Heteroatom-centered complexes

Other radical reactions not covered in this chapter are mentioned in the chapters that follow. These include additions to systems other than carbon-carbon double bonds [e.g. additions to aromatic systems (Section 3.4.2.2.1) and strained ring systems (Section 4.4.2)], transfer of heteroatoms [eg. chain transfer to disulfides (Section 6.2.2.2) and halocarbons (Section 6.2.2.4)] or groups of atoms [eg. in RAFT polymerization (Section 9.5.3)], and radical-radical reactions involving heteroatom-centered radicals or metal complexes [e g. in inhibition (Sections 3.5.2 and 5.3), NMP (Section 9.3.6) and ATRP (Section 9.4)]. 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

Indeed, the extent of disproportionation of NO according to equation (89) clearly depends on the donor strength of the aromatic hydrocarbon.240 For example, hexamethylbenzene which is a strong donor (IP = 7.85 V) promotes the ionization of NO to an extent of 80% whereas the weaker donor durene (IP = 8.05 V) affords less than 25% ion-pair formation. Furthermore, the resulting NO+ cation is a powerful electron acceptor (Erea = 1.48 V versus SCE) in contrast to NO (Ered = 0.25 V versus SCE) and thus readily forms donor/acceptor complexes with a variety of aromatic, olefinic and heteroatom-centered donors. Accordingly, the donor/acceptor complexation and electron-transfer activation are the critical steps in various transformations in Chart 8 as described below. 

Interestingly, homolytic substitution at boron does not proceed with carbon centered radicals [8]. However, many different types of heteroatom centered radicals, for example alkoxyl radicals, react efficiently with the organoboranes (Scheme 2). This difference in reactivity is caused by the Lewis base character of the heteroatom centered radicals. Indeed, the first step of the homolytic substitution is the formation of a Lewis acid-Lewis base complex between the borane and the radical. This complex can then undergo a -fragmentation leading to the alkyl radical. This process is of particular interest for the development of radical chain reactions. 

In the presence of a catalytic amount of methanethiolate-bridged diruthenium complex (la abbreviated as met-DIRUX), reactions of propargylic alcohols (2) with a variety of heteroatom-centered nucleophiles such as alcohols, thiols, amines, amides, and diphenylphosphine oxide gave the corresponding propargylic substituted... 

Only few theoretical studies have been devoted exclusively to the coordination of Lewis bases to silylenes. Most calculational evidence for the formation of silylene-Lewis base complexes was obtained from the computational investigation of the insertion reaction of HiSi into various H-X a bonds, where X is an heteroatom center possessing one or more free... 

The rate of complex formation of dimethylsilylene with a variety of Lewis bases was found to be close to the diffusion limit in cyclohexane at room temperature.33,34 The results of Yamaji et al.34 indicate that the rate of this reaction is governed not so much by electronic factors such as the HOMO energy of the Lewis base as by steric hindrance around the heteroatom center. In contrast, Baggott et al.35 found a satisfying correlation between rate constants and ionization energies of the nucleophile for the reaction of dimethylsilylene with various oxygen-containing substrates in the gas phase. 

Propynylnaphthalene 15 furnishes dinaphthylacetylene 20 in near quantitative yield. On the other hand, the Mori system performs somewhat less well if heteroatoms are present in the substrates, suggesting that the catalytic system is poisoned by the presence of the heteroatom through complexation and/or chelation of the active molybdenum center. Cyano groups and bromides/iodides inhibit the reactivity of the catalyst system. However, both propynylated phenols (16) and esters (17) give satisfactory dimerization results (21, 22). 

Because of their basic resemblance to porphyrins, it was initially expected that the sapphyrins would mimic, at least on some level, the rich coordination chemistry displayed by the porphyrins. However, the larger core size ca. 5.5 A inner N-N diameter vs. ca. 4.0 A for porphyrins), the greater number of potentially chelating heteroatom centers, and the fact that pentaazasapphyrins when fully deprotonated are potentially trianionic ligands made sapphyrin a likely candidate for large metal chelation, particularly as a potential ligand for the trivalent lanthanides and actinides. Unfortunately, in spite of extensive effort, this hope remains largely unrealized. Nonetheless, some metal complexes of sapphyrins and heterosapphyrins have been successfully prepared and characterized. Their preparation and properties are reviewed in this section. 

Mono-Cp complexes supported by the Cp ligands bearing neutral pendant, heteroatom-containing sidearms - the rf -if chelating ligand - fundamentally differ from those discussed under section 8.8, where the Cp pendant arms are anionic in nature and covalently bound to the metal center. Complexes discussed in this section belong to those with... 

Some of the newer compounds may contain both saturated and unsaturated rings, heteroatoms such as oxygen, nitrogen, or sulfur, and halogen substituents. Others, such as synthetic pyrethroids, may have one or more chiral centers, often needing stereospecific methods of synthesis or resolution of isomers (42). Table 4 Hsts examples of some more complex compounds. Stmctures are shown ia Eigure 1 (25). 

The Diels-Alder cycloaddition is the best-known organic reaction that is widely used to construct, in a regio- and stereo-controlled way, a six-membered ring with up to four stereogenic centers. With the potential of forming carbon-carbon, carbon-heteroatom and heteroatom-heteroatom bonds, the reaction is a versatile synthetic tool for constructing simple and complex molecules [1], Scheme 1.1 illustrates two examples the synthesis of a small molecule such as the tricyclic compound 1 by intermolecular Diels-Alder reaction [2] and the construction of a complex compound, like 2, which is the key intermediate in the synthesis of (-)chlorothricolide 3, by a combination of an intermolecular and an intramolecular Diels-Alder cycloaddition [3]. 

The reaction with sulfides occurs efficiently only when the resulting carbon-centered radicals are further stabilized by a a-heteroatom. Indeed, (TMSfsSiH can induce the efficient radical chain monoreduction of 1,3-dithiolane, 1,3-dithiane, 1,3-oxathiolane, 1,3-oxathiolanone, and 1,3-thiazolidine derivatives. Three examples are outlined in Reaction (12). The reaction of benzothiazole sulfenamide with (TMS)3SiH, initiated by the decomposition of AIBN at 76 °C, is an efficient chain process producing the corresponding dialkylamine quantitatively. However, the mechanism of this chain reaction is complex as it is also an example of a degenerate-branched chain process. 

Polymeric forms have also been reported. One example, which also includes germanium heteroatoms terminating the chain, is the oligomer (RO)Ge(RO)2Co(RO)2Co(RO)2Ge(OR), (97) where each Co center is surrounded by four bridging tert-butoxide ions.416 These form via a photochemically induced labile solvent complex, or else through thermally induced substitution... 

The development of the chemistry of carbene complexes of the Group 8a metals, Ru, Os, and Ir, parallels chemistry realized initially with transition metals from Groups 6 and 7. The pioneering studies of E. O. Fischer and co-workers have led to the characterization of many hundreds of carbene complexes in which the heteroatoms N, O, and S are bonded to the carbene carbon atoms. The first carbene ligands coordinated to Ru, Os, and Ir centers also contained substituents based on these heteroatoms, and in this section the preparation and properties of N-, O-, S-, and Se-substituted carbene complexes of these metals are detailed. 

Cationic Fp (olefin) complexes [Fp = f/5-C5H5Fe(CO)2] undergo regio-specific addition of heteroatomic nucleophiles.32 Subsequent ligand transfer (carbonyl insertion) occurs with retention of configuration at the migrating center (R—Fe—CO -> RCOFe).33 A combination of these processes has provided a novel stereospecific azetidinone synthesis which can also be applied to condensed systems.34... 

Heterocyclic carbenes have been applied as ligands to gold(i) and gold(m) with a wide variety of heteroatoms and substitution patterns.256,257 Gold(i) forms 1 1 and 1 2 complexes with a linear coordination geometry, while at gold(m) centers, carbenes appear only in combination with other ligands in a square-planar array. 

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