No matter how much degassing I do, I always get huge amounts of alkyne dimer whenever I do a Sonogashira. My conditions are: dienyl iodide in benzene, add tetrakis-triphenylphoshpine palladium, then 45 min later add BuNH2, followed by CuI, flush well with nitrogen, then slowly add alkyne. With newly opened, high-purity reagents, I still get at least 40-50% dimer. Everytime. What am I doing wrong?