Carbenes—continued structure

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. 

Carbene delivery in catalytic reactions remains a challenge. Although diazocarbonyl compounds are relatively safe, and numerous commercial processes have used and continue to employ these materials, methods for diazo transfer using azides are of concern, and cost-effective alternatives are not evident. Also elusive are structures that could deliver stabilized carbenes, not unlike those of Fischer carbenes, in catalytic processes. 

In a logical continuation of this work, carbene addition to an iron-iron double bond has also been exploited for the simple synthesis of the first /. -methylene complex in the nitrosyl series. The readily available /x-nitrosyliron complex [(Tj5-C5H5)Fe(/u.-NO)]2 (26) exhibits the same structural features as the rhodium dimer 21 (157) and reacts with diazomethane in the temperature range -80-25°C to give the expected /z-methylene derivative 27 (Scheme 14) as a black, air-stable compound in... 

Rh(II) carboxylates, especially Rh2(OAc)4> have emerged as the most generally effective catalysts for metal carbene transformations [7-10] and thus interest continues in the design and development of dirhodium(II) complexes that possess chiral51igands. They are structurally well-defined, with D2h symmetry [51] and axial coordination sites at which carbene formation occurs in reactions with diazo compounds. With chiral dirhodium(II) carboxylates the asymmetric center is located relatively far from the carbene center in the metal carbene intermediate. The first of these to be reported with applications to cyclopropanation reactions was developed by Brunner [52], who prepared 13 chiral dirhodium(II) tetrakis(car-boxylate) derivatives (16) from enantiomerically pure carboxylic acids RlR2R3CC OOH with substituents that were varied from H, Me, and Ph to OH, NHAc, and CF3. However, reactions performed between ethyl diazoacetate and styrene yielded cyclopropane products whose enantiopurities were less than 12% ee, a situation analogous to that encountered by Nozaki [2] in the first applications of chiral Schiff base-Cu(II) catalysts. 

His proposal involved a metal carbene and a metallocyclobutane intermediate and was the first proposed mechanism consistent with all experimental observations to date. Later, Grubbs and coworkers performed spectroscopic studies on reaction intermediates and confirmed the presence of the proposed metal carbene. These results, along with the isolation of various metal alkyli-dene complexes from reaction mixtures eventually led to the development of well-defined metal carbene-containing catalysts of tungsten and molybdenum [23-25] (Fig. 2). After decades of research on olefin metathesis polymerization, polymer chemists started to use these well-defined catalysts to create novel polymer structures, while the application of metathesis in small molecule chemistry was just beginning. These advances in the understanding of metathesis continued, but low catalyst stability greatly hindered extensive use of the reaction. 

The maturing field of cyclic carbene chemistry has continued to generate surprises, as new carbenes containing imusual structural and electronic features continue to be devised. The renewal of interest in acydic carbenes is also leading... 

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