Lewis acid moieties

Studies of catalytic asymmetric Mukaiyama aldol reactions were initiated in the early 1990s. Until recently, however, there have been few reports of direct catalytic asymmetric aldol reactions [1]. Several groups have reported metallic and non-metallic catalysts for direct aldol reactions. In general, a metallic catalysis involves a synergistic function of the Bronsted basic and the Lewis acidic moieties in the catalyst (Scheme 2). The Bronsted basic moiety abstracts an a-pro-ton of the ketone to generate an enolate (6), and the Lewis acidic moiety activates the aldehyde (3). 

In this review, the synthesis and structure of PBs are presented, followed by their coordination to transition metals and the properties of the ensuing complexes. The related ambiphilic derivatives combining phosphines with other Lewis acid moieties (heavier group 13 or group 14 elements) are also discussed. 

The four coordination modes E-H (Figure 18) have been substantiated spectroscopically and structurally for PBs and related ambiphilic ligands. They differ in the participation of the Lewis acid moiety that may remain... 

A few years later, Landis drew similar conclusions for the l,l -ferro-cenyldiphosphines 53a-c featuring pendant benzoxaborolidine moieties.63 Combined with rhodium precursors, these ambiphilic ligands lead to catalytically active species in hydrogenation and hydroformyla-tion reactions (Tables 7 and 8), but the presence of the Lewis acid moiety has no significant effect on activity and selectivity (compared to ligands 51 and 52). 

The rate acceleration and the TT-face selectivity were explained in terms of the coordination of the Lewis acid to the acyl oxygen atom. The binding Lewis acid moiety interacts with the cis- 8-substituent, which forces the olefinic bond to adopt the conformation XXII, approximately orthogonal to the acyl group. The olefinic bond is thus rendered more nucleophilic, accounting for the rapid reaction, and the electrophile E preferentially approaches the face not shielded by the iron auxiliary (Sch. 25). 

As described in Sections 2.3.1.2 and 2.2.3, Choudary et al. recently revealed nanocrystalline magnesium oxide (NAP-MgO) as a recyclable heterogeneous catalyst [40, 45]. These authors extended the use of this new type of heterogeneous catalyst for the asymmetric Michael reaction of different acyclic enones with nitromethane and 2-nitropropane [69a]. In a Michael reaction of chalcone with nitromethane in THF solvent at -20°C, NAP-MgO/(lR,2R)-(-)-diaminocyclohexane (DAC) was found to be an excellent catalyst system (96% ee, 95% yield) (Scheme 2.32). This Michael reaction proceeds via the dual activation of both substrates (nucleophiles and electrophiles) by NAP-MgO. The Lewis basic site (O /O ) of the NAP-MgO activates the nitroalkanes, while the Lewis acid moiety (Mg /Mg )... 

Ambiphilic Ligands Unusual Coordination and Reactivity Arising from Lewis Acid Moieties... 

By design, ambiphilic ligands combine donor and acceptor sites on the same skeleton (Figure 1). The basic idea is to use donor sites as anchors to introduce Lewis acids in the coordination sphere of transition metals. Fine tuning of the structure of the ambiphilic ligands (coordination sites and linker) gives the possibility to control the position of the Lewis acid moiety and the way it participates in bonding and/or reactivity. 

Ambiphilic ligands were investigated early on in Si—Si and C—C coupling with the aim to substantiate the beneficial influence the introduction of boranes or alanes may have. Although the exact role of the Lewis acid moiety during catalysis remains generally unclear, comparison with Lewis acid-free systems has often revealed significant improvments in terms of activity and/or selectivity. 

The phosphine-alane is expected to displace PPh at Ni, but no reaction takes place in the absence of substrate due to strong intramolecular P Al interactions. Lewis bases such a triethylamine are able to split the head-to-tail phosphine-alane dimer and consistently, further increase catalytic activity. The precise role of the Lewis acid moiety in the catalytic cycle remains unknown. It is supposed to interact with the methyl group at Ni and to facilitate methyl/silyl exchange. The key active species 36a could not be characterized, but its Lewis base adduct 36b was identified by NMR (Figure 19). 

Since platinum(II) and - to a lesser extent - palladium(II) complexes are kinetically stable, most of the complexes described herein are prepared by reacting a preformed platinum(n) or paUadium(II) complex with a salt of a d ° or s cation. In some cases, the Lewis-acidic moiety is also a preformed complex containing labile ligands. There are some examples of more complicated reactions in which coordination to a Lewis acid induces changes in the coordination sphere of Pt to form oligomeric structures that cannot be isolated otherwise (see Sect. 4.2.1). 

Terminally bound alanyle complexes of the type L M-A1R2 contain an electron-deficient A1 center that, in principle, may act as a Lewis acid moiety. Such complexes therefore tend to form intra- or intermolecularly coordinated structures as observed in base-stabilized complexes such as [( 7 -C5H5)(CO)2Fe-Al (CH2)3NMe2)/-Bu] and in dimeric complexes such as [(C5H4Me)(p-77 77 -C5H3Me)Mo(p-Al(H)/-Bu)]. Compound [Cp(CO)2Fe-Al(tmp)2] is the only... 

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