Carbenes conformation

Cyclopropylchlorocarbene [20] has been generated by UV photolysis (A = 335 nm) of cyclopropylchlorodiazirine [21] frozen in a nitrogen matrix at 12 K (Ho et al., 1989). IR and UV spectra of [20] have been recorded. The reaction of [20] with HCl resulted in the formation of (dichloromethyl)-cyclopropane [22], and annealing of the matrix gave (dicyclopropyl)dichloro-ethene [23]. Subsequent irradiation (A = 450 nm) of the carbene [20] led to its isomerization to 1-chlorocyclobutene [24], which was partialy destroyed to give ethene and chloroacetylene. Ab initio calculations predict the existence of two carbene conformers, but attempts to distinguish them in IR or UV spectra were unsuccessful. 

Photolysis of diazirine 75 in N2 at 9 K produced carbene 76, whose IR and UV/Vis spectra fit predictions by DFT calculations. The IR spectra were most consistent with a carbene conformation with Cl aligned 90° to the adjacent C-CH bond. The carbene was found, by IR spectroscopy, to rearrange to chloroadamantene 77 slowly at 9 K in the dark. The rate of rearrangement was somewhat faster in Ar matrices at 9 K or at higher temperatures in N2. 

W=X is N=N or C=N, never if it is C=C. The ring stability of 3-furyl-carbenes conforms with this rule. Ring opening is again the main reaction in a biradical which is generated by extruding carbon dioxide from a lactone at 675°C and which then collapses to an acetylenic ketone.277... 

For the catalytic activity of ruthenium-carbene complexes, the relative stability of the carbene conformations plays a pivotal role. After the dissociation of a victim ligand, followed by alkene coordination, the carbene has to adopt an active conformation. Only then can the new C-C bond in the ruthena(IV)cyclobutane evolve (green traffic light row in Figure 5.2) [19, 20]. 

Figure 5.2 Role of the carbene conformation in the C-C bond formation [19, 20].

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... 

H-Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4H-Pyran, 4-benzylidene-synthesis, 3, 762 4H-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4H-Pyran, 5,6-dihydro-synthesis, 2, 91 4H-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4H-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4H-Pyran, 2-methoxy-synthesis, 3, 762 4H-Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 4H-Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

With the enthusiasm currently being generated by the (so-called) stable carbenes (imidazolylidenes) [7], it is surprising that there are few reports of imidazolium-based ionic liquids being used to prepare metal imidazolylidene complexes. Xiao et al. have prepared bis(imida2olylidene)palladium(II) dibromide in [BMIMJBr [8]. All four possible conformers are formed, as shown in Scheme 6.1-3. 

Simple 1,3-dienes also undergo a thermal monocyclopropanation reaction with methoxy(alkyl)- and methoxy(aryl)carbene complexes of molybdenum and chromium [27]. The most complete study was carried out by Harvey and Lund and they showed that this process occurs with high levels of both regio-and diastereoselectivity. The chemical yield is significantly higher with molybdenum complexes [27a] (Scheme 7). Tri- and tetrasubstituted 1,3-dienes and 3-methylenecyclohexene (diene locked in an s-trans conformation) fail to react [28]. The monocyclopropanation of electronically neutral 1,3-dienes with non-heteroatom-stabilised carbene complexes has also been described [29]. 

However, exo-selective Diels-Alder reactions are found when a,/J-unsatu-rated exocyclic carbene complexes are used as dienophiles. The fixed s-cis conformation of the vinylcarbene moiety of the complex seems to be responsible for the exo selectivity observed in this reaction. Moreover, the reaction of optically active carbene complexes with 2-morpholino- 1,3-butadienes allows the asymmetric synthesis of spiro compounds [99] (Scheme 53). 

Davies [30] studied the PyBOx-induced conformational effects by testing several ligands sterically hindered on the oxazoUne moieties (Scheme 11, structures 18 and 19). However, these new ligands gave poorer results in terms of yields and enantioselectivities than ligand 16 for the Ru-catalyzed cyclopropanation reaction, indicating unfavorable steric interactions between styrene and the carbene complex. 

Despite the planar conformation of nitrogens, several possibilities are available for the introduction of chirality. It is possible to prepare benzim-idazolylidenes, triazolylidenes, imidazolylidenes or unsubstituted-backbone imidazolinylidenes with a stereogenic center on one or two N-substituents (carbenes I and II). The other possibility is to relay the imidazolinylidenes backbone stereogenecity via the N-substituents or to combine stereogenic N-substituents with a chiral backbone (carbenes III or IV). It is at least possible to prepare bis-carbenes of type V with one (or two) stereogenic link between the two carbenes (Fig. 7). 

The conformational disorder and packing structure of diazirine 91N2 is shown on the left of Scheme 7.25. The formation of azine 92 can easily be explained by the head-to-head crystallization of the chloro-diazirine, which experiences close N---C1 interactions that ultimately bring the carbene carbon close to the diazirine nitrogen... 

It was proposed that the more labile IR bands corresponded to the exo-carbene, 71-exo, which has the methyl and fluoro substituents aligned properly for rearrangement. The slower decaying IR bands were assigned to c do-carbene, 71-endo, which was presumed to require rotation to the exo-conformer before undergoing ring expansion. 

Owing to the low barriers to bond formation, reactant conformation often plays a decisive role in the outcome of these reactions. Carbocations, carbene, and radicals frequently undergo very efficient intramolecular reactions that depend on the proximity of the reaction centers. Conversely, because of the short lifetimes of the intermediates, reactions through unfavorable conformations are unusual. Mechanistic analyses and synthetic designs that involve carbocations, carbenes, and radicals must pay particularly close attention to conformational factors. 

Carbene Complex Geometries. Molecular orbital studies of the various conformations of several transition metal-carbene complexes have been undertaken by the groups of Fenske and Hoffmann (8,13). Of the two... 

The carbene ligand in 59 is coplanar with the equatorial plane of the complex, the phenyl ring also lying approximately in the same plane. This is the conformation that would be predicted on both steric and electronic grounds. 

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