Other Carbon-Heteroatom Multiple Bonds

Addition of silyl radicals to carbon-nitrogen multiple bonds has mainly been investigated by EPR spectroscopy [9,66]. 

The adduct of silyl radicals to 4-substituted pyridines and pyrazine monitored by EPR results from the attack at the nitrogen atom to give radicals 52 and 53, respectively [68,69]. The rate constant for the addition of Et3Si radical to pyridine is about three times faster than for benzene (Table 5.3) [24]. 

The addition of the Et3Si radical to the C=N bond of nitrones also occurs readily. A rate constant of 7.1 x 10 M s at 27 °C has been obtained for Reaction (5.35), whereas information on the structure of the adduct radicals has been obtained by EPR spectroscopy [13,70]. 

The EPR technique has also been employed to investigate the adducts of the reaction of silyl radicals with various nitrile-A-oxides [71]. As an example, the 

The addition of silyl radicals to thiocarbonyl derivatives is a facile process leading to a-silylthio adducts (Reaction 5.37). This elementary reaction is the initial step of the radical chain deoxygenation of alcohols or Barton McCombie reaction (see Section 4.3.3 for more details). However, rate constants for the formation of these adducts are limited to the value for the reaction of (TMS)3Si radical with the xanthate c-C6HuOC(S)SMe (Table 5.3), a reaction that is also found to be reversible [15]. Structural information on the a-silylthio adducts as well as some kinetic data for the decay reactions of these species have been obtained by EPR spectroscopy [9,72]. 

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Addition reactions to other carbon-heteroatom MULTIPLE BONDS (see also Grignard reaction, Hydrolysis, Reduction reactions)... 

The hydrosilylation of carbon-heteroatom multiple bonds had received little attention until it was found in 1972 that Rh(PPh3)3Cl is an extremely effective catalyst for the hydrosilylation of carbonyl compounds. This is a new and unique reduction method since the resulting silicon-oxygen bond can easily be hydrolyzed. Other transition metal complexes including platinum, ruthenium , and rhodium also have good catalytic activity in the selective and asymmetric hydrosilylation of carbonyl compounds "". 

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... 

Owing to its tendency to undergo nucleophilic addition with carbonyl groups and other electrophilic carbon-heteroatom multiple bonds (C=NR, C=N, C=S), n-BuLi is usually not the reagent of choice for the generation of enolate anions or enolate equivalents from active hydrogen conpounds. This is done most conveniently using the less nucleophilic lithium dialkylamides (e.g. Lithium DUsopropylamide (LDA), Lithium 2,2,6,6-Tetra-... 

Addition to Carbon-Heteroatom Multiple Bonds. The behavior of r-BuLi in reactions with carbon-heteroatom r-bonds is relatively unremarkable and parallels that of other organolithium reagents. Even in cases where steric hindrance might be expected to lead to difficulties, product yields are reasonable. Thus, for example, tri-f-butylcarbinol (3-r-butyl-2,2,4,4-tetramethyl-3-pentanol) may be prepared by addition of r-BuLi to di-r-butyl ketone (2,2,4,4-tetramethyl-3-pentanone), although there is a significant amount of reduction in this case, and V-lithio-di-r-butylimines may be generated by addition of r-BuLi to r-butyl cyanide. ... 

Hetero-Diels-Alder reactions starting with unsaturated compounds with heteroatom-carbon or heteroatom-heteroatom multiple bond(s) are also enhanced by Lewis acids [374-381]. Aldehydes and imines work as dienophiles under the influence of TiCU- Electron-rich dienes are generally a preferable partner, as shown in Eq. (149), in which the product was obtained virtually as a single isomer [382,383]. The importance of the choice of the Lewis acid in determining the stereochemical outcome of the reaction is illustrated in Eq. (150) [384]. The notion of chelation and of Felkin-Anh models, respectively, is valid for these Diels-Alder reactions. Diastereoi-somers other than those shown in Eq. (150) were not detected. The stereochemistry of the product in Eq. (149) could be also explained by the chelation model. 

Carbon may also form multiple bonds with many other elements. It is this ability that is one of the reasons for the richness of the chemistry of carbon. One of the commonest heteroatomic multiple bond systems in which carbon partakes involves oxygen. Draw the dot and cross structure of this carbon/ oxygen double bond system. 

Other examples of condensed structures with heteroatoms and carbon-carbon multiple bonds are given in Figure 1.5. You must learn how to convert a Lewis structure to a condensed structure, and vice versa. 

The protonation of organo-rare-earth metal species through a-bond metathesis plays a key role in many catalytic applications described below. The high reactivity of rare-earth metals for insertion of unsaturated carbon-carbon multiple bonds [18], in conjunction with smooth o-bond metathesis, allows to perform catalytic small molecule synthesis. This route is atom efficient, economic, and opens access to nitrogen-, phosphorous-, silicon-, boron-, and other heteroatom-containing molecules. The most important catalytic applications of organo-rare-earth metals involving the o-bond metathesis process will be discussed in this review. 

Organic chemistry is the study of carbon (C) compounds, all of which have covalent bonds. Carbon atoms can bond to each other to form open-chain compounds, Fig. 1.1(a), or cyclic (ring) compounds, Fig. 1.1(c). Both types can also have branches of C atoms, Fig. 1.1(b) and (d). Saturated compounds have C atoms bonded to each other by single bonds, C—C unsaturated compounds have C s joined by multiple bonds. Examples with double bonds and triple bonds are shown in Fig. 1.1(c). Cyclic compounds having at least one atom in the ring other than C (a heteroatom) are called heterocyclics, Fig. 1.1 (/). The heteroatoms are usually oxygen (O), nitrogen (N), or sulfur (S). 

At the other extreme, metal carbenes that are electrophilic at carbon are called Fischer-type complexes, and they generally contain jt-donating heteroatom substituents [4], Fischer reported the first example in 1964 [5], In these cases, the metal-carbene interaction can be represented by three resonance structures, the first with a formal M=C double bond, the second with a M-C single bond and charge separation, and the third with additional multiple bond character between the carhon and the heteroatom substituent. 

Heterocyclic compounds make up the third and largest class of molecular frameworks for organic compounds. In heterocyclic compounds, at least one atom in the ring must be a heteroatom, an atom that is not carbon. The most common heteroatoms are oxygen, nitrogen, and sulfur, but heterocyclics with other elements are also known. More than one heteroatom may be present and, if so, the heteroatoms may be alike or different. Heterocyclic rings come in many sizes, may contain multiple bonds, may have... 

This chapter is not intended to provide comprehensive details, but rather to summarize the organocatalytic multiple-bond forming reactions from the last 10 years in the context of their utility in the synthesis of spirocycles with emphasis on the different strategies. The chapter will be structured according to the nature of the spiroatom synthesized and the nature of the spirocycle formed. First, we will discuss the synthesis of all-carbon spirocenters, focusing on the synthesis of spirooxindoles and other heterocycles. Next, we will focus on the synthesis of spirocenters with at least one heteroatom. 

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