Carbenes—continued

Carbenes (Continued) substituents on, 115 Carbocations, 105-108 acids and electrophiles, 97 dialkoxy, 105 ethyl, 84 fluoro, 105 norbornyl, 84 reactions, 107 substituents on, 106... 

Despite the unpromising UV-visible spectra and flash photolysis studies, the carbene complexes presented in this chapter have a rich photochemistry at wavelengths exceeding 300 nm. A wide range of synthetically useful transformations has been developed, and continued studies are likely to reveal more. 

The fact that only ethylene and tetramethylethylene are evolved from exp-[8]rotane 168 and permethyl-exp-[6]rotane 173 upon thermal decomposition leads to the conclusion that the spirocyclopropane moieties in these expanded [n]rotanes fragment only externally and leave carbene moieties behind. Indeed, the MALDI-TOF mass spectra of several exp-[ ]rotanes show fragment ions with M minus 28. Thus, if this fragmentation in an exp-[n]rotane were to continue n times, a cyclic C carbon cluster would be left over. So far, however, a fragment ion with m/z = 480 corresponding to 182 has not been recorded in the mass spectrum of exp-[8]rotane 168 and it remains to be seen whether a Cgo cluster 183 will be detected in the mass spectrum of exp-[12]rotane 171 (Scheme 35). 

Fragmentation of 29 or 29 accounts for r-butylethene formation during the photolysis of 29. In thermolytic experiments, however, the yield of this olefin decreases as TME is added and carbene 20 is trapped. The inference is that fragmentation during thermolysis at 100°C is, at least partly, a reaction of carbene 20,46 with continued contribution from the fragmentation of 29. ... 

Our understanding of carbene chemistry has advanced dramatically, especially in the last two decades, and new developments continue to emerge. Some of the recent exciting findings have been collected in the hrst and second volumes of Advances in Carbene Chemistry. With the third volume, the series will continue to provide a periodic coverage of carbene chemistry in its broadest sense. 

Quite evidently, changing the structure of the aromatic carbene from BA to XA has a profound effect on AGST. There is a difference of more than lOkcalmol-1 in this physical property for these two structures. This difference, in turn, appears to control and determine the chemical properties of the carbenes. With the assumptions outlined earlier, this effect can be wholly attributed to a perturbation of the electronic character engendered by replacement of the boron in BA with the oxygen of XA. It will be seen shortly that these two carbenes represent extremes of a nearly continuously tunable range of carbene properties. 

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

Synthetic Applications of Heteroatom-Substituted Carbene Complexes 61 Table 2.19. continued. 

Synthetic Applications of Non-Heteroatom-Suhstituted Carbene Complexes 153 Table 3.18. continued. 

Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes 159 Table 3.19. continued. 

Synthetic Applications of Acceptor-Substituted Carbene Complexes 185 Table 4.5. continued. 

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