Francis Carré

Pentacoordinate silicon compounds: Stereochemical non-rigidity of chelates formed by intramolecular ring-closure. Crystal structure of 8-dimethylamino-1-trifluorosilylnaphthalene☆

The application of dynamic NMR spectroscopy to the study of stereochemical non-rigidity in pentacoordinate chelated organosilicon compounds is described. It is shown that in the compounds Me2NCH(Me)C6H4SiXYZ, non-dissociative ligand permutation at silicon can be distinguished unambiguously from processes associated with rupture of the chelate ring and nitrogen inversion. The crystal and molecular structure of 8-Me2NC10H6SiF3 has been determined. Pentacoordination of the silicon atom is confirmed, with the donor nitrogen atom and a fluorine atom occupying axial sites in an overall trigonal bipyramidal geometry. The N → Si separation is 2.3 A (average of two distinct but closely related molecular conformations), which is less than the C1C8 distance in the naphthalene nucleus, indicating a substantial bonding interaction. NMR studies of the dynamic behaviour of the Me2N group, and where possible (19F, 1H) of the monodentate ligands in 8-dimethylamino-1-silylnaphthalene compounds, together with the results for the chelated benzylaminosilicon compounds, confirm that inversion of the absolute configuration at the silicon atom is not achieved by this process. The free energies of activation for non-dissociative ligand permutation at a silicon range from less than 7 kcal mol−1 [SiH3, Si(OR)3], which is below the limit of direct measurement, to 13 kcal mol−1 for Me2NCH(Me)C6H4SiF3; difunctional silicon chelate compounds (Cl, F, OR) display values from 9–12 kcal mol−1. These are comparable with those determined for fluxional processes in acyclic pentacoordinate silicon compounds.

Intramolecular coordination at phosphorus: donor-acceptor interaction in three- and four-coordinated phosphorus compounds

Abstract The phosphorus derivatives P(X)(O2C6H4−1,2)2[C6H4(CH2NMe2)2−2] (9–11) undergo ready extension of coordination by N → P intramolecular donor-acceptor interaction as shown by 31P NMR and dynamic 1H NMR spectroscopy. This extension of coordination does not occur with P(X)(OEt)2[C6H4(CH2NMe2)2−2] (7 and 8). The ΔG≡ of pseudo-rotation was calculated by dynamic 1H NMR spectroscopy to be 57–58 kJ mol−1 for the three compounds P(X)O2C64−1,2)2[C10H6(NMe2−8)] (19–21).

Problème du cation siliconium V. Mécanisme de la racémisation de chlorosilanes et de chlorogermanes diversement encombrés

Abstract Evidence is provided for an extension of coordination about the silicon (or germanium) atom in the racemization of chlorosilanes (or chlorogermanes). Furthermore, our results allow to say that the siliconium ion is not involved as an intermediate in reactions.

Synthesis, chemical behaviour and crystal structure of cobalt-stabilized carbenes, (Ph3Ge)(CO)3CoC(OEt)R

Abstract Cobalt stabilized carbenes are readily synthesized by reaction of organolithium compounds with (CO) 4 CoGePh 3 . The neutral carbenes are formed from the anion thus obtained by treatment with an ethylating reagent. Anions α to the carbene atom can be generated by treatment with a strong base (NaH, RLi) and trapped with Et 3 OBF 4 . However attempts to trap both types of anions with chlorosilanes failed, and instead the compound (CO) 4 CoGePh 3 was recovered in good yield. An X-ray structural determination of one of these complexes, (Ph 3 Ge)(CO) 3 Co(OEt)Et, shows unusually short bond lengths in the carbene moiety and the linkage trans to the triphenylgermyl group. The unusual chemical behaviour can be related to special features of the structure of the complex.

Etude de l'activation catalytique des reactifs de grignard par les complexes du nickel dans la serie des organogermanes

Abstract A new synthetic method for the formation of the carbongermanium bond is described. Grignard reagents activated by catalytic quantities of nickel(II) complexes can substitute the germaniumhydrogen bond: The yields depend on both the catalyst and the Grignard reagent. In every case, the stereochemistry of the original organogermane is retained.

Stereoelectronic control of oxazolidine ring-opening: structural and chemical evidences

Abstract Ring opening of oxazolidines derived from tris(hydroxymethyl)aminomethane, l -serine and l -threonine was investigated. It was shown that n(N)→σ∗(C–O) electron delocalization (endo-anomeric effect) occurring in the five-membered ring plays a major role in the cleavage of the intracyclic C–O bond. The present work establishes that when the nitrogen lone pair is conjugated with a carbonyl group (n(N)→π(CO) delocalization) as happens in N-acyloxazolidines, both hydrolysis and reductive ring-opening become much more difficult as a consequence of a concomitant decrease of oxygen basicity and of an increase of the intracyclic C–O bond strength.

Stéréochimie des réactions du triphénylgermyllithium et des germyllithiums asymétriques sur les chloro- et les alkoxy-germanes : Synthèse de digermanes optiquement actifs

Abstract The stereochemistry of nucleophilic substitution of chloro- and alkoxy-germanes by germyllithium reagents, R3GeLi(R3 = Ph3, MePhNp or i-PrPhNp) has been studied. The behaviour of the electrophilic asymmetric germanium atom (in R3GeCl or R3GeOR′) and that of the nucleophilic germanium atom (in R3GeLi) during the reaction are examined. A selective route to the threo and meso isomers of the digermanes (MePhNpGe)2 and (i-PrPhNpGe)2 is described. The germyllithium reagents retain their configuration whilst inversion occurs for the chloro- and alkoxy-germanes.

N → P intramolecular stabilization of phosphenium ions and preparation of hypercoordinated phosphanes with unusual properties

The reaction of ArLi 1 (Ar = [C6H3(CH2NMe2)2-2,6]) with Ph2PCl affords the phosphane 2 which, on treatment with HCl, gives rise to the monoprotonated ammonium salt 3, even in the presence of an excess of HCl. 2 reacts with MeI to give exclusively the phosphonium salt 4. Reaction of 1 with PhPCl2 gives the stabilized phosphenium ion [ArPPh]+Cl−6. This ion is reduced by LiAlH4 in ArPHPh 9, which undergoes hydride abstraction on treatment with trityl cation, giving the same ionic phosphorus species. This unusual reaction is an example of increased reactivity of a PH bond resulting from hypercoordination at the phosphorus atom.

Stéréochimie des réactions de substitution des alcoxygermanes par les organolithiens et par les réactifs de Grignard

Abstract The reactions of organolithium reagents and Grignard reagents with optically active alkoxygermanes R 3 GeOR′ have been studied [R 3 GeOR′ = ()-i-PrPhNpGeOCH 3 , ()-i-PrPhNPGeOMen and ()-MePhNpGeOMen]. Saturated reagents (e.g. butyllithium) give retention of configuration at germanium whilst unsaturated reagents (e.g. allyl- or benzyllithium) lead to inversion.

Synthèse et comportement d'un nouvel organogermane optiquement actif, l'isopropylphényl-1-naphtylgermane

Abstract The preparation and the resolution of a new asymmetric organogermanium compound are described. Its absolute configuration is shown to be “ R ”. Nucleophilic substitutions at germanium seem to occur with a high degree of stereospecificity.