrast, the phosphino(silyl)carbene 14 derived from N,N'-di-tert-butyl-1,2-ethanediamine has been isolated in high yield. By using the enantiomerically pure (S,S)-, and (R,R)-N,N'-di-tert-butyl-1,2-diphenyl-1,2-ethanediamines, the first optically pure phosphino(sily)carbenes (S,S)-17 and (R,R)-17 have been prepared. They react with methyl acrylate to give the corresponding cyclopropanes (S,S,R,R)-19 and (R,R,S,S)-19 with a total syn diastereoselectivity and an excellent enantioselectivity (de>98 %).

Journal Article↗
PubMed2004

Mono- and diaminocarbenes from chloroiminium and -amidinium salts: synthesis of metal-free bis(dimethylamino)carbene.

Bis(trimethylsilyl)mercury cleanly reacts at low temperature with chloroamidinium and -iminium chlorides, generating persistent metal-free cyclic and acyclic diaminocarbenes, as well as transient aryl-, chloro-, and hydrogenoaminocarbenes; with the latter, the corresponding olefin dimers were isolated, whereas with the former, no dimerization processes were observed.

Journal Article↗
PubMed2003

Theoretical study of rhodium(I) carbene complexes: the structural versatility of phosphino- compared with aminocarbenes.

Density functional calculations are reported for complexes of general formula [(carbene)RhClL(2)] featuring model phosphino- and aminocarbenes. Both the cis and trans isomers of the rhodium(I) eta(1)-complexes (1-9) were investigated, and the influence of the rhodium co-ligands (L=ethylene, phosphine, or carbon monoxide) was evaluated. In the case of phosphinocarbenes and carbon monoxide as a ligand, a somewhat unusual coordination mode was observed, in which a significant intramolecular Cl-->C(carbene) interaction is present. The propensity of phosphino- and aminocarbenes to behave as four electron donors was also investigated both structurally and energetically on the related eta(2)-complexes 10-18. These results as a whole emphasize the structural versatility of phosphino- compared with aminocarbene complexes.

Journal Article↗
PubMed2003

Stereoselective synthesis of phosphoranyl aryloxiranes through the addition of a nucleophilic stable carbene to aromatic aldehydes.

The [2+1] addition of the stable (phosphanyl)(silyl)carbene 1 to aromatic aldehydes affords phosphoranyl aryloxiranes, a new class of polyfunctional epoxides, in high yields and excellent diastereoselectivity. No reaction is observed for aldehydes bearing strongly electron-donating groups. Theoretical calculations show a good correlation between Gibbs activation energy and the electronic nature of the substituent on the phenyl ring.

Journal Article↗
PubMed2003

Synthesis of carbenes through substitution reactions at a carbene center.

An (amino)(phosphino) carbene can be transformed into (amino)(phosphonio) carbenes, which undergo nucleophilic intermolecular as well as intramolecular substitution reactions at the carbene center. A variety of carbenes can be synthesized starting from a single carbene precursor. The resulting gamut of electronic and steric effects possible should open the way not only to a detailed study of the mechanism, but also to the subsequent improvement of catalytic reactions that involve carbene-transition metal complexes.

Journal Article↗
PubMed2003

On the electronic structures of the 1,3-diboracyclobutane-1,3-diyls and their valence isomers with a B2E2 skeleton (E=N, P, As).

The concept of through-space versus through-bond interactions on the stabilization of biradical structures with a singlet or triplet ground state is evaluated for the 1,3-diboracyclobutane-1,3-diyls and related congeners. Singlet biradicals are favored when the intermediate units E feature singlet character (PH(2) (+), AsH(2) (+)), while E fragments with triplet character (NH(2) (+)) induce small energy separations between the lowest singlet and triplet states. These considerations are supported by quantum chemical calculations with energy optimization at 1) MCSCF level plus MR-MP2 correction, 2) MR-MP2 level, and 3) two different types of density functional levels for the planar (D(2h)) geometries. The singlet-triplet energy separations in the planar compounds increase with increasing singlet stability of the corresponding E fragments. In addition to this newly developed principal features for singlet stabilization, which primarily occurs in bonded structures with higher main-group elements, the corresponding valence isomers with bicyclobutane, cyclobutene and cis-butadiene structures are investigated.

Journal Article↗
PubMed2003

Transient azomethine-ylides from a stable amino-carbene and an aldiminium salt.

Catalytic amounts of a protic reagent such as tert-butyl alcohol promote the isomerization of a stable amino-aryl-carbene into a transient azomethine ylide. Deprotonation of an alkyl-aldiminium salt also leads to a transient azomethine ylide, but labeling experiments rule out the transient formation of the corresponding amino-alkyl-carbene. The potential hypersurface between model amino-carbene, aziridine, and azomethine ylide is investigated.

Journal Article↗
PubMed2003

(Phosphino)(aryl)carbenes: effect of aryl substituents on their stabilization mode.

A broad range of (phosphino)(aryl)carbenes, 1b-d, 10a,b, and 14a,b, were prepared by photolysis of their diazo precursors. The influence of the steric and electronic properties of the aryl ring on the structure and stability of these carbenes was studied both experimentally and theoretically. Among the different stabilization modes investigated, those featuring an acceptor as well as a spectator aryl substituent result in stable or at least persistent carbenes that could be completely characterized by classical spectroscopic methods. In marked contrast, the new substitution pattern featuring a donor aryl ring results in a very fleeting carbene.

Journal Article↗
PubMed2002

A rare example of a rearrangement involving four structural isomers: alpha-phosphinonitrile/C-phosphinoketenimine/1-aza-4- phosphabutadiene/1,2-dihydro-1,2-azaphosphete.

The stable compound [bis(dicyclohexylamino)phosphino](trimethylsilyl)-carbene (1) reacts with dimethyl cyanamide to afford the original 1,2-dihydro-1,2-azaphosphete 4a (51% yield). The surprising formation of this heterocycle involves the transient formation of a nitrile, a keteneimine, and a 1-aza-4 lambda 3-phosphabutadiene derivative. By using substituent effects and different synthetic routes, all of these structural isomers have been isolated.

Journal Article↗
PubMed2002

Stable (aryl)(phosphino)carbenes: new ligands for transition metals.

The first direct complexation of a stable phosphino carbene to a transition metal center is described. Both eta1- and eta2-coordination modes have been characterized spectroscopically. The Fischer-type structure of the eta1-carbene rhodium complex was revealed by an X-ray diffraction study.

Journal Article↗