SEGPHOS ligand

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. ... 

Indenols. Coupling between o-formylarylboronic acids with alkynes leads to indenols. The catalyst system includes Pd(OTf)2 2H2O and Me4-SEGPHOS ligand. ... 

Fig. 3 Dimeric and monomeric palladium complexes for enantioselective halogenation. (5)-BINAP ligands and (5)-Segphos-ligand...

The asymmetric Friedel-Crafts reaction of trifluoromethyl pyruvate 53 with aromatic compounds is catalyzed by cationic Pd(II) complexes with BINAP or SEGPHOS [184]. The reaction proceeded at -30 °C to afford the product 63 in 89% ee with (S)-BINAP and in 82% ee with (S)-SEGPHOS (Scheme 45). In sharp contrast to the situation of the carbonyl-ene reaction, the BINAP ligand provides higher enantioselectivity than the SEGPHOS ligand. 

The hydrogenation/dynamic kinetic resolution shown in Scheme 32 for the production of a penem antibiotic intermediate is carried out by Takasago [72] on a scale of 50-120 t/year, and recently, it was reported that an optimized segphos ligand [81] can achieve even higher stereoselectivities. Similar results were claimed by Chemi [82] with a Ru/tmbtp catalyst. 

Later on, Carretero and co-workers reported the first enantioselective procedure for the Cu(I)-catalyzed 1,3-dipolar cycloaddition involving a-silylimines as azomethine precursors to provide a variety of 5-unsubstituted a-quatemary proline derivatives in good yields with excellent levels of diastereoselectivity and enantiocontrol (Scheme 11) [23]. The use of the bulky DTBM-SEGPHOS ligand proved to be crucial for obtaining this high enantioselectivity. Moreover, a,-P-unsaturated sulfones could also be employed as dipolarophiles in this catalytic system. 

Optically active furyl cyclopropane 402 was prepared from acetylene dicarboxylate and alkenes (Scheme 1.187) [261]. The acetylene dicarboxylate underwent dimerization to form metallocyclopentadiene 403, which decomposes to give cyclopropanes containing rhodium carbene complex 404. A good level of chiral induction was achieved using Segphos ligand with Rh(cod)2Bp4. 

Scheme 2.25 shows a probable mechanism for the formation of (R,R)-(—)-isomer 2.69 on the basis of l,2-bis(arylpropiolyl)benzenes 2.68 and but-2-yn-l-ol (2.60c, see Scheme 2.21) [65]. Enantio- and stereoselectivity is confirmed also by the formation of intermediate A due to the steric interactions between the two methyl groups of compound 2.68 and two axial PPh2 groups of the (S)-Segphos ligand and...

The in situ generation of CuH from organosilanes in the presence of either a BIPHEP (99) or a SEGPHOS (100) type ligand represents a general method for the asymmetric hydrosilylation of aryl alkyl ketones at low temperatures. 

Vinylsilane to copper transmetallation has entered the literature,93 93a,93b and a system suitable for catalytic asymmetric addition of vinylsilanes to aldehydes was developed (Scheme 24).94 A copper(l) fluoride or alkoxide is necessary to initiate transmetallation, and the work employs a copper(ll) fluoride salt as a pre-catalyst, presumably reduced in situ by excess phosphine ligand. The use of a bis-phosphine was found crucial for reactivity of the vinylcopper species, which ordinarily would not be regarded as good nucleophiles for addition to aldehydes. The highly tailored 5,5 -bis(di(3,5-di-tert-butyl-4-methoxyphenyl)phosphino-4,4 -bis(benzodioxolyl) (DTBM-SEGPHOS) (see Scheme 24) was found to provide the best results, and the use of alkoxysilanes is required. Functional group tolerance has not been adequately addressed, but the method does appear encouraging as a way to activate vinylsilanes for use as nucleophiles. 

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. 

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