Donor intramolecular reactions

Intramolecular reactions of electron donor and acceptor sites in cyclic starting materials produce spirocyclic, fused, or bridged polycyclic compounds. 

Intramolecular reactions between donor and acceptor centres in fused ring systems provide a general route to bridged polycyclic systems. The cts-decalone mesylate given below contains two d -centres adjacent to the carbonyl function and one a -centre. Treatment of this compound with base leads to reversible enolate formation, and the C-3 carbanion substitutes the mesylate on C-7 (J. Gauthier, 1967 A. Belanger, 1968). 

The cyclization reactions discussed here either involve the intramolecular reaction of a donor group D with an acceptor group A or a cyclizing dimerization of two molecules with two terminal acceptors and two donors. A polymerization reaction will always compete with cyclization. For macrolides see p. 146 and p. 319 — 329. 

Krische et al. demonstrated intramolecular reaction with Co(dpm)2 (5mol%) and PhSiH3 (120 mol %) as a hydride donor (Scheme 8) [14-16]. Addition of aldehyde-enone 17 to a solution of the Co catalyst and phenylsi-lane resulted in the formation of the corresponding aldol cyclization product... 

The contribution of the frontier orbitals would be maximized in certain special donor-acceptor reactions. The stabilization energy is represented by Eqs. (3.25) and (3.26). Even in a less extreme case, the frontier orbital contribution maybe much more than in the expression of the superdelocalizability. If we adopt the approximation of Eq. (6.3), the intramolecular comparison of reactivity can be made only by the numerator value. In this way, it is understood that the frontier electron density, /r, is qualified to be an intramolecular reactivity index. The finding of the parallelism between fr and the experimental results has thus become the origin of the frontier-electron theory. The definition of fr is hence as follows ... 

In the direct transfer mechanism, the metal ion coordinates both reactants enabling an intramolecular reaction, and activates them via polarization. Consequently, strong Lewis acids including Alln and the Lnln ions are the most suitable catalysts in this type of reactions. In the hydride mechanism, a hydride is transferred from a donor molecule to the metal of the catalyst, hence forming a metal hydride. Subsequently, the hydride is transferred from the metal to the acceptor molecule. Metals that have a high affinity for hydrides, such as Ru, Rh and Ir, are therefore the catalysts of choice. The Lewis acidity of these metals is too weak to catalyze a direct hydride transfer and, vice versa, the affinity of Alm and Lnm to hydride-ions is too low to catalyze the indirect hydrogen transfer. Two distinct pathways are possible for the hydride mechanism one in which the catalyst takes up two hydrides from the donor molecule and another in which the catalyst facilitates the transfer of a single hydride. 

The benzoin reaction typically consists of the homocoupling of two aldehydes, which results in the formation of inherently dimeric compounds, therefore limiting the synthetic utility. The aoss-benzoin reaction has the potential to produce four products, two homocoupled adducts and two cross-benzoin products. Several strategies have been employed to develop a selective cross-benzoin reaction, including the use of donor-acceptor aldehydes, acyl silanes, acyl imines, as well as intramolecular reactions. 

Alkyl alkanoates are reduced only at very negative potentials so that preparative scale experiments at mercury or lead cathodes are not successful. Phenyl alkanoates afford 30-36% yields of the alkan-l-ol under acid conditions [148]. Preparative scale reduction of methyl alkanoates is best achieved at a magnesium cathode in tetrahydrofuran containing tm-butanol as proton donor. The reaction is carried out in an undivided cell with a sacrificial magnesium anode and affords the alkan-l-ol in good yields [151]. In the absence of a proton donor and in the presence of chlorotrimethylsilane, acyloin derivatives 30 arc formed in a process related to the acyloin condensation of esters using sodium in xylene [152], Radical-anions formed initially can be trapped by intramolecular addition to an alkene function in substrates such as 31 to give aiicyclic products [151]. 

The most spectacular application of the donor/acceptor-substituted carbenoids has been intermolecular C-H activation by means of carbenoid-induced C-H insertion [17]. Prior to the development of the donor/acceptor carbenoids, the intermolecular C-H insertion was not considered synthetically useful [5]. Since these carbenoid intermediates were not sufficiently selective and they were very prone to carbene dimerization, intramolecular reactions were required in order to control the chemistry effectively [17]. The enhanced chemoselectivity of the donor/acceptor-substituted carbenoids has enabled intermolecular C-H insertion to become a very practical enantioselective method for C-H activation. Since the initial report in 1997 [121], the field of intermolecular enantioselective C-H insertion has undergone explosive growth [14, 15]. Excellent levels of asymmetric induction are obtained when these carbenoids are derived... 

The anti-Markownlkov orientation of addition in the presence of electron-acceptor sensitizers applies also to intramolecular reaction, and 5,5-dipheny pent-4-en-1-ol gives a tetrahydrofuran (2.SI) when irradiated in solution with 9,10-dicyanoanthracene, whereas its thermal reaction under proton-acid catalysis leads to 2,2-diphenyltetrahydropyran by Markownikov addition. Sometimes an added sensitizer is not required, if the alkene itself can act as a good electron-donor or electron-acceptor, and this is likely to be the reason why 1-lo-methoxyphenyl)propene adds photochemically to acetic acid (2.52), whereas l-phenylpropene does not. 

Figure 6-5. A metal-ion-templated cyclisation of a compound containing two donor atoms (filled circles) and two reactive groups (open circles). The co-ordination of the open-chain ligand to the metal ion brings the reactive sites into close proximity and favours the intramolecular reaction.

There are several fundamental reasons why the GMH and adiabatic formulations are to be preferred over the traditionally employed diabatic formulation. The definition of the diabatic basis set is straightforward for intermolecular ET reactions when the donor and acceptor units are separated before the reaction and form a donor-acceptor complex in the course of diffusion in a liquid solvent. The diabatic states are then defined as those of separate donor and acceptor units. The current trend in experimental design of donor-acceptor systems, however, has focused more attention on intramolecular reactions where the donor and acceptor units are coupled in one molecule by a bridge.The direct donor-acceptor overlap and the mixing to bridge states both lead to electronic delocalization, with the result that the centers of electronic localization and localized diabatic states are ill-defined. It is then more appropriate to use either the GMH or adiabatic formulation. 

Intramolecular Reactions Donor-Bridge-Acceptor Systems... 

The carbocyclic [6-7] core of guanacastepenes was prepared by. D. Trauner et al. using the intramolecular reaction between carbenoids derived from diazo carbonyl compounds and furans. The required diazo carbonyl substrate was synthesized using p-acetamidobenzenesulfonyl azide (p-ABSA) as the diazo-donor component in the Regitz diazo transfer reaction. 

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