Polymer-bound hydrides

At the start of this complex process, RhCls coordinates with PVA. An oxonium product is formed, which is further transformed via an aUcoxide intermediate into a polymer-bound hydride complex. Finally a colloid forms, resulting from the disproportionation of these rhodium hydrides and subsequent growth of particles (scheme 18). 

Polymer-bound reducing agents have received significantly less attention, presumably because main group metal hydride complexes predominate as reducing... 

Reaction of Resin-Bound Iron Complex (54) with Alkyl Mercaptans, Thiophenols, and Phenols (Fig. 9)31. Sodium thiolates are prepared analogously to the alkoxides from thiol and sodium hydride, except that dry DMF is used as a solvent. The substitution on the polymer-bound arene (54) is performed at 70° in DMF within 16 h. The resin is filtered and washed with DMF (2 x 50 ml), MeOH (2 x 50 ml), H20 (2 x 50 ml), MeOH (2 x 50 ml), and CH2CI2 (3 x 50 ml) and then dried in vacuo at 40° to yield a red resin. 

Stannaries have become prominent in multifunctional anchoring groups. A polymer-bound tin hydride 41 has been used to hydrostannylate alkynes under the action of palladium-catalysis to give polymer-bound alkenylstannanes 42. These alkenyl stannanes have been employed in intermolecular [45] and intramolecular Stille reactions [46]. Alkenylstannanes can also undergo protonation to give alkenes 44 in a traceless fashion. This linker is therefore multifunctional (Scheme 6.1.12). 

In this study, trimethoxylsilane was found to be an effective reagent for the conversion of the polymer-bound phenoxytin intermediate 14 back to tin hydride 13. Using this reagent system, secondary thionocarbonates were reduced in good yield using 10 mol % of the tin hydride polymer 13 (Scheme 3). 

An alternative solution is to use organotin reagents supported on an insoluble, but porous, polymer at the end of the reaction, the polymer can be filtered off, and the catalyst can be regenerated.72 Most of the work in this field has involved polymer-bound tin hydrides (for hydrogenolysis of halides, thiocarbonyl compounds, or hydrostannation of ketones),73-76 but also tin chlorides (for use as Lewis acids),77 allylstannanes (for allyla-... 

Polymer supported tin hydrides ensure complete removal of tin by-products, but their synthesis is not trivial. They have been used for reductions of halides and for ring closures, and the resulting polymer-bound tin halide can be easily filtered from the reaction mixture, regenerated, and reused. 

The first traceless linker was developed by Kamogawa and coworkers in 1983 [82]. Starting from a polymer-bound sulfonylhydrazine, sulfonylhydrazone resin 88 was formed by reaction with ketones or aldehydes. The cleavage step was conducted either by reduction with sodium borohydride or lithium aluminium hydride to yield alkanes 89 or by treatment with base to give the corresponding alkenes 90 in a Bamford-Stevens reaction (Scheme 16.20). This work was a pioneering approach in the field of traceless tinkers. 

Tin hydride reagents are versatile tools for the functionalization of alkenes and alkynes. Based on this concept, Nicolaou and coworkers developed a polymer-bound tin hydride (118) that reacts via Pd-catalyzed hydrostannylation (or nucleophilic attack on the tin chloride with a vinyl lithium) with alkynes to give alkenylstannanes [116]. After further transformation to derivatives 119, the resin-bound substrates undergo proteolytic traceless cleavage to yield unsubstituted alkenes 120. Alternatively, the stannane can be employed for intramolecular Shlle coupling to produce macrolactones 121 in the cleavage step (Scheme 16.28). 

This procedure assures quantitative conversion to the resin-boro-hydride form, obtained as a purified dry resin. Quantities of polymer-bound borohydride of up to 500 g have been prepared successfully by this procedure. 

D. Drying of Polymer-Bound Borohydride and Analysis for Hydride Content. Residual solvents were removed from the polymer-bound borohydride for hydride content analysis. For this purpose, a sample (2-3g) of the polymer-bound borohydride was transferred to a suitable vessel and dried in a vacuum desiccator under a pressure of about 20 torr at ambient temperature. Percent hydride of the dry polymer-bound borohydride was determined by measuring the volume of hydrogen evolved on hydrolysis of the borohydride with acid. 

However, in the 2-ethylhexanol, the sodium and tetraethylammonium borohydrides are, in general, more reactive than the polymer-bound boro-hydrides. Of the resins, those polymers having the styrene-DVB skeletal backbone (A-26 and IRA-400) offer some advantage over the acrylate-DVB-based resin (XE-279 and IRA-458). Of the styrene-DVB resins, the macroreticular resin (A-26) gives the best reduction, particularly at an elevated temperature. Both sodium borohydride and tetraethylammonium borohydride appeared to have limited solubility at this level in 2-ethylhexanol. However, since 2-ethylhexanol is more viscous and less polar than ethanol, reactions involving the porous ion-exchange resins would be somewhat slower than the corresponding reduction with soluble borohydrides. 

Solvolytic Stability. Solvolytic stability of all four polymer-bound borohydrides was investigated in 100% ethanol as a function of temperature and in water as a function of pH. Comparative studies for sodium borohydride and tetraethylammonium borohydride were conducted. Solvolytic decomposition of the borohydride group results in the generation of hydrogen gas. Stability measurements were obtained by observing volume of hydrogen gas evolved as a function of time. Percent loss of hydride as a function of time was calculated from resin weight and initial hydride content. 

Figure 2. Stability of polymer-bound borohydride in 100% ethanol at 40°C percentage loss hydride vs.

In addition to offering convenient and effective removal of boro-hydride-reducible impurities, this system offers several unique advantages over sodium borohydride. First, the polymer-bound borohydride is remarkably stable in alcohols (with the exception of methanol). Second, since the hydride capacity is on the order of 12 meq of hydride per gram of dry resin, a small amount of polymer-bound borohydride will remove trace carbonyl impurities from a substantial volume of alcohol. Third and most importantly, no new contaminants such as Na+ or B02" are added to the alcohol since the borate ion remains bonded to the resin. 

Generation of Volatile Hydrides (11). The use of commercial sodium borohydride as a reducing agent for the generation of volatile arsine (AsH3) in trace arsenic analysis is often complicated by trace (ppb) arsenic impurities in the borohydride. The following procedure using polymer-bound borohydride has eliminated these problems ... 

---