Jr-complex

On the basis of these correlations, Gold and Satchell463 argued that the A-l mechanism must apply (see p. 4). However, a difficulty arises for the hydrogen exchange reaction because of the symmetrical reaction path which would mean that the slow step of the forward reaction [equilibrium (2) with E and X = H] would have to be a fast step [equivalent to equilibrium (1) with E and X = H] for the reverse reaction, and hence an impossible contradiction. Consequently, additional steps in the mechanism were proposed such that the initial fast equilibrium formed a 7t-complex, and that the hydrogen and deuterium atoms exchange positions in this jr-complex in two slow steps via the formation of a a-complex finally, in another fast equilibrium the deuterium atom is lost, viz. 

Transition Metal Arene jr-Complexes in Organic Synthesis and Catalysis... 

P. B. Venuto and P. S. Landis On Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of jr Complex Multicenter Processes G. N. SCHRAUZER... 

An indication of the nature of the transition state in aromatic substitution is provided by the existence of some extrathermodynamic relationships among rate and acid-base equilibrium constants. Thus a simple linear relationship exists between the logarithms of the relative rates of halogenation of the methylbenzenes and the logarithms of the relative basicities of the hydrocarbons toward HF-BFS (or-complex equilibrium).288 270 A similar relationship with the basicities toward HC1 ( -complex equilibrium) is much less precise. The jr-complex is therefore a poorer model for the substitution transition state than is the [Pg.150]

Some derivatization methods mentioned in other sections of this review include chemical ionization by nitric oxide (MS) or epoxidation (MS), formation of jr-complexes for NMR (shift agents) etc. Also, the Diels-Alder reaction, which was mentioned several times as a tool for derivatization of conjugated dienes and polyenes, was extensively described and reviewed in the literature. 

As mentioned earlier, products of disproportionation often accompany the rearrangement products. This reaction is also acid-catalysed and it is a reasonable assumption that reaction proceeds via the protonated species. Experiments with the 4,4 -diiodohydrazobenzene (19) showed that there were significant nitrogen and para-carbon kinetic isotope effects23. This implies that disproportionation must take place after C—C bonding has occurred, i.e. that the intermediate must be the quinonoid form 20 (and cannot, for example, be a jr-complex), which is then believed to react with another reactant molecule to give the disproportionation products (Scheme 4). 

The formation of the bridged intermediate has been represented as an SN2-like displacement of the leaving group from the sulfenyl sulphur of 85116, or alternatively, as reported in equation 90 in agreement with the addition of other electrophiles to alkenes, it has been proposed that the reaction involves the initial formation of jr-complex 86 in a rapid equilibrium with the reagents76. 

C4H6)Fe(CO)3. The elucidation of the structure of ferrocene eventually lead Hallam and Pauson2 to propose a jr-complex (1) for (C4H6)Fe(CO)3 and this was eventually confirmed by crystal structure analysis at low temperature3. Since that time interest in... 

As may be seen from the summaries, all basicity gradations quoted here have predominantly been obtained from jr-complexes or EDA-complexes. If the pronounced differences in the nature of the acceptors are considered, it is nevertheless possible to recognize a satisfactory, even though only qualitative, agreement of gradations. In particular, the effect of substitution of methyl groups and halogen atom in benzene, olefins and condensed aromatic substances may be observed to be similar. 

Although the transition state for the exchange reaction may be described as the critical complex for the conversion of the half-hydrogenated state to either a jr-complexed olefin or an eclipsed vicinal diadsorbed alkane, the stereochemistry of hydrogenation of cycloalkenes on platinum at low pressures can be understood if the transition state has a virtually saturated structure. 

H. Rao, M. J. Steel, R. Scarmozzino and R. M. Osgood, Jr., Complex propagators for evanescent waves in bidirectional beam propagation method. Journal of Lightwave Technology 18, 1155-1160 (2000). 

Regioselective syntheses of 1,3,5-unsymmetrically substituted benzenes (309) are catalyzed by Pd(dba)2/PPh3 mixed alkyne/diyne reactants give mixtures containing homocoupled and mixed products (24 21 from HC CPh + HC=CC= CC Hn). The probable mechanism involves oxidative addition to the Pd(0) center, insertion of the second diyne into the Pd—H bond, reductive coupling and subsequent jr-complexation of this product to Pd(0), followed by Diels-Alder cycloaddition of the third diyne and elimination of product. 

Farona and Kraus have postulated the existence of both a- and rr-bonded dinitrile complexes of manganese carbonyl halides and suggest that the monomeric jr-complexes are the kinetically favoured species, whereas the CT-bonded complexes are thermodynamically favoured. ... 

FIGURE 15. (a) X-ray structure of the jr-complexed lithium in the monomeric benzene complex of 2,6-bis(2,4,6-tri-isopropylphenyl)phenylhthium . (b) X-ray structure of the jr-complexed lithium in the dimeric 2,6-bis(2,6-di-isopropylphenyl)phenyUithium . It is evident from the Figure that the local environments around the lithium cations are similar... 

Dicyclopentadienyliron finds limited application as a catalyst. It was used earlier as an antiknock additive for gasoline. The complex also is used to synthesize other metal jr-complexes and their derivatives. 

The Lewis acid complex 4 can cleave into an ion-pair that is held together by the solvent cage, and that consists of an acyUum ion and a Lewis acid-bound phenolate. A jr-complex 6 is then formed, which further reacts via electrophilic aromatic substitution in the ortho- or para-position ... 

Synthetic operations involving ozonolysis lead to formation of aldehydes, ketones or carboxylic acids, as shown in Scheme 16, or to various peroxide compounds, as depicted in Scheme 7 (Section V.B.5), depending on the nature of the R to R substituents and the prevalent conditions of reaction no effort is usually made to isolate either type of ozonide, but only the final products. This notwithstanding, intermediates 276 and 278 are prone to qualitative, quantitative and structural analysis. The appearance of a red-brown discoloration during ozonization of an olefin below — 180°C was postulated as due to formation of an olefin-ozone complex, in analogy to the jr-complexes formed with aromatic compounds however, this contention was contested (see also Section V1I.C.2). 

Formation of a jr-complex between 1-hexene or 2,4,4-trimethylpent-l-ene and ozone was tentatively proposed based on the appearance of a sharp band at ca 980 cm , assigned to one of the ozone vibrations . However, the existence of olefin-ozone jt-complexes has been contested . 

Tetraalkylammonium superoxide, 736 1,1,2,2-Tetraarylcyclopropanes, photooxygenation, 205-6 Tetrachloroethene, jr-complex with ozone, 731-2... 

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