Conjugate aluminum enolates

Catalytic effect of transition metals on the smooth conjugate addition reaction was next tested [38]. Alkynylation at the / position was followed by capture of the aluminum enolate in the presence of Ni(acac)2 (Scheme 6.18). Yields of 1,4-ad-ducts were moderate, however, indicating the methods were still unsatisfactory for enolate generation. 

DIBAL was used for the conjugate reduction to produce aluminum enolates in the presence of MeCu catalyst [39]. Unlike strong bases fhat readily deprotonate the a-hydrogen of carbonyl compounds, fhis mefhod tolerates a ketone carbonyl and its a hydrogen, and was fhus chemoselective as well as quantitatively reducing fhe a,/ -unsaturated ester (Scheme 6.19). 

Anionic polymerization of methacrylates involves enolate intermediates of diverse molecular weight. These distinctive enolates are readily formed via a number of consecutive conjugate addition steps. As discussed in Section 6.1, control of reactivity and selectivity of enolates should directly reflect the stereoselective synthesis of poly(methyl methacrylate)s (PMMA). Thus it is advisable to compare the nature of aluminum enolates involved in bimolecular and polymolecular reactions. 

Selenenyl halides also react similarly with copper enolates, aluminum enolates, and zirconium enolates [101]. Scheme 15.38 illustrates the selenenylation of copper enolates generated by conjugate addition of lithium diphenylcuprate to cyclo-pentenone [100 d]. 

Aluminum enolates can be formed by conjugate addition with diisobutylaluminum hydride (DIBAL-H) and a catalytic amount of methylcopper in a mixture of THF and HMPA (Scheme 28). " The role of copper and HMPA is crucial, for without these 1,2-reduction of the carbonyl group takes place. The effect of copper(l) on conjugate addition is not unexpected. In regard to the solvents it is suggested that HMPA functions not as a cosolvent but as an essential ligand. Treatment of an a. -un-saturated ketone with trimethylaluminum and a catalyst leads to a dimethylaluminum enolate with moderate ( )/(Z) selectivity. The (Z)-enolate reacts with diphenylketene to give another enolate (Scheme... 

Alkyl-2-cyclohexenones. The alkylation of 2-cyclohexenone can be initiated by conjugate addition and trapping of the aluminum enolate with an aldehyde. Regeneration of the double bond and removal of the hydroxyl group are accomplished by mesylation. Actually, the liberated thiolate displaces the allylic mesylate in the process thus the final operation involves desulfurization with Raney nickel. 

Scheme 2.40 Enantioselective conjugate addition catalyzed by bimetallic complex 135 and consecutive aldol addition of aluminum enolate 136.

To overcome the limitation of the high stability of the aluminum enolates, the oxygen atom has been transformed to silyl enol ethers, enol acetates, and allyl enol carbonates. Silyl enol ethers and enol acetates are precursors to lithium enolates. Enol acetates and allyl enol carbonates are precursors of cx-allylated adducts via the Tsuji-Trost rearrangement [75-77]. The silylation of aluminum enolates using TMSOTf is well established [78], although in some cases the isolation is difficult [33]. Silyl enol ethers allow further modification to be performed as they behave as lithium enolates (Scheme 15). A recent application can be found in the silylation of the conjugate addition adduct (/ )-((3-(but-3-en-l-yl)-3-methylcyclopent-l-en-l-yl)oxy)triethylsilane which allows aldol condensation to form an intermediate in the synthesis of Clavirolide C [79], a diterpene with a trans-bicyclo[9.3.0] tetradecane structure (Scheme 16) [80]. 

Scheme 14 Vinyl oxirane ring-opening using aluminum enolates from asymmetric conjugate addition. Application to the synthesis of [6,7]bicyclic adducts...

In recent years more effort has been devoted to study the possibilities offered by the trapping of the aluminum enolate in the form of enol acetates (Scheme 18). As for silyl enol ethers, both cyclic and linear aluminum enolates can be easily transformed. For the linear enolates both E- and Z-isomers can be formed. Interestingly the use of aluminum enolates favors the formation of Z-enolates [81]. The main advantages of this methodology to the previous ones are that (a) 0-acylation can usually be achieved using diethyl ether as solvent of choice, which corresponds appreciably to the most general experimental conjugate addition conditions ... 

The ene reaction is strongly catalyzed by Lewis acids such as aluminum chloride and diethylaluminum chloride204 Coordination by the aluminum at the carbonyl group increases the electrophihcity of the conjugated system and allows reaction to occur below room temperature, as illustrated in Entry 6. Intramolecular ene reactions can be carried out under either thermal (Entry 3) or catalyzed (Entry 7) conditions 205 Formaldehyde in acidic solution can form allylic alcohols, as in entry 1. Other carbonyl ene reactions are carried out with Lewis acid catalysts. Aromatic aldehydes and acrolein undergo the ene reaction with activated alkenes such as enol ethers in the presence of Yb(fod)3 206 Sc(03SCF3)3 has also been used to catalyze ene reactions.207... 

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