Activation carbon-element bonds

Although tremendous ground has already been broken by researchers in this field, the type of transformations that can be unlocked via the activation of heteroaromatic C—H bonds must still be expanded upon. An ideal scenario would be the disconnection of any carbon-element bond via a C—H activation reaction. 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

The chemistry of 1,3-diketone complexes of nonmetallic elements (B, Si, Ge, Sb, Te) has been reviewed.68 Condensation of acetylacetone with TeCl4 yields Te(CH2COCH2COCH2)Cl2 (8a), which may be reduced to Te(CH2COCH2COCFI2) (8b). The crystal structures of these compounds demonstrate an unusual coordination of tellurium to the terminal carbons of the diketonate.61 72 In the dichloride complex, Teiv exhibits a distorted trigonal bipyramidal geometry with a stereochemi-cally active lone electron pair in the equatorial plane. In S2(MeCOCHCOMe)2 the acetylacetonate methine carbons are bonded to a disulfur unit.73... 

Carbon is the most versatile element in the periodic table. Due to various bond structures such as sp3, sp2, sp hybrids, and multiple pK-pK bonds, it can form one-, two-, and three-dimensionally bond-structured substances and provide a wide range of applications.1 Carbon materials such as graphite, diamond, activated carbons, carbon fibers, and C-C composites have been extensively investigated and used for many years. Since the discovery of carbon nanotubes in 1997, carbon materials have been newly focused as frontier materials in various fields.2-15... 

Acetyl hypofluorite also cleaves the carbon-mercury bond which provides an easy entry to many fluoroethers. Since the electrophilic fluorine attacks the electrons of the C—Hg bond, the reaction proceeds with a full retention of configuration. Several l-fluoro-2-methoxy derivatives were prepared from the corresponding olefins271, formally accomplishing the addition of the elements of MeOF across a double bond (equation 153)272. Such reactions were also used for the fluorination of very activated aromatic compounds (equation 154)273. 

Oxidative additions can occur at all sorts of X-Y bonds, but they are most commonly seen at H-H (also H-Si, H-Sn, or other electropositive elements) and carbon-halogen bonds. In the oxidative addition of H2 to a metal, a d orbital containing a lone pair interacts with the cr orbital of Ik, lengthening and breaking the H-H cr bond. The two pairs of electrons from the metal and the II2 cr bond are used to form two new M-H bonds. C-H bonds can oxidatively add to metals in the same way, and this process, called C H bond activation, is of enormous current interest for its potential applications to petrochemical processing and green chemistry. Si-H, Si-Si, B-B, and other bonds between electropositive elements undergo oxidative addition, too. 

Addition reactions of the Si-Si bonds across carbon-carbon triple bonds have been most extensively studied since the 1970s by means of palladium catalysts. In the early reports, palladium complexes bearing tertiary phosphine ligands, mostly PPh3, were exclusively employed as effective catalysts, enabling the alkyne bis-silylation with activated disilanes, i.e., disilanes with electronegative elements on the silicon atoms such as hydro [36], fluoro [37], chloro [38], and alkoxy-disilanes [39,40] and those with cyclic structure (Scheme 4) [41-44]. The bis-silylation reactions could be successfully applied to terminal alkynes and acetylenedicarboxylates to give (Z)-l,2-bis(silyl)alkenes, which are otherwise difficult to synthesize. 

An obvious extension of the AD-process would be the asymmetric transfer of heteroatoms other than oxygen to a carbon carbon double bond. Indeed, the osmium catalyzed [3] or palladium mediated [4] aminohydroxylation of alkenes has been known for 20 years. The resulting jff-amino alcohols are an important structural element in biologically active compounds as well as the starting point in the design of many chiral ligands. However, to develop this reaction into a catalytic, asymmetric process several problems had to be overcome. 

IV.4.D. ACTIVATION OF CARBON-ELEMENT, ELEMENT-ELEMENT, AND ELEMENT-HYDROGEN BONDS... 

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