Carbenes diaryl

To conclude the details on zero-valent nickel carboxylation catalysts, some recent synthetic approaches worthy of note showed that this area of research still has a rich chemistry. For example, Louie and coworkers reported on the use of N-heterocyclic carbenes (diaryl-imidazolylidene) as new efficient ligands in the Ni-catalyzed coupling of various symmetrical di-ynes with C02 (Scheme 5.19) [60a]. 

In 2003, Sigman et al. reported the use of a chiral carbene ligand in conjunction with the chiral base (-)-sparteine in the palladium(II) catalyzed oxidative kinetic resolution of secondary alcohols [26]. The dimeric palladium complexes 51a-b used in this reaction were obtained in two steps from N,N -diaryl chiral imidazolinium salts derived from (S, S) or (R,R) diphenylethane diamine (Scheme 28). The carbenes were generated by deprotonation of the salts with t-BuOK in THF and reacted in situ with dimeric palladium al-lyl chloride. The intermediate NHC - Pd(allyl)Cl complexes 52 are air-stable and were isolated in 92-95% yield after silica gel chromatography. Two diaster corners in a ratio of approximately 2 1 are present in solution (CDCI3). 

The carbene moieties of methyl diazoacetate 353), dimethyl diazomalonate 3S3) and diazomethane 3541 have been inserted into the Se—Se bond of diaryl diselenides. 

The absolute rate constants for oxygen and sulfur transfer to a range of carbenes (dialkyl, cycloalkylidene, alkylchloro, diaryl, arylchloro, arylalkoxy, and dialkoxy), generated by laser flash photolysis of diazirine or oxadiazoline precursors, were determined. No evidence was seen for ylide formation and a concerted mechanism via an ylide-like transition state was proposed. 

Calculations predict that the lowest state of PN has an open-shell electronic configuration." " The Salem-Rowland Rule for ISC promoted by spin-orbit coupling (SOC) predicts that singlet to triplet relaxation will have its maximum rate when the singlet state is closed-shell. This is the case with diaryl carbenes where the absolute rate constants of ISC are in the order of Michl has recently pointed out the importance of donor-... 

The 5,7-diaryl-2-fluoro-4/7-l,3-diazepines have been synthesized from 3-aryl-substituted 277-azirines and difluoro-carbene (Scheme 13). The reaction involves isomerization of azirinium ylide into a 2-aza-l,3-diene, which undergoes [4+2] cycloaddition with the starting azirine, followed by ring expansion and dehydrofluorination <2006TL639>. 

Nolan reported the use of the 2,6-diisopropylphenyl imidazolium carbene precursor, which contains an unsaturated backbone, for the reaction of aryl chlorides with a variety of amines at 100 °C [165, 166]. This temperature is lower than those conventionally used for reactions of aryl chlorides, but is higher than those used with P(tBu)3 or the 2-biphenylyl di-tert-butylphosphines. Reaction yields were high when 2 mol % palladium was used. Reactions of primary amines occurred in good yield, even when unhindered aryl halides were used. The monoarylamine was obtained in 86 % yield, and only a 5 % yield of the diaryl-amine by-product was isolated. Notably, reactions of both aryl bromides and iodides proceeded at room temperature. 

Although mostly alkoxy carbene complexes have been benzannulated, other types of carbene complexes are equally well suited. These include aryloxy carbene complexes as well as acyla-mino and thioalkylidene complexes, and even complexes with no heteroatoms, such as diaryl carbene complexes, are suitable (see below). Besides the commonly used methoxy and ethoxy carbene complexes, alkoxy carbene complexes with a longer alkyl chain have also been successfully reacted. The benzannulation of aryloxy carbene complexes has recently been studied to probe electronic effects [28a]. Aryloxy alkylidene complexes of type 21 have been used to prepare diaryl ethers 22, which constitute a common substructure in many important types of natural products [28b]. The benzannulation methodology provides an access to phenyl naphthyl ethers in yields of 60-93 % under mild conditions (Scheme 11). 

Scheme 11. Benzannulation of aryloxy carbene complexes affording diaryl ethers.

Representatives of Fischer carbene complexes possessing no heteroatom at the carbene carbon are diaryl chromium carbene complexes. Due to the lack of -stabilization, they react more readily than the heteroatom-functionalized carbene complexes (already at ambient... 

Scheme 13. Benzannulation of diaryl chromium carbene complexes.

P. Schwab, N. Mahr, J. Wolf, and H. Werner, Carbenerhodium Complexes with Diaryl-and Aryl(alkyl)carbenes as Ligands The Missing Link in the Series of the Double Bond Systems trans-[RhCl =C(=C), RR (L)2] Where n = 0, 1, and 2, Angew. Chem. Int. Ed. Engl. 32, 1480-1482 (1993). 

Polycyclic aromatic compounds are obtained upon irradiation of diaryl-substitnted chromium carbenes under a carbon monoxide atmosphere. The reaction probably proceeds via the formation of a biaryl-aUcoxy ketene followed by cyclization. For example, the pyrrole-substituted carbene (22) gave the tricyclic compound (23) (Scheme 34) Related intramolecular Friedel-Crafts-type reactions of carbenes having tethered electron-rich aromatic rings are feasible, usually in moderate yields (Scheme 35). A Lewis acid catalyst such as zinc dichloride is required for optimum yields. 

Pentavalent silicon adducts (78,79) can be isolated from the reaction of diaUcyl- or diaryl-dichlorosilanes with (1) (R = Et, i-Pr R = Me), whereas when Me3SiI is reacted with(l), ionic imidazolium salts are formed (77). Unfortunately, no information regarding the structure of these compounds is available. A crystalline carbene-silylene adduct 1,2-C6H4[N(R)]2C-Si[N(R)]2C6H4-l,2 (80, R = CH2-t-Bu) has... 

Arylcyclopropanes can also be obtained in reasonable yields by heating mixtures of aryl triphenylphosphoranylidenehydrazones 9 and chalcones 10 in a solvent (usually dibutyl ether).Presumably, the reaction does not involve a free carbene instead dipolar addition of the in situ generated diazo compound to the C-C double bond of the enone takes place. The resulting 4,5-dihydro-3/f-pyrazole is thermally unstable and extrudes nitrogen, thus affording 2,3-diaryl-l-benzoylcyclopropanes 11. °... 

Bis(a-chloroarylidene)hydrazines or 1,2-diaryl-1,2-dichloroethenes [a formal dimer of aryl(chloro)carbene] were sometimes observed as side products when 3-aryl-3-chloro-3/7-di-azirines were decomposed in alkenes. 

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