Transition metals catalyzed vinylic substitution

These reactions involve metallate rearrangements [231], migratory insertion and transition metal-catalyzed vinylic substitution reactions. They also perform well in applications in natural product synthesis [204,205,209,233]. 

This preparation illustrates an efficient two-step process for the transformation of a cycloalkenone to the corresponding a-substituted derivative. The first step involves the installation of an a-iodo substituent by a process thought to involve nucleophilic addition of pyridine, iodine capture of the resulting enolate, and pyridine-promoted elimination of pyridine.5 The resulting vinyl iodides are superior to other vinyl halides as participants in a variety of transition-metal catalyzed coupling reactions, illustrated here by the Suzuki coupling with an arylboronic acid. Other coupling partners that... 

On the other hand, the transition metal-catalyzed isomerization of terminal olefins into internal olefins has been extensively studied, and in general a mixture of 1-alkenes and E- and Z-2-alkenes, reflecting the thermodynamic equilibrium, is obtained [67]. Some low-valent titanocene derivatives are highly effective and stereoselective in favor of the E-2 isomer [68]. When non-conjugated dienes such as 92, containing one or two substituted vinyl groups, are treated with the zirconocene 21, a regioisomerization of the less-substituted... 

Substituted styrenes and vinylic compounds are versatile intermediates in organic synthesis, so various methods have been published in the literature [33, 34]. Among them, the Heck reaction is one of the best-studied methods for preparing these compounds [34]. However, for this reaction, the use of the halogen-carbon bond is essential for making C-C bonds. If direct addition of otherwise unreactive C-H bond to acetylenes takes place, this method will become one of the simplest methods for preparing substituted styrenes and vinylic compounds. In this section, we will describe the transition metal-catalyzed vinyla-tion of aromatic compounds by using acetylenes. 

Suitably substituted carbosilanes allow the introduction of reactive vinyl and Si-H side groups which can be used for transition metal catalyzed crosslinking [104] (Scheme 11). 

A parallel development was initiated by the first publications from Sawamoto and Matyjaszweski. They reported independently on the transition-metal-catalyzed polymerization of various vinyl monomers (14,15). The technique, which was termed atom transfer radical polymerization (ATRP), uses an activated alkyl halide as initiator, and a transition-metal complex in its lower oxidation state as the catalyst. Similar to the nitroxide-mediated polymerization, ATRP is based on the reversible termination of growing radicals. ATRP was developed as an extension of atom transfer radical addition (ATRA), the so-called Kharasch reaction (16). ATRP turned out to be a versatile technique for the controlled polymerization of styrene derivatives, acrylates, methacrylates, etc. Because of the use of activated alkyl halides as initiators, the introduction of functional endgroups in the polymer chain turned out to be easy (17-22). Although many different transition metals have been used in ATRP, by far the most frequently used metal is copper. Nitrogen-based ligands, eg substituted bipyridines (14), alkyl pyridinimine (Schiff s base) (23), and multidentate tertiary alkyl amines (24), are used to solubilize the metal salt and to adjust its redox potential in order to match the requirements for an ATRP catalyst. In conjunction with copper, the most powerful ligand at present is probably tris[2-(dimethylamino)ethyl)]amine (Mee-TREN) (25). 

The generation of ethers through the formation of oxygen-carbon(sp ) bonds (Williamson approach) is rare due to the sluggish reactivity of vinyl and aryl halides in nucleophilic substitution. Furthermore, while nucleophihc substitution chemistry tends to work well for primary systems, significant amounts of elimination products are often observed with secondary alkyl hahdes, and tertiary substrates are typically unresponsive. As a result, a number of transition metal-catalyzed routes to the formation of ethers through the formation of oxygen-carbon(sp ) bonds have been developed. 

Over the past few years, transition metal-catalyzed C-H functionalization methods for the formation of the C-C bonds have been well explored, many of which have included a handful of impressive strategies and synthetically valuable heterocycles when the starting materials contain heteroatoms. Among them, iron salts showed particular fascination for such purposes. In 2010, Liang and coworkers reported a new iron-catalyzed dual C-H activation between aryl C(sp )-H and vinyl C(sp )-H bond activation to build substituted indoles in the presence of Cu(OAc)2-CuCl2 (Scheme 9.13) [16]. This reaction showed highly functional group tolerance and exhibited different properties from the palladium-catalyzed reactions of this type. 

In the cases of aromatic aUcynes, the radical addition of PhSeH induced by oxygen (or air) is a very fast process (the addition is finished immediately after mixing the alkynes and PhSeH), giving a n-Markovnikov adduct regioselectively. The key species, PhSe-, adds to alkynes to generate vinyl radicals, a-Aryl-substituted vinyl radicals (formed by the addition of PhSe- to aromatic alkynes) are among 7t-radicals and more stable than a-alkyl-substituted vinyl radicals as a-radicals (formed by the addition of PhSe- to aliphatic alkynes) [86]. Therefore, it is relatively difficult, compared with the hydrothiolation, to control the selectivity in transition-metal-catalyzed hydroselenation of aromatic alkynes. 

Cyclopropane formation occurs from reactions between diazo compounds and alkenes, catalyzed by a wide variety of transition-metal compounds [7-9], that involve the addition of a carbene entity to a C-C double bond. This transformation is stereospecific and generally occurs with electron-rich alkenes, including substituted olefins, dienes, and vinyl ethers, but not a,(J-unsaturated carbonyl compounds or nitriles [23,24], Relative reactivities portray a highly electrophilic intermediate and an early transition state for cyclopropanation reactions [15,25], accounting in part for the relative difficulty in controlling selectivity. For intermolecular reactions, the formation of geometrical isomers, regioisomers from reactions with dienes, and enantiomers must all be taken into account. 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

Thus, our initial goal was to develop new synthetic procedures to allow high yield selective synthesis of vinylborazine. Since our previous work - has demonstrated that transition metal reagents, similar to those widely employed in organic and organometallic chemistry, may be used to catalyze a variety of transformations involving B—H activation reactions, we investigated the use transition metals to catalyze the alkyne-addition and olefin-substitution reactions of borazine. As a result we found, as indicated below, that B-vinyl... 

Changing the identity of the ortho substituent of aryl azides dramatically influences its reactivity toward transition metals. Shen and Driver discovered that substitution of a vinyl group with an iminyl moiety enables Fe(II)-catalyzed benzimidazole (65) formation (Scheme 16.29) [48]. It was proposed that coordination of iron(II) bromide to the imino nitrogen of 64 facilitates the C—N bond formation through nucleophilic attack of an azido nitrogen to the activated imino carbon, leading to B. Expulsion of N2 followed by tautomerization affords the benzimidazoles 65. 

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