Thermal and Photodegradation of Photochromic Spiroindolinenaphthooxazines and -pyrans:  Reaction with Nucleophiles. Trapping of the Merocyanine Zwitterionic Form

Adducts of the zwitterionic (ZW) form of merocyanine (MC) of five photochromic spirooxazines (SO) and spiropyrans (SP) have, for the first time, been prepared photochemically or thermally through a trapping reaction of metastable MC with trimethylsilyl cyanide (TMSCN) in dichloromethane. The adducts (ZWAs) form easily and are obtained in almost quantitative yields. X-ray diffraction analysis of ZWA of spirooxazine 2 shows that in the adduct the planes of the indolyl and naphthyl moieties form a ca. 29° angle. The adduct crystallizes in the monoclinic space group P21/n with a = 22.663(4) Å, b = 8.234(1) Å, c = 14.455(2) Å, β = 104.72(1)°, V = 2637.4(7) Å3, and Dcalcd = 1.1476 Mg/m3 for Z = 4. Least-squares refinement of the model based on 1986 reflections (F > 4σ(F)) converged to a final R = 0.066 and Rw = 0.068. ZWAs form through an unprecedented, clean, and regioselective Michael-type 1,6-addition of TMSCN to either a strongly polarized quinoidal MC form or its charge-separated zwitterionic limit structure. No evidence for the more common 1,2- and/or 1,4-addition has been found. The observed high reactivity of SOs and SPs with nucleophiles may contribute to the reported limited durability of these classes of photochromes.