Polymethylhydrosiloxane

Sergey A. Kosarev AMRI Singapore Research Centre Pte Ltd, Singapore 

Alternate Name PMHS methylhydrogensiloxane polymer methylhydrosilicone homopolymer 1,1,1,3,5,7,7,7-octamethyltetrasiloxane (PMHS-dimer) [16066-09-4] polymethylhydrosiloxane trimethylsilyl terminated [63148-57-2] and [178873-19-3] and poly(dimethysiloxane-co-methylhydrosiloxane) (96% wt methylhydrosiloxane monomer units) [63148-57-2] are sold as PMHS . 

Physical Data colorless free flowing liquid average molecular weight 1500-2200 g mol (supplier dependent) effective mass per hydride of 60 g mol d= 1.006. 

Solubility most ethereal, chlorinated, or hydrocarbon solvents as well as EtOH, i-PrOH, warm DMF, and warm NMP insoluble in MeOH, DMSO, acetonitrile, and water. 

Preparative Methods hydrolysis of methyldichlorosilane followed by heating (60-150 °C) the resultant mixture of cyclic silanes in the presence of hexamethyldisiloxane generates the linear polysiloxane.  

---

Ben2yl azide 1855 and N-benzyloxycarbonylbenzylamine 1859 are both transformed by the cheap polymethylhydrosiloxane (PMHS) 1856, in the presence of (B0C)20 and Pd/C, into 92-94% N-BOC-benzylamine 1857 and the polymer 1858 [81]. (Scheme 12.22). Aromatic and aliphatic amine oxides are readily reduced by 1856/Pd/C into their corresponding amines. Thus, e.g., pyridine-N-oxide 860 and quinohne-N-oxide 877 give pyridine and quinohne in 90 and 92% yield, respectively. Analogously, benzyldimethylamine-N-oxide is converted in 88% yield into free benzyldimethylamine [82]. 

Beller and coworkers reported hydrosilylation reactions of organic carbonyl compounds such as ketones and aldehydes catalyzed by Fe(OAc)2 with phosphorus ligands (Scheme 21). In case of aldehydes as starting materials, the Fe(OAc)2/PCy3 with polymethylhydrosiloxane (PMHS) as an H-Si compound produced the corresponding primary alcohols in good to excellent yields under mild conditions [67]. Use of other phosphorus ligands, for instance, PPhs, bis(diphenylphosphino) methane (dppm), and bis(diphenylphosphino)ethane (dppe) decreased the catalytic activity. It should be noted that frans-cinnamaldehyde was converted into the desired alcohol exclusively and 1,4-reduction products were not observed. 

In 2005, Riant et al. reported the synthesis of a new air-stable S/N-chelating zinc catalyst, depicted in Scheme 10.50, which was fully characterised by all spectroscopic methods. This complex, prepared from the corresponding ferrocene oxazoline, was applied to the enantioselective hydrosilylation of ketones in the presence of polymethylhydrosiloxane, PMHS, providing modest enan-tioselectivities (<55% ee). ... 

Chiu et al. developed a catalytic reductive aldol cyclization of alkyne-diones such as 115 and 117 using [Ph3PCuH]6 (10mol%) as catalyst and polymethylhydrosiloxane PMHS (200 mol %) as terminal reductant. The... 

Equation 11.29 One-pot hydrostannylation and Stille coupling (PMHS = polymethylhydrosiloxane). 

Recently, the silane-mediated reductive cyclization of activated alkynes with tethered ketones using Stryker s reagent as a catalyst was reported.112,90b Alkynyl ketone substrate 84a was treated with a catalytic amount of Stryker s reagent in the presence of polymethylhydrosiloxane (PMHS) to afford the cA-fused hydrindane 84b as a single diastereomer. This method is applicable to both five- and six-membered ring formation, but often suffers from competitive over-reduction of the reaction products (Scheme 59). 

The situation for the hydrosilylation of C = N functions with regard to ecology and economy is somewhat similar as for the hydride reduction, except that fewer effective catalytic systems have been developed [91]. Despite some recent progress with highly selective Ti-based [92] and Cu-based [93] catalysts using cheap polymethylhydrosiloxane as reducing agent, hydrosilylation will see its major applications in small-scale laboratory synthesis. 

The complex of Me2Zn with (5, 5 )-ebpe, 107, has been applied successfully as catalyst in the enantioselective reduction of ketones by polymethylhydrosiloxane and combines excellent product yields with high ee values . Its structure comprises the iV,iV-chelate coordination of the ebpe ligand to the MeiZn unit (Figure 51). It is remarkable that in this case the two secondary amine functionalities are coordinated to zinc and leave the Zn—C bonds unaffected. Indeed, usually secondary amines undergo a fast deprotonation reaction with dialkylzinc compounds. 

Direct reduction of an aldehyde or ketone to the corresponding ether could potentially telescope two reactions, reduction and protection, into one step. S. Chandrasekhar of the Indian Institute of Chemical Technology, Hyderabad, reports (Tetrahedron Lett. 2004,45,5497) that in the present of polymethylhydrosiloxane (PMHS) and catalytic B(C6F,), TMS ethers of alcohols will convert aldehydes to the corresponding dialkyl ethers. The reaction works well for both saturated and benzylic alcohols. This may prove to be a useful alternative to Williamson synthesis for the preparation of complex ethers. 

Polymethylhydrosiloxane (PMHS) has been reported to be a more selective reducing agent when coupled with Pd° catalysts than R3SnH, permitting, for example, the reduction of allylic acetates in the presence of enones and acyl halides (equation 104).300... 

Reduction with LiAlH(OBu )3293 or LAH292 also gives selective hydride addition to the less substituted allyl end (equations 317 and 318). In contrast, formate reductions selectively deliver hydride to the more substituted allyl terminus (equations 319 and 320).302-303 Si—H-mediated reduction, conveniently performed with polymethylhydrosiloxane (PMHS), demonstrates no clear pattern of regioselectivity (equation 321).320 LiHBEt3 delivers hydride regioselectivity to the less substituted allyl terminus (equation 322)289-291... 

---