Diynes, transition metal-catalyzed

Transition metal-catalyzed carbocycUzation reactions of tethered diene, enyne, diyne, and vinylallene derivatives represent an important class of transformations in synthetic organic chemistry. This may be attributed to the abihty to significantly increase molecular complexity through the highly selective combination of acyclic components, thereby facilitating the synthesis of complex polycychc products. Recently, rhodium-catalyzed carbocyclization reactions have attracted significant attention due to their immense synthetic versatility and the unique selectivities observed over a range of different transformations. This chapter provides an account of recent developments in rhodium-catalyzed [4-1-2] and [4-i-2-t2] carbocyclization reactions. 

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)). 

Tsuda, T., Transition Metal Catalyzed Diyne Cycloaddition Copolymerization , in The Polymeric Materials Encyclopedia, CRC Press, Boca Raton, Vol. 3, pp. 1905-1915. 

Transition metal-catalyzed [2 + 2 + 2] cocyclization of two molecules of an alkyne with an alkene is a powerful method for forming 1,3-cyclohexa-dienes [29-31]. These compounds are of course valuable partners for Diels-Alder reactions [32]. Through the right choice of substrates, both [2 + 2 + 2] and [4 + 2] cycloadditions can be performed in a single chemical operation [33]. Indeed, the reaction of electroneutral diyne 14 with electron-deficient maleimide 15 in the presence of lmol% of a Ru(I) catalyst exclusively afforded the highly symmetrical 1 2 adduct 17 in 74% yield (Scheme 9). 

Transition-metal-catalyzed hetero-[2 + 2 + 2]-cy-cloaddition of alkynes with carbon—heteroatom multiple bonds, such as isocyanides, carbon dioxide, nitriles, aldehydes, and ketones, provides heteroare-nes and unsaturated heterocycles. This reaction can be categorized into two groups one is the reaction of l,a>-diynes 397 with carbon—heteroatom multiple bonds, and the other is reaction of the alkynes 399, having a carbon—heteroatom multiple bond with alkynes as illustrated in Scheme 127. The reaction of 1,6 -diynes 397 proceeds through formation of the metalacyclopentadiene intermediate 398 followed by insertion of a carbon—heteroatom multiple bond, such as heterocumulenes (route a),189 nitriles (route b),190 and carbonyls (route c).191 On the other hand, the... 

A synthetically important breakthrough in the application of transition metal catalyzed alkyne trimerization was made in the selective cocyclotrimerization of diynes with silylated alkynes using dicarbonyl(t/5-cyclopentadienyl)cobalt as precatalvst. The silylated alkynes do not show any tendency to undergo self-trimerization thus, depending on the nature of the chain between the two alkyne units, benzo-fuscd carboeyclic or heterocyclic rings are formed3,31 34... 

Transition metal-catalyzed carbocyclization of nitrogen- and oxygen-tethered 1, n-enynes and diynes to give five- or six-membered heterocyclic compounds 12CC10271. 

Zhang, D. H., Zhang, Z. and Shi, M. 2012. Transition metal-catalyzed carbocycUzation of nitrogen and oxygen-tethered l,n-enynes and diynes Synthesis of five or six-membered heterocychc compounds. Chem. Commun. (Camb.) A% %3) 10271-10279. 

Schulte [285, 286] and later Chalk [287] described the Cu(I)-catalyzed synthesis of symmetrical 2,5-diarylpyrroles 261 from conjugated diynes 257 and primary amines 258. The reaction is believed to proceed via the transition metal-catalyzed hydroamination [33, 288-291] leading to tautomeric aminoenyne 259 or homopro-pargylic imine 260 intermediates, which further undergo 5-endo-dig cyclization to furnish pyrrole product 261 (Scheme 8.95). 

The transition metal-catalyzed [2+2+2] cycloaddition has been applied to the atroposelective biaryl synthesis. The first example is the chiral cobalt(I) complex-catalyzed [2+2+2] cycloaddition of aryl-diynes with nitriles to produce axially chiral arylpyridines [19a]. Subsequently, the enantioselective synthesis of axially chiral biaryl phosphine oxides has also been achieved (Scheme 21.16) [19b]. 

Not only ruthenium but also rhodium was used in the transition metal-catalyzed cycloaromatization via transition metal-vinylidene complexes. For example, the reaction of an acyclic (Z)-3-ene-l,5-diyne in the presence of RhCl(P(/-Pr)j)2 (5mol%) afforded the corresponding allyUc benzene presumably through the rhodium-vinylidene complex (Scheme 21.56) [63]. 

Synthesis of Polyynes. Conjugated diyne and polyyne units, which exhibit unusual electrochemical, optical, and structural properties, can be efficiently constructed from 1-haloalkynes via homologation by one acetylene unit with TIPS-acetylene through a transition metal-catalyzed cross-coupling reaction, 2 which allows an access to a new class of polyyne framework. 

An early contribution to use of the transition-metal-catalyzed pyridine formation reaction was the synthesis of vitamin Be (124) via the crossed-cyclotrimerization reaction of the bis-stannylated diyne 122 with acetonitrile under cobalt catalysis (Scheme 7.26) [36a andb]. The underlying crossed [2 - - 2 - - 2] cycloaddition reaction here provided the fused pyridine 123 in 76% yield after a regioselective destannylation effected by treatment of the cycloaddition product with aluminum oxide. 

Axially chiral biaryls are found widely in the key structure of useful chiral ligands and catalysts [1] and biologically active compounds [2], The conventional method for their catalytic asymmetric synthesis is based on an asymmetric cross-coupling approach [3], As a conceptually new approach to axially chiral biaryls, asymmetric aromatic ring construction via transition-metal-catalyzed [2 - - 2 + 2] cycloaddition has recently appeared [4], In 1999, Sato et al. reported two types of nickel-catalyzed [2-1-2-1-2] cycloaddition for the synthesis of functionalized biaryls [5], One was the cycloaddition of aryl-substituted monoyne 1 with acetylene, leading to biaryl 2 (Scheme 9.1), and the other was the cycloaddition of aryl-substituted 1,6-diyne 3 with acetylene, leading to biaryl 4 (Scheme 9.2) [5],... 

Transition metal-catalyzed cycloisomerizations have been the subject of particular attention as atom-efficient methods for the construction of carbo- and heterocyclic molecules, which are fundamental constituents of natural products, pharmaceutical compounds, and functional materials. In view of their importance, several studies concerning their metal-catalyzed cyclization mechanisms have been undertaken in a,oo-bifunctional substrates such as dienes, enynes, and diynes. 

Transition metal catalyzed hetero-[2+2+2] cycloaddition of alkynes to compounds with a multiple carbon-heteroatom bond such as isocyanates, carbon dioxide, nitriles, aldehydes, and ketones leads to heterocyclic arenes and unsaturated heterocycles. These reactions are classified into two groups coupling of diynes with multiple carbon-heteroatom... 

Yamamoto, Y., Takagishi, H. and Itoh, K. (2002) Ruthenium-catalyzed cycloaddition of 1,6-diynes with isothiocyanates and carbon disulfide first transition-metal catalyzed [2+2+2] cocyclotrimerization involving C=S double bond. Journal of the American Chemical Society, 124(1), lS-19. 

The reductive cyclization of non-conjugated diynes is readily accomplished by treatment of the acetylenic substrate with stoichiometric amounts of low-valent titanium52 523 and zirconium complexes.53 533 Hence, it is interesting to note that while early transition metal complexes figure prominently as mediators of diyne reductive cyclization, to date, all catalyzed variants of this transformation employ late transition metal complexes based on nickel, palladium, platinum, and rhodium. Nevertheless, catalytic diyne reductive cyclization has received considerable attention and is a topic featured in several review articles. ... 

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. 

Hashmi et al. investigated a number of different transition metals for their ability to catalyze reactions of terminal allenyl ketones of type 96. Whereas with Cu(I) [57, 58] the cycloisomerization known from Rh(I) and Ag(I) was observed (in fact the first observation that copper is also active for cycloisomerizations of allenes), with different sources of Pd(II) the dimer 97 was observed (Scheme 15.25). Under optimized conditions, 97 was the major product. Numerous substituents are tolerated, among them even groups that are known to react also in palladium-catalyzed reactions. Examples of these groups are aryl halides (including iodides ), terminal alkynes, 1,6-diynes, 1,6-enynes and other allenes such as allenylcarbinols. This che-moselectivity might be explained by the mild reaction conditions. 

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