The structure and reactivity of aromatic carbenes

Genuine exposition of the chemical properties of an aromatic carbene comes from the fusion of all of the types of experiment described above. The significance of each type becomes clear when considered within the context of the whole array of theoretical and experimental findings. The chemical properties of a particular carbene, in turn, become categorizable only with respect to other related examples. Finally, a pattern connecting structure to reactivity emerges when an entire host of clearly understood cases are compared. A pattern of structure and reactivity for the carbenes listed in Table 1 will be developed. The analysis begins at one extreme with BA, and then jumps, for contrast, to another extreme with an account of XA. With the boundaries defined, the other examples fall clearly into place. 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet ( BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

Irradiation of a benzene solution of DABA at room temperature with a nitrogen laser (Horn and Schuster, 1982) gives the transient absorption spectrum shown in Fig. 3. This spectrum was recorded 50 ns after irradiation of the diazo-compound and decays over a period of ca 250 (is by a path exhibiting complex kinetic behavior. This transient spectrum is essentially identical with the low temperature optical spectrum described above, and thus is similarly assigned to BA. 

Irradiation of DABA at room temperature with a mode-locked frequency 

Examination of DABA photolysis in cyclohexane instead of benzene solution leads to predictably different results. The laser spectroscopy shows that BA is formed but, in this solvent, the triplet carbene undergoes an additional rapid reaction to generate the mesitylbora-anthryl radical (BAH ). This radical is identified by comparison of its spectrum with that of an authentic sample prepared from dihydrobora-anthracene. The half-life of 

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In Chapter 11 we will discuss the structure and reactivity of carbenes. These are traditionally extremely unstable structures, where carbon only has six electrons. However, there are cases of stable carbenes, typically possessing resonance structures with stabilizing features such as zwitter-ionic and aromatic character. For example, for moderately large R, carbene A can be isolated and does not dimerize to a tetraaminoethylene derivative. Yet, carbene B dimerizes irreversibly, presumably due to the lack of additional aromatic stability. 

The results from a series of early studies of the products formed from irradiation of DAX appeared to show that XA does not react with hydrocarbons or simple olefins (Reverdy, 1976a,b,c.). However, later reports (G. W. Jones et al., 1978, 1979) seemed to contradict many of these claims. Our more recent investigation of XA resolves the apparently conflicting results and provides further information on the forces that relate structure to reactivity for aromatic carbenes (Lapin and Schuster, 1985). 

The acid-catalysed and thermally induced rearrangements of cyclohexadienones have been surveyed recently, and an attempt was made to reduce the plethora of apparent sigmatropic shifts to a smaller number of independent reactions in these blocked aromatic molecules . The electronic structure and reactivity of cyclohexa-dienone carbenes has also received coverage in a review. 

The diazo-compounds and corresponding aromatic carbenes that form the basis for our dissection of structure and reactivity are shown in Table 1. The carbenes in this group are carefully chosen so that the variation in structure is systematic the theory identifies the carbene bond angle and certain electronic factors as controlling chemical and physical properties, and as far as possible, these two features are varied independently of each other for these carbenes. Table 2 lists some other aromatic carbenes that have been studied. In general, the structures of these carbenes are not simply related to each other. Nevertheless, the principles uncovered by analysis of the compounds of Table 1 can be readily extended to those of Table 2. 

The subject of this chapter is carbenes with aryl substituents (aromatic carbenes). These materials are short-lived reactive intermediates in which the normal tetravalency of carbon is reduced by two. Carbenes have been the object of speculation and investigation for more than 80 years. Nevertheless, there still is considerable uncertainty about their chemical and physical properties. In the last five years the pace of research in carbene chemistry has quickened. This is a consequence of the development of high-speed pulsed lasers that permit, for the first time, direct observation of carbenes under the conditions in which they react. This research has provided new information on the effect of structure on the chemical and physical properties of carbenes. 

There are many other kinds of reactive intermediates, which do not fit into the previous classifications. Some are simply compounds that are unstable for various possible reasons, such as structural strain or an unusual oxidation state, and are discussed in Chapter 7. This book is concerned with the chemistry of carbocations, carbanions, radicals, carbenes, nitrenes (the nitrogen analogs of carbenes), and miscellaneous intermediates such as arynes, ortho-quinone methides, zwitterions and dipoles, anti-aromatic systems, and tetrahedral intermediates. This is not the place to describe in detail the experimental basis on which the involvement of reactive intermediates in specific reactions has been estabhshed but it is appropriate to mention briefly the sort of evidence that has been found useful in this respect. Transition states have no real hfetime, and there are no physical techniques by which they can be directly characterized. Probably one of the most direct ways in which reactive intermediates can be inferred in a particular reaction is by a kinetic study. Trapping the intermediate with an appropriate reagent can also be very valuable, particularly if it can be shown that the same products are produced in the same ratios when the same postulated intermediate is formed from different precursors. 

Structurally related species that exhibit C-H activation include the bis(pyrazolyl) borate complex in Scheme 21, for which (as in the above dmdpb system) protonation (or methide abstraction) generates an intermediate that reacts readily with benzene [74] the bis(azaindolyl)methane complex in Scheme 22, which activates both aromatic and benzylic C-H bonds [75,76] (some stable Pt(TV) complexes based on the same architectiRe have also been isolated and shown to undergo reductive elimination of MeX [77]) and complexes based on anionic bidentate ligands such as 2-(2 -pyridyl)indolide [78]. Intramolecular C-H activation was observed for one example of a series of A -heterocyclic carbene complexes of Pt(II) distortions induced by steric crowding appear to influence reactivity strongly [79]. 

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