Jack Besancon

Tandem carbonyl coupling-rearrangements promoted by the niobium(III) reagent. Dual reductive and Lewis acid properties of NbCl3(DME)

Abstract Benzaldehyde (1a) and acetophenone (6) are shown to undergo highly stereoselective reductive couplings by NbCl3(DME). When the reaction is performed at −10°C, the threo diols can be isolated. The intermediate niobiopinacols reveal the ability of subsequent transformations, due to the Lewis acid properties of the reagent used. Starting from 1a stereospecific acetalization leads to threo-2,4,5-triphenyl-1,3-dioxolane (3a), whereas 3,3-diphenyl-2-butanone (7) is obtained from 6 in a pinacol-pinacolone type rearrangement. Alkenes may also be formed in the competing, temperature controlled deoxygenation The stereoselectivity and the easy control of NbCl3 (DME) mediated reactions make it a promising reagent for further synthetic applications.

Pinacol coupling of aliphatic aldehydes promoted by niobium (III) reagent

Abstract NbCl3 (DME) was found to be a useful reagent for the intermolecular pinacol coupling of aliphatic aldehydes. The high anti diastereoselectivity of this reaction (dl / meso ≥ 9 / 1) did not depend on the variation in the aldehyde alkyl chain. With respect to intramolecular pinacolization, the loss of the stereoselectivity observed is consistent with the mechanism involving an insertion of the oxo-group into the metal-carbon bond of the intermediate niobiooxirane.

Recherches dans la série des métallocènes : XXVIII. Action des réctifs de grignard sur les cétones benchrotréniques chirales

Abstract Benchrotrenyl chiral ketones beating an inductive group (CH 3 or OCH 3 ) were treated with Grignard reagents. The percentage of the diastereoisomeric tertiary alcohols obtained in each case depends on the natures of the inductive group and the benchrotrenyl ketone. In some cases the yield consists mainly or solely of the two secondary alcohols, obtained via reduction process. The structure of the alcohols is established by either spectroscopic or crystallographic means. Models complying with the observed stereoselectivity are proposed in each case.

Pseudoasymetrie dans la serie du benchrotrene I. Synthese des trois glycols ortho-substitues (OC)3CrC6H4(CHOHCH3)2

Abstract The three stereoisomeric glycols (CHOHCH 3 ) 2 C 6 H 4 Cr(CO) 3 have been prepared via 1,2-diformyl- and 1,2-diacetyl-benchrotrenes. The racemic (pseudosymmetric) form and the two meso (pseudoasymmetric) forms of the glycols have been identified by chemical derivatization and analysis of the spectroscopic data. The photochemical decomplexation of these glycols is a specific route for the preparation of the two corresponding benzenic glycols. The stereoselectivity which has been observed in some reactions is discussed.

Synthesis of bent titanocene metalloligands with the (diphenylphosphino)tetramethylcyclopentadienyl moiety. X-ray structure of [(η5-C5Me4PPh2)2TiCl2]Mo(CO)4

The reactions of lithium(diphenylphosphino)tetramethylcyclopentadienide with CpTiCl 3 and secondly with TiCl 3 followed by CCl 4 oxidation lead to the formation of two titanocene phosphines: (η 5 -C 5 H 5 )[η 5 -C 5 Me 4 P(C 6 H 5 ) 2 ]TiCl 2 ( 2 ) and [η 5 -C 5 Me 4 P(C 6 H 5 ) 2 ] 2 TiCl 2 ( 3 ), respectively. The metalloligand 3 reacts readily with Mo(CO) 4 cod, Mo(CO) 5 THF and Mo(CO) 6 to give in each case [(η 5 -C 5 Me 4 o(CO) 4 ( 6 ) as a sole product. The structure of 6 has been determined by X-ray diffraction. Crystal data: P , a = 11.716(1), b = 11.753(2), c = 16.110(2) A, α = 99.06(1), β = 92.61(1), γ = 104.20(1)°, Z = 2. The molybdenum-titanium distance of 5.194(1) A rules out any metal-metal interaction. The chlorine substitution reactions by CO in 2 and 3 and by thiolate group (pH 3 C-C 6 H 4 -S) in 16 are reported.

Problems de stereochimie dynamique en serie du titanocene ☆: II. Stereochimie dynamique de reactions déchange de ligands σ-lies

Two ligand exchange reactions at the titanium atom in quasi-tetrahedral titanocene complexes have been studied. The first is substitution of a Cl ligand by an aryloxy group starting from substrates η5-Cp-η5-Cp′Ti(Cl)OPh which have an planar chirality on the Cp′ ring. The second is the substitution of one of the aryloxy groups of the complexes η5-Cp-η5-Cp′Ti(OPh′)OPh by the action of HCl. In this case, the reaction is generally selective and has a high degree of stereo-specificity with retention at the titanium atom. This retention has been established by crystallographic analysis of two suitable substrates: diastereoisomer F. 171°C of η5-C5H5[η5-(1-Me-3-CHMe2C5H5)](2-ClC6H4O)(2,6-Me2C6H3O)Ti and diastereoisomer F. 134°C of η5-C5H5[η5-(1-Me-3-CHMe2C5H3)](2-ClC6H4O)TiCl.

Problemes de stereochimie dynamique en serie du titanocene : I. Synthese de diastereoisomeres chloroaryloxy et diaryloxy derives de complexes presentant une chiralite planaire sur Lún des ligands cyclopentadienyle

Abstract From the two dichlorides η5-Cp-η5-Cp′TiCl2 (Cp = C5H5; Cp′ = C5H3MeCHMe2-1,2 or -1,3), couples of diastereoisomeric chloroaryloxy [η5-Cp-η5-Cp′Ti(Cl)OPh] or of diastereoisomeric diaryloxy [η5-Cp-η5-Cp′Ti(OPh′)OPh], titanocene derivatives, are prepared in several ways. In each case, the diastereoisomeric complexes are separated and identified. These complexes are starting substrates which can be used for dynamic stereochemistry of the ligand substitution process which will be discussed in a following paper.

Recherches dans la serie des metallocenes

Abstract Three methods of obtaining asymmetric diaryloxy complexes of titanium ( h 5 -Cp)( h 5 -Cp′)Ti(OAr)(OAr′) are proposed. Each of these methods leads to secondary products, selective substitution of the ligand (OAr) or (OAr′) by chlorine atom occurs by the action of hydrochloric acid in C 6 H 6 solution.

Structure cristalline des deux éthylphényl-o-méthylbenchrotrényl méthanols

Abstract The crystalline structure of the two diastereomeric ethyl-phenyl-o-methylbenchrotrenylmethanols (racemic forms) have been determined by X-ray diffraction in support of the discussion on asymmetric induction presented in J. Organometal. Chem, 59 (1973) 267.

Pseudoasymetrie dans la serie du benchrotrene II. Structure cristalline de la forme pseudoasymetrique F 144° des glycols ortho-substitutes (OC)3CrC6H4(CHOHCH3)2 ☆

Abstract The crystalline structure of one of the pseudoasymmetric forms of the benchrotrenic glycols 1,2-(CHOHCH 3 ) 2 C 6 H 4 Cr(CO) 3 (m.p. 144° C) has been determined by X-ray diffraction to provide a complete identification of these isomeric glycols.