NOTE pubs.acs.org/joc
CuCl/Bipyridine-Catalyzed Addition Reactions of Arylboroxines with Aldehydes, r,β-Unsaturated Ketones, and N-Tosyl Aldimines Yuan-Xi Liao and Qiao-Sheng Hu* Department of Chemistry, College of Staten Island and the Graduate Center of the City University of New York, Staten Island, New York 10314, United States
bS Supporting Information ABSTRACT: CuCl/bipyridine-catalyzed addition reactions of arylboroxines with aldehydes and R,β-unsaturated ketones at elevated temperatures were described. By using the microwave energy, CuCl/bipyridine-catalyzed addition reactions of arylboroxines with aldimines were also realized.
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ver the past decade, transition-metal-catalyzed addition reactions of arylboron reagents with aldehydes have emerged as useful transformations in organic synthesis.19 Transition metal catalysts including Rh(I)/(II),2 Pd(II),3 Ni(0)/(II),4 Cu(I)/(II),5 Fe(III)6 complexes, and more recently Ru(II)7 and Co(II)8 complexes have been reported to catalyze this type of addition reaction. Although enormous success, including promising enantioselectivity, has been achieved, most of the transition metal catalysts are expensive and/or require air-free handling operation. The search for operationally convenient and cost-effective catalysts for this type of addition reaction continues. In our laboratory, we are interested in employing readily available transition metal complexes as catalysts for this type of addition reaction. In this context, we have recently documented air/moisture-stable anionic four-electron donor-based (type I) metalacycles (Figure 1),10 a large family of cyclic organometallic compounds, as catalysts for the addition reactions of arylboronic acids with aldehydes, R,β-unsaturated ketones, R-keto esters, and aldimines.11,12 We have also reported [Rh(COD)Cl]2 and Ni(COD)2/4-RCOC6H4Cl-catalyzed addition reactions of arylborons with aldehydes.13 During our study, we became interested in using Cu(I)/Cu(II) complexes as catalysts for this type of addition reaction because Cu(I) or Cu(II) salts such as CuCl or CuCl2 are inexpensive. So far, two Cu catalysts have been reported for addition reactions. CuF2/(R)-5,50 -bis[di(3,5-di-tert-butyl-4-methoxyphenyl)phosphino]-4,40 -bi-1,3-benzodioxole with tetrabutylammonium difluorotriphenylsilicate (TBAT) as additive was reported for the addition reaction of arylborates with aldehydes.5a Recently, the Cu(OAc)2/dppf complex was also reported as the catalyst for the addition reaction of arylboronic acids with activated aromatic aldehydes.5b While these protocols are useful, there are drawbacks associated with them, such as the requirement of additives and/or limited substrate scope. In our early study, we found low conversions (
