Al-salens

Scheme 2-59. Chiral Al-salen-catalyzed Strecker reaction. 

The compound [Al(Salen)(H20)2]Cl is not air-sensitive. It has mp 162-165°C dec. The cation is soluble only in water. IR (KBr) 3057 s (br) 1639 s, 1560 m, 1473 s, 1296 s, 814 m, 760 s cm . The synthesis and properties of the SalenCl and Acen derivatives are described in Ref. 7. Salophen, Salen( Bu), Salpen( Bu), Salben( Bu), Salophen Bu, and Salomphen Bu derivatives can also be prepared in a similar manner. In the solid state these compounds have been shown by X-ray crystallography to contain six-coordinate octahedral aluminum. The solvent molecules are trans to one another and the hgand copla-nar with the aluminum atom. The chloride anions are connected by hydrogen bonds to the solvents on the aluminum as well as additional solvent in the crystal lattice. 

D. DIMETHANOL-1,2-BIS(2-HYDROXY-3,5-BIS(tcri-BUTYL)BENZYLIDENEIMINO)ETHANEALUMINUM TETRAPHENYLBORATE, [Al(Salen Bu)(MeOH)2] [BPh4]... 

The Michael addition of N-nucleophiles to a./ -unsaturated carbonyl compounds is of obvious synthetic importance, e.g. for the preparation of / -amino acids [50a]. Several metal-containing catalysts have been devised, e.g. the chiral Al-salen... 

Asymmetric cyanohydrin synthesis from aldehydes using trimethylsilyl cyanide (TMSCN) has been carried out using a chiral Al(salen) complex-triphenylphosphine Lewis acid-base combination.263... 

Aluminum salen complexes have been identified as effective catalysts for asymmetric conjugate addition reactions of indoles [113-115]. The chiral Al(salen)Cl complex 128, which is commercially available, in the presence of additives such as aniline, pyridine and 2,6-lutidine, effectively catalyzed the enantioselective Michael-type addition of indoles to ( )-arylcrolyl ketones [115]. Interestingly, this catalyst system was used for the stereoselective Michael addition of indoles to aromatic nitroolefins in moderate enantiose-lectivity (Scheme 36). The Michael addition product 130 was easily reduced to the optically active tryptamine 131 with lithium aluminum hydride and without racemization during the process. This process provides a valuable protocol for the production of potential biologically active, enantiomerically enriched tryptamine precursors [116]. 

The supposed mechanism for Al(salen)Cl/amine catalyzed 1,4-addition involves a crucial stereo controlled formation of an intermediate octahedral Schiff base-aluminum enolate 132 as depicted in Fig. 2. 

Fig. 2 Intermediate for Al(salen)Cl/amine-catalyzed Michael addition...

Jacobsen et al. have reported that peptide Schiff bases and an Al-salen complex are valuable for asymmetric hydrocyanation of imines with TBSCN [660] and TMSCN [661], respectively. It is, however, most likely that HCN arising from these cyanosilanes and adventitious water is the reactive nucleophile. Hoveyda et al. also have used TMSCN as a source of HCN in the hydrocyanation of imines catalyzed by a Ti-peptide Schiff base complex [662],... 

Chiral Al-salen complexes enable pyranoquinolines to be obtained with high diastereoselectivity by an inverse electron demand DA reaction between dihydropyran and benzylidene aniline <04BMCL2035>. Sulfamic acid effects the one-pot reaction between benzaldehyde, an aromatic amine and dihydropyran which leads to the same products (Scheme 3). An intramolecular version of the latter variant involves the imines derived from O-prenylsalicylaldehyde and gives isochromanoquinolines <04S69> and sulfamic acid is also a suitable catalyst for the Pechmaim synthesis of coumarins <04SL1909>. 

Al-salen complex, the addition of hydrazoic acid to mixed imides in which one of the acyl groups is a,p-unsaturated, excellent enantioselectivity results. Thus, this process is very valuable for the synthesis of p-amino acids. 

Al-Salen NA, Empsall HD, Markham R, Shaw BL, Weeks B (1979) J Chem Soc Dalton Trans 1972... 

Mason R, Textor M, Al-Salen N, Shaw BL (1976) J Chem Soc Chem Commun 292... 

Transition metal catalyzed ring expansions of cyclic ethers to lactones under pressures of CO [51, 52] have been reported for tetrahydrofuran [53], oxetanes, and epoxides [54—56]. Carbonylation of epoxides is particularly important since P-lactone products are challenging synthetic targets (see Section 2.2.5). Using Co(CO)4 in combination with a Lewis acidic Al-salen counterion, the reaction of (R)-propylene oxide and CO occurs with stereochemical retention (Scheme 2.23) [57]. The mechanism is believed to involve Lewis acid activation of the epoxide followed by nucleophilic ring opening with Co(CO)4 [58]. 

N2O2 Ligand Architecture Mono- and Dinuclear Al-Salens and Others... 

Aluminium(iii)-based salens seem, however, used with a cocatalyst, to be limited in most cases to cyclic carbonate formation, the higher Lewis acidity of aluminium(iii) being probably partly responsible for this. The first successful aluminium-salen, Al(salen)Cl (Scheme 18.37), used in the formation of cyclic carbonates was tested by the group of Ren He in 2002, with the rather uncommon substrate, ethylene oxide and was further optimised by Xiao Bing Lu in 2004. ... 

Scheme 18.42 Bifunctional dinuclear oxo-bridged Al-salens including ammonium or phosphonium groups.

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