Persistent carbenes

The reaction scheme to realize the coupling necessary to generate the persistent carbene precursor 122 is illustrated in Scheme 9.39. " Diazo groups are generally labile, and, hence, these groups are usually introduced at the last step of the... 

Some remarkably persistent carbenes have been synthesized. This was achieved either by substitution with sterically demanding groups that hinder dimerization (triplet carbenes)399 400 or by substitution with strong Jt-electron donors (singlet carbenes)401 (Scheme 5.5). The latter are probably better described as ylides (zwitterions). 

A range of (phosphino)(aryl)carbenes (20 R = i-Pr, c-hexyl V, W, X, Y, Z = various combinations of H, F, Me, CF3, NMe2) have been generated by photolysis of their diazo precursors. The substituent effects on carbene stability were investigated both experimentally and theoretically. The presence of an acceptor and also a spectator aryl substituent resulted in stable or at least persistent carbenes, which could be characterized by standard spectroscopic techniques, whereas a donor aryl ring produced a very fleeting carbene. 

Most carbenes are very short lived, although persistent carbenes are known. A persistent carbene (also known as a stable carbene or a Arduengo carbene) is a type of carbene demonstrating particular stability. The best-known examples are diaminocarbenes with the general formula (R2N)2C , where the R represents various functional groups. The groups can be bridged so that the carbon with unfilled orbitals is part of a heterocycle, such as imidazole or triazole. Persistent... 

Phosphinocarbene or 2 -phosphaacetylene 4, which is in resonance with an ylide form and with a form containing phosphoms carbon triple bond, is a distillable red oil. Electronic and more importantly steric effects make these two compounds so stable. Carbene 4 adds to various electron-deficient olefins such as styrene and substituted styrenes. Bertrand et al. have made excellent use of the push-pull motif to produce the isolable carbenes 5 and 6, which are stable at low temperature in solutions of electron-donor solvents (THF (tetrahydrofuran), diethyl ether, toluene) but dimerizes in pentane solution. Some persistent carbenes are used as ancillary ligands in organometallic chemistry and in catalysis, for example, the ruthenium-based Grubbs catalyst and palladium-based catalysts for cross-coupling reactions. 

Persistent carbenes were introduced into synthetic chemistry by Bertrand, who isolated the first carbene Rj P-C-SiR 3 in 1988 [13], and by Arduengo [14], who synthesized the N-heterocyclic carbenes (NHCs) in 1991. Stable carbones were actually isolated much earlier than carbenes. In 1961, Ramirez et al. [15] reported on the synthesis of the compound hexaphenyl-carbodiphosphorane ClPPhjlj- The authors assumed that the molecule has a linear structure PhjP=C=PPh3, which they described with resonance structures in terms of double bonds between phosphorous and carbon and as a double ylide Ph3P=C=PPhj Phj P(-l-)-C(2—[-(-l-jPPhj. 

Persistent carbene AZ-Heterocyclic carbene R = alkyl, aryl... 

Persistent carbenes, the most common of which are NHCs, are distinguished by the presence of one or more heteroatoms capable of jt-donation on either side of the carbene nucleus, one of which is usually nitrogen. 

As to other carbenes, some react as triplets, some as singlets, and others as singlets or triplets, depending on how they are generated. There are, however, molecules that generate persistent triplet carbenes. 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

Development of the chemistry of metal carbenes commenced with the characterization of complexes such as (CO)sW=C(OMe)Ph by Fischer and his co-workers (e.g., Ref. 14 and references therein). The series of hydrocarbon-substituted carbene compounds discovered later by the Schrock group (e.g., Ref. 15 and references therein) appeared to be so different from the other known carbene complexes that they were placed in a different class altogether. Overemphasis of this distinction has persisted in the literature of carbene complexes, even to the present day. 

If Scheme 2 accurately represented the PhCH2CCl chemistry, curvature in the addn/rearr vs. [alkene] correlation would persist when the carbene was generated from 37. The absence of curvature in this case counts against Scheme 2 (and the CAC mechanism), but accords with the RIES mechanism, Scheme 3. Elimination of the diazirine precursor eliminates the diazirine excited state. From 37, both cyclopropane formation and 1,2-H rearrangement proceed from a single (carbene) intermediate, and addn/rearr vs. [alkene] is linear.25... 

The ruthenium carbene complexes 1 discussed in the previous chapter have set the standards in the field of olefin metathesis and are widely appreciated tools for advanced organic synthesis [3]. A serious drawback, however, relates to the preparation of these compounds requiring either 3,3-diphenylcyclopropene or diazoalkanes, i.e. reagents which are rather difficult to make and/or fairly hazardous if used on a large scale [60]. Therefore, a need for metathesis catalysts persists that exhibit essentially the same activity and application profile as 1 but are significantly easier to make. 

Another interesting allenic intermediate is generated when di(9-anthryl)diazo-methane (256) is deazotized by irradiation in benzene (Scheme 5.39). The carbene produced, 257, belongs to the long-sought persistent triplet carbenes, although its... 

It should also be noted that persistent triplet carbenes have been discovered by Tomioka.12... 

Since this discovery, a few persistent triplet carbenes have been prepared and other types of stable singlet carbenes have been isolated. Triplet carbenes are discussed in Chapter 9 by H. Tomioka in this volume, and therefore this chapter will be focused on singlet carbenes. 

Up to 2000, the number and variety of stable carbenes have been limited by the perceived necessity for two strongly interacting substituents. The preparation of stable or persistent (aryl)- or (alkyl)-(phosphino)carbenes as well as (aryl)(amino)-carbenes demonstrates that a single electron-active substituent allows the spectroscopic characterization of singlet carbenes under standard laboratory conditions. It has been shown that an amino substituent is more efficient than a phosphino substituent to stabilize a carbene center and that the steric bulk of the spectator substituent can be decreased even to the size of a methyl group in the phosphino series, so that a broad range of observable carbenes will be readily available. 

The reaction of triplet carbenes with a persistent nitroxide such as 2,2,6,6-tetra-methylpiperidine A -oxide (TEMPO, 84) to form benzophenone would be spin allowed and >100-kcal/mol exothermic (Scheme 9.26). The reaction has a few parallels in free radical chemistry, such as the reaction of tert-butoxyl with carbon monoxide (to yield CO2) or with phosphorus (111) substrates to yield P(V) products. " ... 

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). 

To probe the stability of 16b under normal conditions, the precursor diazomethane was photolyzed in degassed benzene at room temperature. The bands due to 16b decayed cleanly, showing isosbestic points, persisting for more than 3h before disappearing completely. The decay curve was analyzed in terms of second-order kinetics 2k/zl = 5.2 x lO s ). The half-life for 16b was estimated to be 19 min. This carbene is the longest-lived triplet thus far generated in our research group. 

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