Optically active isonitriles

Enantioselective intramolecular bis-silylation of homoallylic alcohols has been achieved in the presence of a catalyst prepared from Pd(acac>2 and optically active isonitriles. The bulky substituents of the substrate on the silicon atom proximal to the ether... 

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. 

Acetylated Meldrum s acid 172 and a-aminoesters 173 have been used as the building blocks to prepare the substituted imidazoles 174, which are then cyclized to give optically active dihydro products 175 <20030L3907> (Scheme 7). A modified four-component LJgi reaction has been used to prepare dihydro products 177 from the intermediate functionalized imidazole 176, isonitriles, and amines (Equation 24) <2005EJ04670>. 

Polymerization of optically active isonitiiles, 72, also leads to the formation of helical polymers with a preferential screw sense (219-222). Various factors distinguish this system from the preceding ones In the isonitrile case no new stereogenic atoms are formed during polymerization the helices are rigid and there is no indication of conformational equilibrium in the system the formation of a preferential screw sense is very probably a kinetically controlled process. 

The optically active Mo complexes 13a and 13b are configurationally stable28,29 Their optical rotations in solution remain constant over long periods of time. If, however, a trace of free R—(—)-a-phenyl ethyl isonitrile30,31 is added to solutions of 13a at room temperature, its optical rotation decreases within about one hour to values around 0 28,29). We explain this behavior by a backside attack of the free isonitrile on the optically active complex 13a according to Eq. (11). 

The optically active Mo isonitrile complex 13a may also be reacted with nucleophiles other than isonitriles. Triethylphosphine, a much better nucleophile than the isonitrile bound in 13a, also attacks complex 13a at the backside and replaces the isonitrile via an S 2 transition state in a reaction which at room temperature is completed within a few minutes28,29 ... 

System 12 is very favorable, since the optically active phosphine complex 14b is relatively stable towards the free nucleophiles, the excess phosphine, on the one hand and the isonitrile formed in reaction 12 on the other. Therefore, if the reaction of 13a with P(C2H5)3 is stopped after a few minutes, only the inversion represented... 

Rupe, H., Glenz, K. Influence of constitution upon the rotatory power of optically active substances. XVI. Acetylene derivatives, ketones and isonitriles. Ann. 1924, 436, 184-204. 

Whereas the methyl-substituted cyclopropyl carbanion (861 R = Me) maintains its configuration, the cyanato and acetylenic derivatives (861 R = CNO or C=CR) racemize, an sp carbon atom evidently lowering the barrier to inversion. The effect of an sp nitrogen atom has now been assessed by preparation of the isonitrile (861 R = NC) from an optically active precursor. Its methylation below — 50°C gives (862) in 98 % optical purity. 

The combination of the Pd-catalyzed bis-silylation and subsequent Peterson-type elimination was applied to the synthesis of aUenylsilanes from propargylic ethers. Scheme 7 shows a preparation of optically active aUenylsilane and its use in the synthesis of syn-homopropargylic alcohol. Use of the palladium-isonitrile catalyst is crucial to promote the bis-silylation effectively. 

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