Lewis basic phosphine

Shibasaki et al. developed a polymer-supported bifunctional catalyst (33) in which aluminum was complexed to a chiral binaphtyl derivative containing also two Lewis basic phosphine oxide-functionahties. The binaphtyl unit was attached via a non-coordinating alkenyl Hnker to the Janda Jel-polymer, a polystyrene resin containing flexible tetrahydrofuran-derived cross-Hnkers and showing better swelling properties than Merifield resins (Scheme 4.19) [105]. Catalyst (33) was employed in the enantioselective Strecker-type synthesis of imines with TMSCN. 

In the presence of 168 a (9mol%) and a phosphine oxide (Bu),P(O) and Ph2P(O)Me for aromatic and ahphatic aldehydes, respectively, 36 mol%), slow addition of TMSCN achieves excellent enantioselectivity with a wide range of aldehydes (86-100%, 83-98% ee). The Al complex has been proposed to work as a bifunctional catalyst for dual activation of the two reactants - the Lewis acidic Al center enhances the electrophilicity of aldehydes and the Lewis basic phosphine oxide induces cyanide addition by nucleophihc activation (Scheme 10.240). This catalytic asymmetric cyanosilylation has been used for the total synthesis of epothilones [652]. 

Oshima and coworkers have reported that treatment of tertiary alkyl fluorides with diphenylphosphine in the presence of a stoichiometric amount of BF3 OEt2 gives rise to the corresponding tert-alkyldiphenylphosphine tetrafluoroborate salts, which can be trapped by elemental sulfur to give the phosphine sulfides (144) (Equation 86) [88]. Remarkably, this reaction works despite the presence of the highly coordinating Lewis basic phosphine. 

Shibasaki has described the use of bifunctional catalysis in asymmetric Strecker reactions, using BlNOL-derived Lewis acid-Lewis base catalyst 160 (Equation 24) [114]. The aluminum complex had previously been shown to catalyze enantioselective cyanohydrin formation (Chapter 2, Section 2.9) [115]. In the proposed catalytic cycle, the imine is activated by the Lewis acidic aluminum while TMSCN undergoes activation by association of the silyl group with the Lewis basic phosphine oxide. Interestingly, the addition of phenol as a putative proton source was beneficial in facilitating catalyst turnover. The nature of the amine employed for the formation of the N-substituted aldimine proved to be vital for enantioselectivity, with optimal results obtained for N-fluorenyl imines such as 159, derived from aliphatic, unsaturated, and aromatic aldehydes (70-96% ee) [114],... 

Highly Lewis basic and nucleophilic functional groups are not compatible with zinc carbenoids. The methylation or ylide formation of heteroatoms is one of the most important side reactions of these reagents. For example, amines, thioethers and phosphines readily react with the zinc reagents to generate ammonium salts", sulfonium" and phosphonium ylides" ". Terminal alkynes generally lead to a large number of by-products". ... 

The dithienophospholes 74 are obtained in good yields by lithiation of dibromo-bithiophenes 73, followed by addition of dihalo (aryl) phosphines (Scheme 4.22) [57]. The Lewis basicity of the P center of 74 enables a broad range of facile chemical modifications to be undertaken, as exemplified with the synthesis of derivatives 75-77 (Scheme 4.22) [57]. 

The P=0 —> Sn bonds in phosphine oxide complexes (Table 24) are somewhat shorter than those found in the same carbonyl (Table 23), sulfoxide (Table 25) and A-oxide (Table 26) complexes, reflecting the greater Lewis basicity of the former. 

In one of the few examples of a chiral ligand-controlled asymmetric reaction mediated by Sml2, Mikami reported the use of the chiral, bis-phosphine oxide, BINAPO (4), as a Lewis basic additive.23 For example, reductive coupling of methyl acrylate and acetophenone using Sml2 in THF with 2 equiv of BINAPO gave lactone 5 in 46% yield and 67% ee (Scheme 2.8).23... 

The first example of reversible spUtting of H2 on a nonmetal center has been found [19]. The phosphine borane in Eq. (5) has a strong Lewis acidic center (boron) linked to a Lewis basic site (phosphoms). 

For cyclopropylboron compounds, the initial search for 7r-donation from cyclopropyl to boron using H, and FNMR as well as UV spectroscopy did not provide any evidence for a preference for a bisected conformation for the cyclopropyl. Furthermore, tricyclopropylboron was a stronger Lewis acid than triisopropylboron in the formation of complexes with amines and phosphines . This latter behavior would be favored not only on steric grounds due to the presumed smaller size of cyclopropyl but also due to flT-withdrawal by cyclopropyl, but Ti-donor effects of cyclopropyl would decrease the Lewis basicity. 

It is known from previous work that diazoalkanes can form carbene(alkylidene)-metal complexes [17, 18], cf. eq. (5). It is thus reasonable to assume that a metal-carbene 9 is formed from the (phosphine oxide-stabilized) species CH3Re 02) (eq. (5)). High oxidation-state metal carbene complexes have ample precedent, especially through the work of Schrock et al. [19], Isolation of type-8 species may be facilitated by sterically more-demanding auxiliary groups (e. g., C5H5 in place of CH3) or, by using heterocyclic carbenes of pronounced Lewis basicity (e. g., 1.3-imidazolin-2-ylidene [20]). 

Other catalytic reactions may require an electron-rich metal to facilitate the ratedetermining (slowest) step of the catalytic cycle - Lewis basicity of the catalyst then plays a key role in substrate activation. For example, oxidative addition of is frequently rate determining in olefin hydrogenation reactions. The rate of alkene hydrogenation catalyzed by Rh(I)-diphosphinoferrocene complexes of type 2a (Figure 2) are known to increase the basicity of the phosphine moieties. In conventional approaches this can be achieved by... 

---