Segphos chiral ligand

An interesting approach to investigating the relationship between the position of enantiodescriminating sites in a number of chiral ligands and enantioselec-tivity in enantioselective hydrogenation has been proposed by Saito et al. [50]. In this report, (aS,S,S)-MPL-SEGPHOS (21) was used for the reduction of... 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

Furthermore, the first catalytic synthesis of allenes with high enantiomeric purity [15c, 25] was applied recently to the pheromone 12 by Ogasawara and Hayashi [26] (Scheme 18.7). Their palladium-catalyzed SN2 -substitution process of the bromo-diene 16 with dimethyl malonate in the presence of cesium tert-butanolate and catalytic amounts of the chiral ligand (R)-Segphos furnished allene 17 with 77% ee. Subsequent transformation into the desired target molecule 12 via decarboxylation and selenoxide elimination proceeded without appreciable loss of stereochemical purity and again (cf. Scheme 18.5) led to the formation of the allenic pheromone in practically the same enantiomeric ratio as in the natural sample. 

Besides BINAP, other axial chiral diphosphines (e.g., Hg-BINAP, SEGPHOS, MeO-BIPHEP, P-Phos, etc.) as well as planar chiral ligands (e.g., PhanePhos) became very popular as a result of their high activity and selectivity in a variety of asymmetric hydrogenations (Figure 30. IB). 

The ruthenium catalyst generated in situ from H2Ru(CO)(PPh3)3, (S)-SEGPHOS and the TADDOL-derived phosphoric acid (169) promoted butadiene hydrohydroxyalkylation to form enantiomerically enriched products (168) (Scheme 61). Match/mismatch effects between the chiral ligand and the chiral TADDOL-phosphate counterion have been described. For the first time, single-crystal X-ray diffraction data for a ruthenium complex modified by a chiral phosphate counterion has been reported. ... 

The copper-catalyzed chiral reduction of -substituted ,/Tunsaturated lactones with PMHS and (S)-/ -Tol-BINAP in the presence of a hindered alcohol can be carried out in moderate to good yields with moderate ee values.599 The reaction is useful for both butenolides and pentenolides. Inferior results are realized with diphenylsilane as the reducing agent. Excellent results employing PMHS and the DTBM-SEGPHOS ligand are possible (Eq. 354).598... 

Molecules containing allene and aUcene units that ate separated by several bonds are subject to cycloisomerization. With a catalyst derived from AuCl and a SEGPHOS ligand 16, chiral products of formal [2+2]cycloadditon are obtained. ... 

The chiral biaryl bisphosphine ligand SEGPHOS 34, developed by Takasago, possesses a smaller dihedral angle than BINAP. The ligand has provided greater enantioselectivity over BINAP in Ru-catalyzed hydrogenation of a wide variety of carbonyl compounds.31... 

Currently, a significant body of work deals with the use of chiral cationic palladium complexes bearing ligands of the BINAP type or related bisphosphine ligands such as SEGPHOS (Fig. 3). These are based on the pioneering work from Sodeoka on the direct formation of chiral palladium enolate complexes from the palladium precursors and 1,3-dicarbonyl compounds [10, 23]. Within this context, the combination of cationic BINAP-Pd complexes and N-fluoro-bis(phenylsulfonyl)imine (NFSI) was introduced by Sodeoka for the realization of an extremely efficient a-fluorination of 3-keto esters (Scheme 5). 

Duan et al have synthesized a series of chiral alkylphosphonates (449) bearing a )5-stereogenic center in good enantioselectivities (up to 95% ee) via the CuH-catalyzed asymmetric, conjugate reduction of jS-substituted a,j8-unsaturated phosphonates (448) under optimal conditions using Cu(0Ac)2 H20 as the copper source, (R)-SEGPHOS as the ligand, polymethylhydrosiloxane as the siloxane, and /-BuOH as the additive (Scheme 111). 

The sense of chirality in these hydrosilylations is such that (7 j-(-)-DTBM-SEGPHOS predictably delivers hydride to aryl alkyl ketones fi om the si face to give alcohols of R absolute stereochemistry, which is also true for imines. Stereocontrol at a p-site due to asymmetric 1,4-additions is controlled by either the nature of the geometrical isomer (i.e., or Z) or the axial chirality of the ligand. Thus, by switching from the (/ )- to the (5)-enantiomer of the ligand, or from the E- to the Z-activated olefin isomer, the observed central chirality can be inverted. 

Moreover, deconstructing the tridentate architecture of the triphos ligand into chiral diphosphines such as xyl-BINAP or SEGPHOS provided an enantios-elective cycloisomerization with enantio-ratios up to 98 2 (Scheme 31) (68). 

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