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 CoBr 2 /tertiary phosphine/Zn/ZnI 2 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 [CoBr 2 (PPh 3 ) 2 ]/Zn/ZnI 2 -catalyzed reaction of 3-hexyne ( 1a ) with 1-(4-methoxyphenyl)butadiene ( 2a ) at room temperature in CH 2 Cl 2 exclusively produces a [2 + 2] cycloaddition product ( E )-2-(2,3-diethylcyclobut-2-ene-1-yl)vinyl-4-methoxybenzene ( 3aa ). When [CoBr 2 (dppp)]/Zn/ZnI 2 is used as a catalyst, a bicyclic compound 6-(4-methoxyphenyl)-2,3-diethylbicyclo[3.1.0]hex-2-ene ( 4aa ) is dominantly formed in a 77% yield. The CoBr 2 /dppe/Zn/ZnI 2 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.