Diphenylcarbenes

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

Benzannulation of the diphenylcarbene ligand by 2-ethynylglucose derivative 34 results in only low diastereoselection, albeit it represents a rather rare example of a low-temperature protocol [38] (Scheme 22). 

These tetrathiametallacyclohexanes are probably formed via the initial attack of diphenylcarbene, generated from the corresponding diazo compound, on a sulfur atom and subsequent ring expansion (Scheme 55). The... 

Using the pseudo-first-order equation A obsd = 0 + co2 [COiKwhere kcoi is the second-order rate constant for the reaction of carbene with CO2 and ko is the rate of carbene decay in the absence of CO2), solution-phase values of kcoi for phenylchlorocarbenes 9 and 12, and diphenylcarbenes 14 and 15 in dichloromethane were estimated (Table 4.1). (The concentration of CO2 in saturated dichloromethane solution at 25°C and 1 atm is 196mmol/L. ) The trend of these estimated second-order order rate constants agrees with that observed in low-temperature matrices by Sander and co-workers. ... 

Interest in carbenes in rigid media can be dated to the 1960s when Murray et al. reported diphenylcarbene as the first organic species with a triplet ground state. ... 

Shortly after, Doetschman and Hutchison reported the first example of a reactive carbene in the crystalline solid state, by preparing diphenylcarbene from diphenyldi-azomethane in mixed crystals with 1,1-diphenylethylene 84 (Scheme 7.23). When the mixed crystals were irradiated, carbene 85 was detected by electron paramagnetic resonance (EPR) and the disappearance of the signal was monitored to determine its kinetic behavior. Two reactions were shown to take place under topochemical... 

Fig. 10.6. Dimeric (Ar = 2,6-dimethylphenyl) (a) and monomeric (Ar = 2,4,6-trimethylphenyl) (b) copper complexes with diphenylcarbene. Reproduced from J. Am. Chem. Soc., 126, 10085 (2004), by permission of the American Chemical Society.

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. 

In comparison with other catalysts, (diphenylcarbene)tungstenpenta-carbonyl is surprisingly sluggish (26). This observation is significant as it relates to diverse views regarding the carbene-to metallocyclobutane interconversion. Whereas Casey emphasizes a need to accommodate the incoming olefin within the coordination sphere of the metal prior to rearrangement to a metallocycle [Eq. (5)], Katz (28) has described the process essentially as a dipolar attack of a polarized carbene-metal (R2C+—M-) on the olefin [Eq. (6)]. The latter does not specify a need for w-complexation of the olefin as a precondition to metathesis. 

Suggestive evidence for the protonation of diphenylcarbene was uncovered in 1963.10 Photolysis of diphenyldiazomethane in a methanolic solution of lithium azide produced benzhydryl methyl ether and benzhydryl azide in virtually the same ratio as that obtained by solvolysis of benzhydryl chloride. These results pointed to the diphenylcarbenium ion as an intermediate in the reaction of diphenylcarbene with methanol (Scheme 3). However, many researchers preferred to explain the O-H insertion reactions of diarylcarbenes in terms of electrophilic attack at oxygen (ylide mechanism),11 until the intervention of car-bocations was demonstrated by time-resolved spectroscopy (see Section III).12... 

Diphenylcarbene also undergoes a ring expansion of the above kind. When it was generated in the gas phase at 350°C/3Torr, small quantities of tetraphenylethylene, the triphenylheptafulvene 563 and the diphenylheptafulvalene 564 were formed in addition to fluorene (565), which was obtained in 29% yield. As illustrated in... 

Scheme 6.114, a carbene-carbene rearrangement transforms diphenylcarbene to o-phenylphenylcarbene, which is the progenitor of 565. Two phenylbicydo[4.1.0]-hepta-2,4,6-trienes and l-phenyl-l,2,4,6-cycloheptatetraene (562) have to be assumed as further intermediates. The participation of 562 is supported by the structure of the products 563 and 564, which should result from the addition of 562 to diphenylcarbene and the dimerization of 562, respectively. By thermolysis of the sodium salt of 2-phenyltropone tosylhydrazone, 562 was generated directly. At 100 °C in diglyme as solvent, 564 was identified as the only product and at 340°C/4Torr in the gas... 

Fig. 8 Molecular structure of diphenylcarbene [15 / = 1] showing the orthogonal orbitals at the divalent carbon atom.

The theory had never been tested on a logical model system. Let us consider in detail one representative case, the superimposable stacking of the two benzene rings, one from each triplet diphenylcarbene molecule. These are considered to represent idealized modes of dimeric interaction of the aromatic ring parts of open-shell molecules in ordered molecular assemblies like crystals, liquid crystals and membranes. 

Fig. 19 n-Spin-density distribution in diphenylcarbene [15 m= ] determined by ENDOR experiments and MO calculations. The o-spin density at the divalent carbon is in parentheses. 

Fig. 20 Idealized stacking modes of two diphenylcarbene molecules. Two benzene rings, one each from the two carbenes, are always in a superimposable disposition.

To test the first theory of McConnell as exemplified by the stacking overlap between two triplet diphenylcarbene molecules (see Fig. 20), Izuoka et al. (1985, 1987) took advantage of the [2.2]paracyclophane skeleton. It was Forrester and Ramasseul (1971a,b, 1975) who first tried to demonstrate... 

The arylcarbenes are also bent, the angle being about 150—155° for phenyl-and diphenylcarbenes 63,64) jj g zero-field splitting parameters were shown to be appreciably dependent on the host matrix used >. However diphenylcarbene prepared from different precursors proved to be identical and is believed to be the triplet species of diphenylcarbene. 

Electron absorption and emission spectra of triplet diarylcarbenes have also been obtained. The emission of diphenylcarbene occurs around 480 nm. The ex-... 

Table 5. Spectra of diphenylcarbenes (in 2-methyltetrahydro-furan) and cyclic carbenes...

The addition of the diphenylcarbene, generated by the photolysis of diazo-diphenylcarbene, to cis- or trans-butene gives mostly the abstraction product, and a small amount of the addition product. However, the abstraction processes are... 

If identical addition rates for singlet and triplet diphenylcarbene are assumed, this ratio corresponds to 70% of the triplet species. 

In the photolysis of diphenyl-diazo-methane in olefins the ratio cyclopro-pane/abstraction products depends on the structure of the olefin. The nonstereospecific addition of diphenylcarbene to cis-2-butene is temperature-dependent i8b) ... 

This experimental fact indicates that the singlet and triplet diphenylcarbene are in thermal equilibrium and that Aigc is fast because the Ph2C-state is the ground state. The relatively low nonstereospecifity can be explained by assuming a) a faster addition of the singlet or b) most of the triplet undergoing the abstraction reaction and only little of it the addition reaction. 

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