Carbenes, from

Phosphinidenes differ from carbenes because of the additional lone pair. This lone pair enables interactions with, e.g., a transition metal group for increased stability, while maintaining carbene-hke behavior. These terminal /] -complexed phosphinidenes differ from the p2-> fi3-> and p4-complexes, which are not part of this survey. Phosphinidenes that are stabilized by a transition metal group also relate to carbene complexes. A distinction in Fischer and Schrock-type complexes has been advanced to distinguish phosphinidene complexes with nucleophilic properties from those that are electrophiHc [ 13 ]. In this survey we address this topic in more detail. 

An attempt to measure the IR spectrum of the hydroxy carbene [28] after UV photolysis (A>220 nm) of formaldehyde isolated in an argon matrix was unsuccessful (Sodeau and Lee, 1978). Instead of [28] only hydroxyacetal-dehyde resulting from carbene insertion into the C—H bond of the starting formaldehyde was found in the reaction products. Due to its small size, the... 

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. 

By analogy with cyclopropane formation from carbenes and C=C bonds, azo compounds might be expected to give diaziridines in their reaction with carbenes. Although acyclic ADC compounds react readily with diazoalkanes... 

Now the 2-butenes comprise only 73% of the products, while 1-butene, a minor thermolytic product, increases to 23%. It is at least conceivable that some of the 1-butene here derives from an excited state of diazirine 8, rather than exclusively from carbene 6.3,15 Computational studies indicate that the 1,2-H shifts of 6 leading to the 2-butenes are preferred to the 1,2-H shift that yields 1-butene by a AAG of 2.6-3.3 kcal/mol. This again suggests that the substantial yield of 1-butene obtained from the photolysis of 8 is not simply derived from carbene 6.16... 

In the cyclobutylfluoro system, the excited diazirine, 21-F, is considerably less involved. A parallel analysis indicates that only about 12% of 22-F and 23-F arise from excited diazirine, while 88% of those products descend from carbene 17-F.28 The increased carbene involvement in the photolysis of 21-F presumably reflects the greater stability of fluorocarbene 17-F over its chloro analogue 17-C1 and, consequently, a more favorable partition (ki/k3) of excited diazirine 21-F to the carbene. 

In pentane, the distribution of 1,3-insertion product 25 to 1,2-Me shift product 26 is 91 9. Upon addition of 2-methyl-1-butene, the yield of 25 smoothly decreases (to 19% with 4 M alkene), but the yield of 26 is unaffected 1 Moreover, correlation of addn/l,3-CH insertion (to 25) for 18 is nicely linear. The simplest interpretation is that 25 comes directly from carbene 18, whereas the 1,2-Me shift product 26 comes from the excited diazirine.27 Interestingly, thermolysis of 24 at 79°C produces 90% of 25 and 10% of 26, but now the yields of both products smoothly decrease in the presence of an alkene. In thermolysis the (electronically) excited diazirine is unavailable, both 25 and 26 stem from the carbene, and their formation is suppressed by the alkene s interception of the carbene. A pyridine ylide kinetic study gave the 1,3-CH insertion rate constant (18 - 25) as 9.3 x 10s s"1.27-47... 

Of all the carbenes considered in this chapter, benzylchlorocarbene (10a) has produced the most debate, from carbene-alkene complexes vs. excited state... 

Laser flash photolysis (LFP) of quinone diazide 2d in Freon-113 at room temperature produces carbene Id, which could be monitored indirectly by addition of trapping reagents.25 At 2.0 xs the lifetime of Id is slightly longer than that of la (1.65 xs), otherwise the reactivities of these carbenes are very similar. The Id —> 11 rearrangement is not observed in the LFP experiments. All trapping products with a variety of reagents (O2, acetonitrile, pyridine etc.) are derived from carbene Id. 

K. S. Peters. Time-Resolved Photoacoustic Calorimetry From Carbenes to Proteins. Angew. Chem. Int. Ed Engl. 1994, 33, 294-302. 

Scheme 9 Ylide complexes obtained from carbene derivatives...

Closely related with the synthesis of ylides from carbenes is the use of ylides as carbene transfer reagents (CTR), that is processes in which the ylide is cleaved homolytically, liberating the nucleophile and the carbene, which could remain both coordinated to the metal or not (Scheme 10). Diphosphirane (34) can be obtained from the diphosphene by reaction with sulfur ylide Me2S(0)=CH2, which behave as a carrier of the CH2 unit [95]. Recent work of Milstein et al. shows that sulfur ylides decompose in the presence of Rh derivatives with vacant coordination sites affording Rh(l)-carbene complexes [96, 97]. Complexes (35-37) can be obtained from... 

Concerning the possible rearrangement of the lithiooxirane into the alkoxy carbene 155, calculations have also shown that the activation energies of the 1,2-H shifts (to cyclopentanone enolate or cyclopentenol) are extremely high (at least 23 kcalmol" ) from 155, whereas they are much lower (between —0.4 kcalmol" and 8.8 kcalmol" ) from carbene 154. This is explained by a strong intramolecular stabilization of the carbene by the alcoholate moiety, as depicted in Scheme 66. This stabilization could signify that the formation of a carbene from the carbenoid is a disfavored process, and that the carbenoid itself is involved in the rearrangement reaction. 

As shown in Scheme 8.67, the cyclization of diazoalkenes 273 requires thermal activation and not only affords 3/7-pyrazoles 274, but also cyclopropenes 275 that are formed from carbene intermediates (319). The activation parameters for cyclopropene formation (i.e., N2 elimination from 273) have been determined (320). A novel example involves the cyclization of the 3-nitro-l-diazoprop-2-ene derivative 276 into pyrazolopyridine derivative 277 (45). 

In addition to the rather trivial differences mentioned above, laser irradiation can also lead to products as a result of reexcitaion of the carbenes. Thus, excitation of 30 in isooctane with a pulse of the 249-nm line from a KrF excimer laser results in the formation of 9,10-diphenylanthrancene (103), 9,10-diphenylphenanthrene (104), and fluorene, in addition to tetraphenylethylene (Scheme 9.31). Conventional lamp irradiation of 30 results in the formation of benzophenone azine as a major product. None of the products mentioned above are detected. Moreover, the yield of both 103 and fluorene increased markedly with increased laser power. While the details of the mechanism of this reaction are not certain yet, it is clear from the dependence on laser power that some of these products arise from carbene photochemistry. " ... 

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