Cu-catalyzed reaction

In 2010, Nicholas and co-workers studied the enantioselective ben lic amination reactions.As shown in Table 1.11, they tested different kinds of ligands including (S)-histidine, (5)-proline, diimine ligands, and chiral phenanthroline. Although the corresponding product can be prepared with high yields, the enantioselectivity is low. The preliminary results from the mechanistic studies support a stepwise C—H bond insertion process, most likely through the intermediacy of carbon-centered radicals. Subsequently, they expanded this reaction to an intramolecular version with up to 18% ee.  

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The reaction was studied for all coinage metal nanoparticles. In the case of GMEs the rate follows zero-order kinetics with IT for all the coinage metal cases. The observed IT for the Cu catalyzed reaction was maximum but its rate of reduction was found to be minimum. Just the reverse was the case for Au and an intermediate value was obtained for the Ag catalyzed reaction (Figure 7). The adsorption of substrates is driven by chemical interaction between the particle surface and the substrates. Here phe-nolate ions get adsorbed onto the particle surface when present in the aqueous medium. This caused a blue shift of the plasmon band. A strong nucleophile such as NaBH4, because of its diffusive nature and high electron injection capability, transfers electrons to the substrate via metal particles. This helps to overcome the kinetic barrier of the reaction. 

Scheme 31 Diastereoselective and enatioselective Cu-catalyzed reaction of keto-unsatur-ated esters...

In 1974, Vermeer et al. described formation of allenic alcohols 61 by the reaction of alkynyl epoxides 60 with Grignard reagents in the presence of 10mol% of Cul (Scheme 3.33) [71]. In the absence of Cul, a complicated mixture of products was obtained. Furthermore, the Cu-catalyzed reactions exhibited higher yields and higher selectivity than analogous reactions of alkynyl epoxides with lithium dialkylcup-rates [72], This method was applied to a reaction of allylmagnesium bromide with an alkynyl epoxide [73]. 

The palladium/copper-catalyzed coupling reaction of 2-iodo-3-methoxy-6-methylpyridine and terminal alkynes leads to the formation of o-methoxyalkynylpyridines which undergo electrophilic cyclization reactions to afford furo[3,2-3]pyridines in moderate yields <2005JOC10292>. A similar Pd/Cu-catalyzed reaction with hydroxypyridines and trimethylsilyl (TMS)-acetylene leads to the formation of alkynyl pyridines which cyclize to form furo[2,3- ]-pyridines in good yields <1998JME1357>. 

Although not discussed in this chapter, the Tsuji-Trost reaction159 is undoubtedly the most extensively investigated Pd-catalyzed allylation with allyl electrophiles. There have also been some uncatalyzed and Cu-catalyzed reactions of allyl electrophiles with alkyl metals and metal cyanides. On the other hand, the Pd- or Ni-catalyzed reactions of allyl electrophiles with organometals containing allyl-, benzyl-, propargyl- and other alkylmetals do not appear to have been extensively investigated. 

A series of chiral bis(oxazoline) ligands, A, B, and C, differing in the length of the chain connecting the chiral oxazoline subunits and in the nature of the substituent at the chiral center, were examined in the M(OTf)2 (M = Zn, Mg, Cu) catalyzed reaction of A-crotonyl oxazolidinone with cyclopentadiene [34] (Eq. 8A.18). A l,4-bis(oxazoline) ligand C proved best for... 

Oxidative coupling of phenylethynesis another Cu-catalyzed reaction that involves one-electron redox steps. Substituted, conjugated alkynes are valuable components for liquid crystal applications. Auer et al. (181) used a calcined CuMgAl-LDH-C03 to catalyze the air oxidation of phenylethyne to give l,4-diphenylbuta-l,3-diyne ... 

ZnBr2 also is an effective catalyst for the carbonyl insertion (Equation (75)).290 The Zn-catalyzed reaction is applicable to various aldehydes and ketones including aliphatic compounds. In sharp contrast to the Cu-catalyzed reaction, the carbonyl insertion occurs on the less substituted side with high regioselectivity. ZnBr2 most likely serves as electrophilic activation of carbonyl compounds. 

Chapter 5 includes complete coverage of the transition metals-mediated carbon-carbon bond forming reactions. Pd-, Ni-, Cr-, Zr- and Cu-catalyzed reactions such as Heck, Negishi, Sonogashira, Suzuki, Hiyama, Stille, Kumada reactions are covered in adequate details including the applications of these reactions in organic synthesis. 

A one-step synthesis with a good yield has been reported in which the conditions are mild, with Et4NBr as catalyst. Experiments on Cu and Ni catalysts with labeled compounds proved that the Cu-catalyzed reaction takes place with retention. Work has also been carried out on the kinetics of carbonate formation. ... 

Ligands of type 1 have also led to interesting results in kinetic resolutions during Cu-catalyzed reactions of dialkyIzinc with cycloalkene oxides (eq. (1)) [18] and cyclohexenones [19]. On the other hand, monophosphonites bearing a fused 1,4-dioxane ring behave moderately in the Rh-catalyzed hydrosilylation of ketones (up to ca. 56 % ee) [20] cf. Section 2.6 Finally, a phosphapallada-cyclic complex has been reported as an exceptionally fast catalyst for the hydroarylation of norbomene (TONs up to 10 °) but with low ees (< 25 %) [21]. 

Amide insertion into trans- and ctx-187a is effectively catalyzed by a Cu salt at or below room temperature (Scheme 10.252) [683]. Aromatic aldehydes, a, -unsa-turated aldehydes, formates, and formamide can be used as the carbonyl donor. Carbonyl insertion into 187b favors the more substituted side, and the regioselectivity is very high (Scheme 10.252). It has been proposed that the Cu-catalyzed reaction proceeds by transmetalation, forming an organocopper intermediate. 

Intermolecular alkyl-alkyl couplings have been performed with a complex molecule, e g., a fragment of vitamin D, attached to a sohd support via a sulfonate linkage, and an alkyl Grignard reagent such as (89) [207]. The Cu-catalyzed reaction was designed to attach various alkyl chains at the 22-position of atom Tl of vitamin D3 analogs (88) (Scheme 20). 

The most versatile carbene precursors are a-diazocarbonyl compounds such as diazoacetic acid esters because they are readily prepared, easy to handle and much more stable than ordinary diazoalkanes [10,38]. Nevertheless, one should always be aware of the potential hazards of diazo compounds in general [39],but if the necessary precautions are taken, they can be safely handled even on an industrial scale [18]. The most frequently used reagent is commercially available ethyl diazoacetate. Besides a-diazocarbonyl reagents, diazomethane [40,41 ] and a y-diazoacrylate derivative [42] have been used in enantioselective Cu-cata-lyzed cyclopropanations but the scope of these reactions has not been studied systematically. It has been shown in certain cases that diazo compounds can be replaced by other carbene precursors such as iodonium ylides, sulfonium yUdes, or lithiated sulfones [8,43],but successful applications of these reagents in enantioselective Cu-catalyzed reactions have not been reported yet. 

It is possible to prepare allylic boronates by a Cu-catalyzed reaction of allylic carbonates with bis(pinacolato)diboron. A chiral version of the reaction uses a QuinoxP ligand (36)/ ... 

Arylamines. Arylzinc compounds are converted into arylamines on Cu-catalyzed reaction with the reagent. 

Thus, the Cu++-catalyzed reaction between H2 and Cr207 may be formally represented by the sequence of steps... 

The Pd/Cu-catalyzed reaction of iodopyrimidinones and propargyl alcohol has been reported to give products in which cyclization has occurred on the carbonyl oxygen (Eq. 9) [33]. 

Beilina and Rossi also reported a ligand-free approach for die C2-arylation of NH-free indoles (Scheme 15, 06JOC1379, 07T1970). However, this Pd/Cu catalyzed reaction occurring under base-free conditions gave low yields with indoles (although this procedure was very efficient for other substrates, such as NH-free imidazoles). 

Redirecting reactions via conformational control In the final example of this chapter, we will show how understanding of enzyme stereoelectronics allows one to redirect reactivity of known substrate into a new direction. When the stractural constraints of enzyme active sites are removed, a new set of stereoelectronic constraints can be imposed on the same substrate to enforce a different reactivity. For example, the effect of stereoelectronic factors on the relative order of steps in the deprotonation/decarboxylation sequence were recently analyzed for an enzyme-catalyzed vs. Cu-catalyzed reactions of the similar substrates (Figure 11.66). The first process is involved in the biosynthesis of polyketides and fatty acids where enzymatic activation of... 

Figure 11.66 Decarboxylation precedes condensation in the enzyme mechanism whereas the Cu-catalyzed reaction starts with deprotonation.

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