Jt-complex intermediates

SCHEME 1.17 Calculated (HF/STO-3G level) x-electron populations at ring positions of substituted 

In the initial stages of an S Ar reaction, a jc-complex often forms between the electrophile and the arene. Similar donor-acceptor complexes have been long known from solntion-phase smdies. These complexes were observed to be noncondnctmg, colored solutions formed from mixing an aromatic compound with a x-acceptor, such as HCl, Ag salts, or I.  

The involvement of x-complexes in S Ar reactions was first proposed by Dewar to explain relative reaction rates for some conversions [48]. For example, the relative stabilities of arene x-complexes (with HCl) have been shown to correlate with the relative rates of nitration (Table 1.3) [49]. The x-complex for m-xylene is estimated to be only about twice as stable as that for benzene. The relative rates of nitration for these two arenes are similar, suggesting a role of the x-complex in the rate-determining step of the nitration. In contrast, chlorination exhibits a markedly greater rate of reaction with m-xylene compared to benzene. This suggests that the rate-determining step for chlorination involves a transition state resembling the a-complex. Thus, the importance of x-complexes varies among different S Ar reactions. 

TABLE 1.3 Relative Stabilities of HCl—Arene Ji- and T-Complexes and Relative Rates of Reactions 

FIGURE 1.4 X-ray crystal structures of the [Br, C HJ jt-complex (57) and the [Br, Cj.H5CHj] x-complexes (58 and 59). From Vasilyev et al. [42]—Reproduced with permission from the Royal Society of Chemistry. 

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The mechanism of mercuration is shown in Scheme 2. In the first step, the mercury salt forms a Jt-complex with the aromatic substrate [15, 16]. In 1982, Lau, Huffman, and Kochi [17] reported the first isolation and full characterization (including X-ray molecular and crystal structure) of such an intermediate, a complex of hexamethylbenzene with Hg(TFA)2. The X-ray structure revealed a Hg2( -TFA)i framework with a molecule of C6Me6 -coordinated to each of the Hg atoms. Analogous Jt-complexes have also been observed and studied by Dean and co-workers [18] and more recently by Barron s [19] and Gabbai s [20] groups. The Jt-complex intermediate can rearrange to the ej-complcx (a Wheland intermediate) directly, or sometimes via electron transfer, to produce a radical ion pair which then collapses (Scheme 2) [21,22]. 

Alternatively, copper may activate the iodoalkyne via the formation of a Jt-complex intermediate (Scheme 7.13b, which then engages the azide, producing complex 54. Cyclization then proceeds via a now familiar vinylidene-like intermediate 55, to give iodotriazole 52. A similar transition state has been proposed to explain the involvement of di-copper intermediates in the CuAAC reaction [114, 115]. The distinctive feature of this pathway is that the C—I bond is never severed during the catalysis. 

The widely known Repp s synthesis of cyclo-octatetraene by tetra-merization of acetylene undoubtedly involves a metal jt-complex intermediate. However, a clear-cut mechanism for this reaction has not yet been advanced. The formation of a cyclobutadiene ring by cyclodimerization of... 

Similar models explain the 1,8-, 1,10- and 1,12-addition reactions to the extended Michael acceptors 91, 93 and 95, respectively (Schemes 2.32 and 2.33). Again, these transformations start with the formation of a cuprate Jt-complex at the double bond neighbouring the acceptor group [61a]. Subsequently, an equilibrating mixture of a-copper(III) intermediates is presumably formed and the regioselectivity of the reaction may then be governed by the different relative rates of the reductive elimination step of these intermediates. Consequently, the exclusive formation of allenic prod-... 

In general the syn,syn complex is the most stable isomer but it is in equilibrium with the other three diastereomers (Scheme 17.3). The interconversion between the jt-complexes proceeds via the less stable rj-complexes and is rather slow. Usually more than one Jt-allyl intermediate is present in the reaction mixture, which makes chirality transfer from an enantiomerically enriched allene to the product complicated. 

The remaining double bond has a strong preference for a -configuration in the product. This is due to a slow equilibrium between the different jt-allyl intermediates, as shown in Scheme 17.7. When the attack of the second nucleophile is relatively slow then the intermediate complex has enough time to rearrange to the thermodynamically preferred isomer that will lead to the product with a Z double bond. 

This process probably takes place through a Jt-complex-type intermediate without the complete detachment of the migrating tert-butyl group from the aromatic ring.80 82... 

Reactions of the HNiL3CN complex with 1,3-cyclopentadiene, 1,3-cyclo-hexadiene, and 1,3-cyclooctadiene gave intermediates with decreasing stabilities in that order the 1,3-cyclooctadiene intermediate was not spectroscopically observable. The cyclohexadiene adduct was shown to be the cyclohexadienyl complex 12 by its proton spectra, with resonances of H , Hb, and —(CH2)3— at 14.53, 6.06, and 8.47, respectively these values are close to the chemical shifts found earlier (51) for 13 14.52,5.86, and 8.48. The reaction of DNi[P(OMe)3]X with cyclopentadiene gives 13-d, with addition of D and Ni to the same side of the ring (52). Backvall and Andell (55) have shown, using Ni[P(OPh)3]4 and deuterium cyanide (DCN), that addition of D and CN to cyclohexadiene is stereospecifically cis, as expected for jt-allyl intermediate 12. 

In the competition between allylic intramolecular cyclopropanation and macrocyclization (Eq. 5.20), the more electrophilic catalyst favors macrocyclization. Doyle has explained this differential selectivity as due to the formation of an intermediate Jt-complex between the C-C double bond and the carbene center. The more electrophilic the carbene carbon or the more electron-rich the double bond, the more that this Jt-complex is favored and the more favorable the pathway to macrocyclization [97]. However, thus far few systems have been examined... 

Distinction between enantiodiscrimination by complexation and by alkylation of equilibrating intermediates is less clear in a number of related cases. It is likely that more than one type of chiral discrimination may be involved. For example, when a conformational ly flexible four-membered ring substrate is used for the same reaction, the enantioselectivity was only 56% ee (Eq. 8E.15) [175]. In this case, it has been proposed that equilibration via a tertiary e-palladium species may be possible, switching the origin of enantio-discrimination to the alkylation step. A more contrasting example involves the formation of an asymmetric diene via selective P-elimination of similar diastereomeric Jt-allyl intermediates (Eq. 8E.16). Evidence suggests that the enantio-determining elimination process occurs after the equilibration of the 7t-allyl intermediates [176]. 

MO studies of aromatic nitration cast doubt on the existence of jt-complexes and electron-transfer complexes in liquid-phase nitrations.14 The enthalpy of protonation of aromatic substrates provides a very good index of substrate reactivity to nitration. Coulomb interaction between electrophile and substituent can be a special factor influencing regioselectivity. A detailed DFT study of the reaction of toluene with the nitronium ion has been reported.15 Calculated IR spectra for the Wheland intermediates suggest a classical SE2 mechanism. MO calculations of cationic localization energies for the interaction of monosubstituted benzenes with the nitronium ion correlate with observed product yields.16... 

H, Me, r-Bu, or Ph or R = H and R = Me, r-Bu, or Ph), was performed. Two possible reactions were investigated (a) the reactions suitable for the gas-phase interactions, which start from a 1 1 Br2-alkyne r-complex and do not enter into a 2 1 Br2-alkyne jt-complex and (b) the processes passing through a 2 1 Br2-alkyne 7r-complex, which appear more realistic for the reactions in solutions. The structures of the reactants and (g) the final products and also the possible stable intermediates have been optimized and the transition states for the predicted process have been found. Both trans- and cw-dibromoalkenes may ensue without the formation of ionic intermediates from a n-complex of two bromine molecules with the alkyne (solution reactions). The geometry around the double bond formed in dibromoalkenes strongly depends on the nature of the substituents at the triple bond. The cluster model was used for the prediction of the solvent influence on the value of the activation barrier for the bromination of the but-2-yne.35... 

The only targeted preparation of a lA3,2A3-diphosphete complex is based on a thermal-induced chloride abstraction by a transition metal and rearrangement of the isolable intermediate (/ra t-l,2-dichloro-lA3,2A3-diphosphete)Fe2(CO)8 cr-complex lg which leads to the (lA3,2A3-diphosphete)Fe(CO)3 Jt-complex 112 <1999OM2021>. Bis-trimethylsilyl-l,3-diphosphetane-2,4-diyl 6e <1999AGE3028> may be reduced by K or Li to form aromatic 1,3-diphosphetediide salts, for example, 20 (Scheme 36) <2004AGE637, 2004PS779>. 

A very reasonable role of copper(I) in the coupling process seems to be intermediate formation of non-reactive copper-jt-complexes. Coordination of cuprous ions activates the corresponding alkyne units toward deprotonation (Scheme 5). 

In the Au(I) catalysis of electron-poor alkynes such as 4, the catalytically active species is likely to be a cationic ligand-stabilized gold(I) Jt-complex, as in previously reported additions of oxygen nucleophiles to alkynes [5], Gold catalysts are very soft and thus carbophilic rather than oxophilic. On the basis of this assumption a plausible mechanism can be formulated as shown in Scheme 6. The cationic or strongly polarized neutral Au(I)-catalyst coordinates to the alkyne, and nucleophilic attack of the electron-rich arene from the opposite side leads to the formation of a vinyl-gold intermediate 7 which is stereospecifically protonated with final formation of the Z-olefm 8 [2, 4]. Regioselectivity is dominated by elec-... 

Jt-allyl complex can be generated after cyclization, as suggested by Takacs in a Fe(0)-catalyzed cyclization of polyenes. It also can be preformed if an active functional group is present in the allylic position. The palladium-catalyzed intramolecular cycloisomerization reaction of allylic acetates is an efficient method for constructing five- or six-membered rings [56, 57]. An asymmetric approach to this transformation has been studied and so far only poor enantioselectivity has been achieved (0-20% ee) [58]. Very recently, Zhang et al. also reported a Rh-catalyzed cycloisomerization involving a Jt-allylrhodium intermediate formed from an allylic halide [59]. 

For many years the question has been discussed as to whether other intermediates are involved in electrophilic aromatic substitutions in addition to a-complexes. Most claims that jt-complexes or radical pairs are intermediates are ambiguous. It is not possible to differentiate between an intermediate on the direct way from reagents to products and an adduct of the reagents in a side equilibrium, if in the formation or dissociation of such a compound no additional particle is added or transferred to another particle. Only in such a case can the steady-state equations be tested by checking the dependence of the overall rate constant on the concentration of such particles. 

In this chapter, the synthesis, structure, and reactivity of several Jt-allylmthenium complexes, and characteristic C-C bond-forming reactions mediated and catalyzed by ruthenium complexes via Jt-allylruthenium intermediates are described. 

A similar ruthenium complex (C5H5)RuCl(cod) catalyzes a totally different reaction pathway for alkynes and allylic alcohols to produce y,d-unsaturated ketones, which involves a ruthenacyclopentene intermediate, rather than a jt-allylmthenium intermediate [39]. 

The oligomerization and cooligomerization of conjugated dienes are representative reactions that proceed via transition-metal Jt-allyl intermediates. When (CsMesjRuCljt/ -butadiene) in dichloromethane was treated with an acetone solution of an equimolar amount of silver trifluoromethanesulfonate (AgOTf) in the presence of excess butadiene at ambient temperature, after which the mixture was allowed to react with carbon monoxide (1 atm), a cationic 1,5-cyclooctadiene carbonyl complex, [(C5Me5)Ru(CO)( -l,5-C8Hi2)]OTf, was isolated in 95% yield (Eq. 

Compared to the other reaction pathways. Scheme 6 illustrates the most plausible mechanism for chain termination. The reaction barrier of 8.3 kcal/mol is higher than the insertion barrier of 5.7 kcal/mol. Keim and co-workers have successfully trapped the nickel hydride as evidence to support their catalytic mechanism in which the nickel hydride is considered as the active catalyst [15]. We have found in the present study that the nickel hydride actually exists as an intermediate of the chain termination process. The premise for Scheme 6 to be practically competitive to the ethylene insertion reactions is the formation of the 7t-complex 4c. Based on our calculations, 4c is only a shallow minimum with the stabilization energy of 0.2 kcal/mol. Higher ethylene concentration is thus expected to facilitate the formation of the jt-complex and hence to increase the possibility of chain termination in order to generate dimers and trimers. 

The bond between the carbon atoms a and (3 to a C-C double bond can be broken by a transition metal with formation of a Jt-allyl intermediate providing the driving force. Whereas stoichiometric reactions of this sort are yet to appear, jt-(allyl)metal intermediates are occasionally involved in catalytic C-C bond cleaving reactions. The nickel catalyzed skeletal rearrangement of 1,4-dienes involves the formation of an olefin coordinated Jt-(allyl)nickel complex (99) [118]. 

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