Aromaticity copper enolates

The Tol-BINAP complexes of CuFa, CuF, and CuOt-Bu also work as efficient chiral catalysts of the aldol reactions of aromatic and a, -unsaturated aldehydes wifh dienolate 64 (Scheme 10.62) [164]. IR spectroscopy has revealed fhat the stoichiometric reaction of 64 wifh Cu(Ot-Bu)(S)-Tol-BINAP forms a Cu(I) enolate, and fhat subsequent reaction wifh an aldehyde gives a copper aldolate. The copper enolate is also obtained by stepwise treatment of 64 with Bu4NPh3SiF2 and Cu(C1O4)(S)-To1-BINAP. These results, with the known reduction of Cu(II) to Cu(I) by SEE, indicate that fhe Cu-catalyzed aldol reactions proceed fhrough a transmetalation mechanism involving a chiral Cu(I) enolate. 

The effect of neighbouring aromatic groups on the stereochemistry of alkylation of copper enolates [generated either by 1,4-addition to enones or by addition of copper(i) halides to lithium enolates] has been investigated by Posner. ... 

The radicals generated from esters of halogenodifluoroacetic acid or halogenodi-fluorophosphonic acid add onto olefins and enolates. When these reactions are intramolecular, they afford tetrahydrofurans. In the presence of copper dust, ethyl bromodifluoroacetate can couple with aromatic and vinyl halides or can add onto Michael acceptors (Figure 2.13). ... 

Among several chiral cyclic and acyclic diamines, (R,R)-cyclohexane-l,2-diamine-derived salen ligand (which can adopt the gauche conformation) was most effective in providing high enantioselectivity [38]. Further, the introduction of substituents at the 3,4, 5 and 6 positions on the aromatic ring of catalyst 39c was not advantageous, and resulted in low enantioselectivity [32,37,39]. The metal ions from first-row transition metals - particularly copper(II) and cobalt(II) - that could form square-planar complexes, produced catalytically active complexes for the asymmetric alkylation of amino ester enolates [38]. 

Examples of thermochemical considerations of cupric enolates include the study of the binding of Cu + with kojic acid (16), a cyclic a-ketoenol. Comparison was made between the divalent cations of U02 +, Cu +, Zn +, Ni +, Co +, Cd +, Ca + where these metals are listed in decreasing order of binding constants over 6 powers of 10. In this case carbon-bonded metal seems most unreasonable because it would ruin the chelation as well as any aromaticity in the pyrone ring. It is admittedly an assumption that pyrones are aromatic. There are no one-ring pyrones for which there are enthalpy of formation data for gas phase species, as opposed to the benzoannelated compounds coumarin (I7)i07a, I07b chromone (is) " " "and xanthone (19) . Plausible, but unstable, Cu(II) enolates eliminate copper and form the 1,4-dicarbonyl compounds as shown in equation 8. 

Template reactions between malonaldehydes and diamines in the presence of copper(II), nickel(II) or cobalt(II) salts yield neutral macrocyclic complexes (equation 15). Both aliphatic and aromatic diamines can be used. In certain cases, non-macrocyclic intermediates can be isolated and subsequently converted into unsymmetrical macrocyclic complexes by reaction with a different diamine (Scheme 11). These methods are more versatile and more convenient than an earlier template reaction in which propynal replaces the malonaldehyde (equation 16). This latter method can also be used for the non-template synthesis of the macrocyclic ligand in relatively poor yield. A further variation on this reaction type allows the use of an enol ether (vinylogous ester), which provides more flexibility with respect to substituents (equation 17). The approach illustrated in equation (15), and Scheme 11 can be extended to include reactions of P-diketones. The benzodiazepines, which result from reaction between 1,2-diaminobenzenes and P-diketones, can also serve as precursors in the metal template reaction (Scheme 12). The macrocyclic complex product (46) in this sequence, being unsubstituted on the meso carbon atom, has been shown to undergo an electrochemical oxidative dimerization (equation 18). ... 

Phenols have been condensed with alkenoylesters to give chromans by an oxa-Michael addition/electrophilic aromatic addition sequence with magnesium(II)- or copper(II)-bis-oxazoline complexes as chiral Lewis acid catalysts (Scheme 17b) [97]. This reaction may be initiated by an oxa-Michael reaction, followed by a hydroarylation of a carbonyl group. The authors suggest that the initial stereodetermining oxa-Michael addition is followed by a fast diastereoselective aromatic substimtion [97]. A nickel Lewis acid, derived from Ni(hfacac)2 (hfacac = 1,LL5,5,5-hexafluoro-3,5-dioxopentane enolate) and chiral Al-oxide ligands, catalyzes the enantioselective oxa-Michael cyclization of 2-tert-butyloxycarbonyl-2 -hydroxy-chalcones to 3-ferf-butoxycarbonyl flavanones, which can be decarboxylated to flavanons in a separate step (Scheme 17c) [98]. A Lewis acid activation of the unsaturated p-ketoester unit can be assumed. 

Because the reactions of related in -cyclohexadienyl complexes are synthetically valuable, the reactions of this ligand have been studied extensively. An outline of how this chemistry can be conducted on the Fe(CO)j fragment is shown in Equation 11.51. A variety of cyclohexadienes are readily available from Birch reduction of substituted aromatics. Coordination and abstraction of a hydride, typically by trityl cation, leads to cationic cyclohexadienyl complexes. These cyclohexadienyl complexes are reactive toward organolithium, -copper, -cadmium, and -zinc reagents, ketone enolates, nitroal-kyl anions, amines, phthalimide, and even nucleophilic aromatic compounds such as indole and trimethoxybenzene. Attack occurs exclusively from the face opposite the metal, and exclusively at a terminal position of the dienyl system. This combination of hydride abstraction and nucleophilic addition has been repeated to generate cyclohexa-diene complexes containing two cis vicinal substituents. The free cyclohexadiene is ttien released from the metal by oxidation with amine oxides. ... 

Kobayashi and coworkers showed that Lewis add surfactant-combined catalysts such as scandium tris(dodecyl sulfate), Sc(03SCi2H25)3, or copper bis(dodecyl sulfate), Cu(03SCi2H25)2, were efficient catalysts for the three-component Mannich-type reaction, with 73-95% yield being obtained in neat water.Neutral salts such as sodium triflate and sodium iodide catalyzed the condensation reaction in water between preformed imines and silicon enolates, or the three-component Mannich-type reaction using aromatic amines, with 49-93% yields and 0-80% diastereoselectivities. Mechanistic studies indicated that both sodium triflate and the Mannich adduct itself cooperatively promote the reaction. 

However, the formation of the pyrroles 234 by copper(II)-catalyzed reaction of alkenyl azides 229 with ethyl acetoacetate was explained by an attack of the enolate 231 at the jS-carbon atom of the polarized C,C double bond of intermediate 230 (Scheme 5.29). The proposed mechanism included ring closure by a second nucleophilic attack followed by hydrolysis, dehydration, and tautomerism to get the aromatic final product 234. 

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