New synthetic transformations via organoboron compounds

It was previously reported that the palladium-catalyzed cross-coupling reaction of 1-alkenylboron compounds with various vinylic or aryl halides in the presence of base provides the corresponding coupling products in high yields, stereoand regioselectively. The recent progress of such reactions, including the reaction of sterically hindered arylboronic acids with sterically hindered haloarenes, the palladium-catalyzed carbonylative cross-coupling to provide a versatile ketone synthesis, the synthesis of functionalized organotin compounds, the stereoselective synthesis of exocyclic alkenes, and the syntheses of allylic and benzylic boronates, is described. INTRODUCTION The cross-coupling reactions of various organoboranes with a number of organic halides in the presence of a catalytic amount of palladium complexes and bases were reported to give versatile and useful synthetic methods for conjugated alkadienes and alkenynes, arylated alkenes, 1.4-alkadienes, allylic benzenes, a$-unsaturated carboxylic acids, and 2,4-alkadienoates (ref. 1). Thereafter, a modified method for the synthesis of conjugated alkadienes, syntheses of stereodefined trisubstituted alkenes, benzo-fused heteroaromatic compounds, and a,&unsaturated ketones were presented (ref. 2). Although the palladium-catalyzed cross-coupling reactions of 1-alkenyland arylboranes with organic halides proceed readily and stereoand regioselectively to give expected coupling products in high yields, organoboranes with alkyl groups on boron were not used successfully for the coupling under similar conditions. Recently, it was found that the reaction between 9-alkyl-9-BBN derivatives and l-halo-lalkenes or haloarenes in the presence of dichloror l,l'-bis(diphenylphosphino)ferrocene]palladium(II) and base, such as sodium hydroxide, potassium carbonate, and phosphate gives the corresponding alkenes or arenes in excellent yields (ref. 2). By using the reaction, cycloalkenes, benzo-fused cycloalkenes, and exccyclic alkenes are readily synthesized (ref. 2). The palladium-catalyzed coupling reactions of 1-alkenyl-, aryl-, and alkylboron compounds with aryl or 1-alkenyl triflates, instead of organic halides, take place with ease to give expected coupling products in high yields under mild conditions (ref. 2). Most recently, we have found new synthetic methods related to the palladium-catalyzed cross-coupling reactions of organoborane derivatives, which are described in this review. CARBONYLATIVE CROSS-COUPLING OF 9-ALKYL-9-BBN DERIVATIVES WITH 1-HALO-1-ALKENES (ref. 3) AND IODOALKANES (ref. 4) Carbonylative cross-coupling of organometallic compounds with organic halides are reported to give a method for the synthesis of ketones. Among a variety of organometallic reagents, a method using organoboron compounds was first examined by Kojima and his coworkers (ref. 5 ) for the synthesis of 213