Aluminum, lithium complexes

The structurally well-defined aluminum-lithium complex 124 was readily prepared by mixing two equivalents of BINOL with LiAlH4 (Scheme 12.15) [117]. It catalyzes the addition of dimethyl malonate (123) to cyclohexenone (122) to furnish adduct 125 in 99% ee and 95% yield. The substituted cyclohexanone 125 sei-ved as a key intermediate en route to the Sttychnos alkaloid tubifolidine (126) [118]. 

Shibasaki showed that an aluminum-lithium-BINOL complex (ALB) also catalyzes the asymmetric addition of dialkyl phosphites to aldehydes, with ees ranging from 55 to 90% for aryl or unsaturated aldehydes (Scheme 5-37). 

Reduction of Poly(2-cyano-l,3-phenylene arylene ether), 20 Twenty-five mL of a 1.0 M solution of lithium aluminum hydride (LAH) in THF was cooled to 0° C before adding a solution of 1.64 g (5.0 meg) of 20 in 120 mL of THF. The resultant slurry was stirred for 24 h at 0° C, refluxed for 1 h, recooled to 5° C, and the excess LAH decomposed with 2 mL of water. The volume of the solution was reduced to 25 mL before pouring the mixture into 500 mL of 5% HC1 to dissociate the amine aluminum salt complex and precipitate the polymer. The polymer was recovered by filtration, reslurried in 20 mL of water and the pH adjusted to 9.0 with NaOH. After recovery of the neutralized polymer was recovered, it was dried in vacuo redissolved in CHC13, and reprecipitated using water as the nonsolvent. Final drying in vacuo for 24 h at 35° C left 1.2 g (72.3%) of poly[oxy-l,4-phenylene-(l-methylethylidene)-l, 4 -phenylene-oxy-(2"-aminomethyl)-l",3"-phenylene], 21, [n] (CHCI3) 0.3 dl/g. 

R)-aluminum-lithium-BINOL complex (0.024 g, 0.04 mmol) was dissolved in toluene (0.4 ml), and to this solution was added dimethyl phosphite (0.044 g, 0.4 mmol) at room temperature the mixture was stirred for 30 min. Benzaldehyde (0.042 g, 0.4 mmol) was then added at -40°C. After having been stirred for 51 h at -40°C, the reaction mixture was treated with 1 N hydrochloric acid (1.0 ml) and extracted with ethyl acetate (3 x 10 ml). The combined organic extracts were washed with brine, dried over sodium sulfate, and concentrated under reduced pressure. The residue was purified by flash chromatography (silica, 20% acetone/hexane) to give the diethyl (S)-a-hydroxybenzylphosphonate (78 mg, 90%) with 85% enantiomeric excess as a colorless solid of mp 86 to 87°C. 

Moreover, these rare earth heterobimetallic complexes can be utilized for a variety of efficient catalytic asymmetric reactions as shown in Scheme 7 Next we began with the development of an amphoteric asymmetric catalyst assembled from aluminum and an alkali metal.1171 The new asymmetric catalyst could be prepared efficiently from LiAlH4 and 2 mol equiv of (R)-BINOL, and the structure was unequivocally determined by X-ray crystallographic analysis (Scheme 8). This aluminum-lithium-BINOL complex (ALB) was highly effective in the Michael reaction of cyclohexenone 75 with dibenzyl malonate 77, giving 82 with 99% ee and 88 % yield at room temperature. Although LLB and... 

A tricyclic aromatic system closely related to borole, 9-boratafluorene, might be expected to form ( -coordinated transition metal sandwich complexes, but so far only the lithium complex has been characterized.19 Monohapto aluminum adducts of neutral 9-borafluorene, in which A1 is bound only to the boron atom, have been prepared.20... 

The concept of using group I metal initiators was applied in order to minimize the toxicity generated by heavy metal residues in the end product PLAs when using metals like aluminum, tin, and lanthanides as initiators. In recent years, dinuclear lithium and macro-aggregates with phenolate ligands have attracted substantial interest, mainly due to uncommon strucmral feamres and their ability to catalyze formation of polyester and various other polymeric materials via ROP [28]. A series of lithium complexes supported with 2, 2-ethylidene-bis (4, 6-di-tert-butylphenol) (EDBP-H2) 2-6, (Scheme 6) are excellent initiators for the ROP of L-lactide in CH2CI2 at 0 °C and 25 °C [33-35]. In this case, the PDIs of the obtained PLAs were quite narrow (1.04—1.14) and a Unear relationship between and the monomer-to-initiator ratio ([M]o/[I]o) existed at 0 °C. Dimeric complexes 4 and 6 were the... 

A catalyst system with a zinc complex that also induces the chain-transfer reaction has been developed by Jerome et al When the polymerization of PO is conducted by using a mixture of zinc/aluminum bimetallic complex (Bu0)2A10Zn0Al(0Bu)2 and phenoxyethanol ([PO]/[Zn]/[phenoxyethanol] = 1000/1/20), the conversion reaches 97% to give PPO that contains low and high molecular weight fractions. An additive of lithium chloride or... 

Discovery. These catalysts were discovered during a study of the use of transition metal cyanides in combination with metal alkyl and hydride reducing agents in polymerizations. The combination of nickel cyanide and lithium aluminum hydride complexed very strongly with tetrahydrofuran. A similar complexing action occurred with propylene oxide and nickel hexacyanoferrate(II)-lithium aluminum hydride. This led to speculation as to the role of the double-metal cyanide itself. 

This idea was realized very successfully by Shibasaki and Sasai in their heterobimetallic chiral catalysts [17], Two representative well-defined catalysts. LSB 9 (Lanthanum/Sodium/BINOL complex) and ALB 10 (Aluminum/Lithium/BINOL complex), are shown in Figure 8D.2, whose structures were confirmed by X-ray crystallography. In these catalysts, the alkali metal (Na, Li, or K)-naphthoxide works as a Br0nsted base and lanthanum or aluminum works as a Lewis acid. 

Another highly useful heterobimetallic catalyst is the aluminum-lithium-BINOL complex (ALB) prepared from LiAlH4 and 2 equiv. of (/ )-BINOL. The ALB catalyst (10 mol %) is also effective in the Michael reaction of enones with various malonates, giving Michael products generally with excellent enantioselectivity (91-99% ee) and in excellent yields [23]. These results ate summarized in Table 8D.3. Although LLB and LSB complement each other in their ability to catalyze asymmetric nitroaldol and Michael reactions, respectively, the Al-M-(/ )-BINOL complexes (M = Li, Na, K, and Ba) are commonly useful for the catalytic asymmetric Michael reaction. 

Aluminum ate complexes, (CH3)3SiCH -CHCH2 Al(C2H5)3Li+ (1). The ate complex is prepared by reaction of the lithium anion of allyltrimethylsilane with A1(C2Hs)3. In contrast with the anion of allyltrimethylsilane, which reacts with carbonyl compounds mainly at the y-position, 1 reacts selectively at the a-position (equation T). 

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]. 

Asymmetric reduction of oi, -enones. Prochiral cyclic and acyclic a,p-enones are reduced by lithium aluminum hydride complexed with 1 to (S)-allylic alcohols in optical yields of 30-100% (equation 1). 

Although the Michael addition of metal ynolates to a,/ -unsaturated carbonyl compounds is expected to give six-membered cycloadducts, 1,2-addition to carbonyl groups usually precedes 1,4-addition. The cycloaddition of the lithium-aluminum ate complex of silyl-substimted ynolate 112 with ethyl benzylideneacetoacetate (113), which is doubly activated by the ester and keto functions, gives the y-lactone 114 via a [4 4- 2] type cycloaddition (equation 46). Diethyl benzylidenemalonate (115) affords the uncyclized ketene 116 by reaction with 112 (equation 47). This could be taken as evidence for a stepwise mechanism for equation 46. ... 

Lithium aluminum hydride (LAH) reacts with pyridines and their analogs in aprotic solvents to give dihydro- and tetrahydro-pyridines. In the absence of proton sources dihydropyridines normally predominate, solutions of pyridine and LAH form lithium complexes (32 Scheme 7), which likely consist of both 1,2- and 1,4-dihydropyridlnes. This intermediate has been used as a reducing agent for ketones, and reaction with alkyl halides generates 3-substituted pyridines (33) in good yield. 

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-... 

After hydrogen evolution has subsided, the solution is refluxed for 2 hrs. The complex has been shown to eifect asymmetric reduction of ketones optically active alcohols of up to 40% optical purity have been obtained and they all have the (S)-configuration. On the other hand, if increasing quantities of ethanol are added to the lithium aluminum hydride complex, the configuration of the secondary alcohol formed changes from (S) to (R). Thus the stereoselectivity increases to a maximum and then decreases as more ethanol is added. Furthermore, maximum selectivities are substantially increased... 

The lithium aluminum hydride complex is prepared as in ihe procedure above. The diethyl ether solution is then refluxed for 10 min, then allowed to stand at 25 CC for 24 h. The solid dissolves after 2-3 min at reflux, and remains in solution. Reduction is then performed as in the procedure above. 

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