Lewis acids pendant

Subsequently, Landis studied the coordination of the ferrocene-based diphosphine-benzoxaborolidine ligands 53 with the aim of supporting secondary interactions between the pendant Lewis acid and an external Lewis base.63 Stable platinum complexes 63a-c were formed by reacting... 

A. Pendant Lewis acids as potential anchoring sites in catalysis... 

The pioneering studies of Kagan and Landis pave the way to the use of remote Lewis acids, but strong evidence for substrate anchoring under catalytic conditions is still missing. Further developments in this area would probably be possible by finely tuning the position and electron-deficiency of the pendant Lewis acid. 

More recently, complexes featuring pendant Lewis acids were prepared from C -bridged phosphine-boranes (Figure 6). As mentioned above, the orf/io-phenylene linker usually favors the participation of the Lewis acid in coordination, but in the Pd complex 13, this effect is counterbalanced by the bulky Mes which prevents interaction of the boron center with the metal fragment. Despite smaller substituents at boron, the Ru complex 14 adopts the same coordination mode. = The flexibility of the CH CH linker probably comes into play here (the phosphine-borane ligand adopts antiperiplanar conformation). 

Figure 5 Rh, Pd and Pt complexes featuring pendant Lewis acids...

To date, the ability of external Lewis bases to bind to the pendant Lewis acid (anchoring effect) has not been structurally substantiated, but H/"B NMR evidence has been reported in a few instances using A -based donors. ... 

The synthesis of oxygen heterocycles in which cyclization onto a pendant alkyne is a key step has also been achieved. Reaction (7.36) shows an example of iodoacetal 29 cyclization at low temperature that afforded the expected furanic derivative in moderate Z selectivity [47]. A nice example of Lewis acid complexation which assists the radical cyclization is given by aluminium tris(2,6-diphenyl phenoxide) (ATPH) [48]. The (3-iodoether 30 can be com-plexed by 2 equiv of ATPH, which has a very important template effect, facilitating the subsequent radical intramolecular addition and orienting the (TMS)3SiH approach from one face. The result is the formation of cyclization products with Z selectivity and in quantitative yield (Reaction 7.37). 

The simplest bonding situation conceivable upon coordination of an ambiphilic ligand to a transition metal is that involving the sole coordination of the Lewis base moiety, the Lewis acid fragment remaining pendant. Several examples of such complexes have been described in the recent years, most frequently upon coordination of preformed ambiphilic ligands. 

According to X-ray diffraction data, in the solid-state complex 87 adopts a six-membered metallacyclic structure, as the result of P-Pd-Cl-B bridging coordination. The interaction of the borane moiety with the chlorine atom is apparent from the short Cl-B distance (2.165(2) A) and the pyramidalization of the boron environment (EBa = 349.1°). The bridging coordination of the Pd-Cl bond by the PB 7b contrasts with the B-pendant coordination mode adopted by complex 72, deriving from the related ligand 7e featuring mesityl groups at boron (see Section IV.A). This illustrates the role of steric factors in the participation, or not, of the Lewis acid in the coordination assembly. 

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 reaction is based upon the two components condensation between an aldehyde or ketone 6 (or their synthetic equivalents) and alcohol 95, which contains an allylsilane (or vinylsilane) moiety. The IMSC reaction is mediated by Lewis or Bronsted acids, which activate the carbonyl group of 6 towards nucleophilic attack. After addition of alcohol 95 on the activated carbonyl, the oxonium cation 96 is formed, which is intramolecularly captured by the pendant allylsilane function, leading to oxygen-containing rings 97 (Scheme 13.38). This process typically requires a stoichiometric (or more) amount of Lewis acid. 

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