Aluminum complexes salen

Malonate and related activated methylene compounds have also been used as the nucleophile in conjugate addition/Michael reactions. Taylor and co-workers have developed a new methodology that utilizes (salen)aluminum complexes such as 43 as a catalyst to effect the enantioselective conjugate addition to a,p-unsaturated ketones by a variety of nucleophiles.25 For example, nitriles, nitroalkanes, hydrazoic acids, and azides have found utility in this reaction. Additionally, cyanoacetate (42) has been demonstrated to undergo a highly enantioselective conjugate addition to 41. The Krapcho decarboxylation is then necessary to produce cyanoketone 44, an intermediate in the synthesis of enantioenriched 2,4-cw-di substituted piperidine 45. 

Evans, D.A., Janey, J.M., Magomedov, N. and Tedrow, J.S. (2001) Chiral salen-aluminum complexes as catalysts for enantioselective aldol reactions of aldehydes and 5-alkoxyoxazoles an efficient approach to the asymmetric synthesis of syn and anti P-hydroxy-a-amino acid derivatives. Angewandte Chemie - International Edition, 40, 1884—1888. 

R. Luo, X. Zhou, S. Chen, Y. Li, L. Zhou, H. Ji, Highly efficient synthesis of cyclic carbonates from epoxides catalyzed by salen aluminum complexes with built-in "CO2 capture" capability under mild conditions. Green Chem. 16 (2014) 1496-1506. 

In principle, rac-lactide, a racemic mixmre of d- and L-lactide, may be polymerized in a stereoselective fashion. Depending on the stereoselection as the ROP proceeds, the resulting polymer may thus exhibit different stereoregularities these directly influence the thermal and mechanical properties of the produced PLAs. In this regard, isotactic PLA stereoblocks and PLA stereocomplexes, which are of interest for their thermal and mechanical properties, may be produced via the ROP of rac-lactide initiated by an achiral derivative, provided the polymerization proceeds via a chain-end stereocontrolled mechanism i.e., the last inserted lactide unit stereo-controls the insertion of the incoming monomer. This strategy has been first validated using salen-based aluminum complexes such as 16 (Scheme 16, top) to produce PLLA-PDLA isotactic stereoblocks [95, 96]. Alternatively, the chiral racemic salen aluminum complex 17 was found to be suitable for the parallel stereoselective synthesis of isotactic poly(D-lactide) and poly(L-lactide) from rac-... 

In 1998, Jacobsen reported an asymmetric metal-catalyzed Strecker reaction that employs aluminium salen complex 145 (Equation 20) [105], Interestingly, the salen(aluminum) complex 145 proved superior to a wide range of other salen(metal) complexes both in terms of conversion and of enantioselectivity. This constituted the first example of successful use of a main group salen(metal) catalyst for asymmetric synthesis. The N-allylated imine 144 was thus converted into the trifluoroacetyl derivative 146 in 91 % yield and 95 % ee. 

Figure 10.11 Chiral SALEN-aluminum complexes leading to isotactic PLAs by site-controlled ROP of rac-lactide.

A few examples of syndiotactic PLA preparations have been reported by the site-controlled ROP of meso-lactide with chiral complexes. Whilst only moderate selectivities were obtained with the trisindazolyl-hydroborate-magnesium complex 20 (Figure 10.14) [17a], better results were achieved with the SALEN-aluminum complex 8a (toluene, 70 °C, Pr 0.96) [19e]. Strikingly, the enantiomerically pure... 

Five-coordinate aluminum alkyls are useful as oxirane-polymerization catalysts. Controlled polymerization of lactones102 and lactides103 has been achieved with Schiff base aluminum alkyl complexes. Ketiminate-based five-coordinate aluminum alkyl (OCMeCHCMeNAr)AlEt2 were found to be active catalyst for the ring-opening polymerization of -caprolactone.88 Salen aluminum alkyls have also been found to be active catalysts for the preparation of ethylene carbonate from sc C02 and ethylene oxide.1 4 Their catalytic activity is markedly enhanced in the presence of a Lewis base or a quaternary salt. 

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]. 

A 3-allyloxy-2-bromo ester 264 (Y=0, R2=C02Et) cyclized similarly using 20 mol% of (salen)Co complex 267 as a catalyst under reductive electrochemical conditions at a carbon fiber cathode and a sacrificial aluminum anode under constant current conditions. Tetrahydrofurancarboxylates 266 were isolated in 70% yield and a good 6 1 CK//r[Pg.266]

The catalyzed hydrocyanation of imines (Strecker reaction) has the option of employing a stable (salen)aluminum chloride or a Zr complex of 6,6 -dibromo-BINOL, with BujSnCN. It is important to derive the imines firom o-aminophenol for the present purpose. 

Another type of activation of aluminum alkyl was found in the asymmetric-selective polymerization of epichlorohydrin (ECH) with an optically active cobalt-salen type complex [Co (II)]. The structure of the salen-type cobalt complex was shown previously (13, 14). In a benzene solution of the binary system consisting of [Co (II)] and AlEt, no evolution of ethane or ethylene was observed at room temperature. The NMR signals of the methyl protons for AlEt shifted down field on mixing with [Co (II)]. These observations together with a circular dichroism study indicated that AlEt and [Co (II)] formed a molecular complex in benzene, none of Al-Et bonds being cleaved by this complexation. 

The aluminum complex 4 of the salen prepared from BINAMINE catalyzes a very interesting and useful reaction. It turns propargylsilanes into a-silylallylidenation agents for aldehydes such as glyoxamides. ... 

Cycloadditions. Asymmetric Simmons-Smith reaction of allylic alcohols performed in the presence of an aluminum complex of the salen 127 has been reported. ... 

Yang et tested two aluminum complexes with salen ligands prepared from trimethyl aluminum (AlMes), methanol, and (R,R)-N,N -bis(salicylidene)-l,2-diaminocyclohexane as catalysts for the ROP of TMC and DTC as well as cyclic esters (Scheme 49). 

Scheme 49 Aluminum complexes with salen ligands used for polymerization of six-membered cyclic carbonates.

Hydrocyanation to imines with HCN, the Strecker reaction, is one of the most direct and efficient methods for natural and unnatural a-amino acids. Asymmetric Strecker-type reaction with chiral aluminum Lewis acids has been developed. As shown in Scheme 6.48, the research group of Jacobsen reported chiral Al(salen)Cl complex (67a) as an effective asymmetric catalyst for catalytic enantioselective Strecker reaction of aromatic N-allylimines with HCN [62]. Compared to the reactions of aromatic imines, that of a-branched aliphatic imines (R = Cy and t-Bu) gave Strecker products in only moderate optical yield. Additionally, the use of TMSCN instead of HCN dramatically reduced in the enantioselectivity. 

Alternatively, chain-endaluminum complexes. An exploration of various ligands has begun... 

To a solution of hexa hydrated aluminum chloride (1.99g, 8.28 mmol, 1.0 equiv.) in tetrahydrofuran (25 mL), precatalyst (R,R)-salenCo (5.0 g, 8.28 mmol, 1.0 equiv. ) was added and stirred in at open atmosphere at room temperature. As soon as the chiral Co (salen) was added color of the solution changes from brick red to dark olive green. The mixture was stirred for 1 h. The resulting solution was concentrated imder reduced pressure. The crude solid was worked up with H2O and CH2CI2. Yield = 98-99 % as a dark green solid powder. The complex have been analyzed by A1 NMR with reference to [A1(D20)6] and... 

Chiral aluminum SALEN complexes have been used by Kee for asymmetric addition of dimethyl phosphite to benzaldehyde derivatives (Scheme 5-43). 

The dramatic acceleration observed for the ROP of epoxides when (255) is present has also been observed with several other aluminum-based initiators, including salen- (325)-(327), tetraazaan-nulene- (328) and phthalocyanine- (329) ligated systems.946 For example, (325) polymerizes PO extremely slowly at room temperature (4% conversion of 50 equivalents over 8 days), but in the presence of MAD (255), 200 equivalents require just 70min to attain 43% con version.794,795,947 Complex (328) shows a similar increase in activity upon addition of MAD 200 equivalents PO reach 74% conversion within 90 seconds in the absence of MAD such a yield requires several days. 

Several aluminum biphenolate complexes have been investigated as initiators for the ROP of PO.810,935 Unlike the TPP and salen-based systems, a cis coordination site is realistically accessible and in theory an alternative cis-migratory mechanism to the backside attack pathway might operate. However, NMR analyses on the resultant PPO show that stereochemical inversion still occurs when the biphenolate initiators are used (Scheme 22). It has also been confirmed that the same process occurs with the Union Carbide calcium alkoxide-amide initiator for both PO and CHO.810... 

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