Carbenes persistent carbene

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. 

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... 

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