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

Abstract

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 FcSiMe2OTf (5 a) and Fc(3)SiOTf (5 b) (Fc=(eta5-C5H4)Fe(eta5-C5H5)) were synthesized by means of HOTf-induced ring-opening protonolysis of strained sila[1]ferrocenophanes fcSiMe2 (3 a) and fcSiFc2 (3 b) (fc=(eta5-C5H4)2Fe). Reaction of 3 a and 3 b with HBF4 yielded fluorosubstituted ferrocenylsilanes FcSiMe2F (6 a) and Fc3SiF (6 b) and suggested the intermediacy of a highly reactive silylium ion capable of abstracting F- from the [BF4]- ion. Generation of the solvated silylium ions [FcSiMe2THF]+ (7a+), [Fc3SiTHF]+ (7b+) and [FcSiiPr2OEt2]+ (7c+) at low temperatures, by reaction of the corresponding sila[1]ferrocenophanes (3 a, 3 b, and fcSiiPr2 (3 c), respectively) with H(OEt2)(S)TFPB (S=Et2O or THF; TFPB=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) was monitored by using low-temperature 1H, 13C, and 29Si NMR spectroscopy. In situ reaction of 7a+, 7b+, and 7c+ with excess pyridine generated [FcSiMe2py]+ (8a+), [Fc3Sipy]+ (8b+), and [FcSiiPr2py]+ (8c+), respectively, as observed by 1H, 13C, and 29Si NMR spectroscopy. A preparative-scale reaction of 3 b with H(OEt2)(THF)TFPB at -60 degrees C and subsequent addition of excess pyridine gave isolable red crystals of 8b-[TFPB]CHCl3, which were characterized by 1H and 29Si NMR spectroscopy as well as by single-crystal X-ray diffraction.

Taming the Silylium Ion for Low‐Temperature Diels–Alder Reactions

“…We were pleased to see that the 29 Si NMR spectrum of 2 showed a diagnostic signal at δ =114.5 ppm, a significant downfield shift (Figure 2). In comparison with the reported 29 Si NMR chemical shifts (Figure 3),3–6, 10, 18 the observed value indicates to us that the iron‐stabilized silylium ion is not “quenched” by a solvent molecule and is therefore still a reasonably strong Lewis acid.…”

“…Inspired by a seminal attempt by Corey et al 17. and a recent contribution by Manners and co‐workers,18 we set out to employ an electron‐rich late transition metal for intramolecular stabilization of a trivalent silicon cation; our goal was realized by the ferrocene‐based 2 (see the Supporting Information for straightforward preparation of its precursor 1 ), which is decorated with both a small and a large group (Scheme ). Whereas the iron–silicon interaction attenuates Lewis acidity, the deliberate choice of the substituents at the silicon atom also seems to be vital 19.…”

Strained Metallocenophanes and Related Organometallic Rings Containing π‐Hydrocarbon Ligands and Transition‐Metal Centers

“…[92][93][94] Notably, the FeÀCp bond in sila [1]ferrocenophanes has been shown to be susceptible to a reduction of hapticity and ultimately cleavage with photoirradiation in the presence of weak bases such as [C 5 H 4 R] À (R = H, Me). [48,61,63] Germanium and Tin…”

Ein gezähmtes Silyliumion für Diels‐Alder‐Reaktionen bei niedrigen Temperaturen

“…Angeregt durch einen richtungweisenden Versuch von Corey et al [17] und einen Beitrag jüngeren Datums von Manners et al [18] setzten wir uns das Ziel, ein dreiwertiges Siliciumkation durch ein elektronenreiches Übergangsmetall intramolekular zu stabilisieren. Dies konnten wir im ferrocenbasierten 2 verwirklichen, in dem das Siliciumatom mit einer kleinen und einer großen Gruppe substituiert ist (Schema 1); die einfache Herstellung der Vorstufe 1 ist in den Hintergrundinformationen beschrieben.…”