Hydrosilylative cyclization

Very recently, Wiedenhoefer272 has devised the first asymmetric 1,6-enyne hydrosilylation/cyclization tandem process using a rhodium(l) catalyst with (R)-276 as chiral ligand where rhodium-BINAP complexes were not effective (Scheme 70). More developments on this reaction are covered in Chapter 11.13. 

Widenhoefer has also disclosed an interesting extension consisting of hydrosilylative cyclization of a diene catalyzed by palladium. High enantioselectivity (up to 95% ee) was achieved by using palladium catalysts with Ci-symmetric pyridine-oxazoline ligands351,364 and recent mechanistic studies have confirmed the involvement of an intramolecular carbometallation step.365... 

C-C bond formation mediated by silane.6,6a 6f With respect to the development of intramolecular variants, these seminal studies lay fallow until 1990, at which point the palladium- and nickel-catalyzed reductive cyclization of tethered 1,3-dienes mediated by silane was disclosed. As demonstrated by the hydrosilylation-cyclization of 1,3,8,10-tetraene 21a, the /rarcr-divinylcyclopentanes 21b and 21c are produced in excellent yield, but with modest stereoselectivity.46 Bu3SnH was shown to participate in an analogous cyclization.46 Isotopic labeling and crossover experiments provide evidence against a mechanism involving initial diene hydrosilylation. Rather, the collective data corroborate a mechanism involving oxidative coupling of the diene followed by silane activation (Scheme 15). 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

A solitary report of enantioselective hydrosilylative cyclization appears in the literature.64 Here, a variety of 1,6-enyne substrates are cyclized in good yields and enantioselectivities, using chirally modified cationic rhodium... 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

The stoichiometric reaction of low-valent rhodium salts with l, -diynes to afford rhodacyclopentadiene complexes is well established and has been reviewed.73 733 The first rhodium-catalyzed reductive cyclization of a non-conjugated diyne has been reported only recently.74 743 The stereochemical outcome of the rhodium-catalyzed hydrosilylation-cyclization is dependent upon the choice of catalyst. Whereas reductive cyclization of 1,6-diyne 54a catalyzed by Rh4(CO)i2 provides modest yields of the Z-vinylsilane 54c, exposure of 54a to Wilkinson s catalyst... 

Rhodium complexes catalyze hydrosilylation-cyclization of 1,6-allenynes in the presence of (MeO SiH.77 To avoid complex product distributions, the use of substrates possessing fully substituted alkyne and allene termini is imperative. As shown in the cyclization of 1,6-allenyne 62a, the regiochemistry of silane incorporation differs from that observed in the rhodium-catalyzed hydrosilylation-cyclization of 1,6-enynes (see Section 10.10.2.3.2). For allenyne substrates, allene silylation occurs in preference to alkyne silylation (Scheme 40). 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

A single example of the reductive cyclization of allenic carbonyl compounds is reported, which employs a rhodium-based catalyst in conjunction with Et3SiH as terminal reductant.113 This protocol promotes hydrosilylation-cyclization to form both five- and six-membered rings with exceptional levels of yy -diastereocontrol. As revealed... 

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

Palladium-catalyzed enantioselective hydrosilylation of carbon-carbon double bonds is well known." Widenhoefer s group recently extended this reaction to cyclization of functionalized 1,6-dienes. The PyrOx ligands were excellent ligands for this reaction, and comparative evaluations of ligand performance for the hydrosilylation-cyclization of dimethyl diallylmalonate 613 have been reported (Scheme 8.193)." ° " The trans-isomer of 614 is the preferred product and... 

Hydrosilylative cyclization of 1,7-diynes.1 Hydrosilylation of 1,7-diynes with HSiXj promoted by a Ni(0) catalyst, prepared from Ni(acac)2 and DIBAH, results in... 

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... 

Hydrosilylation/cyclization of hindered polyenes can be efficiently catalyzed by sterically open anra-lanthanocenes, but trimethylsilylcyclopentadienyl derivatives are particular active and allow cyclization of these challenging substrates at room temperature (23) [74],... 

The Rh complex of spiro diphosphine (R)-21a was proven to be an efficient catalyst for the hydrosilylation/cyclization of 1,6-enynes and afforded unprecedented high enanti-oselectivity (89-99% ee) for a wide range of substrates (Scheme 38) [101]. 

In a similar fashion, palladium- or nickel-catalyzed hydrosilylative cyclization of bis-diencs 6 stereoselectively leads to //( / .< -1,2-disubstituted cyclopentanes 761. This reaction resembles telomerization of dienes with 1,4-carbosilylation of one of the diene subunits, leading to allylsilanes. Both stereoisomers are formed with the same relative configuration of the ring substituents, solely differing with respect to the Z/E configuration of the (wo different unsaturated side chains. 

Additionally, the first NHC-Rh-catalyzed hydrosilylation/cyclization of enynes was achieved using Rh catalyst 93 by Chung and co-workers (Equation (13.6)). "... 

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