Diynes silylated

In addition to their role as a source of lithiated diynes or terminal diynes silyl-protected diynes have found direct application in the preparation of Group 8 diynyl complexes. Reactions of Me3SiC=CC=CR (R = Ph, C=CPh, C=CC=CPh) with RuCl(PPh3)2Cp in the presence of KF afford the corresponding poly-ynyl complexes Ru(C=CC=CR)(PPh3)2Cp." ... 

Cationic palladium complex 121 reductively coupled enynes (Eq. 20) using trichlorosilane as the stoichiometric reductant [71]. This combination of catalyst and silane afforded silylated methylenecyclopentanes such as 122 in good yield from enynes such as 123. Attempts to develop an enantioselective version of this reaction were not successful [71]. When enediyne 124 was cyclized in the presence of trichlorosilane, the reaction favored enyne cycli-zation 126 by a 3 1 ratio over diyne cyclization to 125 (Eq. 21). In contrast, when the more electron-rich dichloromethylsilane was used as the reductant, diyne cyclization product 125 was preferred in a ratio of 4 1 [71]. Selectivities of up to 10 1 for enyne cyclization were observed, depending on the substrate employed [72],... 

Itoh and coworkers [223] have shown that fullerene derivatives as 6/2-113, which to date have been prepared in a stepwise procedure, can be obtained in a three-component domino process by treatment of diynes 6/2-109, dimethylphenylsilane 6/2-110 and fullerene (C60) in the presence of a Rh-catalyst [223]. Interestingly, using maleic anhydride as dienophile failed to give the desired cycloadduct, whereas Cso -in spite of its strong tendency to form complexes with various transition metals [224] - never suppressed the catalytic silylative cyclization step to give the diene 6/2-112 (Scheme 6/2.24). 

The reaction of Ba[P(SiMe3)2]2(THF)2 with diphenylbutadiyne in toluene for 12 days induces a m-addition of the diyne to the phosphide, followed by a 1,3-silyl group shift and ring closure. The dinuclear complex 132 is then isolated in good yield.283 Its complex structure contains Ba-C a bonds (2.881(5), 2.899(5) A), side-on Ba-alkyne (3.003(6), 3.363(6) A) and arene interactions, and Ba-phospholide bonds (Ba-P = 3.487(2) A) (Figure 65). 

The palladium-catalyzed addition of silylstannanes to 1,6-diynes invokes intramolecular cyclization, giving rise to 1,2-dialkylidene cyclopentanes (Equation (110)).269 The resulting Z,Z-l-silyl-4-stannyl-l,3-diene moiety fixed in an i -m-conformation makes the molecule axially chiral. Rapid equilibrium between the two helical forms is observed by NMR spectroscopy.270,161... 

The vinylsilane C-Si bond can also be formed from a silane by reductive cyclization/hydrosilylation of a 1,6- or 1,7-diyne. Reductive cyclization of diynes is an important ring-forming method catalyzed by transition metals, and silanes are common reductants in this process. However, in many cases the silane serves only as a hydride source, and the silyl group is not retained in the isolated product.95 Here, the focus is on the more rare methods which allow simultaneous C-C bond formation and vinylsilane installation. 

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... 

As shown in Eq. 2.59, zirconacyclopentadienes can be inserted into such a polymer chain (lq) from the beginning. The polymer can then be converted into polyphenylenes 88 in good yields [43]. Silyl-bridged diynes react with Cp2ZrBu2 to afford a macrocyclic tri-mer (89), as shown in Eq. 2.60 [43]. 

Vollhardt118) employed the organocobalt-promoted reaction between Wjftrimethyl-silyl)acetylene (322) and the diyne (338) to construct the steroid estrone 118). 

In 1999, Sekiguchi et al. prepared the first silyl acetylene dendrimers 255 and 256 with up to 22 Si atoms and 21 acetylene units (Schemes 35 and 36).358 The preparation of 255 and 256 has been achieved by the reaction of the triple Grignard reagent 257 with the trisila diyne 258 and the heptasila hexayne 259, respectively. 

When the terminal alkynes 96 are treated with the trimethylsilylalkyne 97 in the presence of HfCl4 as a Lewis acid, the silylated vinylallenes 98 are produced in acceptable yields. In an intramolecular variant of this process, 100 was obtained from the diyne 99 [32]. Vinylallenes, incorporated into a cyclic framework and hence of restricted conformational mobility, are of interest for photochemical studies [33] and are among the photoproducts in ring-enlargement reactions of polycyclic allenes [34]. 

Iridium-phosphine complexes were found to be efficient carbonylative alkyne-alkene coupling catalysts [62]. Although frequently applied in other transformations, the dimeric complex [ Ir( x-Cl)(cod) 2] appeared to be a very active catalyst in the coupling of silylated diynes with CO [63], giving bicyclic products with a carbonyl moiety (Scheme 14.12). 

Metal-promoted 1,2-migration of silyl groups in silylalkynes results in the formation of silylvinylidenes which are subsequently readily desilylated. " Alkynyl-substituted silylvinylidenes have been obtained from silylated diynes and the Fe(N2)(CO)2 P(OMe)3 2/ Fe(CO)2[P(OMe)3]2 2(/t-N2) reagent and similar species are implicated in the reactions of several Group 8 metal complexes with mono- and bis-trialkylsilyl diynes. " ... 

The cascade silylcarbocyclization is applicable to diynals. For example, the reaction of undec-l,6-diyn-ll-al 72 with PhMe2SiH, catalyzed by Rh(acac)(CO)2 affords the corresponding silylated exo-silylmefhylene(hydroxy)bis(cyclopentylidene) 73 (Eq. 20) [48]. 

A serious side reaction is the formation of the bis-silylatcd diyne, which may result from trans-metallarion of the primary product by LiCsCOCH and subsequent silylation. This reaction can be suppressed to some extent by using a 100% excess of diacetylene. 

To a mixture of 30 g of zinc powder (Merck, analytical grade) and 30 ml of absolute ethanol is added 3.5 ml of 1.2-dibromoethane. The mixture is heated until an exothermic reaction (evolution of ethene and temporary reflux) starts. The activation is completed by heating the mixture for an additional 10 min under reflux. After cooling to about 50 C, the trimethylsilylated diyne (0.05 mol, see Chap. VI for silylation methods) is added in one portion. The introduction of N2 is started and the mixture is heated for 30 min under reflux. After cooling to room temperature, the work-up is carried out in a way similar to that in exp. 2 (no aqueous ammonia is nsed). Z-CjH- CI CHC CSiMe, b.p. 75 020 mmHg, njy(20 ) 1.4592, is obtained in a high yield. 

Tamao and Ito have reported a nickel-catalyzed protocol for the cyclization/hydrosilylation of 1,7-diynes to form (Z)-silylated dialkylidene cyclohexane derivatives.For example, reaction of 1,7-octadiyne with triethoxysilane catalyzed by a mixture of Ni(acac)2 (lmol%) and DIBAL-H (2mol%) in benzene at 50°G for 6h gave the corresponding silylated dialkylidene cyclohexane in 70% yield as a single isomer (Table 1). The reaction of 1,7-octadiyne was also realized with mono- and dialkoxysilanes, trialkylsilanes, and dialkylaminosilanes (Table 1). Diynes that possessed an internal alkyne also underwent nickel-catalyzed reaction, albeit with diminished efficiency (Table 1), while 1,6- and 1,8-diynes failed to undergo nickel-catalyzed cyclization/hydrosilylation. 

Tamao and Ito proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of 1,7-diynes initiated by oxidative addition of the silane to an Ni(0) species to form an Ni(ii) silyl hydride complex. Gomplexation of the diyne could then form the nickel(ii) diyne complex la (Scheme 1). Silylmetallation of the less-substituted G=C bond of la, followed by intramolecular / -migratory insertion of the coordinated G=G bond into the Ni-G bond of alkenyl alkyne intermediate Ila, could form dienylnickel hydride intermediate Ilia. Sequential G-H reductive elimination and Si-H oxidative addition would release the silylated dialkylidene cyclohexane and regenerate the silylnickel hydride catalyst (Scheme 1). 

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). 

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