Hydrosilylation carbocyclization

Initial studies on the hydrosilylation/carbocyclization of terminal dienes carried out in the early 1990s have demonstrated that five-membered ring formation proceeds smoothly in the presence of lutetocene [40], neodymocene [43], and samarocene [29] catalysts (18). 

Rare-earth metal catalysts tolerate a variety of aprotic functional groups for example, sterically encumbered mesityl sulfonamides sufficiently shield the oxygen from the metal center in order to allow hydrosilylation/carbocyclization, albeit at a reduced rate (20) [72],... 

So far, the only example of an asymmetric hydrosilylation/carbocyclization sequence of a,co-hexadienes and heptadienes utilize the (i )-BINOL-derived yttrocene catalyst 16 to produce cyclopentanes and cyclohexanes in high yields but only low to moderate enantioselectivities of up to 50% ee (25) [75]. 

To give some new examples of the remarkable catal5dic activity of SMA, we report here some recent results obtained on the sUyl-carbocyclization of alkynes, promoted by arene-solvated Rh atoms, and on the hydrosilylation of functionalized alkenes, catalyzed by arene-solvated Pt atoms. 

Hydrosilylation of dienes accompanied by cyclization is emerging as a potential route to the synthesis of functionalized carbocycles. However, the utility of cycliza-tion/hydrosilylation has been Umited because of the absence of an asymmetric protocol. One example of asymmetric cycUzation/hydrosilylation has been reported very recently using a chiral pyridine-oxazoUne ligand instead of 1,10-phenanthroline of the cationic palladium complex (53) [60]. As shown in Scheme 3-21, the pyridine-oxazoUne Ugand is more effective than the bisoxazoUne ligand in this asymmetric cyclization/hydrosilylation of a 1,6-diene. 

Hydrosilylation of 1,6-dienes accompanied by cyclization giving a five-membered ring system is emerging as a potential route to the synthesis of functionalized carbocycles.81,81a,81b 82 As its asymmetric version, diallylmalonates 86 were treated with trialkylsilane in the presence of a cationic palladium catalyst 88, which is coordinated with a chiral pyridine-oxazoline ligand. As the cyclization-hydrosilylation products, //ww-disubstituted cyclopentanes 87 were obtained with high diastereoselectivity (>95%), whose enantioselectivity ranged between 87% and 90% (Scheme 25).83 83a... 

In analogy with the strategy of carbocyclic construction, 1,6-enynes containing an oxygen heteroatom in the carbon atom sequence have been used for 3,4-disubstituted tetrahydrofuran synthesis. The simplest example is given by the hydrosilylation of enyne at room temperature (Reaction 7.38) [49]. Tetrahydrofurans with an exocyclic methylene functionality can also be prepared from the appropriate alkynes, such as 32, with (TMS)3SiH in refluxing benzene which afforded exclusive formation of the exomethylene in the Z conformation (Reaction 7.39) [50]. 

Analogously to the carbocycle and oxycycle synthesis, cyclic amines can be obtained by the hydrosilylation of a suitable enyne, such as 46 (Reaction 7.54), which gave the six-membered ring via a 6-endo cyclization of the vinyl radical onto the C=N bond [63]. In another example, the isothiocyanide functionality of compounds 47 or 48 reacts with silane under radical conditions... 

Yu and co-workers have expanded upon Ojima s work through development of an effective Rh-catalyzed protocol for the cyclization/hydrosilylation of allenyl carbonyl compounds to form silylated vinylcycloalkanols and heterocyclic alcohols.For example, reaction of tosylamide 44 (X = NTs, R = H, n= ) and triethylsilane catalyzed by Rh(acac)(GO)2 (1 mol%) under GO (10 atm) at 70 °G for 8h gave the silylated vinyl pyrrolidinol 45 (X = NTs, R = H, n= ) in 74% yield with exclusive formation of the m-diastereomer (Equation (29)). The rhodium-catalyzed reaction was also effective for the cyclization of alleneones and for the formation of carbocycles, oxygen heterocycles, and six-membered cyclic alcohols (Equation (29)). However, Rh-catalyzed cyclization/hydrosilylation of allenyl carbonyl compounds that possessed substitution on an allenyl carbon atom was not established (Equation (29)). The efficiency of the Rh-catalyzed reaction of allenyl carbonyl compounds depended strongly on GO pressure. Reactions run under 10 atm GO were more efficient than were... 

In contrast to the reactions catalyzed by Group Vm transition metal complexes (see Section II.A. 1), the hydrosilylation of 1,5- or 1,6-dienes with H3SiPh catalyzed by Cp2 NdCH(SiMe3)2 results in the formation of (silylmethyl)cyclopentanes (336) via 5-exo carbocyclization (equation 137)46. 

Synthesis of heterocycles through hydrosilylation, silylformylation, silyl-carbocyclization, and cyclohydrocarbonylation reactions 06COC1341. 

Esters of various carbocyclic acids are readily hydrosilylated to give products that are used as silane coupling agents according Eq. 2. [1, 2]. 

Recent advances in the cyclizations catalyzed by transition metals and their complexes are reviewed. The catalytic cyclizations discussed here include various carbocyclizations, for example, cycloisomerization, cycloaddition, reductive cyclization, and so on cascade carbocyclizations, for example, cyclotrimerization, silylcarbocyclization, and Heck reaction carbonylative carbocyclizations cyclohy-drocarbonylations intramolecular hydrosilylations intramolecular silylformyla-tions and aldol cyclizations. These reactions serve as efficient and useful methods for the syntheses of a variety of heterocycles and carbocycles that are important... 

The hydrosilylation of unsaturated carbon-rhodium-catalyzed silylcarbocyclizations. In the presence of Rli4(CO)i2 and triethoxysilane, a rigid triyne backbone can undergo a silylcarbotricyclization cascade reaction to yield [5,6,5]-tricycles (eq 16). Similar to the results observed by Sieburth for the hydrosilylation of enamines, the alkoxysilane functionality provides significant rate enhancement in comparison to silylcarbocyclizations using alkyl- and arylsilane reagents. The incorporation of carbonyl functionality as terminal electrophiles into these cyclizations has also been successful. Rhodium-catalyzed carbonylative silylcarbocyclizations proceed in the presence of carbon monoxide (10 atm) to incorporate a carbonyl unit, usually as the aldehyde. Both of these tandem ad-dition/cyclization strategies produce functionalized carbocycles with simultaneous incorporation of sUyl functionality as aryl- and vinylsilanes. These alkenylsilanes can then be exploited for further synthetic manipulations as discussed above. "" ... 

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