Carbenes special

The vinylcyclopropane rearrangement is an important method for the construction of cyclopentenes. The direct 1,4-addition of a carbene to a 1,3-diene to give a cyclopentene works only in a few special cases and with poor yield. The desired product may instead be obtained by a sequence involving the 1,2-addition of a carbene to one carbon-carbon double bond of a 1,3-diene to give a vinylcyclopropane, and a subsequent rearrangement to yield a cyclopentene ... 

Wanzlick showed that the stability of carbenes is increased by a special substitution pattern of the disubstituted carbon atom [12-16]. Substituents in the vicinal position, which provide n-donor/a-acceptor character (Scheme 2, X), stabilize the lone pair by filling the p-orbital of the carbene carbon. The negative inductive effect reduces the electrophilicity and therefore also the reactivity of the singlet carbene. 

Aziridines can be prepared directly from double-bond compounds by photolysis or thermolysis of a mixture of the substrate and an azide. The reaction has been carried out with R = aryl, cyano, EtOOC, and RSO2, as well as other groups. The reaction can take place by at least two pathways. In one, the azide is converted to a nitrene, which adds to the double bond in a manner analogous to that of carbene addition (15-62). Reaction of NsONHC02Et/ CuO [Ns = A(/7-toluenesulfonyl-inimo)] and a conjugated ketone, for example, leads to the A-carboethoxy aziridine derivative.Calcium oxide has also been used to generate the nitrene.Other specialized reagents have also been used." ... 

REMP, the acronym for ring expansion metathesis polymerisation is a special case of ROMP, where the growing polymer chain stays attached to the catalyst at both ends nntil a macrocycle is released. This requires that the active carbene be tethered... 

From the reactions presented in this section one can conclude that cyclic acetal formation via addition to a carbene intermediate is a general reaction for type I cleavage of cyclobutanones, tricyclic compounds, and certain bridged bicyclics as minor products. No acetal has been isolated from photolyses of cyclopentanones or cyclohexanones except for the special case of an a-sila ketone previously discussed. 

Since the successful exploration of silver(i) oxide usage as a multifunctional precursor for the synthesis of silver(i) A-heterocyclic carbene complexes, there has been an increasing number of reports related to silver(i) A-heterocyclic carbene chemistry. Silver(i) oxide can act as a weak base to deprotonate imidazolium salts, generating the free A-heterocyclic carbene ligands in situ, which then forms the silver(i) carbene complexes readily. This reaction can take place in the presence of air and moisture, and as a result, no special treatment in regard to the solvents has to be undertaken. More importantly, its basicity is rather specific toward the deprotonation at the G2 position of the imidazole moiety. Exploration of using silver(i) carbonate as a milder precursor in place of silver(i) oxide has also been pursued, but longer reaction times are usually required. 

Under special conditions carbenes have been observed to dimerize to alkenes as illustrated in the synthesis of tetramesitylethylene (2.19)234). 

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

The 3ai and b orbitals, which are symmetric (a) or antisymmetric (n) with respect to the molecular plane, comprise the special carbene orbitals. 

Carbenes play important roles both as reactive intermediates and also as ligands consequently, considerable effort has been devoted to understanding their molecular and electronic structures. Special interest is associated with carbenes that feature the attachment of donor groups to the carbenic carbon since they behave as nucleophiles and, in some instances, can be isolated. Pioneering work on nucleo-... 

Finally, recall that AGST is ca lOkcalmoP1 for methylene. The corresponding value is smaller for BA. This can be viewed as the major consequence of the structural change. It will be a point of special interest to show how this parameter is affected in the other carbenes that are discussed. 

The Lewis-like picture suggests20 that saturated metal alkylidenes (carbenes) H M=CH2 of special stability should correspond to duodectet-rule-conforming... 

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

In C-NMR spectra, the signals for the carbene carbon are usually shifted upheld by about 20-30 ppm upon complexation of the free NHC to a transition metal. Cr-NMR data of [LCr(CO)s] complexes underline that NHC are a special case of carbene ligands because of their lack of tt-acceptor ability. Photoreactions of metal complexes containing NHCs by laser flash and continuous photolysis show that NHCs are quite inert ligands in photolysis reactions. He I and He II photoelectron spectra of platinum(O)- and palladium(O) bis(imidazolin-2-ylidene)... 

Recent research on aminocarbenes has led to the development of a very fruitful field. The synthesis of relevant complexes (Scheme 19) such as aminobis(yhde) carbene species (69) [147], cyclic C-amino P-ylides (70) (easily transformed into carbenes) [148] and their corresponding complexes (71) [149], and special ylides (72), which also transform very easily into carbenes by loss of pyridinium group, has been reported. Emphasis has been made on the transformation between ylides and carbenes and on the donor properties of the ylides. From the results obtained the ylides have shown a stronger a-donor behavior compared with the carbenes. 

Carbodiphosphoranes C(PR3)2 and related ligands CL2 which are also termed double ylides differ from the other carbon ligands which are discussed in this volume in the number of lone electron pairs at the carbon donor atom. Ylides, carbenes, allenylidenes, and cumulenylidenes have one lone electron pair but carbodiphosphoranes have two lone electron pairs with a and n symmetry. The bonding situation in a carbodiphosphorane (a special class of CL2 compounds, abbreviated as CDP) is best described in terms of donor-acceptor bonding between the phosphane ligands which serve as a donor and a naked carbon atom in an... 

Alternatively, NHC ligands can also be transferred from triethylborane-carbene adducts [103] or complexes of the type [M(NHC)(CO)5] (M = Cr, Mo, W) [104], but these procedures are limited to special cases and are of less importance. The coordination chemistry of silver NHC complexes [105] and the advantages and applications of the Ag20 method [106] have recently been reviewed. 

With the exception of thermodynamically stabilized [64] or sterically protected [65] carbenes, these species and their hetero-analogs, nitrenes, are very reactive and therefore special conditions are required for their direct observation. Fast spectroscopic techniques capable of characterizing species with lifetimes of a few picoseconds have been used [1-3]. More recently, time-resolved IR (TRIR) experiments have been used to characterize species with lifetimes of microseconds and even nanoseconds [4-6]. 

Here again, all these reactions have been widely reviewed and just general trends will be discussed here, with a special focus on the mechanisms. These mechanisms can involve a carbenoid rearrangement or a free carbene rearrangement. The question of the carbenoid or carbene intermediacy has been largely addressed and, in most cases. 

Intramolecular carbene addition reactions have a special importance in the synthesis of strained ring compounds. Because of the high reactivity of carbene or carbenoid species, the formation of highly strained bonds is possible. The strategy for synthesis is to construct a potential carbene precursor, such as diazo compounds or di- or trihalo compounds, which can undergo intramolecular addition to the desired structure. Section E of Scheme 10.5 gives some representative examples. 

A. J. L. Pombeiro, J. Organometal. Chem. 2005, in press (special issue on Carbene Chemistry. 

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