Reactions Involving Heteroatoms

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)]. 

Intramolecular anodic olefin coupKng reactions involving heteroatomic aromatic rings were used to prepare fused, bicyclic ring skeletons (Scheme 122) [159]. 

Because many of the reactions involving heteroatom-substituted carbene complexes are cascade processes involving several different reaction types, they are... 

Allylic reactions (Continued) Substitution reactions involving heteroatoms... 

In the following, a general formulation of the X model is given suitable for treating chemical reactions involving heteroatoms. This generalization is based upon the following assumptions ... 

The arylation of nucleophiles by reaction with diaryliodonium salts can be greatly facilitated by copper catalysis. This effect was observed by Beringer et al in the thermal decomposition of diaryliodonium halides as well as by Caserio et al in the hydrolysis of diaryliodonium salts. 2 jhe thermal decomposition of diphenyliodonium chloride shows a reduced activation energy upon copper catalysis Ea = 19 kcal/mole in Methylene glycol in the presence of CuCl instead of 31 kcal/mole in the absence of catalyst.From the synthetic point of view, the copper-catalysed arylation with diaryliodonium salts has been particularly useful in the case of a number of reactions involving heteroatomic nucleophiles, in particular for 5-, 5e-, O- and N-arylation reactions. 

The most important of the concerted cycloaddition reactions is the Diels-Alder reaction between a diene and an alkene derivative to form a cyclohexene. The alkene reactant usually has a substiment and is called the dienophile. We discuss this reaction in detail in Section 10.2. Another important type of [2+4] cycloaddition is 1,3-dipolar cycloaddition. These reactions involve heteroatomic systems that have four rr electrons and are electronically analogous to the allyl or propargyl anions. 

Domino Reactions Involving Heteroatom-Directed C-H Activations... 

Transition-metal-catalyzed (Pd, Rh, and Ru) heteroatom-directed C-H activations have been successfully employed in domino strategies for constructing multiple carbon-carbon/carbon-heteroatom bonds. This area has already been extensively reviewed [46]. Selected recent examples of domino reactions involving heteroatom-directed C-H activations are chosen in this section to highlight this concept. 

In general many of the features which govern the cyclization of carbon radicals are met with again in reactions involving heteroatomic radicals. But it is also true that the nature of Z may introduce subtle, or dramatic, changes in I I... 

Displacement reactions involving a heteroatom and a carbon atom are very common in the synthesis of heterocyclics of many sizes. Most often, the heteroatom functions as a... 

The Diels-Alder reaction is of wide scope. Not all the atoms involved in ring formation have to be carbon atoms the hetero-Diels-Alder reaction involving one or more heteroatom centers can be used for the synthesis of six-membered heterocycles. The reverse of the Diels-Alder reaction—the retro-Diels-Alder reaction —also is of interest as a synthetic method. Moreover and most importantly the usefulness of the Diels-Alder reaction is based on its 5y -stereospecifi-city, with respect to the dienophile as well as the diene, and its predictable regio-and c ifo-selectivities. °... 

Furthermore, the strongly metallic character of selenium weakens the C-Se bond and thus favors reactions involving opening of the ring. The basicity of the three heterocycles is approximately in the same order, the nitrogen atom of selenazole and thiazole possessing much the same properties as the heteroatom of pyridine. Of the two carbon atoms ortho to nitrogen, that is, the 2-carbon and the 4-carbon, only the one in the 2-position is fairly active as a result of its interaction with selenium or sulfur. The 4- and 5-positions of thiazole and selenazole are more susceptible to electrophilic substitution than the 3- and 5-positions of pyridine. This is particularly true of the 5-position of selenazole. Thus it can be said that the 2- and 5-positions of the selenazoles and thiazoles... 

Catalysts such as Fe(BuEtCHC02)3 have been d eloped that are effective for the heteroatom Diels-Alder reaction. Indium trichloride (InCls) is a good catalyst for imino-Diels-Alder reactions. Hetero-Diels-Alder reactions involving carbonyls have been done in water. Ultrasound has been used to promote the Diels-Alder reactions of 1-azadienes. ... 

In summary, we may add that bacterial utilization of quinoline and its derivatives as a rule depends on the availability of traces of molybdate in the culture medium [363], In contrast, growth of the bacterial strains on the first intermediate of each catabolic pathway, namely, the lH-2-oxo or 1 II-4-oxo derivatives of the quinoline compound was not affected by the availability of molybdate. This observation indicated a possible role of the trace element molybdenum in the initial hydroxylation at C2. In enzymes, Mo occurs as part of the redox-active co-factor, and all the Mo-enzymes involved in N-heteroatomic compound metabolism, contain a pterin Mo co-factor. The catalyzed reaction involves the transfer of an oxygen atom to or from a substrate molecule in a two-electron redox reaction. The oxygen is supplied by the aqueous solvent. Certainly, the Mo-enzymes play an important role in the initial steps of N-containing heterocycles degradation. 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

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