Diaminocarbenes cyclic

NHCs possess a lone pair of electrons and an accessible vacant orbital. This combination resembles the situation in many transition metal centers and could thus mimic their chemical behavior. While cyclic diaminocarbenes are inert towards hydrogen, it has been demonstrated that cyclic (aIkyl)(amino)carbenes of type 16... 

Much less coordination chemistry of acyclic diaminocarbenes 4 as compared to that of cyclic ones has been described (Scheme 6). Interestingly, in the case of... 

Within the last decade many variations of the basic imidazole-2-ylidene structure (Scheme 2,A) have been synthesized [14-19]. They are not limited to sterically hindered unsaturated cyclic diaminocarbenes like 1, also 1,2,4-triazolin-5-ylidenes (Scheme 2,B), saturated imidazolidin-2-ylidenes [6,7,20] (Scheme 2,C), tetrahydropyrimid-2-yhdenes [21,22] (Scheme 2,D), acychc structures [23,24] (Scheme 2,E), systems with larger ring sizes [25,26] (Scheme 2,F) or constrained geometry [27,28] (Scheme 2,G). Reviews on the different possible synthetic routes from various precursors can be found in the literature [29-31]. 

Recently, spectacular achievements have been reported in the catalysis arena using stable cyclic diaminocarbenes III, IV, and VII, now called V-heterocyclic carbenes... 

Summary Bis(trimethylsilyl)mercury cleanly reacts at low temperature with chloroiminium chlorides to form stable metal-free cyclic and acyclic diaminocarbenes as well as aryl-, oxy-, chloro-, hydrogeno- and alkyl-amino carbenes. The aryl-, chloro-and hydrogeno-amino carbenes were formed as transient intermediates that undergo dimeri2ation into the corresponding alkenes, which were isolated in good yields. Alkyl-and oxy-amino carbenes were observed at low temperature by C NMR spectroscopy. 

The template-controlled generation of ylidene ligands from p-functionalized isocyanides constitutes an alternative access to complexes with cyclic diaminocarbenes. It proceeds via the complexes with NH,NH-stabilized ylidene ligands which are easily alkylated at the heteroatoms. Such complexes are valuable and reactive intermediates for the generation of complexes with macrocyclic ligands possessing NHC donors (see Sect. 4). 

When compared to nitrogen, phosphorus is much more reluctant to achieve a planar configuration with sp hybridization. [31,32] The ensuing smaller stabilizing effect of phosphorus compared to nitrogen is illustrated by the small singlet-triplet gap predicted for the (phosphino)(silyl)earbenes (5.6-13.9 kcaPmol) [29] compared to that calculated for acyclic as well as cyclic diaminocarbenes (58.5-84.5 kcal/mol). [33] This means that the commitment of the lone pair to donation into the vacant orbital on the divalent carbon atom is less definitive for phosphorus than for nitrogen and thus, the phosphinocarbenes retain more of a divalent-carbon behavior. This is well illustrated by their reactivity. 

Acyclic diaminocarbenes (ADCs) as a promising alternative to N-hetero-cyclic carbenes (NHCs) in transition metal catalyzed organic transformations 12CCR2029. 

Bertrand and coworkers have reviewed the topic of stable carbenes based on scaffolds other than diaminocarbenes [47]. Diphosphinocarbenes, aminophosphi-nocarbenes, cyclic (alkyl)amino carbenes, cyclic (amino)ylidic carbenes, cycloprope-nylidenes, cyclic bent allenes, carbodiphosphoranes, and vinylidenephosphoranes have all been reported (examples 37-44, respectively, in Figure 1.24a). Use of the [RhCl(CO)2(L)j system for measuring TEPs allows the electron-donating ability of these systems to be ranked and compared to those of NHCs and abnormally bound NHCs (Figure 1.24b). 

Cyclic (amino). Abnon l (ylidic)carbenes diaminocarbenes... 

This Chapter is devoted to the development of cobalt-, rhodium- and iridium-based catalysts that contain N-heterocyclic carbene (NHC) ligands. It will cover their most relevant catalytic applications, along with stoichiometric model reactions, except for catalytic oxidations and reductions such as hydrogenations, hydrosilylations and hydroborations, which are treated in detail in Chapters 12 and 13. Since the NHC chemistry of Group 9 metals is one of the most developed areas in this field, this overview will only cover the chemistry of classical NHCs, namely cyclic diaminocarbenes. Chapter 5 reviews the chemistry of the non-classical NHCs, to which the reader is referred. Finally, this Chapter does not pretend to be exhaustive and further details may be found in previous overviews. ... 

For stable cyclic carbenes and related species beyond diaminocarbenes, see M. Melaimi, M. Soleihavoup and G. Bertrand, Awgem Chem., Int. Ed., 2010,49,8810-8849. 

Calculations have been carried out in order to assess the reactivity of cyclic and acyclic diaminocarbenes with CO2 in the presence of alcohols These calculations indicate that the formation of carbonate salts is favoured compared to the direct interaction with CO2. Because of their high values, these carbenes represent efficient systems for the capture and sequestration of carbon. Computations on the stereoselective transformation of 2-benzyloxyphenyl trimethylsilylketone to substituted benzofuran (Scheme 1) have been carried out. They revealed an exergonic 1,2-Brook rearrangement to an oxycar-bene, exhibiting two unpaired antiparallel electrons that feature in H migration followed by diradical coupling. 

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