Diazoalkanes carbene reactions

Fusion of a cyclohexyl ring at C1-C2 also produced only the pyrazole, but when a cyclopentyl ring was fused at C1-C2 the diazo compound 71 reacted via all three possible modes of reaction (Scheme 23). This was the first example of a 3-aryl-l-diazoalkane reacting by both 1,5- and 1,7-electrocycliza-tion. This effect can be attributed to the fact that in 71a the termini separation is slightly greater than in the acyclic case 64 but not as great as in 69. So, in contrast to 69, both 6m- and 8w-electrocyclizations are possible, and the carbene reactions are also competitive [79JCS(P1)1433]. 

The complications that occasionally arise in the use of diazoalkanes reflect the possible further reactions of carbene ligands, which will be dealt with subsequently, e.g. insertion into adjacent M-H or M-halide bonds and the formation of bimetallic complexes supported by bridging carbene ligands. In some cases, transition metals may catalyse reactions of diazoalkanes, leading to products which are suggestive of the reactions of free carbenes, i.e. dimerization, addition to alkenes (cyclo-propanation) and insertion into C-H bonds (Figure 5.9). In such cases, however, the actual mechanism does not involve free carbenes but rather transient diazoalkane/carbene complexes. This is supported by the obser-... 

D. Carbene Reactions of Diazoalkanes The disconnections available from carbene additions are... 

As mentioned briefly in Section 6.5, it should be emphasized that there is no clear evidence available whether cyclopropanes, including these methanofullerenes, are formed via dihydropyrazoles, i.e., by a 1,3-dipolar cycloaddition, or by the primary dediazoniation of the diazoalkane to a carbene that subsequently reacts with 50-It may be that the mechanism is a dipolar cycloaddition followed by azo-extrusion at low temperature (20°C, i.e., Suzuki s conditions), but a carbene reaction in boiling toluene (Isaacs and Diederich), as shown in Section 6.5, Scheme 6-37, pathways C and A, respectively. In addition, the dihydropyrazole may be the product of a side-equilibrium only, but the reagents form the cyclopropane-type methanofullerene via pathway C. A mechanism via primary dediazoniation is, however, unlikely as dediazoniation of diazoacetate without C o in boiling toluene is much slower than it is in the presence of 50 (Diederich, 1994). 

Interestingly, Dixneuf has shown recently that vinyl metal carbenoid intermediate 176 generated with an electrophilic ruthenium catalyst [RuCl(cod)Cp ] (Cp = CsMes) could be trapped by a diazoalkane carbene to yield acetoxy dienes 186 through carbene dimerization, in good yields (Scheme 77) [162], No dimerization product and no cyclopropane formation could be noticed even when the reaction was run in the beneficial presence of 5 equiv. of styrene. 

The reaction of thiocarbonyl compounds with diazoalkanes (alkyl, aryl substituted) frequently gives good to excellent yields of thiiranes. The mechanism may involve addition of a carbene across the thiocarbonyl group, especially in the presence of rhodium(II) acetate... 

Diazirine, fluoromethoxy-nitrogen extrusion, 7, 224 Diazirine, methylvinyl-rearrangement, 7, 221 Diazirines addition reactions to Grignard compounds, 7, 2 0 as carbene precursors, 7, 236 IR spectra, 7, 203 microwave spectrum, 7, 199 molecular spectra, 7, 202-204 nitrogen extrusion, 7, 223 NMR, 7, 202 photoconversion to diazoalkanes, 7, 234 photoisomerization, 7, 221 photolysis, 7, 225-227 quantum chemical investigations, 7, 197 reactions... 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

At this point the catalytic process developed by Dotz et al. using diazoalkanes and electron-rich dienes in the presence of catalytic amounts of pentacar-bonyl(r]2-ds-cyclooctene)chromium should be mentioned. This reaction leads to cyclopentene derivatives in a process which can be considered as a formal [4S+1C] cycloaddition reaction. A Fischer-type non-heteroatom-stabilised chromium carbene complex has been observed as an intermediate in this reaction [23a]. 

Diazoalkanes can also be converted to ethers by thermal or photochemical cleavage in the presence of an alcohol. These are carbene or carbenoid reactions. Similar intermediates are involved when diazoalkanes react with alcohols in the presence of /-BuOCl to give acetals. ... 

Diazoalkanes are u.seful is precursors to ruthenium and osmium alkylidene porphyrin complexes, and have also been investigated in iron porphyrin chemistry. In an attempt to prepare iron porphyrin carbene complexes containing an oxygen atom on the /(-carbon atom of the carbene, the reaction of the diazoketone PhC(0)C(Ni)CH3 with Fe(TpCIPP) was undertaken. A low spin, diamagnetic carbene complex formulated as Fe(TpCIPP)(=C(CH3)C(0)Ph) was identified by U V-visible and fI NMR spectroscopy and elemental analysis. Addition of CF3CO2H to this rapidly produced the protonated N-alkyl porphyrin, and Bit oxidation in the presence of sodium dithionitc gave the iron(II) N-alkyl porphyrin, both reactions evidence for Fe-to-N migration processes. ... 

Whereas the Rh2(OAc)4-catalyzed addition of diazoalkanes to propargyl alcohols readily gives the insertion of the carbene into the 0-H bond, with only a small amoimt of cyclopropenation of the resulting propargylic ether [54] the 2-diazopropane 59 reacts at 0 °C with l,l-diphenyl-2-propyn-l-ol 62a in dichloromethane and exclusively gives, after 10 h of reaction, only the adduct 63a isolated in 75% yield and corresponding to the regioselective 1,3-dipolar cycloaddition of the 2-diazopropane to the alkyne C - C bond (Scheme 15). 

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. 

Other synthetic approaches have been explored for binding an alkylidene functionality to a metalla-calix[4]arene. Among them, the reaction of diazoalkanes with coordinatively unsaturated metalla-calix[4]arenes deserves particular mention. The synthesis of an unusual high-spin (5.2 BM at 292 K) iron(II)-carbene, 192, is displayed in Scheme 39,13 and its structure is shown in Fig. 22. 

The reaction conditions should be as mild as possible to prevent uncontrolled decomposition of the diazoalkane and carbene complex product. 

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. 

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