This paper describes the reactivity of permethylzirconocene and permethyltitanocene toward different
1,3-butadiynes. A pointed dependence on the metals and the diyne substituents was found. Unusual, but still
stable, five-membered zirconacyclocumulenes (η4-diyne complexes, zirconacyclopenta-2,3,4-trienes) Cp*2Zr(η4-1,2,3,4-RC4R), R = Ph and SiMe3, were prepared using two new and effective synthetic routes. One starts
with the permethylzirconocene bisacetylides Cp*2Zr(C⋮CR)2, R = Ph (1a), SiMe3 (1b), which rearrange in
sunlight to form the stable five-membered zirconacyclocumulenes Cp*2Zr(η4-1,2,3,4-RC4R), R = Ph (2a),
SiMe3 (2b). The alternative route to 2a and 2b is the reduction of Cp*2ZrCl2 with Mg in the presence of the
adequate disubstituted butadiynes RC⋮C−C⋮CR. Both methods failed to produce the analogous titanacyclocumulenes, which seemed extremely unstable. Nevertheless, we were able to obtain distinct products
employing the reduction pathway with permethyltitanocene. For R = SiMe3, the novel titanacyclopropene
(η2-complex) Cp*2Ti(η2-1,2-Me3SiC2C⋮CSiMe3) (3) was isolated. For R = Ph, an activation of both
pentamethylcyclopentadienyl ligands was observed resulting in the complex [η5-C5Me4−(CH2)−]Ti[−C(CHPh)C(CHPh)CH2-η5-C5Me4] (4). The reaction of 4 with carbon dioxide led to the Cp*-substituted
titanafuranone Cp*Ti[−OC(O)C(Ph)C(−)C(CHPh)CH2-η5-C5Me4] (5). The zirconacyclocumulene 2b surprisingly inserted two molecules of CO2 to give the unprecedented cumulenic dicarboxylate Cp*2Zr [−OC(O)C(SiMe3)CCC(SiMe3)C(O)O−] (6). The η2-complex 3 (titanacyclopropene) took up one molecule
of carbon dioxide to afford the titanafuranone Cp*2Ti[OC(O)C(SiMe3)C(C⋮CSiMe3)−] (7).
This contribution gives an overview of the synthesis of chiral -amino acids via asymmetric hydrogenation of the corresponding dehydroamino derivatives. Literature results are discussed regarding substrate synthesis and catalyst performance and how it is affected by substrate and catalyst structure as well as experimental parameters. A tentative mechanistic concept for the hydrogenation step is also presented.
IntroductionEnantiomerically pure -amino acids and their derivatives not only exhibit broad biological activity but are also the building blocks for the synthesis of -peptides. The latter are characterized by a high enzymatic stability and show interesting three-dimensional structures. 1 The cyclization of -amino acids leads to the important family of the -lactams. Scheme 1 shows examples of pharmaceutically interesting structures containing a -aryl-substituted -amino acid as a common structural component. 2 Methods for the preparation of optically enriched -amino acids are predominantly based on stoichiometric reactions with chiral auxiliary agents and to a clearly smaller extent on stereoselective catalytic reactions. 2d,e,3 One of the most promising methodologies, also regarding industrial application, is the asymmetric hydrogenation of the appropriate -dehydroamino acid precursors catalyzed by homogeneous Rh or Ru complexes containing chiral phosphane ligands. In contrast to the synthesis of R-amino acid precursors where it is a standard method with many industrial applications, 4
The complexes Cp 2 Hf(PMe 3 )(η 2 -Me 3 SiC 2 SiMe 3 ) (3) and Cp* 2 Hf(η 2 -Me 3 SiC 2 SiMe 3 ) (4), as the first examples of welldefined hafnium alkyne complexes with a simple intact alkyne, are described; 4 displays an unusually strong interaction of the alkyne with hafnium, in comparison to analogous compounds of titanium and zirconium.
The reactions of the titanium(III) complex Cp*2TiCl with lithium acetylides RC⋮CLi
depend strongly on the solvents used. In THF only the lithium tweezer compounds [Cp*2Ti(C⋮CR)2Li(THF)
n
] (Cp* = η5-C5Me5; R = SiMe3, n = 1 (1); R = t-Bu, n = 1 (2); R = Ph, n
= 2 (3)), not the expected mono(σ-alkynyl) titanocene(III) complexes, were obtained, whereas
in n-hexane these complexes Cp*2Ti(C⋮CR) were isolated for R = Me (4), t-Bu (5). The
reaction of the complex [Cp*2Ti(C⋮CSiMe3)2Li(THF)] (1) with carbon dioxide results in the
titanafuranone 6, which presumably is formed by insertion of carbon dioxide into the η2-butadiyne complex Cp*2Ti(η2-Me3SiC⋮CC⋮CSiMe3). The complex [Cp*2Ti(C⋮C-t-Bu)2Li(THF)] (2) reacts with carbon dioxide to yield the permethyltitanocene carboxylate
Cp*2Ti(O2CC⋮C-t-Bu) (7) via insertion into the σ-acetylide bond of the in situ formed
titanium(III) intermediate [Cp*2Ti−C⋮C-t-Bu]. Compound 7 was also obtained in the reaction
of the isolated titanium(III) complex Cp*2Ti−C⋮C-t-Bu (5) with carbon dioxide. The
interaction of [Cp*2Ti(C⋮CPh)2Li(THF)2] (3) with carbon dioxide gives the stable bis(σ-acetylide) permethyltitanocene Cp*2Ti(C⋮CPh)2 (8) without coupling of the σ-acetylide groups
(as found for 1) and without insertion into the σ-acetylide bond (as found for 2). The syntheses
and reactions of these compounds are compared to those of similar complexes of unsubstituted
titanocene compounds. Complexes 1, 5, and 7 were investigated by X-ray crystal structure
analysis.
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