Transient/reactive intermediates

Cyclobutadiene escaped chemical charactenzation for more than 100 years Despite numerous attempts all synthetic efforts met with failure It became apparent not only that cyclobutadiene was not aromatic but that it was exceedingly unstable Beginning m the 1950s a variety of novel techniques succeeded m generating cyclobutadiene as a transient reactive intermediate... 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

This chapter provides an introduction to several types of homogeneous (single-phase) reaction mechanisms and the rate laws which result from them. The concept of a reaction mechanism as a sequence of elementary processes involving both analytically detectable species (normal reactants and products) and transient reactive intermediates is introduced in Section 6.1.2. In constructing the rate laws, we use the fact that the elementary steps which make up the mechanism have individual rate laws predicted by the simple theories discussed in Chapter 6. The resulting rate law for an overall reaction often differs significantly from the type discussed in Chapters 3 and 4. 

This chapter will begin with a brief overview of the development of carbanion chemistry followed by a section devoted to the structure and stability of carbanions. Methods of measuring carbon acidity and systematic trends in carbanion stability will be key elements in this chapter. Next, processes in which carbanions appear as transient, reactive intermediates will be presented and typical carbanion mechanisms will be outlined. Finally, some new developments in the field will be described. Although the synthetic utility of carbanions will be alluded to many times in this chapter, specific uses of carbanion-like reagents in synthesis will not be explored. This topic is exceptionally broad and well beyond the scope of this chapter. 

From this chapter, it is clear that the common statement that carbenes only occur as transient reactive intermediates is no longer valid. 

Silaallenes 613 have been proposed by Ishikawa, Kumada and coworkers as transient reactive intermediates in the photolytic or thermolytic degradation of alkynyldi-silanes 614 as minor byproducts, the main product being silacyclopropenes 615184,192,287 (equation 203). 

Many reviews have been written recently on the subject of germenes and they have been the subject of numerous computational studies. In the absence of sterically bulky ligands, germenes rapidly dimerize or react with available substrates. For this reason, germenes are frequently studied as transient, reactive intermediates isolated in a matrix,or inferred from trapping reactions. 

One of the most relevant and fruitful areas of structural investigation for synthetic oiganic chemistry during the past decade has been the crystal structure determinations of a variety of enolate and closely related carbanions. Although these species have been considered only as transient reactive intermediates, a number of these enolates can be crystallized out of solution at subambient temperature and stabilized under a stream of cold, dry nitrogen gas during the 24-48 h necessary for X-ray diffraction data collection. A systematic review of these structures known to date begins with the ketone enolates. 

The excellent linear plot (Figure 2) obtained for the chemical oxygenation of 2-methyl-2-pentene demonstrates that a transient reactive intermediate is formed in this reaction and that the fraction of the intermediate which is trapped depends on acceptor concentration in the expected way. [From the intercept of this plot, it can be determined that the yield of 2, based on hypochlorite is 43% under these conditions. At lower temperatures, yields as high as 65% can be obtained repro-ducibly (8)]. 

Many organic, organometallic, and bioorganic reactions involve intermediates that are indeed reactive and transient. Reactive intermediates such as carbocations, radicals, carban-ions, and carbenes are common to organic and bioorganic transformations, whereas coordi-natively unsaturated transition metals, and low and high oxidation state metals are common to organometallic reactions (see Chapter 12). 

Cyclobutadiene escaped chemical characterization until the 1950s, when a variety of novel techniques succeeded in generating cyclobutadiene as a transient, reactive intermediate. 

Unlike most other chain reaction polymerizations involving low concentrations of transient, reactive intermediates, carbanions and organometallic species are often stable enough to be prepared and characterized independently of the polymerization process. 

A number of recent papers have also focussed on the identification of paramagnetic transient reactive intermediates in bio-inspired non-heme iron catalysed oxidations. Makhlynets and Rybak-Akimova investigated the mechanism of the substrate oxidation, using H2O2 as oxidant, in an iron aminopyridine complex for aromatic hydro qrlation (Scheme 1). EPR with stopped-flow kinetic measurements, was used to identify the formation of the key Fe" (OOH) intermediate, with rhombic g... 

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