Cobalt-Catalyzed Divergent Cycloadditions of Alkynes with Conjugated Dienes Yielding 3-Vinylcyclobutenes, Bicyclo[3.1.0]hexenes, and Cyclohexa-1,4-dienes

A traditional cobalt catalyst system still contains undiscovered reactivity. Depending on the tertiary phosphines and substrates used, the catalytic system using CoBr2/tertiary phosphine/ Zn/ZnI2 catalyzes divergent cycloadditions of internal alkynes with conjugated dienes, yielding 3-alkenylcyclobut-1-enes, bicyclo[3.1.0]hexenes, and cyclohexa-1,4-dienes. A [CoBr2(PPh3)2]/Zn/ZnI2- catalyzed reaction of 3-hexyne (1a) with 1-(4-methoxyphenyl)butadiene (2a) at room temperature in CH2Cl2 exclusively produces a [2 + 2] cycloaddition product (E)-2-(2,3-diethylcyclobut-2-ene-1-yl)vinyl-4-methoxybenzene (3aa). When [CoBr2(dppp)]/Zn/ZnI2 is used as a catalyst, a bicyclic compound 6-(4-methoxyphenyl)-2,3diethylbicyclo[3.1.0]hex-2-ene (4aa) is dominantly formed in a 77% yield. The CoBr2/dppe/Zn/ZnI2 system can undergo a [2 + 4] cycloaddition to yield 3-(4-anisyl)-1,2-diethylcyclohexa-1,4-diene (5aa) as the dominant product in 38% yield. The bite angles of the ligands used contribute significantly to this catalytic diversity.