Formation of Carbenes

The domino reaction of a carbonylylide from a diazoketone followed by a 1,3-dipolar cycloaddition has also been investigated using rutbenium(II)porphyrins 6/2-39 as catalyst [196]. Moreover, Che and coworkers [197] have used the Ru-cata- 

The examples described so far clearly show the value of the rhodium-catalyzed carbene transfer obtained from diazo compounds onto carbonyl and imino groups. However, the scope is even broader, as the formed carbene can also undergo an ad- 

Maas and coworkers [199] showed that the rhodium(II)-catalyzed decomposition of vinyldiazoacetate 6/2-44 in the presence of semicydic enaminocarbonyl compounds 6/2-43 gives betaines as 6/2-45, with formation of a spiro compound as in- 

These authors were also able to perform this domino process in an enantioselec-tive fashion using the Rh-proline derivative 6/2-57 (Rh2(S-DOSP)4) as chiral catalyst for the cyclopropanation [202]. Reaction of 2-diazobutenoate 6/2-56 and alkenes 6/2-55 in the presence of the catalyst 6/2-57 led primarily to the cyclopropane derivative 

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The olefins that undergo metathesis include most simple and substituted olefins cycHc olefins give linear high molecular-weight polymers. The mechanism of the reaction is beheved to involve formation of carbene complexes that react via cycHc intermediates, ie, metaHacycles. Industrial olefin metathesis processes are carried out with soHd catalysts (30). 

Preliminary investigations on the formation of carbenes from diazirines have already been made available. Frey and Stevens recently reported the photolysis of cyclic diazomethane. Cyclic diazomethane was irradiated in the gaseous phase with light of wavelength... 

However, formation of the metal carbene complex was not observed in pure, halide-free [BMIM][Bp4], indicating that the formation of carbene depends on the... 

Scheme 5.3-2 Formation of carbene complexes by dialkylimidazolium salt deprotonation.

Scheme 5.3-3 Formation of carbene complexes by oxidative addition to Pt(0).

For a review of formation of carbenes in this manner, see Kirmse, W. Angew. Chem. Int. Ed. Engl, 1965, 4, 1. 

In all cases the formation of carbenes was accompanied by partial photoisomerization of the precursor diazirines [53] into the corresponding diazo compounds [55] which possess a higher photochemical stability than the diazirines. 

Alternatively, enthalpies of formation of carbenes and biradicals can be measured by using the approach shown in Eq. 5.4b. The key to the measurement is the determination of the proton affinity of the substrate, PA(R), which can be obtained... 

TABLE 5.3. Enthalpies of formation and BDEs for formation of carbenes, diradicals, carbynes, and triradicals. ... 

Enthalpies of formation and BDEs for the formation of carbenes and diradicals measured by using negative ion approaches are included in Table 5.3. 

Section D illustrates formation of carbenes from halides by a-elimination. The carbene precursors are formed either by deprotonation (Entries 14 and 17) or halogen-metal exchange (Entries 15 and 16). The carbene additions can take place at low temperature. Entry 17 is an example of generation of dichlorocarbene from chloroform under phase transfer conditions. 

The triplet state is usually the ground state for non-conjugated structures, but either species can be involved in reactions. The most common method for generating nitrene intermediates, analogous to formation of carbenes from diazo compounds, is by thermolysis or photolysis of azides.246... 

Diazirine is a cyclic isomer of diazomethane. According to the organometallic literature, scission of both C-N and N-N bonds can occur when diazirines interact with metal complexes. The formation of carbene ligands arises from selective cleavage of the C-N bond, whereas selective N-N bond scission results in the formation... 

In some, though not necessarily all, cases loss of H and Cl is thought to be concerted, leading directly to the carbene (cf. p. 50) intermediate (75) formation of the product alkene from (75) then requires migration of H, with its electron pair, from the /J-carbon atom. A 1,1-elimination (Ea) will be indistinguishable kinetically from 1,2-(E2), and evidence for its occurrence rests on isotopic labelling, and on inferential evidence for the formation of carbenes, e.g. (75). 

Scheme 6/2.1. Rhodium(ll)catalyzed formation of carbenes from diazo compounds.

Besides the formation of carbenes from diazo compounds and the hydroformyla-tion, rhodium (as described previously for palladium) has also been used as catalyst in domino processes involving cycloadditions. Thus, Evans and coworkers developed a new Rh(I)-catalyzed [4+2+2] cycloaddition for the synthesis of eight-membered rings as 6/2-105 using a lithium salt of N-tosylpropargylamines as 6/2-104, allyl carbonates and 1,3-butadiene (Scheme 6/2.22) [221]. The first step is an al-... 

Palladium(II) acetate was found to be a good catalyst for such cyclopropanations with ethyl diazoacetate (Scheme 19) by analogy with the same transformation using diazomethane (see Sect. 2.1). The best yields were obtained with monosubstituted alkenes such as acrylic esters and methyl vinyl ketone (64-85 %), whereas they dropped to 10-30% for a,p-unsaturated carbonyl compounds bearing alkyl groups in a- or p-position such as ethyl crotonate, isophorone and methyl methacrylate 141). In none of these reactions was formation of carbene dimers observed. 7>ms-benzalaceto-phenone was cyclopropanated stereospecifically in about 50% yield PdCl2 and palladium(II) acetylacetonate were less efficient catalysts 34 >. Diazoketones may be used instead of diazoesters, as the cyclopropanation of acrylonitrile by diazoacenaph-thenone/Pd(OAc)2 (75 % yield) shows142). 

As has already been mentioned for cyclopropanation of olefins, the diazoester should be added slowly to the mixture of alkyne and Rh2(OAc)4, in order to minimize formation of carbene dimers. The reaction works well with mono- and... 

For the formation of stilbenes from aryldiazomethanes, Rh2(OAc)4 was shown to be superior to other catalysts such as CufClO or CuBr2 357), LiBr363 or Ce(NH4)2(N03)6 364) in terms of efficiency, Z-selectivity and compatibility with substituents on the aromatic ring of the diazoalkane 358 . Even higher Z-selectivity was provided by the bulky catalyst iodorhodium(III) mew-tetraphenylporphyrin, but reduced yields had to be acknowledged358 . Contrary to copper catalysts, RhjfOAc failed to induce the formation of carbene dimers from secondary aryldiazoalkanes azines were produced instead 358). 

Besides the advantage of recyclability, reactions in ionic liquids are generally faster and are run under milder conditions than reactions with conventional solvents. Further activation may come from ultrasonic agitation.520 Since the majority of ionic liquids used are imidazolium salts, the effect of these solvents can be at least partly attributed to the in situ formation of carbene complexes (Section 9.6.3.4.10).521 Cross-coupling of ArB(OH)2 can also be efficiently performed in ionic liquids based on long-chain tetraalkylphosphonium salts, in which case aryl bromides and some aryl chlorides can be processed in the presence of the trivial ligand PPh3.522... 

In principle, the heat of formation of carbenes can be determined from PAC heats of reaction and either the carbene precursor or the carbene products heats of formation (Scheme 1). 

The carbene route to bridgehead olefins is a well established reaction and has been developed to a major method for the generation of bridgehead alkenes. The field has been recently reviewed by one of the main contributors to this area.1 The reversed reaction, the formation of carbenes from distorted olefins, has also been known for a long time. Distortion of the tt bond in alkenes is easily affected by photoexitation. In connection with those reactions, carbene chemistry has been observed and the field has also been reviewed some years ago.2... 

In solution, multinuclear NMR spectroscopy is by far the most informative technique for analyzing the structure and bonding of phosphinocar-benes. In fact, prior to the synthesis and single crystal X-ray analysis of the (phosphino)(phosphonio)carbene 2d,28 the only spectroscopic evidence for the formation of carbenes came from NMR. 

The photolysis or thermolysis of diazo compounds results in the formation of carbenes. These reactions were identified as early as 1901 by Hantzsch and Lehmann D and Staudinger and Kupfer who decomposed diazomethane photochemically and thermally, respectively. The subsequent work of Hine 3) and Doering ) started the era of carbene chemistry. Excellent reviews of the chemistry of carbenes are available 5-27),... 

Direct treatment of organic geminal dihalides or trihalides with strongly nucleophilic transition metal complexes can also lead to the formation of carbene complexes, presumably via intermediate a-haloalkyl complexes [484-489]. Examples of such reactions are sketched in Figure 3.17. 

A cycloaddition process between the Rh=C bond of the allenylidene derivative 38 and the C=C bond of the terminal alkyne has been evoked in the formation of the zwitterionic 71-aUyl-allenyl complexes 81 (Scheme 28), the initially formed metaUacyclobutenes 80 evolving into 81 by formation of carbene intermediate [RhCl(P/-Pr3)2(=CHCR=C=C=CPh2)] (R = Ph, p-MeC6H4, SiMe3) and subsequent migration of one of the phosphine ligands from the metal to the carbene carbon atom [205]. 

Flash vacuum thermolysis of 6-chloro-3-diazoindazole (2b) resulted, upon loss of nitrogen, in the formation of carbene 102, which could intra-molecularly rearrange to the nitrene 103, or to the azabenzocyclopropene 104, or to azacycloheptatrienylidene 105 (Scheme 29). The only isolable product was 106, formed by dimerization of the nitrene 103 (78CB2258). 

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