The selective preparation of 1,1,1-tris-(boronates) from vinylarenes and bis(pinacolato)diboron is described. The reactions occur at ambient temperature with excellent selectivity, high yields, and good functional group tolerance. Mechanistic studies suggest that Co(I)-catalyzed double dehydrogenative borylations generate a 1,1-diborylalkene intermediate, which undergoes hydroboration with pinacolborane formed in situ to yield 1,1,1-tris (boronate).