Regioselective Intramolecular Chloride Displacement Leading to Five- or Six-Membered Chelate-Ring Closure and Pentacoordinate Silicon Complexes: A Facile Wawzonek Rearrangement

O-Trimethylsilylated hydrazides with Schiff-base derivatives at the terminal nitrogen atoms[RC(OSiMe3)NNCR′R′′] react with chloro(chloromethyl)dimethylsilane [ClCH2SiMe2Cl] in a regioselective manner, forming either five-membered or zwitterionic six-membered chelate complexes with pentacoordinate silicon. The type of product is determined by the size of the acid-substituent R: bulky R groups (Ph, t-Bu) lead to exclusive formation of the six-membered product, whereas with the less bulky groups (Me, PhCH2) only the five-membered product is obtained. Upon mild heating, the six-membered chelate complex transforms into its five-membered isomer, through a Wawzonek-type rearrangement. This facile transformation is remarkable in comparison to previously reported Wawzonek reactions that take place only at elevated temperatures.