Generation of Carbene

Photolytically generated carbene, as mentioned above, undergoes a variety of undiscriminated addition and insertion reactions and is therefore of limited synthetic utility. The discovery (3) of the generation of carbenes by the zinc-copper couple, however, makes carbene addition to double bonds synthetically useful. The iodo-methylzinc iodide complex is believed to function by electrophilic addition to the double bond in a three-center transition state giving essentially cis addition. Use of the... 

In contrast to carbenes of the AX2 type, which contain three atoms, generation of carbenes with a more complex structure under photolysis or vacuum pyrolysis conditions may be accompanied by intramolecular rearrangements. Thus, the matrix isolation study of the vacuum pyrolysis... 

Nevertheless, a more traditional approach to the stabilization of carbenes and the investigation of their spectral properties deals with the direct generation of carbenes in low-temperature matrices, e.g. by the photolysis of diazo-compounds or ketenes. The method allows stabilization of carbenes in their ground electronic state, prevents intramolecular isomerization and also facilitates direct spectroscopic monitoring of their chemical transformations in low-temperature matrices. 

Different methods have been developed for the generation of carbene and diradical negative ions. One of the most commonly used approaches involves the reaction of an organic substrate with atomic oxygen ion, O , to form water by H2 abstraction (Eq. 5.7). "... 

The photoelimination of nitrogen from diazo compounds provides a simple and versatile route for the generation of carbenes, and in certain instances, insertion reactions of carbenes can be employed in the synthesis of heterocycles. Carbenes are believed to be involved at least in part in the photochemically induced conversion of N,N-diethyldiazoacetamide (439) into the y-lactam 440 and the /Mactam 441,365 and a similar approach has been successfully employed in the synthesis of a carbapen-2-em366 and of 7-methylcephalosporin analogues.367 Carbene insertion of a different type has been observed on irradiation of the 6-anilino-5-diazouracils 442 to give the indolo[2,3-d]pyrimidines 443.368 Ring contractions in heterocycles... 

Photolytic generation of carbene 19 from a precursor diazirine afforded cyclobutenes 27 (68%) and 28 (14%) via CH2 and CMe2 migrations, respectively cf. Eq. 15.45... 

When, however, carbenes are directly generated from diazoalkanes, RIES becomes significant.56 Photolytic generation of carbene 45 from diazoalkane precursors in the presence of >1.5 M pyridine gave values for the derived pyridinium ylides. 

Our approach to the synthesis of model compounds, suited for the generation of carbenes, which are bound to the bridgehead of a strained bicyclic system, used the facile synthesis of [n.l.ljpropellanes, which we developed some years ago. Starting from substituted allyl chlorides 35, dibromocarbene addi-... 

In addition to conventional generation of carbenes from nitrodiazo compounds (22) (79), target intermediates C can be prepared by oxidation of functionalized AN CH2XNO2 with phenyliodonium diacetate. The reactions of Rhodium intermediates with certain olefins afford the corresponding cyclopropanes (23). The cycloaddition reaction was performed in the presence of a catalyst. (The successful synthesis of nitrocyclopropanes from trinitromethane derivatives and nitroacetic ester was also documented (81)). 

Ylides from R N2. This reagent is more effective than bis(acetylacetonate)-copper(II) (5, 244) for generation of carbenes from diazo compounds.2 The decomposition proceeds at a lower temperature, even at room temperature. The mild conditions are particularly useful in the preparation of heat-sensitive ylides, such as those of antimony, bismuth, and tellurium. 

For synthetic purposes, crown ethers have been used frequently as phase-transfer catalysts under conditions where the sodium or potassium hydroxide is present as a concentrated aqueous solution. Various anions of C—H acids have been generated this way (Makosza and Ludwikow, 1974). The use of such conditions for the generation of carbenes will be dicussed in a separate section. 

Fig. 2.9. Generation of carbene complexes from vinylidene complexes.

Fig. 3.25. Generation of carbene complexes by addition of electrophiles to carbyne complexes [533,538,540,543],...

Fig. 3.31. Generation of carbene complexes by [2 -i- 2] cycloreversion of four-membered metallacycles [26,27,605,607-609].

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

Nyce GW, Glauser T, Connor EF, Mock A, Waymouth RM, Hedrick JL (2003) In situ generation of carbenes a general and versatile platform for organocatalytic living polymerization. J Am Chem Soc 125 3046-3056... 

Ahphatic diazo compounds are convenient starting materials for the generation of carbenes by photolytic or thermal methods. Decomposition of diazo compounds can be catalyzed by some metals or metal salts (63,64). Whereas copper salts were first used, rhodium(ll) acetate has recently become recognized as one of the most efficient catalysts and is now used almost exlusively (65). 

Other possible carbene precursors (vicinal dihalides, peresters, ketenes, carbene adducts with stable hydrocarbons, etc.) cannot generally be used for in situ generation of carbenes, because the fragments are likely to recombine. However, they can be used, for example, in experiments involving pyrolysis or other forms of external carbene generation where the fragments get a chance to separate in the gas phase and become trapped in distant matrix sites. AU conceivable halo- and dihalocar-benes were made and smdied in this way (see, e.g., the 1993 review by Sander et al." ) However, such methods can only be applied to carbenes which resist thermal rearrangement to more stable products. 

More recent results support the view that Lewis acids, on the interaction with metal carbonyls, may facilitate dissociation of a ligand to leave a vacant site for alkene coordination.75 Photochemical generation of carbene species [Eq. (12.20)], the crucial step in photoinduced metathesis, is a complicated, multistep process 74,76,77... 

Photochemical reactions, indicated in reaction schemes by the symbol hv, are considered in Chapter 13. For the present, it is sufficient to note that absorption of light transforms a molecule to an excited state, which, in the case of diazo compounds, has sufficient energy for rupture of the C—N bond. Generation of carbenes from diazo compounds is reviewed by W. J. Baron, M. R. DeCamp, M. E. Hendrick, M. Jones, Jr., R. H. Levin, and M. B. Sohn, in Jones and Moss, Eds., Carbenes, Vol. I, p. 1. 

Apart from the significance of diazoalkanes for the generation of carbenes,195 these compounds also play a dominant role in dipolar cycloaddition chemistry.196197 Recent advances in the synthesis of di-... 

With respect to the complications of method B, it is remarkable that generation of carbene 3a according to method C in the presence of c -but-2-ene or various cycloalkenes gave the corresponding (trimethylsilyl)cyclopropanes in yields up to 30%73,74. Similarly,... 

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