Copper complexes Lewis acid catalysis

As anticipated from the complexation experiments, reaction of 4.42 with cyclopentadiene in the presence of copper(II)nitrate or ytterbium triflate was extremely slow and comparable to the rate of the reaction in the absence of Lewis-acid catalyst. Apparently, Lewis-acid catalysis of Diels-Alder reactions of p-amino ketone dienophiles is not practicable. 

The addition of an enolsilane to an aldehyde, commonly referred to as the Mukaiyama aldol reaction, is readily promoted by Lewis acids and has been the subject of intense interest in the field of chiral Lewis acid catalysis. Copper-based Lewis acids have been applied to this process in an attempt to generate polyacetate and polypropionate synthons for natural product synthesis. Although the considerable Lewis acidity of many of these complexes is more than sufficient to activate a broad range of aldehydes, high selectivities have been observed predominantly with substrates capable of two-point coordination to the metal. Of these, benzy-loxyacetaldehyde and pyruvate esters have been most successful. 

Recently, Hiersemann reported the first catalytic enantioselective Claisen rearrangement (Scheme 2.4) [11]. The 2-alkoxycarbonyl-substituted allyl vinyl ethers 11 are reactive under the Lewis acid catalysis. Therefore, the Claisen rearrangements proceed catalytically [12]. Usually the Lewis-acid-catalyzed Claisen rearrangement does not proceed catalytically because of a higher affinity of the carbonyl product for the Lewis acids than the ether substrate. But this 2-alkoxycarbo-nyl-substituted substrate 11 can coordinate to metals in a bidentate fashion. This 2-alkoxycarbonyl substrate has higher affinity for Lewis acidic Cu complexes than the simple ether substrate. In this system, chiral copper (II) bisoxazoline Cu (box) complex 13 is effective for the enantioselective Claisen rearrangement. 

Arnold, P.L., Rodden, M., Davis, K.M., Scarisbrick, A.C., Blake, A.J. and Wilson, C., Asymmetric lithium(I) and copper(II) alkoxy-iV-heterocyclic carbene complexes crystallographic characterisation and Lewis acid catalysis, Chem. Commun. (14), 1612-1613 (2004). 

The utilization of copper complexes (47) based on bisisoxazolines allows various silyl enol ethers to be added to aldehydes and ketones which possess an adjacent heteroatom e.g. pyruvate esters. An example is shown is Scheme 43[126]. C2-Symmetric Cu(II) complexes have also been used as chiral Lewis acids for the catalysis of enantioselective Michael additions of silylketene acetals to alkylidene malonates[127]. 

This chapter will begin with a discussion of the role of chiral copper(I) and (II) complexes in group-transfer processes with an emphasis on alkene cyclo-propanation and aziridination. This discussion will be followed by a survey of enantioselective variants of the Kharasch-Sosnovsky reaction, an allylic oxidation process. Section II will review the extensive efforts that have been directed toward the development of enantioselective, Cu(I) catalyzed conjugate addition reactions and related processes. The discussion will finish with a survey of the recent advances that have been achieved by the use of cationic, chiral Cu(II) complexes as chiral Lewis acids for the catalysis of cycloaddition, aldol, Michael, and ene reactions. 

Evans and coworkers developed C2-symmetrical copper(II) complexes as chiral Lewis acids that rely on two-dentate substrates for their catalysis. Consequently,... 

The reaction of a-diazocarbonyl compounds with nitriles produces 1,3-oxazoles under thermal (362,363) and photochemical (363) conditions. Catalysis by Lewis acids (364,365), or copper salts (366), and rhodium complexes (367) is usually much more effective. This latter transformation can be regarded as a formal [3 + 2] cycloaddition of the ketocarbene dipole across the C=N bond. More than likely, the reaction occurs in a stepwise manner. A nitrilium ylide (319) (Scheme 8.79) that undergoes 1,5-cyclization to form the 1,3-oxazole ring has been proposed as the key intermediate. 

A catalysis through a double Lewis acid activation of the scissile phosphate (coordination of two oxygen atoms of the phosphate onto the copper instead of one oxygen atom for a single Lewis acid assistance) was proposed by Chin (334) to account for the higher reactivity of 8 compared to 6. The mechanism proposed by Bashkin et al. for 6 is totally different and is based on the fact that the optimal rate of phosphate trans-esterification is at a pH value close to the value of the metal-bound water molecule (332, 335). Because monoaqua complexes as in 6 do not form stable four-membered ring phosphate coordinates, they may just behave as acid/base coreactants like histidine residues in ribonuclease A (336) or like imidazole buffer (337). 

MacMillan has reported examples of synergistic catalysis in which copper salts are used. Although these results were driven by ad hoc hypotheses, most of these transformations are related to a Cu(i)/Cu(m) catalytic cycle. In any case, the superior performances offered by copper(i) salts, compared to strong Lewis acids tested in the processes, is an indication that the Lewis acidity of the metal salt is not playing a decisive role in these transformations. The complexation of the enamine 7i-system with Cu(iii)-R is expected to lead to rjl-iminium organocopper species that, upon reductive elimination, will form a carbon-carbon bond and liberate the active Cu(i) catalyst. Hydrolysis of the resulting iminium will also release the imidazolidinone catalyst to complete the organocatalytic cycle as shown in Scheme 18.7. 

Another interesting coupled system between Lewis acid and micellar catalysis was developed by Engberts et al. with a million-fold acceleration of the Diels-Alder reaction. It was shown that if in the absence of Lewis acids, the reaction is retarded by micelles of CTAB or SDS—because of the different binding locations between the diene and the dienophile—the addition of copper dodecylsulfate leads to dramatic rate accelerations due to the complexation of the dienophile to the catalytically active copper ions, and to the high local concentration of both species at the micellar surface. ... 

Copper(I) catalysis has demonstrated its long-held reputation in asymmetric synthesis over the past decade. The moderate Lewis acidity and coordination property of Cu(l) salts make it a versatile metal center in various metal-ligand complex systems and thereby have broad applications in the area of organic chemistry, especially in the asymmetric catalysis field. This chapter summarizes the recent developments of Cu(l)-catalyzed asymmetric cycloaddition and cascade addition-cyclization reactions since 2010. A wide range of asymmetric transformations catalyzed by chiral Cu(l) complexes are discussed, such as the 1,3-dipolar cycloadditions, including [3+2], [3+3], and [3+6] cycloadditions. Other cycloadditions and cascade addition-cyclization reactions are also discussed. 

The utilization of copper(I) catalysis in asymmetric transformations is universal due to the special valence electron, Lewis acidity, and coordination characteristic of the metal. Copper salts are easily available, cost-efficient, and nontoxic. Copper(l)-catalyzed asymmetric cycloaddition and cascade addition-cyclization reactions are straightforward methodologies for the stereoselective construction of various biologically and medicinally important heterocyclic compounds. In the past 5 years, main endeavors have been paid into catalytic asymmetric [3+2] cycloadditions other types of cycloaddition protocols are relatively less developed. The examples described in this chapter clearly demonstrate the potential of chiral Cu(I) complexes in the synthesis of enantioenriched heterocycles. Further studies may lie in the diversification of catalytic system, reaction type, and catalysis mode. Research in this field is still challenging and highly desirable, and it would be expected that more discoveries will come in the near future. 

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