Chan-Evans-Lam

In 2000, Guy reported the stoichiometric coupling of alkane thiols and arylboronic acids, which was initially thought to be mediated by Cu(ll) [71]. Liebeskind proposed that the reaction was more likely catalyzed by Cu(l), generated by oxidation of the alkane thiols into dialkyl disulfides. Based on this hypothesis, Liebeskind predicted that disulfides and disulfide equivalents should be effective reagents for thioether formation [34]. This process would constitute a modification of the Chan-Evans-Lam, which involves the coupling of arylboronic acids and amines or alcohols in the presence of tertiary amine bases, generating aryl amines and ethers, respectively. Indeed, the coupling of diphenyl disulfide with phenyl boronic acid would yield diphenyl sulfide. 

Equation 4 Thioimide nucleophiles in the Chan-Evans-Lam cross coupling [34]... 

Reactions of Aryl Boronic Acids with Amines and Aicohols (Chan-Evans-Lam Couplings)... 

Oxidative Cross-Coupling (Chan-Evans-Lam Reaction) The coupling of phenols with aryl boronic acids has been accomplished (Scheme 20.37). Initially, Chan and coworkers studied an equivalent amount of CulOAc) for the coupling reaction [58]. Subsequently, Evans and coworkers found the use of MS 4 A led to improvement in the yield of the target product [106]. Later, Lam and coworkers made the process catalytic using oxygen as an oxidant [107]. Their mechanistic aspects have been recently demonstrated [108]. 

SCHEME 20.37 Chan-Evans-Lam reaction oxidative cross-coupling. 

The oxidative coupling of a benzoic acid with phenylboronic acid is achievable (Scheme 4.9) [13]. Thus, in the presence of a palladium catalyst together with a silver salt oxidant, decarboxylative coupling takes place selectively to produce 2,4,6-trimethoxybiphenyl. In contrast, under conditions using a copper catalyst in place of Pd, a Chan-Evans-Lam type reaction proceeds to afford phenyl 2,4,6-trimethoxybenzoate. 

Reports by the groups of Chan, Evans, and Lam in 1998 revealed an alternative method to conduct copper-mediated couplings that form C(aryl)-0 and C(aryl)-N bonds. In this process, arylboronic acids react with compounds containing N-H or 0-H bonds in the presence of a Cu(II) reagent or catalyst. TTiese reactions were initially conducted with stoichiometric amounts of copper reagents. " Amines, anilines, amides, ureas, carbamates, and sulfonamides underwent N-arylation in moderate to excellent yields by this process (Equation 19.124). The commercial availability of boronic acids and the ability to conduct these arylations in air under mild conditions has caused this method to be adopted quickly for synthetic applications on a small scale. 

Evans describes the possibility of a common intermediate for the Ullman and Chan-Lam reactions. D. A. Evans, J. Katz, T. R. West, Tetrahedron Letters 1998, 39, 2937... 

A Chan-Lam-Evans coupling has also been designed as a new method to access O-vinyl-iV-arylhydroxylamines under mild conditions that avoid the use of strong bases as described previously for the Bartoli-indole synthesis and related preparations of oxindoles (see Section 14.2.3). Anderson and coworkers have shown that treatment of iV-arylhydroxamic acids with vinylboronic acids leads directly to the synthesis of a-(o-anilido)ketones such as 90 f Scheme... 

Use of Arylboronic Acids and Derivatives Instead of Arylhalides Chan-Lam-Evans Coupling and Other Variants... 

In 2006, Jacobsen et al. [94] reported this methodology for the efficient Af-arylation of the five DNA/RNA nucleobases. Their strategy relied on applying the Chan-Lam-Evans protocol to protected or masked nucleobases because of their improved solubility and less reactive sites, culminating in the deprotection/unmasking at the end. 

Scheme 2.19 Putative general mechanism for the Chan-Lam-Evans arylation reaction [12d, 98].

Figure 2.21 Scope of the catalytic Chan-Lam-Evans arylation of Cbz-protected histidine, as reported by Daizotto s group [101].

Bruneau et al. [102] showed that it was also possible to conduct this reaction with glucosamines. Pentacetylated aminoglucose and other derivatives could be successfully arylated under Chan-Lam-Evans catalytic conditions (Figure 2.22). The yields were good. 

The reactions in water can be facilitated with an amphiphilic surfactant. Inamoto eial. [105] used these conditions in a catalytic Chan-Lam-Evans arylation of imidazoles with copper(II) acetate and 10 mol% of l,l -dimethyl-2,2 -dipyridyl or its fluorous derivative as ligands at room temperature under air. The best surfactants were BrijSO, Triton-X-100, and the fluorous-type surfactant F-PEG. 

---