Hydrosilylation of enynes

These results clearly demonstrate a remarkable regioselectivity in the hydrosilylation of enyne compounds and that this approach works well for syntheses of siloxane polymers with reactive olefinic groups. 

Scheme 4.68 Palladium-catalyzed asymmetric hydrosilylation of enynes 261 forming allenylsilanes 267.

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]. 

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). 

Cyclization/Hydrosilylation of Enynes and Eneallenes 11.11.3.1 Rhodium Catalysts... 

Gyclization/hydrosilylation of enynes catalyzed by rhodium carbonyl complexes tolerated a number of functional groups, including acetate esters, benzyl ethers, acetals, tosylamides, and allyl- and benzylamines (Table 3, entries 6-14). The reaction of diallyl-2-propynylamine is noteworthy as this transformation displayed high selectivity for cyclization of the enyne moiety rather than the diene moiety (Table 3, entry 9). Rhodium-catalyzed enyne cyclization/hydrosilylation tolerated substitution at the alkyne carbon (Table 3, entry 5) and, in some cases, at both the allylic and terminal alkenyl carbon atoms (Equation (7)). 

Ojima has proposed a mechanism for the rhodium-catalyzed cyclization/hydrosilylation of enynes initiated by oxidative addition of the H-Si bond of the hydrosilane to form the Rh(iii) silyl hydride complex If (Scheme 7). Silylmetallation of the G=G bond of the enyne coupled with coordination of the pendant G=G bond could form... 

Catalytic hydrosilylation of enyne (buten-3-yne) with three kinds of hydrosilanes (HSiMeH2, HSiMe2Ph and HSiEt3) in the presence of RuHCl(CO)(PPh3)3 has given five types of reaction products (Eq. 20), the isolation has not been achieved. 

Scheme 89 Platinum carbene catalyzed hydrosilylation of enyne...

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... 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

Pt(PPh3)4, PdCl2(PPh3)2 and Pd(PPh3)4 were much less efficient than H2PtCl6 and RhCl(PPh3)3. With these less reactive catalysts, the enyne 133 could be selectively prepared. Subsequent hydrosilylation of 133 also afforded the allene 134. In this stepwise route, two different silyl groups could be introduced into the allenic product. A typical example is illustrated in Scheme 3.68. 

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... 

Widenhoefer and co-workers have developed an effective protocol for the asymmetric cyclization/hydrosilylation of functionalized 1,6-enynes catalyzed by enantiomerically enriched cationic rhodium bis(phosphine) complexes. For example, treatment of dimethyl allyl(2-butynyl)malonate with triethylsilane (5 equiv.) and a catalytic 1 1 mixture of [Rh(GOD)2] SbF6 and (i )-BIPHEMP (5 mol%) at 70 °G for 90 min gave the silylated alkylidene cyclopentane 12 in 81% yield with 98% de and 92% ee (Table 4, entry 1). A number of tertiary silanes were effective for the rhodium-catalyzed asymmetric cyclization/hydrosilylation of dimethyl allyl(2-butynyl)malonate with yields ranging from 71% to 81% and with 77-92% ee (Table 4, entries 1-5). Although the scope of the protocol was limited, a small number of functionalized 1,6-enynes including A-allyl-A-(2-butynyl)-4-methylbenzenesulfonamide underwent reaction in moderate yield with >80% ee (Table 4, entries 6-8). 

Molander has developed effective protocols for the cyclization/hydrosilylation of 1,6-enynes catalyzed by lanthanide metallocene complexes/ For example, reaction of cyclohexyl-substituted 1,6-enyne 15a with phenylsilane catalyzed by Cp 2YMe(THF) in cyclohexane at room temperature for 2h gave silylated alkylidene cyclopentane 16a as a 6.5 1 mixture of trans. cis isomers (Table 5, entry 1). The diastereoselectivity of the reaction depended strongly on the nature of the allylic substituent. For example, yttrium-catalyzed cyclization/ hydrosilylation of the ethyl-substituted enyne 15b gave silylated cyclopentane 16b in 88% yield as a single diastereomer (Table 5, entry 2). 

Dienes are less reactive toward transition metals than enynes and diynes, and perhaps for this reason, the development of effective catalyst systems for the cyclization/hydrosilylation of dienes lagged behind development of the corresponding procedures for enynes and diynes. The transition metal-catalyzed cyclization/hydrosilylation of dienes was first demonstrated by Tanaka and co-workers in 1994. Reaction of 1,5-hexadiene with phenyl-silane catalyzed by the highly electrophilic neodymium metallocene complex Cp 2NdCH(SiMe2)3 (1 mol%) in benzene at room temperature for 3 h led to 5- ///76 -cyclization and isolation of (cyclopentylmethyl)phenylsilane in 84% yield (Equation (15)). In comparison, neodymium-catalyzed reaction of 1,6-heptadiene with phenylsilane led to 5- X(9-cyclization to form (2-methylcyclopentylmethyl)phenylsilane in 54% yield as an 85 15 mixture of trans. cis isomers (Equation (16)). 

Suisse and co-workers have studied the asymmetric cyclization/silylformylation of enynes employing catalytic mixtures of a rhodium(i) carbonyl complex and a chiral, non-racemic phosphine ligand. Unfortunately, only modest enantioselectivities were realized.For example, reaction of diethyl allylpropargylmalonate with dimethylphenyl-silane (1.2 equiv.) catalyzed by a 1 1 mixture of Rh(acac)(GO)2 and (i )-BINAP in toluene at 70 °G for 15 h under GO (20 bar) led to 90% conversion to form a 15 1 mixture of cyclization/silylformylation product 67 and cyclization/ hydrosilylation product 68. Aldehyde 67 was formed with 27% ee (Equation (46)). 

Radical reaction cascades can be initiated by (TMS)3Si radical addition to unsaturated bonds. Two recent examples are illustrated in equations 54 and 55. The reaction of (TMS)3SiH with the 1,6-enyne derivative 98 afforded the 6-membered cyclic compound having exclusively the exocyclic double bond in E configuration via a 6-exo mode106. On the other hand, hydrosilylation of 99 afforded the 6-membered ring via a 6-endo cyclization of the vinyl radical onto the C=N bond107. 

Dienylsilanes 3 and 6 are the cis and trans addition products of hydrosilanes across the triple bond of 1, respectively. Allylsilane is the product formed by hydrogenation followed by hydrosilylation of 1. Alkenylsilane 5 is the 1,4-adduct of hydrosilane across the enyne skeleton of 1, andbis(silyl)butenyne 7 is the de-hydrogenative silylation product of 1 [32]. 

As mentioned in Sect. 3.2, the carbon-carbon triple bond has a superior reactivity in hydrosilylation compared to a double bond, presumably due to a faster insertion step. This creates an opportunity to perform regio- and chemoselective transformations of enynes [68]. Regioselective transformations of 1,6- and 1,7-enynes proceed via preferred insertion of the triple bond (24). 

Optically active allenyltrichlorosilanes can be prepared by the Pd-catalyzed asymmetric hydrosilylation of 1,3-enynes with I ISiCb, (Scheme 10.138) [389]. In this reaction use of bisPPFOMe (125) as the chiral ligand achieves high enantioselectivity. The allenylsilanes smoothly react with PhCHO in DMF to give optically active homopropargyl alcohols, and the axial chirality of allenylsilanes is completely transferred to the central chirality of the products by a syn-SE process. 

The hydrosilylation of 1,4-bis(trimethylsilyl)-1,3-butadiyne catalyzed by H2PtCl6, RhCl(PPh3)3 or Pt(PPh3)3 gives an enyne (22) and/or an allene (23) 22 arises from the mono-hydrosilylation and 23 from dowb/e-hydrosilylation191 (equation 54). 

Although it is found that the dowb/e-hydrosilylation proceeds stepwise and thus allenes (23) can easily be obtained selectively, the selective formation of enynes (22) requires a bulky hydrosilane, i.e. triisopropylsilane. Typically, the reaction with RhCl(PPh3)3 and HSiMe3 at 100 °C for 19 h gives 23 in 95% yield while the reaction with H2PtCl6 and HSiPr3 at 90 °C for 8 h gives 22 in 92% yield (the selectivity is 100% in both cases). 

Cyclization and hydrosilylation of nitrogen-containing enynes are achieved in a Cp 2YMe-catalyzed process.- ... 

With the exception of norbomene, internal olefins do not undergo hydrosilylation. Hydrosilylation of 3-phenylpropene with PhSiDj forms a unique product and the process tolerates a variety of fianctional groups, halides, ethers and acetals, despite the well known strong Lewis acidity of the catalysts. Cyclisation/silylation of 1,5-dienes or 1,6-enynes has been reported to give a single product (Scheme 14) [26]. In the case of metallocene complexes bearing a menthyl substituent, ee values near 70% were obtained for the asymmetric hydrosilylation of 2-phenyl-but-l-ene [31]. 

Gratifyingly, (IPr)Pt(AE) 45 has also been used to catalyze the hydrosilylation of an elaborate 1,3-enyne 52 during the total synthesis of lactimidomycin by Gallenkamp and Fiirstner (Scheme 5.14) [36]. The high selectivity observed for the addition of the silane onto the internal alkyne instead of the terminal olefin likely originates from the preferential coordination of the platinum(O) complex... 

Scheme 5.14 Hydrosilylation of a 1,3-enyne during the total synthesis of lactimidomycin.

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