Carbenes reactivity, mechanisms

The TTyir singlet states of the cyclopropenes (101) are reactive and lead to re2irrangement products by way of a carbene (102) mechanism as outlined above. Typical examples of the reaction are shown in the Scheme 8, where it can be seen that the rearrangement affords the isomeric substituted furans (103) and (104) from (101, X = 0) and pyrroles (105) and (106) from (101, X =NMe). This latter reaction also yields the diene-substituted pyrrole (107), which is good evidence for the proposed carbene mechanisms. The thiophene derivative (101, X = S) yields a single product (108) on photoexcitation. 

Of the three classes of divalent carbon species—free carbenes, reactive transition-metal carbene complexes (carbenoids), and stable metal carbenes—we restrict our consideration to the first two. Taking into account the fact that a fine reaction mechanism in planning the synthesis of specific molecules is of secondary importance, we discuss carbene and carbenoid reactions together. We concentrate solely on reactions and reaction sequences that result in a formation of a new heterocycle. Within subsections, the material is organized on the basis of reaction type. 

The elucidation of the mechanism for olefin metathesis reactions has provided one of the most challenging problems in organometallic chemistry. In Volume 1 Rooney and Stewart concluded that the carbene chain mechanism is now generally accepted for olefin metathesis reactions, but much remains to be learned about the formation and reactivity of metal-carbene intermediates, metallocycles, and especially the mechanistic aspects of chain initiations. Since that report, systems have been designed that begin to reveal the important mechanistic features of olefin metathesis. 

Now an analysis (66) of the kinetics of the metal-carbene chain mechanism shows that this product should be 4, and it also allows a precise definition of another notable feature of Fig. 1, the formation initially of more Cjz than Cig although less 2-butene was present than 4-octene. 2-Butene must be more reactive than 4-octene, presumably for steric reasons, and the ratios of the rate constants defined by Eqs. (15)... 

Consider now what the product r, x /-z would be if the conventional mechanism were correct, but the olefin displacement reaction were rate determining. The kinetics of a mechanistic scheme much like that in Scheme 4 shows (66) that ri x at zero time is a function of two ratios, that of the concentrations of butene and octene and that of their reactivities. However, for all reasonable values of these ratios, rj x rz is never greater than 2.94. Accordingly, since in the double cross experiment the product was determined to be about 4 at zero time, the conventional mechanism is excluded no matter which step is ratedetermining. The implication is that the carbene chain mechanism is correct. 

Chiefly in an hydrophobic medium, a base can extract the proton on position 2 leading to a reactive intermediate (able to give subsequent condensation) that could be an ylid (35, 36) or a carbene (37), though no dimer has ever been isolated as is the case with benzothiazolium (32, 38). Two mechanisms have been proposed for explaining the particular reactivity of thiazolium ... 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

S+3C] Heterocyclisations have been successfully effected starting from 4-amino-l-azadiene derivatives. The cycloaddition of reactive 4-amino-1-aza-1,3-butadienes towards alkenylcarbene complexes goes to completion in THF at a temperature as low as -40 °C to produce substituted 4,5-dihydro-3H-azepines in 52-91% yield [115] (Scheme 66). Monitoring the reaction by NMR allowed various intermediates to be determined and the reaction course outlined in Scheme 66 to be established. This mechanism features the following points in the chemistry of Fischer carbene complexes (i) the reaction is initiated at -78 °C by nucleophilic 1,2-addition and (ii) the key step cyclisation is triggered by a [l,2]-W(CO)5 shift. 

They concluded that the reactivity of carbenes toward CO2 is determined by their philicity (more nucleophilic carbenes are more reactive) and that carbene spin state interestingly has little effect. Kovacs and Jackson have suggested that this reactivity pattern may be explained by a nonequilibrium surface crossing mechanism. ... 

As demonstrated in the two previous sections, TRIR spectroscopy can be used to provide direct structural information concerning organic reactive intermediates in solution as well as kinetic insight into mechanisms of prodnct formation. TRIR spectroscopy can also be used to examine solvent effects by revealing the inflnence of solvent on IR band positions and intensities. For example, TRIR spectroscopy has been used to examine the solvent dependence of some carbonylcarbene singlet-triplet energy gaps. Here, we will focns on TRIR stndies of specific solvation of carbenes. 

The textbook definition of a reactive intermediate is a short-lived, high-energy, highly reactive molecule that determines the outcome of a chemical reaction. Well-known examples are radicals and carbenes such species cannot be isolated in general, but are usually postulated as part of a reaction mechanism, and evidence for their existence is usually indirect. In thermal reactivity, for example, the Wheland intermediate (Scheme 9.1) is a key intermediate in aromatic substitution. 

Taking together the results of reactivity and stereoselectivity comparisons, one may conclude that the cyclopropanation mechanism as such is quite similar in all cases and involves a metal carbene, but that the stereoselectivity is determined by the nature of the diazoalkane substituent. Doyle has developed a mechanistic scheme which accounts for these observations (Scheme 44). 

Mechanism and reactivity in reactions of organic oxyacids of sulphur and their anhydrides, 17, 65 Mechanism and structure, in carbene chemistry, 7,153... 

The additional reactive intermediate responsible for the curvature was postulated17,33 to be a CAC.30 The mechanism of Scheme 2 was proposed, in which carbene 10a was in equilibrium with the CAC. Thus, styrenes 11a and 12a can be formed by two pathways from the free carbene (kj) and from the CAC (k-). A steady-state kinetic analysis of Scheme 2 affords Eq. 11, which predicts that a correlation of rearr/addn with l/[alkene] should be linear the behavior actually observed by Tomioka and Liu.17,33 The CAC mechanism also accounts for the observation that the lla/12a product ratio depends upon the identity and concentration of the added alkene both k[ and k2, which define the Y-intercept of Eq. 11, depend on the added alkene. The dependence has been observed,19,33-37 albeit with only small variations in the Y-intercepts. 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

Several of the phenylene-linked carbenes and nitrenes exhibit photochemical or thermal reactivity in the matrix. The photolabile p- and m-phenylene-linked species give products of rather unexpected structures via mechanisms that are not understood yet. The o-phenylene-linked species isomerize rather easily either via ring opening of the phenylene linker or by an apparent direct reaction of the two proximal diradical centers to give ring-closure products. The available data... 

This review will focus on the use of chiral nucleophilic A-heterocyclic carbenes, commonly termed NHCs, as catalysts in organic transformations. Although other examples are known, by far the most common NHCs are thiazolylidene, imida-zolinylidene, imidazolylidene and triazolylidene, I-IV. Rather than simply presenting a laundry list of results, the focus of the current review will be to summarize and place in context the key advances made, with particular attention paid to recent and conceptual breakthroughs. These aspects, by definition, will include a heavy emphasis on mechanism. In a number of instances, the asymmetric version of the reaction has yet to be reported in those cases, we include the state-of-the-art in order to further illustrate the broad utility and reactivity of nucleophilic carbenes. 

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