Cohalogenation of Allyl and Vinylsilanes using Polymer-bound Haloate(I)-Reagents

Polymer-supported electrophilic halogenate(I) complexes 2 and 3 promote smooth addition to vinyl and allylsilanes without loss of the silyl group. In conjunction with Amberlyst A26 (OH− -form) vinyl silanes are converted into epoxysilanes.


Introduction
Recently, the development and applications of polymer-supported reagents have seen a dramatic increase in interest [1]. Functionalized matrices can be used in excess to drive reactions in solution to completion and are finding application in high throughput, automated parallel syntheses [2]. In some cases major differences between reactions on polymer supports and their low-molecular mass analogues have been observed [3]. Apart from differences in reaction rates, altered regio-and stereoselectivity has been described in some cases using functionalized polymers. E. g. Patchornik and coworkers encountered that crosslinked poly(maleimide) in which 70% of the repeat units were Nbrominated gave a different set of bromination products in reactions with cumene than Nbromosuccinimide [4]. These properties give polymer-supported reagents additional attraction. Thus, in this communication we report on the unexpected clean cohalogenation of allyl and vinylsilanes which is promoted by functionalized polmers with electrophilic properties.

Results and Discussion
Recently, our interest in new electrophilic halogen-ate(I) complexes [5] has led to the development of the first polymer-bound iodine azide source (1) [6] as well as of functionalized polymers (2)- (4) which are loaded with synthetic equivalents of acylated hypohalites [7]. All of these electrophilic reagents promote 1,2-cohalogenations of various alkenes [8,9] under very mild conditions and with high efficiency [7]. In this paper, we disclose reactions of vinyl and allylsilanes with polymer-supported electrophilic reagents (2) and (3) which instead to the expected desilylation lead to 1,2-addition products. In fact, numerous methods [10] are known for effecting iododesilylation of vinyl and allylsilanes using different iodonium sources like ICl, IBr, IBF 4 , and NIS [11]. It is generally accepted that the reaction is initiated by attack of the electrophilic iodonium source to the double bond to form a cyclic iodonium ion and formation of β-silicon stabilized cation. After rotation around the C-C-bond a planar orientation between the empty p-orbital and the C-Si bond is achieved which allows facile removal of the silyl group by a nucleophile. In some instances, additional stabilization of the intermediate carbocation by the solvent was accounted [11].

(8), 8d
--------no reaction a Refer to Experimental Section. b All reagents are employed in excess with reference to the amount of polymer-bound halide specified by the commercial provider [13]. c All yields refer to isolated pure products. Yields in parentheses refer to crude products. d Formation of the 1,2-cis-addition byproduct was proven by 1 H-NMR-spectroscopy of the crude product. In conclusion, polymer-supported haloate(I)-complexes promote cohalogenation of allyl and vinylsilanes under very mild conditions without exerting substantial desilylation. These reagents allow easy product isolation, are conveniently recycled [14] and finally are potentially useful for automated solution phase synthesis.

Experimental
General Methods 1 H-and 13 C-NMR spectra were measured with 200 MHz (50 MHz) using tetramethylsilane as the internal standard. CDCl 3 is the solvent for all NMR experiments. All solvents used were of reagent grade and were further dried. Reactions were monitored by TLC on silica gel 60 P254 and detected either by UV-absorption or by staining with H 2 SO 4 / 4-methoxybenzaldehyde in ethanol. Flash column chromatography was performed on silica gel 60 (230-400 mesh).

General Procedure for the Preparation of Polymer-Bound Reagents 2 and 3
A suspension of polymer bound halide (available from Fluka; 3.5 mmol/g bromide and 2.9 mmol/g) and PhI(OAc) 2 (1.8 eq.) in dry CH 2 Cl 2 (2.5 mL/mmol halide anion) under nitrogen was shaken at 300 rpm for 24 h at room temperature. The yellowish suspension was protected from light. Filtration and washing of the resin with CH 2 Cl 2 (3x) and drying in vacuo afforded the light yellow reagents.