Convenient Transformation of Carbonyl Compounds via Their Hydrazones into gem-Difluoro Compounds with N-Bromosuccinimide/Pyridinium Poly(hydrogen fluoride) or Poly-4-vinyl-pyridinium Poly(hydrogen fluoride)1 - Surya Prakash, G K; Reddy, V Prakash; Li, Xing-Ya; Olah, George A (Synlett, 1990)