Indium BINOL

Complexes of other metals such as gallium, indium, lead, and antimony have also been used as Lewis acids. Catalytic enantioselective meso-epoxide ring-opening reactions using a chiral gallium(III) catalyst (Ga-Li-linked-BINOL) have been reported (Scheme 84).348 The chemical yields are much improved by linking two BINOL units. 

Third heterobimetallic asymmetric catalyst reported by Shibasaki et al., gallium-sodium-BINOL complex (GaSB) 26 and indium-potassium-BINOL complex (InPB), are also rather effective catalysts for asymmetric Michael reactions, and GaSB was better than InPB in terms of enantioselectivity. The GaSB catalyst was prepared from GaCl3, NaO Bu (4 mol equiv. to... 

Using a BINOL auxiliary with allylindium and indium metal, hydrazones have been allylated enantioselectively,92 to give homoallylic amines in up to 97% ee. 

A chiral BINOL-indium(in) complex has been used to catalyse the addition of allyltributylstannane to aldehydes in high ee.184... 

Chiral BINOL-indium(ni) complexes have been employed in several enantioselective allylations (i) in the ionic liquid, hexylmethylimidazolium-PF6, for aldehydes,190 (ii) a moisture tolerant version, for a wide variety of aldehyde types,191 and (iii) a recyclable example, useful for aromatic, aliphatic, and a,/S-unsaturated ketones.192... 

The enantioselective addition of allyltributylstannanes to aldehydes and ketones has been performed in the presence of various chiral indium(III) complexes derived from (R)-BINOL,136 (S )-BINOL,137,138 and PYBOX.139,140... 

Catalytic asymmetric Diels-Alder reactions in ionic liquids, using an air- and moisture-stable chiral BlNOL-indium complex prepared from ( S)-BINOL and InClj, have also reported [46]. The reactions of various dienes with 2-methacrolein or 2-bromoacrolein proceeded in an ionic hquid with good yields and excellent enantioselectivities (up to 98% ee). The chiral In(lll) catalyst immobilized in ionic liquid [hmim][PF 5] could be reused seven times without any significant loss in catalytic activity (Scheme 7.13). 

The formation of P-bromohydrins and p thioalcohols from epoxides has also been reported. For example, Shibasaki has shown that the heterobimetalhc gallium lithium BINOL complex (12.138) is a good catalyst for nucleophilic ringopening of epoxides, including the use of thiols. Cyclohexene epoxide (12.116) is converted into the sulfide (12.141) with very good enantioselectivity. Indium complexes of bipyridine (12.126) are very active catalysts in the thiolysis of meso-epoxides providing between 92 and 96% ee in the reaction of ds-stilbene oxides with a range of aromatic and aliphatic sulfides. ... 

A hydroamination/N-sulfonyliminium cyclization cascade was carried out enantioselectively with gold and BINOL-phosphoric acid catalysts to generate fused indole 188. The method can also be extended to furnish lactams with up to 93% ee (13OL4330). Polycyclic indoline 189 was prepared via an indium/TFA-catalyzed tandem cycloannulation of P-oxodithioesters with tryptamine in yields up to 92% (130L1974). Dixon and colleagues utilized an enantioselective Michael addition/iminium ion cyclization of trypt-amine-derived ureas in the synthesis of tetracyclic indole 190 (22 examples, up to 78% yield and 96% ee) (130L2946). 

A simpler preparation of catalytic chiral indium complex based on BINOL ligand were reported by Shibasaki et al. in their asymmetric alkynylation of aldehydes [317]. InBrs was the Lewis acid of choice and the authors proposed a dual role for this bifunctional catalyst, both in activating the alkyne triple bond and the carbonyl moiety. These characteristics, and the inclusion of the chiral BINOL ligand into the In(III) center, had allowed the asymmetric addition of terminal alkynes to aldehydes with just the addition of a mild amine base (Figure 8.150). Positive nonlinear effect was observed with BINOL of varying optical enrichment, and thus the active catalytic species was expected by the authors to be most likely bimetallic in nature. 

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