Carbenes, from diazoalkanes

Nitrenes are the nitrogen analogs of carbenes. The most common method for their formation is basically the same as that of carbenes from diazoalkanes, namely thermolysis or photolysis of azides (8-8). 

Carbenes from Diazo Compounds. Decomposition of diazo compounds to form carbenes is a quite general reaction that is applicable to diazomethane and other diazoalkanes, diazoalkenes, and diazo compounds with aryl and acyl substituents. The main restrictions on this method are the limitations on synthesis and limited stability of the diazo compounds. The smaller diazoalkanes are toxic and potentially explosive, and they are usually prepared immediately before use. The most general synthetic routes involve base-catalyzed decomposition of V-nitroso derivatives of amides, ureas, or sulfonamides, as illustrated by several reactions used for the preparation of diazomethane. 

Carbenes from Sulfonylhydrazones. The second method listed in Scheme 10.8, thermal or photochemical decomposition of salts of arenesulfonylhy-drazones, is actually a variation of the diazoalkane method, since diazo compounds are intermediates. The conditions of the decomposition are usually such that the diazo compound reacts immediately on formation.147 The nature of the solvent plays an important role in the outcome of sulfonylhydrazone decompositions. In protic solvents, the diazoalkane can be diverted to a carbocation by protonation.148 Aprotic solvents favor decomposition via the carbene pathway. 

A Mechanism for Alkylidene Formation. There is no unambiguous example of free-carbene capture by a metal substrate, and the mild reaction conditions used in the generation of these carbene complexes from diazoalkanes suggests that such a mechanism is highly unlikely here. Transition metal diazoalkane complexes, then, are almost certainly implicated as intermediates in these reactions. 

Some examples of carbene dimer formation resulting from diazoalkane decomposition on transition-metal surfaces have been reported. Diazomethane is decomposed to give ethylene and N2 upon passage over a C0O/M0O3 catalyst as well as on Ni, Pd, Fe, Co, Ru and Cu surfaces 367). Similarly, 2-diazopropane is readily decomposed on Raney nickel 368). At room temperature, propene and N2 were the only detectable products, but above 50 °C, the carbene dimer 2,3-dimethyl-2-butene started to appear which reached its maximum yield at 100 °C, where approximately 40 % of the carbene fragments dimerized. It is assumed 367,368), that surface carbenes are formed as intermediates from both diazomethane and 2-diazopropane which either dimerize or desorb by migration of a P-hydrogren, if available (Scheme 40). 

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. 

As in experiments with diazirine 1, the maximum yields of the ylides, and hence of the carbenes, increased with increasing strength of the alkyl group s a-CH bond.56 For example, the relative yields of carbene 45 from diazoalkane photolysis were 100% for R=cyc/o-C3H5, 96% for R=t-C4H9, and 72% for R=CH3 the relative carbene yields decreased to 14% with R=C2Hs and to only 7% with R=i-C3H7. 

RIES from diazoalkanes is also sensitive to the dihedral angle between the migrating a-H and the C-N bond of the diazo moiety.57 For example, the A values for the pyridine capture of the photolytically generated carbenes from 46 and 47 are in the ratio of 1.7 1. Similarly, the carbene from 46 is more efficiently generated and trapped in methanol, whereas the photolysis of 47 in methanol affords twice as much olefin (by 1,2-H RIES) compared to the photolysis of 46. These phenomena are attributed to conformational factors that favor RIES during the photolysis of 47, with the proximal excited state represented as a pyramidalized 1,3-C-N=N diradical.57... 

It has been known for a long time that the decomposition of diazoalkanes can be catalyzed by transition metal complexes [493-496]. Carbene complexes were proposed as possible intermediates by Yates in 1952 [497]. However, because reactions of diazoalkanes with metal complexes tend to be difficult to control, it was not until 1975 [498] that stable carbene complexes could be directly obtained from diazoalkanes (Figure 3.19). 

Fig. 3.20. Examples of the preparation of non-heteroatom-substituted carbene complexes from diazoalkanes [60,504-506].

Electrophilic carbene complexes generated from diazoalkanes and rhodium or copper salts can undergo 0-H insertion reactions and S-alkylations. These highly electrophilic carbene complexes can, moreover, also undergo intramolecular rearrangements. These reactions are characteristic of acceptor-substituted carbene complexes and will be treated in Section 4.2. 

Walling297 suggested that the electronic structure of the carbenes, the photolysis products from diazoalkanes, is somewhat analogous to carbon monoxide, and most of their subsequent reactions, e.g., addition to aromatic58 59 or rearrangements98 are those of electron-deficient electrophilic entities rather than radicals.51 However, radical photoaddition of diazomethane to carbon tetrachloride289 290 should not be overlooked. 

We showed you the formation of a carbene from diazomethane to illustrate how this reaction was different from the (ionic) methylation of carboxylic acids. But this is not a very practical way of generating carbenes, not least because of the explosive nature of diazoalkanes. However, diazocarbonyl compounds are a different matter. 

The four-coordinate sqnare planar iron(n) porphyrins discussed above are not only of great valne in heme protein model chemistry, but also in chemical applications, since they undergo a wealth of ligand addition reactions. For example it has been shown that TPPFe complexes are active catalysts for important carbon transfer reactions in organic chemistry and are found to catalyze the stereoselective cyclopropanation of aUcenes, olefin formation from diazoalkanes, and the efficient and selective olefination of aldehydes and other carbonyl compounds. The active species in these carbon transfer reactions are presumably iron porphyrin carbene complexes. " It was also found that ferrous hemin anchored to Ti02 thin films reduce organic halides, which can pose serious health problems and are of considerable environmental concern because of their prevalence in groundwater. ... 

Many more carbenes can be made safely from diazoalkanes if the diazoalkane is just an intermediate in the reaction and not the starting material. Good starting materials for these reactions are tosylhydrazones, which produce transient diazo compoimds by base-catalysed elimination of toluenesulfinate. The diazo compound is not normally isolated, and decomposes to the carbene on heating. 

In contrast to the wealth of chemistry reported for catalyzed reactions of diazocarbonyl compounds, there are fewer applications of diazomethane as a carbenoid precursor. Catalytic decomposition of diazomethane, CH2N2, has been reported as a general method for the methylenation of chemical compounds [12]. The efficacy of rhodium catalysts for mediating carbene transfer from diazoalkanes is poor. The preparative use of diazomethane in the synthesis of cyclopropane derivatives from olefins is mostly associated with the employment of palladium cat-... 

No matter how they are generated, carbenes and carbenoids undergo four typical reactions. The most widely used reaction is cyclopropanation, or addition to a TT bond. The mechanism is a concerted [2 + 1] cycloaddition (see Chapter 4). The carbenes derived from chloroform and bromoform can be used to add CX2 to a 7T bond to give a dihalocyclopropane, while the Simmons-Smith reagent adds CH2. Carbenoids generated from diazoalkanes with catalytic Rh(II) or Cu(II) also undergo cyclopropanations. 

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