Intramolecular bis-silylation

Synthetic transformations of the products of the intramolecular bis-silylation have been examined. The five-membered ring products derived from homopropargylic alcohols were hydrogenated in a stereoselective manner (Scheme ll).90 Oxidation of the products under the Tamao oxidation conditions (H202/F /base)96 leads to the stereoselective synthesis of 1,2,4-triols. This method can be complementary to the one involving intramolecular bis-silylation of homoallylic alcohols (vide infra). 

Table 1 Intramolecular bis-silylation of disilanyl ethers of substituted homoallylic alcohols in the presence of a palladium/ f-OcNC catalyst... 

Asymmetric intramolecular bis-silylation has been achieved by using optically active isocyanide as chiral ligands on palladium (Equation (36)).107... 

A novel intramolecular bis-silylation of propargylic disilanyl ethers followed by a Peterson-type syn elimination provides access to a variety of allenylsilanes (Table 9.36) [57]. The elimination is initiated by treatment of the presumed four-membered siloxane intermediate with BuLi. This intermediate could not be isolated, but spectral data were in accord with the assigned structure. The cycloaddition and elimination steps were shown to take place stereospecifically (Eq. 9.48). 

Furthermore, intramolecular bis-silylative cyclization of dienes 28 with face selectivity gives a diastereomeric mixture of 29 and 30 (equation 16). The size of the substituents on the nonterminal silicon influences the ratio of 29/30, which follows the order Me < Et < Ph < /-Hu < / -Hr (cf. entry 17 of Table l)27. 

Palladium-catalyzed intramolecular bis-silylation of allyldisilanyl ether produced /ram-siloxctane, which undergoes a fast dimerization <2005CEJ2954>. 

For stereoselective intramolecular bis-silylation of alkenes promoted by a palladium-isocyanide catalyst see reference 32. 

The latter isonitrile-palladium complexes effectively catalyze intramolecular bis-silylation of carbon-carbon triple bonds (Eq. 21) [46,47]. Not only terminal alkynes but also internal alkynes including those having aryl, alkenyl, silyl, and... 

Under extremely high pressure, the isonitrile-palladium catalyst promotes intramolecular bis-silylation of bis(silyl)acetylenes 51 to give tetrakis(silyl) alkenes 52, which are otherwise difficult to synthesize the reaction under atmospheric pressure hardly proceeds even at 200°C (Eq. 22) [48,49]. 

AppHcation of the intramolecular bis-silylation to disilanyl ethers of propar-gylic alcohols 53 provides a new entry into oxasilethanes 54, highly strained four-membered ring compounds including Si-O bonds (Eq. 23) [50,51]. 

Enantioselective intramolecular bis-silylation was accomplished by use of the optically active re rt-alkyl isonitrile ligand 75 on palladium (Eq.38) [71]. The bulky substituents of 76 on the silicon atom proximal to the ether oxygen are crucial to attain good enantioselectivity. 

The intramolecular bis-silylation also proceeds with disilanyl ethers of allylic alcohols 78 with vicinally disubstituted C=C bonds, providing eight-membered ring l,5-dioxa-2,6-disilaoctane derivatives 79, which may arise from dimerization of primarily formed four-membered 1,2-oxasilolanes 80, in good yields (Eq.39) [72,73]. Worthy of note is that the bis-silylation proceeds with nearly... 

Elimination of silyl groups with p-oxy groups, i.e., Peterson-type elimination, is a useful method for preparing stereodefined alkenes [101,102]. The synthetically useful allylsilanes are effectively synthesized in geometrically and enantio-merically pure forms through the Peterson-type elimination of organosilanes prepared by palladium-catalyzed bis-silylation (Eq. 57) [72, 73]. The intramolecular bis-silylation of optically active allylic alcohols in refluxing toluene af-... 

Intramolecular bis-silylation of homoallylic alcohols, which proceeds with high diastereoselectivity, is applied to the stereoselective synthesis of 1,2,4-triols 109 (Eq.59) [67-71]. 

Suginome and Ito have developed a reliable method for the synthesis of highly enantioenriched allyl- and allenylsilanes. The synthetic process involves 1,3-chirality transfer from homochiral allyl and propargyl alcohols through Pd-catalyzed intramolecular bis-silylation and subsequent Peterson-type elimination (Scheme 10.142) [395]. This method provides an efficient route to enantioenriched allylsilanes bearing a hydroxyalkyl group, which are very valuable as synthetic intermediates for diastereo- and enantioselective synthesis of heterocycies and carbocycles [396]. Polymer-supported highly enantioenriched allylsilanes have been prepared from enantioenriched allyl alcohols and a polymer-supported disilanyl chloride [397]. 

Bis-silylation of alkynes and alkenes.1 Palladium(ll) acetate in combination with a large excess of a /-alkyl isocyanidc can effect bis-silylation of terminal alkynes by otherwise unreactive disilancs (equation 1). intramolecular bis-silylation of an alkyne is... 

This intramolecular bis-silylation has been extended to terminal and 2,2-disubstituted alkenes. 1,2-Disubstituted alkenes do not undergo this reaction. The substrates, unsat-urated disilanyl ethers, arc prepared by silylation of allylic and homoallylic alcohols. 

Intramolecular bis-silylation docs not occur if the disilane is tethered to the double bond by chains of more than four atoms. Therefore, it is not surprising that alkenes do not undergo intermolecular addition of disilanes. The bis-silylation is stereoselective when the alkene bears an a- or /3-group (equations HI and IV).2... 

Intramolecular bis-silylation does not 00c by chains of more than four atoms. The undergo intermolecular addition of disiU the alkene bears an a- or j8-group (eqw... 

Borylsilylation of 1-octyne with 120 provided the alkenylboronate 121 in high yield using Pd(OAc)2 and bulky isocyanide (1,1,3,3-tetramethylbutyl isocyanide, XVll-6) as a ligand. The product 121 was subjected to Suzuki coupling catalyzed by PdCBCdppf) [36], Under similar conditions, no intermolecular bis-silylation of internal alkynes took place. However, intramolecular bis-silylation of disilanyl ether of homopropargylic alcohol proceeded smoothly with 5-exo-dig cyclization [36a]. 

As in the case of bis-silylation of acetylenes, intramolecular bis-silylation of alkenes is more effectively performed compared to the intermolecular reactions. The isonitrile-palladium catalyst provides 5-exo cyclization products stereospeciflcally from (Z)- and ( )-alkenes tethered to disilanyl groups by ether linkage (Scheme... 

Diastereoselective intramolecular bis-silylation was applied to the synthesis of highly enantio-enriched allylsilanes (Scheme 14 and Table... 

Intramolecular bis-silulation of alkenes has also been achieved more effectively than intermolecular reactions. The use of optically active isonitrile ligand on palladium catalyst provides us with a synthetic method for an enantioselective intramolecular bis-silylation of homoallylic alcohols. The method has successfully been applied to the synthesis of highly enantio-enriched allylsilanes. 

Intramolecular Bis-silylation of Alkenes Catalyzed by Palladium(O) fert-AUcyl Isocyanide Complex. Stereoselective Synthesis of Polyols. 

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