Directed intramolecular

Here, is the magnetization of spin i at thermal equilibrium, p,j is the direct, dipole-dipole relaxation between spins i and j, a-y is the crossrelaxation between spins i and j, and pf is the direct relaxation of spin i due to other relaxation mechanisms, including intermolecular dipolar interactions and paramagnetic relaxation by dissolved oxygen. Under experimental conditions so chosen that dipolar interactions constitute the dominant relaxation-mechanism, and intermolecular interactions have been minimized by sufficient dilution and degassing of the sample, the quantity pf in Eq. 3b becomes much smaller than the direct, intramolecular, dipolar interactions, that is. 

Xie and co-workers developed a simple route for the synthesis of 3-aryl-l,2,3,4-tetrahydroquinolines 79 using a direct intramolecular reductive ring closure strategy <06TL7191>. The yields for the key reductive ring closure were moderate however, the simplicity of their route leads to an efficient synthesis of a variety of tetrahydroquinolines 79. 

Directed intramolecular transfer hydrogenations are catalyzed by rhodium complexes with the pendant alkene acting as an internal sacrificial olefin (Equation (39)). 

Palladium-catalyzed directed intramolecular activations of aryl C-H bonds have been reported, as in the phenyla-tion of heterocycle analogs. Palladacycles are proposed intermediates, acting as effective catalysts, and the mechanism is likely to proceed via oxidation of Pd(ll) to Pd(iv) by the iodonium salt, as for the Equation (57), which described the activation of benzylic i/-CH bonds (Equations (121)—(123).109... 

Direct Intramolecular Carbon-Carbon Bond Forming Reactions. 66... 

Redox reactions usually lead, however, to a marked change in the species, as reactions 4-6 indicate. Important reactions involve the oxidation of organic and metalloprotein substrates (reactions 5 and 6) by oxidizing complex ions. Here the substrate often has ligand properties, and the first step in the overall process appears to be complex formation between the metal and substrate species. Redox reactions will often then be phenomenologically associated with substitution. After complex formation, the redox reaction can occur in a variety of ways, of which a direct intramolecular electron transfer within the adduct is the most obvious. 

The interconversion of 1- and 2-acids in H2SO4 at 160 could result either from a direct intramolecular isomerisation, or by reversal of sulphonation to yield naphthalene which undergoes new attack at the other position. It should be possible to distinguish between these alternatives by carrying out the reaction in H2 S04, for the former should lead to no incorporation of in the product sulphonic acids, whereas the latter should lead to such incorporation. Experimentally it is found that incorporation of does take place but at a rate slower than that at which the conversion occurs. This could imply either that both routes are operative simultaneously, or that, after reversal of sulphonation, new attack takes place on the resultant naphthalene by the departing H2SO4 molecule faster than by surrounding H2 S04 molecules—the question is still open. 

The only example known for the formation of azetidine 82 by direct intramolecular aza-Wittig reaction is the reaction of the /3-azidoketone 81 with triphenylphosphane (Scheme 41). Attempts to transfer this reaction to 83 and 84 were not successful (87NKK1250). This failure can be attributed to the formation of intermediates with highly energetic transition states, where the rate of intramolecular attack on the carbonyl function is so slow that oligo- and polymeric compounds are preferentially formed. 

Waals radii of two atoms (2.90 A). Nevertheless, each reaction site is far apart the hydrogen abstraction from the benzyl group proceeds and the product was obtained in optically active form. In this case, another mechanism besides the direct intramolecular hydrogen abstraction from the benzyl group may be involved, like intermolecular hydrogen abstraction or hydrogen shift. 

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

This reaction can initiate intramolecular cyclization of unsaturated thioketals and of enol silyl ethers containing a thioketal group to provide an equivalent of a directed intramolecular aldol reaction.3 Examples ... 

A novel hypervalent iodine-induced direct intramolecular cyclization of a-(aryl)alkyl-jS-dicarbonyl compounds 33 has been recently reported (Scheme 15) [30]. Both meta- and para-substituted phenol ether derivatives containing acyclic or cyclic 1,3-dicarbonyl moieties at the side chain undergo this reaction in a facile manner affording spirobenzannulated compounds 34 that are of biological importance. 

The pentamer of tetrafluoroethene (66) (Scheme 29) is an unusual example of type (96) and reacts readily with nucleophiles [129] (Scheme 65). In contrast, (66) undergoes a remarkable reaction with aqueous triethylamine, producing the dihydrofuran derivative (101) and the process formally involves a direct intramolecular displacement of fluorine from a saturated site and a mechanism has been advanced (Scheme 66) which accounts for the product formed [126]. Understandably, this process is not easily accepted [3, 130] because it has essentially no precedent. Indeed, it is well established that nucleophilic displacement from saturated sites in fluorocarbons occurs only in exceptional circumstances. Consequently, other mechanisms have been advanced, which seem no more convincing [3, 130]. It should be remembered that the major point in favour of the step (100) to (101) (Scheme 66) is that the nucleophile is generated in close proximity to the reaction centre because of the special geometry of this situation. Consequently, much of the otherwise high energy/entropy barrier has already been overcome in this case. 

Scheme 4. The direct intramolecular asymmetric aldol reaction using L-proline.

It is only recently that S. Faulkner took advantage of the kinetic inertness of Lnm cy-clen macrocyclic complexes for producing the neutral pure heterotrimetallic compound [Yb(116b)] (fig. 96) in which convergent directional intramolecular Tbm - Ybm processes are responsible for the sensitization of the NIR Yb(2F5/2) emission (Faulkner and Pope, 2003). The complex is stable in water and according to the lifetime measured (1.83 and 4.22 ps in H2O and D2O, respectively), the hydration number of Yb111 is close to zero (heptadentate cyclen derivative. 

Some interesting intramolecular variants have been reported. For example, homoallylic alcohols (e.g. 139) can be treated with sulfamoyl chloride to form the corresponding sulfamates (140), which then engage in a direct intramolecular copper-catalyzed aziridination mediated by iodosylbenzene <02OL2481>. A carbamate tether is also effective in delivering the nitrene center to the olefin, as is the case with the cyclohexenyl derivative 142, which spontaneously cyclizes in the presence of iodosylbenzene <02OL2137>. The acetoxy-aminoquinazolinone 144 is converted to the lactone 145 via intramolecular aziridination upon treatment with lead tetraacetate and hexamethyldisilazane (HMDS) <02TL2083>. 

An even closer parallel is found in the work of Kreilick (33, 34), who prepared the ketal derived from 2,6-di-tert-butyl-4-acetoxyphenol. From a study of the effect of temperature on line broadening in the NMR spectrum of this compound he concluded that rearrangement occurs both by dissociation to radicals and recombination and by a direct intramolecular process. The former corresponds to the key reaction of the redistribution mechanism, while the latter is entirely analogous to the ketal rearrangement mechanism (Reaction 15). 

It is proved that at short lengths of dimethylsiloxane unit, HFC reaction proceeds in two directions intramolecular ring formation giving polycyclic products and intermolecular formation of cyclo-linear copolymers. Formation of polycyclic products is proved by the direct synthesis. 

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