Ring‐Opening Protonolysis of Sila[1]ferrocenophanes as a Route to Stabilized Silylium Ions

The ring‐opening reactions of a series of sila[1]ferrocenophanes with protic acids of anions with various degrees of noncoordinating character have been explored. Ferrocenyl‐substituted silyl triflates FcSiMe 2 OTf ( 5 a ) and Fc 3 SiOTf ( 5 b ) (Fc=(η 5 ‐C 5 H 4 )Fe(η 5 ‐C 5 H 5 )) were synthesized by means of HOTf‐induced ring‐opening protonolysis of strained sila[1]ferrocenophanes fcSiMe 2 ( 3 a ) and fcSiFc 2 ( 3 b ) (fc=(η 5 ‐C 5 H 4 ) 2 Fe). Reaction of 3 a and 3 b with HBF 4 yielded fluorosubstituted ferrocenylsilanes FcSiMe 2 F ( 6 a ) and Fc 3 SiF ( 6 b ) and suggested the intermediacy of a highly reactive silylium ion capable of abstracting F − from the [BF 4 ] − ion. Generation of the solvated silylium ions [FcSiMe 2 ⋅THF] + ( 7a + ), [Fc 3 Si⋅THF] + ( 7b + ) and [FcSi i Pr 2 ⋅OEt 2 ] + ( 7c + ) at low temperatures, by reaction of the corresponding sila[1]ferrocenophanes ( 3 a , 3 b , and fcSi i Pr 2 ( 3 c ), respectively) with H(OEt 2 )(S)TFPB (S=Et 2 O or THF; TFPB=tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate) was monitored by using low‐temperature 1 H, 13 C, and 29 Si NMR spectroscopy. In situ reaction of 7a + , 7b + , and 7c + with excess pyridine generated [FcSiMe 2 ⋅py] + ( 8a + ), [Fc 3 Si⋅py] + ( 8b + ), and [FcSi i Pr 2 ⋅py] + ( 8c + ), respectively, as observed by 1 H, 13 C, and 29 Si NMR spectroscopy. A preparative‐scale reaction of 3 b with H(OEt 2 )(THF)TFPB at −60 °C and subsequent addition of excess pyridine gave isolable red crystals of 8b ‐[TFPB]⋅CHCl 3 , which were characterized by 1 H and 29 Si NMR spectroscopy as well as by single‐crystal X‐ray diffraction.