Palladium‐Catalyzed Oxidative Carbocyclization–Borylation of Enallenes to Cyclobutenes

Abstract

Ah ighly efficient palladium-catalyzed oxidative borylation of enallenes was developed for the selective formation of cyclobutene derivatives and fully-substituted alkenylboron compounds.C yclobutenes are formed as the exclusive products in MeOH in the presence of H 2 Oand Et 3 N, whereas the use of AcOH leads to alkenylboron compounds. Both reactions showed ab road substrate scope and good tolerance for various functional groups,i ncluding carboxylic acid ester,free hydroxy,imide,and alkylgroups.Furthermore, transformations of the borylated products were conducted to show their potential applications. Scheme 1. Previous work and the current work.

Iron‐Catalyzed Cross‐Coupling of Propargyl Ethers with Grignard Reagents for the Synthesis of Functionalized Allenes and Allenols

“…The desired trifluoromethyl allene 2 e was obtained in an excellent 88 % yield. To show-case the potential synthetic utility of the obtained fluoroalkyl allenes, 2 h was subjected to the regio-and stereoselective palladium-catalyzed oxidative borylation reaction [16] to give the trifluoromethyl group-containing borylated triene product 8 in 67 % yield (Scheme 3 b).…”

Palladium‐Catalyzed Oxidative Cascade Carbonylative Spirolactonization of Enallenols

“…Coordination of the allene and the olefin units to the Pd II center would trigger the formation of vinylpalladium intermediate Int-1 through an allene attack on Pd involving allenic CÀHbond cleavage. [19] Insertion of CO into the CÀPd bond of Int-1 would form intermediate Int-2.S ubsequent olefin and CO insertion would then give intermediate Int-4 via Int-3.F inally,i ntramolecular reaction of Int-4 with the hydroxy group provides the spirolactone 2.…”

“…Simultaneous coordination of the olefin and allene units of substrate 1 to the Pd II center triggers an allene attack on Pd II with CÀHb ond cleavage to produce intermediate Int-A. [19] Int-A may then undergo ac ascade carbon monoxide (CO) insertion [4,5] and subsequent carbocyclization [1] via olefin insertion to give Int-B.T he latter intermediate could then undergo an additional CO insertion followed by alactonization, which would provide the desired spirolactone 2.H owever,t he challenge is how to control the selectivity during the whole process,c onsidering that three C À Cbonds and one C À Obond would be selectively formed in aone-pot cascade reaction.…”

Palladium‐Catalyzed Oxidative Cascade Carbonylative Spirolactonization of Enallenols

“…[19] Insertion of CO into the CÀPd bond of Int-1 would form intermediate Int-2.S ubsequent olefin and CO insertion would then give intermediate Int-4 via Int-3.F inally,i ntramolecular reaction of Int-4 with the hydroxy group provides the spirolactone 2. Coordination of the allene and the olefin units to the Pd II center would trigger the formation of vinylpalladium intermediate Int-1 through an allene attack on Pd involving allenic CÀHbond cleavage.…”

“…[19] Int-A may then undergo ac ascade carbon monoxide (CO) insertion [4,5] and subsequent carbocyclization [1] via olefin insertion to give Int-B.T he latter intermediate could then undergo an additional CO insertion followed by alactonization, which would provide the desired spirolactone 2.H owever,t he challenge is how to control the selectivity during the whole process,c onsidering that three C À Cbonds and one C À Obond would be selectively formed in aone-pot cascade reaction. Simultaneous coordination of the olefin and allene units of substrate 1 to the Pd II center triggers an allene attack on Pd II with CÀHb ond cleavage to produce intermediate Int-A.…”