Cyclization — hydrosilation

Cyclization/HydrosUation. Recently, (S)-(—)-4-(2-methylp-ropyl)-2-(2-pyridyl)-2-oxazoline has been applied to the cyclization/hydrosilation process catalyzed by palladium (eq 8). The precatalyst (L -Pd(Me)Cl), purified and characterized after preparation by ligand exchange with (COD)Pd(Me)Cl, afforded product in >95% de and 77% ee. 

The ability of Pd complexes to catalyze tandem cyclization-hydrosilation reactions, that could provide a synthetic route to carbocycles of interest in the fields of biochemistry and medicine, was recently demonstrated. Widenhoefer s group has developed phenanthroline-based complexes 11 and others that quite efficiently bring about this dual transformation for a wide variety of functionalized dienes, with high regio- and stereoselectivities [e.g., Eq. (10)]. ... 

The novel spirosilane (48) can be elegantly prepared in a two-step synthesis which combines both of the general routes described above. Hydrosilation gives the y-chloropropylsilane which is then cyclized with magnesium (Scheme 62) (71DOK(198)112). 

Silacyclohex-2-enes are conveniently made through the formation of l-trichlorosilyl-5-chloro-l-pentene by hydrosilation, followed by CyClization using magnesium (Scheme 230) (65JOM(4)284). The Diels-Alder addition of silenes to 1,3-dienes provides a useful route to the isomeric silacyclohex-3-enes (Scheme 231) (79CR529). 

Analogously, intramolecular hydrosilation reactions have been carried out with alkenes cyclization of alkenyloxy-silanes catalyzed by thiols <1998J(P1)467> and by Pt <1996TL827> has been described. In addition, intramolecular temporary silicon-tethered rhodium-catalyzed [4- -2- -2]-cycloisomerization reactions have been carried out by Evans and Bawn (Equation 50) <2004JA11150>. 

Dimethyl- and 3,3-diethyl-l,3-thiasilacyclobutanes are obtained by intramolecular cyclization of the bis(chloromethyl)silane with hydrosulfide ion, as exemplified by the synthesis of 522 or by an intramolecular hydro-silation, as exemplified by the synthesis of 523. A chloroplatinic acid catalyst (Speiers catalyst) also has been used for the hydrosilation... 

To disfavor unwanted cyclization entropically, this general reaction scheme was extended to include monomers 6-hex-1-enyltris(dimethyl-siloxy)silane and 8-oct-l-enyltris(dimethylsiloxy)silane. The resulting polymers gave no indications of cyclization and resulted in molecular weights of about 12,000. However, SEC traces were multimodal in nature caused by monomer contamination with internal olefins (notoriously sluggish or completely unreactive towards hydrosilation reac-... 

Co2Rh2(CO)i2] and [Co3Rh(CO)i2] catalyze the hydrosilation of isoprene, cyclohexanone, cyclohexenone, and 1-alkynes, the regioseleetive inter- or intramolecular (Eq. 5) silylformylation of alkynes and 1-alkynals, and the silylcarbo-cyclization of enynes, diynes, and alkynals. These synthetically important reactions have been reviewed recently. 

B.iii. Palladium-Catalyzed Cyclization Reactions Via Hydrosilation-Cyclic Carbopalladation... 

Intramolecular Hydrosilation of Allyl Amines. The amino group of an allyl amine is silylated with chlorodimethylsilane, and then the resulting silazane is subjected to standard platinum-catalyzed cyclization and oxidation steps to form a syn-2-amino alcohol (eq 1). Exclusive formation of the four-membered cyclic product and the high stereoselectivity should be noted. ... 

The reaction of terminal allyl alcohols proceeds in a 5-endo fashion to give five-membered ring compounds regioselectively and stereoselectively. Subsequent oxidation affords 2,3-5 y -l,3-diols preferentially, regardless of the nature of the catalyst (eq 1). The stereoselectivity increases with increased bulkiness of the al-lylic substituent and the nature of the alkene substituent (see below). 5-Exo type cyclization occurs with homoallyl alcohols to form five-membered heterocycles and 1,3-diols after oxidation. Two chiral centers are produced in this reaction. The 2,3-relationship (anti) is controlled by the allylic substituent, while the 3,4-relationship is determined by the stereochemistry of the alkene the hydrosilation occurs by cis addition of Si-H to the alkene (eq 2). ... 

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