Ligands sparteine

D. Reactions with the Chiral Diamine Ligand (—)-Sparteine... 928... 

All evidence points to a kineticaUy controlled differentiation between enantiotopic protons, leading to a configurationally stable intermediate 150 , which is stereospecifically substituted with retention of the configuration. Experiments with the deuteriated substrate 149-D (D for H at N-CH2 in 149) and the results of kinetic smdies support this assumption . The ligand (—)-sparteine (11) in 150 contributes to enhanced configurational stability this can be concluded from the lithiodestannylation experiment shown in equation 34. 

Fig. 5 Relationship between the amount of graft polymers and Mn of free polymers. The graft polymerization was carried out under various conditions on silicon wafer (squares), silica particles with varying diameter (d = 12, 130, 290, 740, 1550 nm) (circles), and silica monolith with 50-nm mesopores (triangles). Two types of immobilized initiators, 2 and 5 (n = 6 and R" = CH3) in Fig. 2, two types of copper halides, CuBr and CnCl, and two types of ligands, spartein (Sp) and dipyridyl derivatives (4,4 -diheptyl-2,2 -dipyridyl (dHbipy) and 4,4 -dinonyl-2,2 -dipyridyl (dNbipy)), were used...

The rate-determining step occurs during (3-hydride elimination. One model for enantioselection is shown in Figure 12-4, where the diamine ligand, (- sparteine, exhibits Q symmetry48 instead of C2. C2 symmetry is associated with dia-stereomeric structure 22b, (-)-a-isosparteine). C2 is also the inherent symmetry of chiral bisphosphine ligands used for asymmetric hydrogenation.49... 

Kinetic resolution of some secondary allylic and benzylic alcohols has been shown to occur efficiently in the presence of the chiral ligand (—)-sparteine. For example, partial oxidation of the racemic alcohol 43 with a palladium(II) catalyst under an atmosphere of oxygen in the presence of (—)-sparteine occurs to give a mixture of the ketone 44 and recovered alcohol (5)-43 (6.39). Selective oxidation of the (i )-alcohol occurs with the chiral catalyst system. 

In 123, the internal chiral induction can be overridden by the external ligand (-)-sparteine (Sect. 2.5.4) [78]. 

A chiral ligand mediated approach to lithiation-substitutions of allylic amines has also been well developed. Weisenburger and Beak demonstrated that lithiation of doubly protected allylic amines 141 in the presence of the chiral ligand (-)-sparteine (5), and substitution with a variety of electrophiles provided highly enantioenriched enecarbamate products 142 (Scheme 44) [100]. The authors demonstrated that the intermediate organolithium could be viewed as either an aldehyde P-homoenolate or y-lithioamine synthetic equivalent by hydrolysis or reduction and deprotection of the enecarbamates, respectively. 

These processes could be performed in an enantioselective manner by addition of (-)-sparteine or bisoxazoline ligands in general the yields and ees were found to... 

As far as investigated77, most reactions of the allyllithium-sparteine complexes with electrophiles proceed antarafacially, either as SE2 or anti-SE2 reactions. As a working hypothesis it is assumed that the bulky ligand obliterates the Lewis acid properties of the lithium cation. 

It should also be noted that the 5-exo-trig cyclization of achiral olefinic organolithiums has been found to proceed enantioselectively when conducted in the presence of a chiral ligand that serves to render the lithium atom stereogenic. Thus, for example, R) 1 -allyl-3-methylindolinc has been prepared in 86 % ee by cyclization of an achiral aryllithium in the presence of an equivalent of (-)-sparteine.15... 

Abstract While the use of stoichiometric amounts of sparteine and related ligands in various asymmetric reactions often lead to highly enantioselective transformations, there have been far fewer applications of sparteine to asymmetric catalysis. The aim of this review is to highlight recent advances in the field of asymmetric transformations that use sparteine as chiral auxiliary, emphasizing the use of substoichiometric or catalytic amounts of this ligand. 

An early example of the use of a subcatalytic amoimt of sparteine for the activation of an organolithium nucleophile was reported by Lautens et al. in the carbometallation of a meso-unsaturated oxabicycle 25, with ring opening leading to the substituted cycloheptene derivative 26 (Scheme 4) [4]. Both yield and enantiomeric excess remained virtually unchanged when the ratio n-BuLi sparteine was lowered to 1 0.15. However, when a 3 mol% amount of the ligand 1 was used, a 20% decrease in enantioselectivity was observed. 

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