BINOL-aluminium complexes

Chiral aluminium complexes have been used as catalysts for inverse electron-demand 1,3-dipolar cycloadditions of alkenes with nitrones, and the first contribution to this field was pubhshed in 1999 (344). The chiral AlMe-BEMOL (BINOL = 2,2 -bis(diphenylphosphino)-l,l -binaphthyl) complexes 235 were excellent catalysts for the reaction between nitrone 225a and vinyl ethers 232 (Scheme 12.68). The diastereo- and enantioselectivities are highly dependent on the chiral ligand. An exo/endo ratio of 73 27 was observed, and the exo-product was... 

Other fates are possible for the enolate formed in the initial conjugate addition and an obvious possibility is an aldol reaction. With an asymmetric catalyst, the combination of three simple molecules leads to one enantiomer of one diastereoisomer of the tandem Michael-aldol product14 83. The catalyst 84 is based on a BINOL A1 complex (see chapters 25, 26). It can be drawn either as a lithium salt with an aluminium cation or, better, as a lithium aryloxide with a Lewis-acidic aluminium atom. This is better because both basic ArCT and Lewis acidity are necessary for catalysis. 

Aluminium-BINOL-based complexes have also been shown to be highly selective bifunctional asymmetric cyanation catalysts. For example the bisphosphine oxide (6.65) developed by Shibasaki and coworkers catalyses the cyanation of both aromatic and aliphatic aldehydes with ees ranging from 83-98%. ... 

The development of a catalytic enantioselective process has received much attention and, in common with the all-carbon Diels-Alder reaction, most success has been achieved using metal-based Lewis acids and, most recently, organocatalysts/ The earliest work on an enantioselective variant of this reaction was performed by Danishefsky/ The enantiomerically pure europium complex Eu(hfc)3 provided moderate enantioselectivities for the hetero-Diels-Alder reaction between derivatives of diene (8.123) and benzaldehyde (8.125), performed under solvent-free conditions. Higher ees have been obtained using the hindered BINOL ligand (8.126), as its aluminium complex. Yamamoto s CAB catalysts (see Section 8.1) such as (8.129), with the appropriate substituent on boron, have also been used to good effect. 

Scheme 19.7 Asymmetric cyanosilylation of aldehydes catalysed by aluminium complexes of BINOL-derived ligands.

In 1997, Feringa s group reported an enantioselective Michael addition of a-nitroesters to a,p-unsaturated ketones in the presence of a catalytic amount of (/ )-ALB ent-18. X-ray structure analysis in combination with NMR studies of the aluminium catalyst showed that Al-Li-BINOL is a mixture of aluminium complexes in solution (Scheme 19.26). 

An aluminum-lithiiun catalyst, (R)-ALB, prepared from (R)-BINOL, and lithium aluminium hydride promoted the addition of malonate to 23 giving (R)-44 in 99% ee. X-ray analysis of the ALB catalyst showed an aluminum ate complex structure with li coordination to the oxygen atom. The asymmetric tandem Michael-aldol reaction of 46 was conducted with this catalyst giving a single isomer 47 containing three asymmetric centers. The aluminum enolate under-... 

The bifunctional catalysts developed by Shibasaki and coworkers effective in the asymmetric cyanation of aldehydes and ketones (see Section 6.2) have been applied to good effect in the cyanation of imines. For instance, aluminium BINOL (6.65) catalyses the cyanation of aromatic and a,p-unsaturated N-fluorenylaldimines using TMSCN in good ee, while gadolinium complexes of the glucose-derived ligand (6.71) and derivatives have been used in the enantioselective cyanation of ketimines. ... 

Some of the metal-based catalysts used in the asymmetric hydrophosphonylation of aldehydes (see Section 6.4) can also be applied to the phosphonylation of imines. For instance, Shibasaki s heterobimetallic BINOL complexes work well for the catalytic asymmetric hydrophosphonylation of imines. In this case lanthanum-potassium-BINOL complexes (6.138) have been found to provide the highest enantioselectivities for the hydrophosphonylation of acyclic imines (6.139). The hydrophosphonylation of cyclic imines using heterobimetallic lanthanoid complexes has been reported. Ytterbium and samarium complexes in combination with cyclic phosphites have shown the best results in the cases investigated so far. For example, 3-thiazoline (6.140) is converted into the phosphonate (6.141) with 99% ee using ytterbium complex (6.142) and dimethyl phosphite (6.108). The aluminium(salalen) complex (6.110) developed by Katsuki and coworkers also functions as an effective catalyst for the hydrophosphonylation of both aromatic and aliphatic aldimines providing the resulting a-aminophosphonate with 81-91% ee. ... 

It has been shown that inexpensive aluminium-based catalysts can effect the racemization of secondary alcohols at ambient temperature. The reactivity of the complexes may be increased through the use of a bidentate ligand such as BINOL (Scheme 4.35)[86). 

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