Asymmetric reactions with bidentate phosphines

A short five-step synthesis of a bifuran, namely ( )-2,2 -bis(diphenylphosphino)-3,3 -binaphtho[2,l-I>]furan (BINAPFu) from naphthofuranone via a low-valent titanium-mediated dimerization was reported. The newly developed resolution procedure for phosphines was utilized to provide the optically active bidentate phosphine ligands (BINAPFu), which consistently outperforms BINAP in the asymmetric Heck reaction between 2,3-dihydrofuran and phenyl triflate . Another way in which a benzofuranone can be converted into benzo[7 ]furan is by treatment of the former with 1-BU2AIH at -78°C followed by an acidic work up <00TL5803>. 

In 2000, Mathey described the use of the interesting bidentate phosphine BIPNOR in the rhodium-catalyzed asymmetric isomerization of the meso-diene illustrated in Eq. (4) [5]. Interestingly, the reaction proceeds with much lower enantioselectivity when BINAP is employed as the ligand (<40% ee). 

Molecular mechanics modeling of the asymmetric hydrogenation must begin with the mechanism of the reaction. When the prochiral olefin binds to the catalyst containing chiral bidentate phosphine, two possible diastereomers result one with the re face and one with the si face of the olefin coordinated to the metal (Fig. 3). Work in the Halpern and Brown laboratories has shown that the observed enantiomeric product cannot result from the diastereomer observed in solution (17-20). Thus, the minor diastereomer, which cannot be observed, must be responsible for the dominant chiral product. Any molecular mechanics model of the asymmetric hydrogenation reaction must explain how the minor diastereomer reacts faster than the major. 

Stelzer and co-workers reported a number of chiral water-soluble secondary phosphines [14], prepared by nucleophilic phosphination of primary phosphines with fluorinated aryl sulfonates in the superbasic medium DMSO/KOH. Further reaction with alkyl halides gives bidentate tertiary phosphines with P-chirality, but only racemic versions have been reported so far. Hanson et al. introduced so-called surface-active phosphines into asymmetric aqueous-phase catalysis. One of the main problems inherent to two-phase catalysis is the often very low miscibility of the substrates in the aqueous phase. Insertion of long alkyl chains between phosphorus atoms and phenyl groups in sulfonated phosphine ligands has been proven to increase reaction rates in the Rh-catalyzed hydroformylation of 1-octene [15], This concept was extended to a number of chiral ligands, i.e., the monoden-... 

In 2008, Shibata and co-workers reported a cationic iridium bidentate phosphine complex catalyzed C—H bond alkylation reaction of 2-methy-lacetophenone with alkynes and alkenes (Scheme 5.61). Compared with alkynes, the use of styrenes as acceptors requires the weakly coordinated tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) as a counteranion of the iridium complex and a higher temperature to achieve good yields. When a norbornene was used as the acceptor, the sole example of an asymmetric ortho C—H bond alkylation product 175 was obtained in 58% yield and 70% ee. 

Asymmetric a-arylations of ketones to form products containing quartemary benzyhc stereogenic centers with high enantioselectivities were also reported by Hartwig et al. and Buchwald et al. Initial studies employed the traditional Pd/BINAP or Tol-BINAP complexes, which in many cases resulted in modest enantioselectivity. Later, other combinations of Pd or Ni precatalysts with ligands such as electron-rich monodentate or bidentate phosphines, l,l -bis-substi-tuted ferrocenyl phosphines, or A(-heterocyclic carbenes/-phosphines expanded the scope of this reaction. 

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