Chromium (low valent)
The stereoselective reaction of different aldehydes and ethyl dibromoacetate promoted by SmI2 or CrCl2 gives (E)-α,β-unsaturated esters by an an aldol-type reaction and a subsequent β-elimination reaction.
J. M. Concellon, C. Concellon, C. Mejica, J. Org. Chem., 2005, 70, 6111-6113.
Slow addition of a terminal alkyne and water to a mixture of an aldehyde, CrCl2, NiCl2 and a catalytic amount of triphenylphosphine in DMF at 25°C generates a 1,2-disubstituted allylic alcohol regioselectively.
K. Takai, S. Sakamoto, T. Isshiki, Org. Lett., 2003, 5, 653-655.