Editorial

, Volume: 16( 2)

Metal-Free Photochemical Borylation of Aryl Halides

Charlo William^*

*Correspondence:

Charlo William
Editorial office, Organic chemistry An Indian Journal
E-mail: organicchem@journalres.com

Abstract

Arylboronic acids and esters have a wide range of uses in the chemical, pharmaceutical, and materials sciences. They are flexible synthons for the creation of carbon–carbon or carbon–heteroatom bonds in synthetic organic chemistry. Reactions of arylmetallic intermediates with trialkyl borates, followed by transesterication or hydrolysis, are the traditional procedures for producing arylboron compounds. These reactions have several significant limitations, including restricted functional group tolerance and the requirement of strict anhydrous conditions. Transition metal-catalyzed borylation processes including palladium, nickel, copper, and zinc have developed as very helpful ways for converting C–X bonds to C-B bonds in recent decades [1]. Direct C–H borylation techniques based on transition-metal catalysts have been developed more recently. Several transition-metal-free approaches for C–B bond synthesis have been developed to reduce costs and the amount of heavy metal residue in final products. Ito and colleagues showed that an alkali alkoxide-mediated borylation of aryl halides can be achieved using a silylborane as the only borylating reagent [1,4]. Zhang and colleagues found that aryl iodides could be borylated in reuxing methanol with 4.0 equivalents of bis(pinacolato) diboron and 2.0 equivalents of Ce2CO3 as the promoter. The reaction time ranged from a few hours to several days, with moderate yields [2,3].  Under mild circumstances, Fernandes and Muniz converted diaryliodonium acetates to arylboronates.

 

Introduction

Arylboronic acids and esters have a wide range of uses in the chemical, pharmaceutical, and materials sciences. They are flexible synthons for the creation of carbon–carbon or carbon–heteroatom bonds in synthetic organic chemistry. Reactions of arylmetallic intermediates with trialkyl borates, followed by transesterication or hydrolysis, are the traditional procedures for producing arylboron compounds. These reactions have several significant limitations, including restricted functional group tolerance and the requirement of strict anhydrous conditions. Transition metal-catalyzed borylation processes including palladium, nickel, copper, and zinc have developed as very helpful ways for converting C-X bonds to C-B bonds in recent decades [1]. Direct C–H borylation techniques based on transition-metal catalysts have been developed more recently. Several transition-metal-free approaches for C–B bond synthesis have been developed to reduce costs and the amount of heavy metal residue in final products. Ito and colleagues showed that an alkali alkoxide-mediated borylation of aryl halides can be achieved using a silylborane as the only borylating reagent [1,4]. Zhang and colleagues found that aryl iodides could be borylated in reuxing methanol with 4.0 equivalents of bis(pinacolato) diboron and 2.0 equivalents of Ce2CO3 as the promoter. The reaction time ranged from a few hours to several days, with moderate yields [2,3]. Under mild circumstances, Fernandes and Muniz converted diaryliodonium acetates to arylboronates. Wang invented a mild and efficient Sandmeyer-type borylation process using aryl amines as the starting material. 8a–c Aryl diazonium salts8d–f and aryl triazenes8g have also been observed to be borylated. Furthermore, new procedures for direct C– H borylation under transition metal-free conditions have been reported9, however the substrates were limited to electron-rich arenes or heterocycles, and air and moisture sensitive reagents were required [4]. As a result, a practical, metal-free approach that is quick and successful, operates under mild circumstances with a variety of easily accessible borylating reagents, has a high functional group tolerance, and avoids strong acids, bases, and dangerous chemicals remains highly desirable. We present here our invention and development of a new aryl halide borylation reaction that uses light as a clean reagent. 

Discussion

A solution of 4-iodoanisole and bis (pinacolato) diboron in acetonitrile was first placed in a quartz test tube and irradiated for 4 hours with a 300 W high pressure mercury lamp (maximum at 365 nm). According to 1H NMR analysis of the crude product, the intended aryl-B(pin) product 3a was generated in a 29 percent yield [5]. Other polar solvents, such as tri uoroethanol and methanol, had little effect. In both cases, using water and acetone as co-solvents resulted in a 46 percent increase in yield. After testing a variety of organic and inorganic additions, it was discovered that an organic base, N,N,N0,N0-tetramethyldiaminomethane (TMDAM), could boost the yield to 58 percent. Other bases, on the other hand, yielded inferior outcomes. Surprisingly, a higher amount of TMDAM resulted in a decreased yield. Using two B2 (pin) 2 equivalents could increase the yield to 72%. Changing the reaction concentration of 1a resulted in a greater yield after further adjustment. Traditional methods for generating arylboron compounds include reactions of arylmetallic intermediates with trialkyl borates, followed by transesterication or hydrolysis [1,3,5]. These reactions have a number of drawbacks, including limited functional group tolerance and the need for stringent anhydrous conditions. In recent decades, transition metal-catalyzed borylation techniques such as palladium, nickel, copper, and zinc have emerged as particularly useful methods for converting C–X bonds to C–B bonds. More recently, direct C–H borylation procedures using transition-metal catalysts have been devised [6]. To reduce costs and the amount of heavy metal residue in final products, many transition-metal-free techniques for C–B bond synthesis have been devised. Ito and coworkers demonstrated that silylborane can be used as the sole borylating reagent in an alkali alkoxidemediated borylation of aryl halides.