Cyclization silylation of dienes

Molander et al. described a sequential cyclization—silylation of dienes that was induced by the cation-like permethylzirconocene complex [Cp 2ZrMe(p-Me)B(C6F5)3] [95] (Scheme 8.52). 

A seqnential cyclization silylation of dienes can be induced by a cation-hke permethylzirconocene complex (equation 66). ... 

Scheme 11. Organoyttrium-catalyzed sequential cyclization/silylation of 1,5- and 1,6-dienes...

Reductive cyclizations. The cyclization products from a BF,OEt2-catalyzed reaction are susceptible to reduction in situ by EtjSiH which is added. - The hydrosilane does not interfere with the cyclization itself. Sequential cyclization-silylation of 1,6- and 1,7-enynes is observed in the presence of Cj YMe THF, and for 1,6-dienes, also with 1,10-phenanthroline-Pd(I) complex. ... 

Palladium oxazoline compounds (e.g., (47)) have been used to catalyze the cyclization/hydro-silylation of functionalized 1,6-dienes (Scheme 31). With R = Pr1, >95% diastereomeric excess and 87% ee was achieved at low temperature. Changing the ligand bulk with R = Bu1 gave a higher ee value, but poorer diastereoselectivity. A range of functional groups can be tolerated at both the allylic and terminal alkene positions.135-137... 

For the conversion of silyl-carbon bond in the cyclization products into hydroxy-carbon bond, several functionalized hydrosilanes were examined (Scheme 27).84,84a,84b Of the hydrosilanes examined, benzhydryldimethylsilane (HSiMe2CHPh2) was found to be most enantioselective in the reaction of diene 86a to give the cyclization product 87a with 93% ee. The second highest enantioselectivity (91% ee) was observed with hydrosiloxane HSiMe2OSiPh2Bu-/. The cyclization-hydrosilylation with the HSiMe2CHPh2 and catalyst 88 was very successful... 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

Bercaw has investigated the application of the 6 2-symmetric, enantiomerically pure lanthanide metallocene derivative (i ,A)-BnBpYH 34 as a catalyst for the asymmetric cyclization/hydrosilylation of 1,5- and 1,6-dienes. Although 34 displayed high activity for the reaction of a number of dienes, asymmetric induction was low. In the best case, reaction of 3,3-dimethyl-1,5-hexadiene with phenylsilane catalyzed by 34 gave silylated cyclopentene 35 in 95% yield with 50% ee (Equation (25)). 

Widenhoefer and co-workers have developed an effective Pd-catalyzed protocol for the asymmetric cyclization/ hydrosilylation of functionalized 1,6-dienes that employed chiral, non-racemic pyridine-oxazoline ligands." " " Optimization studies probed the effect of both the G(4) substituent of the pyridine-oxazoline ligand (Table 7, entries 1-6) and the nature of the silane (Table 7, entries 6-15) on the yield and enantioselectivity of the cyclization/ hydrosilylation of dimethyl diallylmalonate. These studies revealed that employment of isopropyl-substituted catalyst (N-N)Pd(Me)Gl [N-N = (i )-( )-4-isopropyl-2-(2-pyridinyl)-2-oxazoline] [(i )-43f and a stoichiometric amount of benzhydryldimethylsilane provided the best combination of asymmetric induction and chemical yield, giving the corresponding silylated cyclopentane in 98% yield as a single diastereomer with 93% ee (Table 7, entry 15). 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

Tsuji has reported the palladium-catalyzed cyclization/disilylation of bis( 1,3-dienes) with disilanes to form disilylated divinylcycloalkanes. For example, reaction of ( , )-6,6-dicyano-l,3,8,10-undecatetraene 89a and diphenyltetra-methyldisilane (1.2equiv.) catalyzed by Pd(DBA)2 gave 90a in 74% yield with exclusive formation of the trans-E,Z-diastereomer (Equation (58)). The stereoselectivity of the palladium-catalyzed cyclization/disilylation of bis(dienes) was substrate dependent, and the Pd-catalyzed reaction of 89b gave the bis(silylated) cyclopentane 90b in 82% yield with 70% selectivity for the /ra //j - ,Z-diastereomer (Equation (58)). In comparison, the reaction of ( , )-6,6-bis(ethoxycarbonyl)-l,3,9,ll-dodecatetraene gave the bis(silylated)cyclohexane 91 in 49% yield with exclusive formation of the /ra //i - , -diastereomer (Equation (59)). 

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. 

The use of isopropylidine acetals (112) as tethers in the intramolecular Diels-Alder reactions of dienes with alkenes facilitates the formation of civ-fused cycloadducts (113) from an endo transition state (Scheme 41).218 The intramolecular Diels-Alder reaction of 4-[tris-(2-mcthylcthyl)silyl]oxy-2//-thiopyran derivatives with potential dienophiles tethered at C(2), C(3), C(5), and C(6) positions yielded cycloadducts when the dienophiles were activated with a carbomethoxy group.219 By the substitution of a phenylsulfonyl group on the dienophile of 2-benzopyran-3-ones, it is possible to enhance exo addition during intramolecular Diels-Alder cyclizations to yield a predominance of trans-fused hexaphenanthrenes related to natural products.220 The intramolecular Diels-Alder reaction of 2-furfuryl fumarates has been investigated by molecular mechanics (SIBFA)/continuum reaction field computations.221 The intramolecular 4 + 2-photo-cycloaddition of A-benzylcinnamamides (114) in the presence of C(,H6 gives 3-azatricyclo[5.2.2.01,5]undeca-8,10-dien-4-ones (115) with high stereoselectivity (Scheme 42).222... 

The molybdenum complex [(CF3)2MeCO]2Mo(NAr)(=CHCMe2Ph) has been observed to be a more efficient catalyst for cyclization of vinyl silyl ether dienes than the ruthenium complex Cl2(PCy3)2Ru(=CHPh), probably because this type of alkene is sterically more demanding (than allyl derivatives) and therefore requires a catalyst less sensitive to steric bulkiness near the reaction center. However, some examples of the RCM of substituted vinylsilanes catalyzed by ruthenium complexes have been reported [127, 131] (Eq. 74). For more examples see Ref. [127]. 

On the other hand, the cyclization reaction of a vinyllithium onto an acetylenic unit provides an efficient route to five- and six-membered bis-exocyclic 1,3-dienes, which react stereoselectively with a wide range of dienophiles157. The 5-exo carbolithiation reaction of vinyllithiums 369, derived from the corresponding vinyl bromides, is syw-stereospecific giving, after hydrolysis, the /(-isomer of five-membered outer-ring dienes 370 and tolerates aryl-, silyl- or alkyl-substituents at the distal acetylenic carbon (Scheme 97). However, the alkyl-substituted alkynes are far more resistant to rearrangement than the aryl- or silyl-substrates and the addition of TMEDA and longer reaction times are needed for the latter... 

Transition-metal catalyzed tandem hydrosilylation/cyclization of dienes is a well known process, hi order to render this reaction environmentally benign, Studer developed metal-free conditions using a silylated cyclohexadi-ene reagent 114 (Scheme 33) [108-110]. Thereby, the silyl radical released upon rearomatization of radical intermediate 115 reacts with a diene to form a /1-silyl radical 116, which undergoes cyclization into 117. Reduction of 117 with cyclohexadiene 114 affords again 115, as well as 118, thus allowing for... 

Asymmetric hydrosilylation of 2-phenyl-1-butene yields enantiomeric excess ee) values as high as 68% [149]. Products obtained by sequential cyclization/ silylation reactions of 1,5-dienes and 1,6-dienes feature in the suggested mechanistic scenario (Scheme 8) [149, 155]. Furthermore, hydrosilylation of terminal olefins achieved both excellent chemoselectivity in the presence of any internal olefin, and functional-group compatibility with halides, ethers, and acetals [155]. 

The sequential cyclization/silylation reactions of 1,5-dienes and 1,6-dienes are catalyzed by Cp 2YMe(THF). The reaction tolerates a number of functional groups and proceeds with good yields and diastereoselectivities to give phenylsilane products which can be converted easily to synthetically more versatile alcohols (Scheme 276). The hydrosilylation of dienes is also effectively catalyzed by the neodymium alkyl complex Cp 2NdCH(SiMe3)2. 

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