Enolates aluminum

Transmetalation of lithium enolate 1 a (M = Li ) by treatment with tin(II) chloride at — 42 °C generates the tin enolate that reacts with prostereogenic aldehydes at — 78 °C to preferentially produce the opposite aldol diastereomer 3. Diastereoselectivities of this process may be as high as 97 3. This reaction appears to require less exacting conditions since similar results are obtained if one or two equivalents of tin(ll) chloride arc used. The somewhat less reactive tin enolate requires a temperature of —42 C for the reaction to proceed at an acceptable rate. The steric requirements of the tin chloride counterion are probably less than those of the diethyla-luminum ion (vide supra), which has led to the suggestion26 44 that the chair-like transition state I is preferentially adopted26 44. This is consistent with the observed diastereoselective production of aldol product 3, which is of opposite configuration at the / -carbon to the major product obtained from aluminum enolates. 

An example of double asymmetric induction has been reported. The resolved enantiomers of rac-4 have been converted to the aluminum enolates and reacted at —78 °C with enantiomer-ically pure ter/-butyl (S)-2-fonnyl-l-pyrrolidine carboxylate46. A comparison of the two reactions reveals that the reaction pair leading to the (5Fe,/ ,5)-product is matched while the alternative reaction pair is mismatched. 

This high diastereoselectivity contrasts dramatically with the nearly nonexistent selectivity of the lithium enolate of the corresponding triphenylphosphane complexes (vide supra). The diastereomer preferentially obtained from the fluorophenyl lithium enolate 9 corresponds to the major product produced by reaction of the aluminum enolate 1 b derived from the parent triphenylphosphane complex. 

The enantiomerically pure (S)-enantiomer of propanoyl complex 1 has been converted to the aluminum enolate and reacted with 4-pentenal (7) to provide the product (Fe5,2 5,3 / )-8 with good diastereoselectivity 51. 

The diastereomeric a-alkoxy complexes (1 )-15 and (S)-15, separable by chromatography, were each converted to the corresponding aluminum enolates and reacted with 2-methylpropanal (17)49. Enolate (/ )-16 selectively provided a mixture of two diastereomers with the (Fe/ ,2, 3 i )-complex (/ )-18 identified as the major constituent of a 94 6 mixture. The two chiral auxiliaries of complex (S j-lS exerted antagonistic effects and an undefined mixture of all four possible diastereomers was obtained. 

Transmetalation of 19 by treatment with two equivalents of diethylaluminum chloride generates the aluminum enolate species 23. The latter reacts with acetaldehyde to produce the stable aluminum aldolates 24 which do not undergo the Peterson elimination23. A protic quench then provides the a-silylated aldol adducts of tentative structures (2 R)-25 and (2 V)-25 with little diastereoselectivity. Other diastereomers are not observed. 

In the Lewis acid mediated reaction the developing carbenium ion in C is stabilized by the nearby 7t-electrons of the titanium or aluminum enolate. This generates as the major diastereomer the 3,3a-/r .v-relationship between the substitution at the ring junction and the vinyl group at C-3 via a synclinal transition state. 

The (TPP)A1X family of initiators has been used to initiate the polymerization of a range of other monomer classes including epoxides, episulfides, and methacrylates.776 In the latter case the propagating species is an aluminum enolate and this too may initiate the ROP of lactones, such as 6-VL, albeit slowly. In this way a block copolymer P(6-VL)-b-PM M A of narrow molecular weight distribution (Mw/Mn= 1.11) has been prepared.787... 

The mechanistic considerations of a catalytic asymmetric Michael reaction suggest that the reaction of a alkali metal enolate derived from a malon-ate derivative with an enone should lead to an intermediary aluminum enolate. Is it possible that such an A1 enolate could then be trapped by an... 

Homologation of ECHO to a-bromo-a, -enones.4 The initial steps of this homologation involve addition of dibromomethyllithium (1) to an aldehyde followed by oxidation of the adduct to a dibromomethyl ketone (2). The aluminum enolate of 2 undergoes an aldol reaction to provide an a-bromo-p-hydroxy ketone... 

Lithium, magnesium, and aluminum enolates appear to afford comparable levels of kinetic aldol diastereoselection for a given enolate of defined structure. 

The block copolymerization of MAN was attempted from a living prepolymer of MMA carrying a (porphinato)aluminum enolate growing terminal (Scheme 6) however, with out 3e, the polymerization of MAN from 2 did not occur. However, when 100 equiv of MAN were added to a CH2CI2 solution of 2 [Mn=12,000, Mw/Mn=l.ll prepared with [MMA]o/[1]q of 100, 100% conver-... 

By using the aluminum porphyrin-Lewis acid system, we attempted the synthesis of a narrow MWD block copolymer from oxetane and methyl methacrylate (MMA). Methacrylic monomers can be polymerized radically and anioni-cally but not cationically, so a block copolymer of oxetane and methyl methacrylate has never been synthesized. As already reported, methacrylic monomers undergo accelerated living anionic polymerization with the (TPP)AlMe (1, X= Me)-3e system via a (porphinato)aluminum enolate as the growing species. 

Aldol reaction. The reagent (I) adds in a 1,4-fashion to an a,/J-unsaturated ketone lo give an aluminum enolate, which undergoes aldol condensation with an aldehyde. The adduct is converted into an a-substituted-a./l-unsaturated ketone on sulfoxide elimination.1... 

A facile synthesis of cyclobutylmethanols has been devised by reacting 2-ethoxy-5-alkyl-3,4-dihydro-2H-pyrans with aluminum alkyls (Scheme 149) (80TL4525). When (648) is reacted with triisobutylaluminum the cyclobutylmethanol (649) is formed quantitatively. While several mechanisms have been proposed for this process, initial rupture of the carbon-oxygen bond of the pyran ring to form an aluminum enolate, which then undergoes ring closure and reduction, appears to be most likely. 

Polyfluoro-l -alkenyl phosphates are reduced under mild conditions by diisobuty-laluminum hydride (DIBAL) to the aluminum enolate, which is denvatized by addition to benzaldehyde [96] (equation 78)... 

Shibasaki has shown that ALB is also effective for the three-component coupling of enones, aldehydes, and malonates [23]. The above-mentioned mechanistic consideration suggested that the reaction of a lithium enolate derived from a malonate derivative with an enone would lead to the formation of an intermediary aluminum enolate. Thus, further studies were carried out to obtain direct evidence for the formation of an aluminum enolate. The larger electronegativity of aluminum (1.5) as compared with that of lithium, sodium, or lanthanoid suggests that the protonation of the aluminum enolate should be slower than that of the corresponding lithium, sodium, and/or lanthanoid enolates. Then, is it possible to trap such an Al-enolate by an... 

Reaction of ester enolates with epoxides.1 Lithium enolates of esters do not open epoxides, but the aluminum enolates do. Li to A1 exchange can be effected with diethylaluminum chloride. The less substituted O-C bond is cleaved and the syn-diastereomer predominates. Reactions of optically active epoxides proceed with high... 

Reductive dehalogenation of fi-halo ketones.1 a-Bromo or a-chloro ketones undergo reductive dehalogenation on reaction with freshly prepared A1I3 in refluxing CH3CN (80-95% yield). The reaction probably involves an aluminum enolate since addition of benzaldehyde results in an aldol condensation. 

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. 

Diisobutylaluminum benzenetellurolate, prepared from diphenyl ditellurium and diisobutylaluminum hydride, adds in a 1,4-fashion to a,/i-unsaturated aldehydes, ketones, and carboxylic acid esters to give as intermediates /i-phenyltelluro-substituted aluminum enolates that can be hydrolyzed to the carbonyl compounds or reacted with aldehydes to produce aldol adducts2. 

Reformatsky-type reaction. Reaction of an a-bromo aldehyde, ketone, or ester with the reagent (I) formed from Bu3SnLi and (CiHjj.AlCl provides an aluminum enolate... 

---