Highly Regio- and Stereoselective Acylboration, Acylsilation, and Acylstannation of Allenes Catalyzed by Phosphine-Free Palladium Complexes:  An Efficient Route to a New Class of 2-Acylallylmetal Reagents

Abstract

A new method for the synthesis of substituted 2-acylallylmetal reagents in a highly regio- and stereoselective fashion involving a three-component assembly of allenes, acyl chlorides, and bimetallic reagents (B-B, Si-Si, and Sn-Sn) catalyzed by phosphine-free palladium complexes is described. Treatment of various allenes (CR(2)R(3)=C=CH(2)) with acyl chlorides (R(1)COCl) and bispinacolatodiboron in the presence of PdCl(2)(CH(3)CN)(2) in toluene at 80 degrees C gave 2-acylallylboronates in moderate to good yields. The acylsilation of allenes with acid chlorides and hexamethyldisilane (5) proceeded successfully in the presence of Pd(dba)(2) in CH(3)CN affording the corresponding allylsilanes (CR(2)R(3)=C(COR(1))CH(2)SiMe(3)) in good to moderate yields. Several chloroformates (R(4)OCOCl) also react with 1,1-dimethylallene (2a) and 5 to afford allylsilanes (CR(2)R(3)=C(COOR(4))CH(2)SiMe(3)) in 66-70% yields. Acylstannation of allenes could also be achieved by slow addition of hexabutylditin (10) to the reaction mixture of acyl chloride (or chloroformate) and allene 2a in CH(3)CN in the presence of Pd(dba)(2) at 60 degrees C; the corresponding 2-substituted allylstannanes were isolated in moderate to good yields. The above catalytic reactions are completely regioselective and highly stereoselective. A mechanism is proposed to account for the catalytic reactions and the stereochemistry.

Singular Metal Activation of Diboron Compounds

“…Cheng has demonstrated that phosphine-free Pd complexes together with alkenyl or aryl iodides are very efficient catalysts for 1,2-diboration of allenes. 67,68 This reaction is completely regioselective and highly stereoselective affording diborated products with mainly Z stereochemistry. This Pd-catalyzed reaction proceeds via a previously unknown mechanism involving the oxidative addition of an IdB bond into the palladium center instead of the oxidative addition of a BdB bond to metal (Scheme 18).…”

Metal‐Catalyzed Multicomponent Reactions

“…In recent years, similar strategies were developed by Cheng and co-workers to prepare allyl azides [63] and allylic silanes [64]. The same group developed efficient palladium-mediated methods [65] for the assembling of acyl chlorides, allenes, N I 9 5 mol% Pd(dba) 2 and bimetallic reagents such as bis(pinacolato)diboron, hexamethyldisilane, and hexabutylditin to form the corresponding 2-acylallylmetal reagents (Scheme 8.24). Related acylation of allenes using acyltetracarbonylcobalt complexes and subsequent intermolecular nucleophilic attack of the resulting p-allyl complexes by alkylnitronates had been developed previously by Hegedus [66].…”

Nickel‐Catalyzed Cyclizative trans‐Carboboration of Alkynes through Activation of Boron–Chlorine Bonds by Using Organometallic Reagents as Donors of Organic Groups

“…Keywords: carbometalation · cross-coupling · nickel · organoboronic acids · trans-addition [11] Herein, we describe the detail of our findings on nickelcatalyzed cyclizative carboboration of chloroboryl ethers of homopropargyl and bishomopropargyl alcohols by using organometallic reagents as donors of organic groups. A preliminary report on this topic appeared previously on the use of alkynylstannanes as the transmetalation reagent.…”