Carbenes, heterocycles from

Presumably, 9 is actually formed from carbene 8 in the pyrolysis zone by a P/C phenyl shift, but then apparently succumbs to fast transformation into the thermodynamically stable final products. Formation of the methane derivative 13 should be preceded by a 1,2-phenyl shift to give the shortlived 10, the production of fluorene (14) by the occurrence of diphenylcarbene (II), and the formation of benzophenone (15) by isomerization to the angle-strained three-membered heterocycle 12, which is followed by elimination of phenylphospbinidene. No direct evidence is available for the intermediacy of 10-12. 

Stable free carbenes derived from four- (13), six-, or seven-membered heterocycles 14 are also known. Carbenes of type 13 can possess a phosphorus [19] or a boron atom [69] within the heterocycle. These carbenes exhibit extreme values regarding their NMR spectroscopic ( carbene-c 285 and 312.6 ppm) and structural properties (angle N-Ccarbene-N 96.72(13) and 94.0(2)°). 

Su and Chu <1999CPL(308)283> have also carried out B3LYP/6-31G level calculations for 1,3-diazetidine, especially for carbene generation from this heterocycle. The carbene generated from 1,3-diazetidine is a 4k electron system. The relative energies of carbene 21 are summarized. 

Ketenimine complexes from carbene complexes and isocyanides as building blocks for N-heterocycles 88AG(E)1456. 

Si- and Ge-Heterocycles, formation from carbene analogues 79ZVK475. 

Carbanions 128, generated from the corresponding Fischer carbene complexes with -butyllithium, reacted with 2-methylthio-l,3-dithiolium salts 123 to give heterocyclic organometallic carbenes 129. Reactions of allylic carbanions 130 afforded a mixture of mono- and diheterocyclic condensation products 131 and 132, respectively (Scheme 11) <2004TL7843>. 

Carbenes with a C=Z bond adjacent to the carbenic center may react with multiple bonds, yielding either three- or five-membered heterocycles. The latter may result from the opening of an intermediate carbocycle or from carbene [3 -I- 2]-cycloaddition (Scheme 16). 

A special case occurs in compounds with a C=Z bond (Z = P, Si) arising from a [l,2]-shift to a carbene center (Scheme 17). The formation of heterocycles from these compounds occurs via [2 + 2]- and [4 + 2]-cycloaddition reactions. 

If the carbene centre is next to a heterocyclic ring carrying a nitrogen atom, nitrene formation takes place. Nitrene formation from carbenes is favoured by the greater stability of the nitrene, even when a competing carbene route is available. ... 

N-Heterocyclic carbene complexes can be generated from the stable, free carbenes or by reaction of carbene precursors. Three examples of the s)mthesis of N-heterocyclic carbene complexes from carbene precursors are shown in Equations 13.14-13.16. The reaction of an imidazolium salt with a complex containing a basic ligand forms N-heterocydic carbene complexes (Equation 13.14), as does transfer of the carbene from a silver-carbene complex (Equation 13.15), - or reaction of an imidazolium-2-carboxylate, with accompanying decarboxylation (Equation 13.16). ... 

Lupton et al. reported the total synthesis of (-) 7-deoxyloganin (82) in 18 steps from 2,5-dimethoxytetrahydrofuran in overall 0.8% yield. The key step of their synthetic strategy was the N-heterocyclic nucleophilic carbene-catalyzed rearrangement of ot,p-unsaturated enol ester (83) to dihydropyranone (84) (Scheme 97.7) [114]. 

Oh and coworkers successively developed a series of highly efficient methods for the construction of various types of fused polycyclic heterocycles from enynals with a pendant unsaturated bond [72]. Enynals with a benzyloxy substituent at the propargylic position 155 were successfully cyclized via platinum-catalyzed Huisgen-type cycloaddition to form polycyclic Pt-carbene complex intermediate 156, which undergo insertion into a C-H bond to afford various types of fused polycyclic heterocycles 157 (Scheme 12.68) [73]. 

Formation of five-membered heterocycles from imino carbene complexes... 

Formation of Five-Membered Heterocycles from Imino Carbene Complexes... 

Furan and thiophene undergo addition reactions with carbenes. Thus cyclopropane derivatives are obtained from these heterocycles on copper(I) bromide-catalyzed reaction with diazomethane and light-promoted reaction with diazoacetic acid ester (Scheme 41). The copper-catalyzed reaction of pyrrole with diazoacetic acid ester, however, gives a 2-substituted product (Scheme 42). 

Cyclopent-2-en-l-one, 2-hydroxy-3-methyl-synthesis, 3, 693 Cyclopentenone, 4-methoxy-formation, 1, 423 Cyclopenthiazide as diuretic, 1, 174 Cyclopent[2,3-d]isoxazol-4-one structure, 6, 975 Cyclophane conformation, 2, 115 photoelectron spectroscopy, 2, 140 [2,2]Cyclophane conformation, 2, 115 Cyclophanes nomenclature, 1, 27 Cyclophosphamide as pharmaceutical, 1, 157 reviews, 1, 496 Cyclopiloselloidin synthesis, 3, 743 Cyclopolymerization heterocycle-forming, 1, 292-293 6H-Cyclopropa[5a,6a]pyrazolo[l,5-a]pyrimidine pyrazoles from, 5, 285 Cydopropabenzopyran synthesis, 3, 700 Cyclopropachromenes synthesis, 3, 671 Cyclopropa[c]dnnolines synthesis, 7, 597 Cyclopropanation by carbenes... 

The third volume of this series covers three specific groups of compounds the carbolines (reviewed by R. A. Abramovitch and I. D. Spenser), the thiatriazoles (K. A. Jensen and C. Pedersen), and the pentazoles (I. Ugi). The remaining four chapters deal with topics of general chemical interest from the heterocyclic viewpoint the quaternization of heterocyclics (G. F. Duffin), carbene reactions (C. W. Rees and C. E. Smithen), applications of the Hammett equation (H. H. Jaffe and H. Lloyd Jones), and some aspects of the nucleophilic substitution of heterocyclic azines (G. Rluminati). 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Diamino-substituted complexes of type 37 were first obtained by Fischer et al. [12] in two steps via the 1,2-addition-elimination product 34 from di-methylamine and 35 (Scheme 6). The (3-aminoallenylidene)chromium complexes 36, which can be prepared either from 33 [47,48] or directly from 35 [33], can also be transformed to l,3-bis(dialkylamino)-substituted complexes of type 37 (e.g., R2=z Pr) by treatment with dimethylamine in excellent yields [33]. Although the complex 37 is accessible by further reaction of the complex 34 with dimethylamine, and 34 itself stems from the reaction of 35 with dimethylamine, the direct transformation of 33 to 37 could not be achieved [12]. In spite of this, heterocyclic carbene complexes with two nitrogens were obtained by reactions of alkynylcarbene complexes 35 with hydrazine [49] and 1,3-diamines [50]. 

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