Carbenes diazoalkane cycloaddition 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]. 

Q ,/i-Unsaturated complexes undergo a number of facile cycloaddition reactions to produce more elaborate complexes. For example, cycloadditions using diazoalkanes, nitrileimines, alkylnitrones, azomethine ylids (see Ylide), and imines are feasible. For example, reaction of the chiral carbene (9) with trimethylsilyldia-zometathane gave the heterocyclic-substituted carbene (10) (Scheme 15). ... 

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). 

The reaction of diazoalkanes with acetylenes can give rise to cyclopropenes by two main routes. Some reactions involve an initial loss of nitrogen to generate a carbene which then adds to the acetylene (see Section 1.2.1.), but this section is concerned only with those reactions where the first step is a cycloaddition leading to formation of a 3//-pyrazole. Unlike the parallel series of reactions in the cyclopropane series, where the C-C double bond of the alkene requires activation by a suitable substituent or by strain. Under pressure even acetylene itself will react with diazoalkanes. For example, diphenyldiazomethane underwent addition in good yield and deazetization gave 3,3-diphenylcyclopropene (1). ... 

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. 

Cycloaddition of 1,3-dipolarophiles to alkynes for the synthesis of diazo compounds can also be applied to reaction of diazoalkanes with alkynes (2-91). 2-Diazopropane and 1,2-diarylethynes readily form 3//-pyrazoles (2.229). These pyrazoles isomerize photochemically to the 4-diazo-2-methyl-3,4-diarylbutenes (2.230), i.e., to a vinyldiazo compound (Pincock et al., 1973 Arnold et al., 1976 Leigh and Arnold, 1979). Some cyclopropene (2.231) is formed in a consecutive dediazoniation, i. e., by cyclization of the carbene formed. The method is not useful for unsymmetrically substituted alkynes because these cycloadditions are not regiospecific. It is, however, applicable to the synthesis of diazoalkenes with alicyclic... 

The reaction system (6-37) includes the thermal azo-extrusion of a cyclic azo compound to a cyclopropane derivative and the direct formation of cyclopropanes, catalyzed by metal complexes. Synthetic routes to cyclopropane derivatives became an important subject in the last two decades, and one frequently used method is the 1,3-dipolar cycloaddition of a diazoalkane to an alkene followed by thermal or photolytic azo-extrusion of the 4,5-dihydro-3//-pyrazole formed to the cyclopropane derivative (6-37 A). This route can be followed in many cases without isolation, or even without direct observation, of the 4,5-dihydro-3//-pyrazole. Therefore, it is formally very similar to cyclopropane formation from alkenes with diazoalkanes, in which a carbene is first formed by azo-extrusion of the diazoalkane (see Sect. 8.3). As shown in pathway (6-37 B), this step can be catalyzed by copper, palladium, or rhodium complexes (see Sects. 8.2, 8.7, and 8.8). There are cases where it is not clearly known whether route A or B is followed. Scheme 6-37 also includes... 

Very active interest in a new addition reaction of aliphatic diazo compounds started in 1991 when WudPs group reported that diphenyldiazomethane forms diphenylmethanofullerene with buckminsterfullerene (C o Suzuki et al., 1991). Although this investigation showed that the reaction proceeds via the formation of a dihydro-pyrazole, i.e., in the mode of a 1,3-dipolar cycloaddition followed by an azo-extrusion, we shall discuss the syntheses of methanofullerenes in its entirety in the chapter on carbenes (Sect. 8.4) because Diederich s recent work (see review of Diederich et al., 1994b) shows that the methano bridge can also be obtained from a carbene. The question whether the dihydro-pyrazoles are intermediates or side-equilibrium products (see earlier in this section) is also open for the reaction of with diazoalkanes. 

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). 

Diazoalkanes were readily decomposed and underwent [2+l]-type cycloaddition to alkenes in the presence of catalytic amounts of rhodium complexes. Rh2(OAc)4 is the simplest complex for the reaction. For example, fluoro-cyclopropane 134 was prepared by the carbene addition to fluoroalkene 133 (Scheme 1.64) [108]. A copper catalyst also catalyzed the addition reaction. 

The treatment of 1,6-enynes with 5mol% amounts of cp RuCl(cod) 382 in the presence of diazoalkanes resulted in the formation of bicyclic cyclopropanes 383 in good yields (Scheme 1.180) [252]. The reaction progressed through the formation of mthenacyclobutene 384, which cleaved to give a mthenium carbene complex 385. A [2-1-2] cycloaddition of the complex 385 with the internal alkene... 

Herzberg and collaborators provided the first experimental evidence that a meta-stable, bent singlet was initially produced from diazoalkane and decayed to a ground-state triplet. - The hquid-phase chemistry of methylene, for example, is simple in comparison with its gas-phase chemistry. However, hot molecules produced by extremely exothermic reactions of methylene play a minor role in the gas phase. Carbenes have been used on countless occasions to make three-membered rings via [1+2]-cycloaddition. 

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