Diazo coupling reactions are capable of forming E,E,E-trienes from cinnamaldehydes in good yield. An efficient methodology is reported for the production of styryldiazomethanes that are subsequently used with catalysis for coupling and for cyclopropanation. A vast difference in product selectivity is seen with styryldiazomethane generated from the corresponding hydrazone via manganese dioxide oxidation and that formed in situ by treatment of the tosylhydrazone sodium salt of cinnamaldehyde with transition metal catalysts. This observation impacts understanding of the reaction mechanism for diazo decomposition.