Aluminum complexes alcohol derivatives

A variety of chelate complexes of Al3+ with N and/or O donor atoms are known through stability constant data.7,8 Their formation illustrates several aspects of A1 coordination chemistry. Chelates and other multidentate ligand systems provide a means of regulating the reactivity of aluminum compounds. For example, A1 alkoxides can be converted to amino alcohol derivatives to confer water solubility and a degree of hydrolytic stability on otherwise water-sensitive materials. 

Asymmetric reduction of alkyl aryl ketones. The complex (2) derived from lithium aluminum hydride and this diamino alcohol (1), prepared from (S)-( — )-... 

Asymmetric Reductions. Asymmetric reductions of prochiral ketones to optically active secondary alcohols have been extensively studied. The most common method involves the use of chiral unidentate or bidentate ligands in conjunction with Lithium Aluminum Hydride. However, an (5)-aspartic acid derived tridentate ligand has been shown to be very effective in certain cases, presumably due to the rigidity of aluminum complex (4) (eq 5-7). ... 

In the laboratory of E.J. Corey, the first synthesis of nicandrenones (NIC), a structurally complex steroid-derived family of natural products, was accomplished. The side chain of NIC-1 was constructed from the known six-membered lactone which was converted to the Weinreb s amide by treating it with excess MeNH(OMe) HCI and trimethyl-aluminum. The resulting primary alcohol was protected as the TBS ether. The ethynylation of this amide was carried out by reaction with two equivalents of lithium trimethylsilylacetylide to afford an ynone, which was reduced enantioselectively to the corresponding propargylic alcohol using CBS reduction. 

Fortunately, continued exploration has provided more effective reagents. The most successful utilize l,l -bi-2-naphthalenol, diamino, or amino alcohol derived modifiers. These ligands apparently effectively complex the aluminum and minimize the disproportionation problem, However, structural information is still lacking and mechanism-based predictions of the absolute configuration of the product usually cannot be made. Nevertheless, these reagents often give over 90% asymmetric induction with a variety of ketones (Table 3). 

Synonyms Aluminum chlorhydroxide alcohol soluble complex Aluminum chlorohydrol propylene glycol complex Aluminum, chloro hydroxy propylene glycol complexes Aluminum, chloro propylene glycol complexes Definition Deriv. of aluminum chlorohydrate (coordination complex of basic aluminum chloride and propylene glycol or polyethylene glycol in which water molecules have been displaced by glycol)... 

Kobayashi and coworkers reported catalytic asymmetric Simmons-Smith type reaction of allylic alcohols (Scheme 6.98). In this reaction, Lewis acid (R,R)-(112) prepared by premixing of (1R,2R)-cyclohexane bis-sulfonamide and i-Bu2AlH was found to realize good enantioselectivity. Since, in the similar reaction catalyzed by chiral Zn complex derived from (1R,2R)-cyclohexane bis-sulfonamide and Et2Zn instead of chiral aluminum complex, the same enantioselectivity was observed, chiral Zn carbenoid species formed from (R,R)-(112) and Et2Zn via Al-Zn transmetallation was proposed as an active species [117]. 

Diphenylacetic acid has been obtained by the reduction of benzilic acid with hydriodic acid and red phosphorus 1 by the treatment of phenylbromoacetic acid with benzene and zinc dust,2 or with benzene and aluminum chloride 3 by the hydrolysis of diphenylacetonitrile 4 by heating a-diphenyldichloroethyl-ene with alcoholic sodium ethylate 5 by heating benzilic acid 6 from diphenylmethane, mercury diethyl, sodium and carbon dioxide 7 by the oxidation of a,a,5,S-tetraphenyl- 8-butine 8 by the decomposition of some complex derivatives obtained from diphenylketene 9 by the hydrolysis of diphenyl-5,5-hydan-toin 10 by the treatment of diphenylbromoacetic acid with copper 11 by the oxidation of dichlorodiphenylcrotonic acid.12... 

Aluminum chloride and its derivatives are the most familiar Lewis acids and are routinely employed in many Lewis acid-promoted synthetic transformations. The first polymer-supported metal Lewis acids to be studied were polymers attached by weak chemical or physical interactions to a Lewis acid. In the 1970s Neckers and coworkers reported the use of styrene-divinylbenzene copolymer-supported AlCl,- or BF3 as catalyst in condensations, esterifications, and acetalization of alcohols [11,12]. This type of polymer-supported AICI3 (1) is readily prepared by impregnation of a polystyrene resin with AICI3 in a suitable solvent. Subsequent removal of the solvent leaves a tightly bound complex of the resin and AICI3. The hydrophobic nature of polystyrene protects the moisture-sensitive Lewis acid from hydrolysis, and in this form the Lewis acid is considerably less sensitive to deactivation by hydrolysis. This polymer complex could be used as a mild Lewis acid catalyst for condensation of relatively acid-sensitive dicyclopropylcarbinol to an ether (Eq. 1) [13],... 

The concept of in situ protection of the less hindered or more Lewis basic of two ketones to enable selective reduction of the usually less reactive groups has been successfully developed. The sterically hindered Lewis acid MAD (78) derived from BHT and trimethyl aluminum was used to coordinate preferentially to the less hindered ketone and DIBAL-H reduced the more hindered ketone that remained un-complexed. An approximate order of comparative reactivity for various classes of ketones has been established. The selectivity was improved by using the more hindered Lewis acid MAB (79) and/or di-bromoalane as the reducing agent. The discrimination between aromatic ketones is good but less successful between two dialkyl ketones. The chemoselectivity was demonstrated in the reduction of diketone (80) to keto alcohol (81) in 87% yield and excellent selectivity (equation 20). 

Thus (XX) reacts with phenol in pyridine to form diphenoxysilicon phthalocyanine (XXII), with benzyl alcohol to form (XXIII), and with triphenylsilanol to form (XXIV) (168,170, 200). These complexes sublime readily without decomposition (cf. corresponding aluminum derivatives). Bis(diphenylmethylsiloxy)silicon phthalocyanine, which melts before subliming, is one of the very few metal phthalocyanines which actually melt (873). The siloxy complex (XXIV) may also be prepared in benzyl alcohol, thus implying that the Si—O—Si(Pc)—0—Si backbone is more stable than C—O—Si(Pc)—O—C. The dibenzyloxy derivative (XXIII) reacts with diphenylsilanediol to form bis(benzyloxydiphenylsiloxy)silicon phthalocyanine (XXV). 

Amino acids are fundamental biological chemicals and most are commercially available. Exceptions and the preparation of derivatives are discussed in Section 2.4. The compounds themselves, as well as their esters, may be reduced with complex hydrides (e.g., lithium aluminum hydride or sodium borohydride) to the corresponding a-amino alcohols (mostly named after the amino acid, e.g., alaninol from alanine). As a typical example of the in situ preparation of an amino acid ester and reduction to the amino alcohol, the synthesis of (- )-(S )-phenylalaninol is given1. 

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