Formation of Carbon-Heteroatom Bonds

vinyl and aryl radicals can give homolytic substitution at a heteroatom either inter- or intramolecularly thus forming C-heteroatom bonds. 

A new entry for the synthesis of vinyl- and arylphosphonates has been achieved by reaction of vinyl or aryl halides with trialkyl phosphites in the presence of (TMS)3SiH under standard radical conditions [72]. An example of this intermolecular C—P bond formation is given in Reaction (7.62). Interestingly, the reaction works well either under UV irradiation at room temperature or in refluxing toluene. 

Reaction (7.63) shows an example of C—S bond formation [73,74]. In fact, the aryl radical formed by iodine abstraction by (TMS)3Si radical rearranged by substitution to the sulfur atom, with expulsion of the acyl radical and concomitant formation of dihydrobenzothiophene (60). This procedure 

An example of C—Si bond formation concludes this overview of carbon heteroatom bond formation. Reflux of bromide 62 in benzene and in the presence of small amounts of (TMS)3SiH and AIBN afforded the silabicycle 63 in 88 % yield (Reaction 7.64) [76]. The key step for this transformation is the intramolecular homolytic substitution at the central silicon atom, which occurred with a rate constant of 2.4 x 10 s at 80 °C (see also Section 6.4). The reaction has also been extended to the analogous vinyl bromide (Reaction 7.65) [49]. 

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Functionalization of pyridines via formation of carbon-heteroatom bond with elements of groups IV, V, and VI 99KGS437. 

Our review of the use of organoboron compounds in radical chemistry will concentrate on applications where the organoborane is used as an initiator, as a direct source of carbon-centered radicals, as a chain transfer reagent and finally as a radical reducing agent. The simple formation of carbon-heteroatom bonds via a radical process is not treated in this review since it has been treated in previous review articles [3,9]. 

When reductive elimination from a late transition metal involves the formation of a carbon-carbon bond, the process is intramolecular and the groups have to be aligned cis to one another in the complex. In the formation of carbon-heteroatom bonds the reductive elimination from palladium might take place via competing pathways.16... 

Examples of the transition metal catalyzed formation of carbon-heteroatom bonds other than carbon-nitrogen are less abundant. In a recent survey of the copper catalyzed carbon-oxygen bond formation between alcohols and organotrifluroborates the coupling of 3-thienyltrifluoroborate and 2-furfuryl alcohol gave the expected thienyl-furfuryl-ether in good yield (6.83.),113... 

Substitutions such as alkylation (Chapter 5) and oxygenation (Chapter 9) are fundamental transformations essential to the chemistry of hydrocarbons. Other heterosubstitutions (i.e., formation of carbon-heteroatom bonds), such as halogenation, nitration, or sulfuration (sulfonation), are also widely used reactions. It is outside the aim of our book to discuss comprehensively the wide variety of substitution reactions (for a scope, see, e.g., March s Advanced Organic Chemistry), but it is considered useful to briefly review some of the most typical selected heterosubstitutions of hydrocarbons. 

The formation of carbon-heteroatom bonds can be effected by reactions of hypervalent iodine reagents with a wide range of organic substrates and inorganic nucleophiles, and represents one of the most popular applications of organoiodine(III) compounds [1-10]. Except for C-I(III) bond forming reactions used for the synthesis of iodanes and iodonium salts, C-heteroatom bond formation is almost always accompanied by reduction of the hypervalent iodine reagents to iodine(I) compounds. 

F. W. Stacey, J. F. Harris, Jr, Formation of Carbon-Heteroatom Bonds by Free Radical Chain Additions to Carbon-Carbon Multiple Bonds, Org. React. 1963,13, 150-376. 

Asymmetric synthesis of heterocycles from olefins via cyclization with the formation of carbon-heteroatom bonds 84MI1. 

Reactions involving the formation of carbon heteroatom bonds include the industrially best known photochemical reactions. In fact, chlorination, bromination and sulfochlorination are major processes in industrial chemistry, and oxygenation has likewise an important role. Due to the focus on fine chemistry of this chapter, the discussion below is limited to laboratory-scale preparations and in particular to some bromination and oxygenation reactions illustrating the advantage of the photochemical approach, as well as to some alkoxylation, hydroxylation and amination reactions. 

Catalytic synthesis of heterocyclic compounds via formation of carbon—heteroatom bond using aryl and... 

In his Nobel lecture, Suzuki has summarized work on the use of organoboranes in the formation of carbon-carbon bonds.A wide-ranging review has been published covering the transmetalation reactions involving the use of organoboron compounds in the formation of metal-carbon bonds.There has also been a review of the use of copper-promoted reactions of arylboronic acids in the formation of carbon-heteroatom bonds. 

Oxidative Dearomatization with Formation of Carbon-Heteroatom Bonds... 

The photocatalytic generation of aryl radicals was also successfully applied to the formation of carbon heteroatom bonds. The aryl pinacolboronates 141 can be easily achieved by visible light irradiation of a solution of aryl diazonium salts 142 and diboron pinacol ester 143 containing 5mol% of eosin (Scheme 29.24) [89]. The proposed meehanism involves the addition of aryl radical 144 to the complex 145 that is generated by interaction of tetrafluoroborate anion and diboran pinacol ester 143. This process leads to the formation of the aryl pinacolboronates 141 and the radical anion intermediate 146. The oxidation of this intermediate by eosin radical cation completes the catalytic cycle. 

The formation of carbon—heteroatom bonds is generally more straightforward than the formation of carbon-carbon bonds because of the difference in electronegativity between carbon and heteroatom. This concept carries over to electrocyclic processes as well, but with a caveat Because of the difference in electronegativity, polar mechanisms may compete with the electrocycUzation. The mechanistic complexity of these systems notwithstanding, there are obvious applications of such reactions for the synthesis of heterocycles. Muller and List s results are shown in... 

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