Carbenes generation/characterization

The related [1,2]-H shift in the isomeric triplet 1-phenylethylidene (53) was also investigated. The triplet carbene, generated from irradiation of the corresponding diazo compound, was characterized in low temperature inert matrices by EPR, IR, and UV/VIS spectroscopy. In this case, the carbene was stable in Ar up to the temperature limits of the matrix (36 K). Irradiation, however, readily converted the carbene to styrene. 

Bis(2,4,6-tribromophenyl)carbene (106f) was easily generated by photolysis of the precursor diazomethane (105f) and was characterized by EPR spectroscopy (Scheme 9.14). The triplet carbene generated in a degassed solution at room temperature decayed very slowly, persisting for at least 30 s. The decay was found to be second order Ik/el = 8.9 s ). The parameter fj/2, is estimated to be Is (Table 9.14). 

The fact that carbenes are characterized by their stereospecific transformations has effectively paved the way for numerous innovative developments. The discovery that rhodium acetate can function as a catalyst for carbene generation from diazoacetates [15, 16] with much higher efficiency than copper salts, which had been traditionally used, provided an opening into the vast possibilities offered by transition metal catalysis. The increasing number of novel, stereoselective reactions of carbenoid species have become an indispensable toolbox for the modern practitioner of chemical synthesis [17-22],... 

Functionalized A -heterocyclic carbenes generated from the pro-ligands 16-R, either in racemic or enantiomeric ally pure form, have been coordinated to Pd through the reaction with either [PdCl2(MeCN)2], to yield products 23-Me, or [PdCl(allyl)]2 to yield products 24-R (R = Me or Mesityl see Scheme 5.15). NMR data of the allylic compounds indicated the presence of two species in a 55 45 (24-Me) or 75 25 (24-Mes) ratio, which we can attribute to exo and endo configurational isomers. Crystallographic characterization was presented for complexes rac-(23-Me), f/ )-(23-Me), rac-(24-Me) and rac-(24-Mes) [80]. 

Dichlorocarbene, generated by the action of 50 % potassium hydroxide on chloroform, adds to ethyl 1 W-azepine-l-carboxylate to furnish the all /rntu-trishomoazepine 12 in 35% yield280 (see Houben-Weyl, Vol. E 19b, p 1523). Subsequently, and as a result of a careful and detailed study of the addition of dichlorocarbene generated by the thermal decomposition of phenyl(trichloromethyl)mercury, it was deduced that carbene addition takes place sequentially in the order C4 —C5, C2—C3 and C6 — Cl. The intermediary mono- 10 and bis(dichlorocar-bene) 11 adducts have been isolated and characterized. 

Van der Schaaf et al. described a synthesis of the 14-electron complex [RuHCl(PPr13)2] (32) from [RuCl2(COD)]A.,PPr31,isopropanol,and abase.Compound 32 is a suitable precursor for ruthenium carbene complex 33, as outlined in Scheme 10. Although 32 was isolated and structurally characterized, it may also be generated in situ for the preparation of the carbene complex 33 [18]. 

Singlet halo(trifluoromethyl)carbenes [33a]-[33c], which were characterized by IR and UV spectroscopies and chemical trapping with HCl, have been generated from respective 3-halo-3-(trifluoromethyl)diazirines [34a]-[34c] frozen in an argon matrix at 12 K and irradiated with UV light at... 

Starting from diaeetyl we prepared and characterized new imidazohum salt 5. After generation of the N-heteroeyelic carbene in situ we formed new eomplex Ic. This complex like the parent eomplexes la and lb is stable in the presenee of air and moisture. 

Interaction of a carbonyl group with an electrophilic metal carbene would be expected to lead to a carbonyl ylide. In fact, such compounds have been isolated in recent years 14) the strategy comprises intramolecular generation of a carbonyl ylide whose substituent pattern guarantees efficient stabilization of the dipolar electronic structure. The highly reactive 1,3-dipolar species are usually characterized by [3 + 2] cycloaddition to alkynes and activated alkenes. Furthermore, cycloaddition to ketones and aldehydes has been reported for l-methoxy-2-benzopyrylium-4-olate 286, which was generated by Cu(acac)2-catalyzed decomposition of o-methoxycarbonyl-m-diazoacetophenone 285 2681... 

Aryl(trimethylsiloxy)carbenes. Acylsilanes (153) undergo a photoinduced C —> O silyl shift leading to aryl(trimethylsiloxy)carbenes (154).73,74 The carbenes 154 can be captured by alcohols to form acetals (157) 73 or by pyridine to give transient ylides (Scheme 29).75 LFP of 153 in TFE produced transient absorptions of the carbocations 155 which were characterized by their reactions with nucleophiles.76 The cations 155 are more reactive than ArPhCH+, but only by factors < 10. Comparison of 154 and 155 with Ar(RO)C and Ar(RO)CH+, respectively, would be of interest. Although LFP was applied to generate methoxy(phenyl)carbene and to monitor its reaction with alcohols,77 no attempt was made to detect the analogous carbocation. 

The methylated carbenes lb-d are easily generated in argon matrices by photolysis of the corresponding diazo compounds.24 The IR and UV/vis spectra are very similar to that of la. In addition, the carbenes were chemically characterized by their reaction with molecular oxygen to give the corresponding quinone O-oxides 7b-d. 

Generation and characterization of high-spin organic molecules 210 Cross-conjugated poly(carbenes) 210 Other high-spin molecules 215... 

In contrast with non-acceptor-substituted carbene complexes, most of which are rather stable compounds, only few acceptor-substituted carbene complexes have been isolated [500,502,947,948]. In particular, acceptor-substituted carbene complexes relevant to organic synthesis (e.g. copper or rhodium acylcarbene complexes) are normally highly reactive and have remained elusive to spectroscopic characterization (for theoretical treatments, see Section 1.2). The inference that these intermediates are indeed carbene complexes is in part based on the observation that the modes of generation and the reactivity of these reactive species correlate well with those of less reactive carbene complexes. 

Furylchorocarbene (11), generated by irradiation of the corresponding diazirine in a nitrogen matrix at 8K, was characterized by IR spectroscopy, which revealed two species, one of which was destroyed on irradiation to form the aldehyde (12). The experimental and calculated spectra were in accord with two conformations of the carbene, one of which decomposes to the aldehyde. 

By now many sorts of reactive intermediates, such as free radicals, carbocations, carbenes, benzynes, and Bredt s rule violating olefins, have been generated and isolated under conditions permitting full structural characterizations. Theory-based structural parameters are generally in nearly perfect agreement with experimentally determined values. 

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