Reduction silanes

Similar reactivity is observed in the cyclization of enynes in the presence of the yttrium-based catalyst 70 and a silane reductant [53,54]. The 1,6- and 1,7-enynes 90 and 91 provide -E-alkylidene-cyclopentancs 92 and -cyclohexanes 93 in very good yield (Eq. 15, Scheme 20) [55]. These transformations likely proceed by syn hydrometallation of the 7r-basic alkyne, followed by insertion of the alkene and a-bond metathesis. The reaction of 1,6-enynes tolerated... 

The mechanistic discussion of silane reductions will be limited to those of cationic reductions, thus excluding the many silane reductions that involve metal... 

Aluminum chloride, used either as a stoichiometric reagent or as a catalyst with gaseous hydrogen chloride, may be used to promote silane reductions of secondary alkyl alcohols that otherwise resist reduction by the action of weaker acids.136 For example, cyclohexanol is not reduced by organosilicon hydrides in the presence of trifluoroacetic acid in dichloromethane, presumably because of the relative instability and difficult formation of the secondary cyclohexyl carbocation. By contrast, treatment of cyclohexanol with an excess of hydrogen chloride gas in the presence of a three-to-four-fold excess of triethylsilane and 1.5 equivalents of aluminum chloride in anhydrous dichloromethane produces 70% of cyclohexane and 7% of methylcyclopentane after a reaction time of 3.5 hours at... 

Standard cyclisation methodology was used to access the cyclic monophosphinic acid derivative 78 by reaction of ammonium phosphonate and ethyldiisopropylamine, followed by the addition of chlorotrimethylsilane, with 2,2 -bis (bromomethyl)-l,l -biphenyl. Silane reduction of 78 gave the secondary phosphine. The secondary phosphine borane complex 79 could be used in alkylation or Michael addition reactions. For example the Michael adduct 80 was produced in high yield by treatment of 78 with a NaH suspension in THF followed by the addition of diethylvinylphosphonate . 

Compound 19 was obtained by direct sulfonation of Compound 1 with concentrated H2S04 at 25°C. The dimethylamino sulfone was prepared by chlorosulfonating and aminating DiPAMP as the bis-oxide followed by silane reduction. 

Samaan used reduction at a mercury cathode to convert the phosphonium salt 33 to the phosphine (34) (79PS89). This gave the opposite stereoisomer from that available as the major product (35) from base hydrolysis followed by silane reduction. The stereochemistry of each compound was established by NMR analysis. 

Figure 3.89 Syntheses of a phosphino functionalised imidazolium salt using the silane reduction route.

The use of polymer-supported tin hydrides is now a well-estabhshed method for minimizing tin contamination in products resulting from radical reactions. A new version of the Barton-McCombie deoxygenation reaction has been developed by Dumartin [4] which combines the use of a polymer-supported tin hydride with a stoichiometric silane reductant, giving an attractive catalytic process. 

Silane reductions of phosphorus compounds are listed in Table 1. Mono- or dichlorophosphines and phosphoryl compounds react with Ph SiH, e.g. ... 

J2J1S Stannane and Silane Reduction of Esters and Carironates... 

When the SiH4/WF6 chemistry is used, the demands on the chemical compatibility of the adhesion layer are relaxed since this chemistry is so much milder than the hydrogen chemistry [Ellwanger et al.14]. In this way Al and Ti become acceptable adhesion layers. Unfortunately,the step coverage of the SiH4/WF6 chemistry is very poor (see section 2.3.2) and is therefore not suitable for contact fill applications. The silane reduction is, however, still applied to start the tungsten deposition especially atop of TiN (see 2.2.1) followed by the tungsten deposition based on the H2/WF6 chemistry. 

The reaction rate of the WF with silane is very fast. The hazard of an explosion may be overcome by low total pressure, low silane partial pressure, and fast flow rate. Silane reduction is only used for a short duration to initiate W nucleation and is followed by WF6/H2 reduction. 

Wightman et al. showed that in fmanose sugars, reduction of C-glycoside lac-tols gives 1,2-anti products predominantly. It seems that silane reduction occurs preferentially from the same face as the vicinal oxygen suhstituent (Scheme 69) [114]. 

Kunz and coworkers reported the application of an immobilized chiral galacto-sylamine in diastereoselective synthesis of piperidine and amino acid derivates [14]. Preparation of the galactosylamine auxiliary from 1,6-hexandiol and P-D-galactopyranosyl azide (1) required a six-step synthesis protocol. The precursor 2 was loaded onto a polymer bound silane. Reduction of the azide function gave the corresponding immobilized amine 3 (Scheme 12.2). 

Alternative methods for both the chelation-controlled and the Felkin Anh selective reduction of a-oxygenated ketones are described in Section D.2.3.4. (silane reductions). 

Polymethylhydrosiloxane (PMHS), a safe and inexpensive polymer co-product of the silicon industry, is an efficient alternative reducing agent for C=0 and C=N bonds when associated with catalysts (1). Mimoun et al. recently reported a new system based on zinc hydride catalysts which enables the chemoselective reduction of unfunctionalized and a,/ -unsaturated- aldehydes, ketones and esters (2). Because gummy silicon residues, which are usually associated with silane reductions, do not form, this PMHS system is attractive for synthetic / industrial purposes. Nevertheless, in contrast to tin-catalyzed reductions of ketones with PMHS... 

Several different reagents are used to reduce /li-oxides of arsole derivatives. These include SO3/HCI/KI (Scheme 31), SnCl2/HCl (Scheme 22), and phenylsilane (Equation (24)). Silane reduction is stereospecific and proceeds with retention of configuration for highly pyramidal arsenic... 

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