Electron transfer and bond formation

Figure 3.9. Two-electron, two-orbital interaction (a) both electrons in lower orbital, dative bond formation and electron transfer from to A (b) one electron in each orbital, large stabilization and covalent bond formation.

In an alternative treatment (17), again desolvation is assumed to precede bond formation, and electron transfer and desolvation are assumed to be... 

Proton transfer from the hydrogen peroxide molecule to His 74 is accompanied by O—H bond break and electron transfer to the oxygen atom. Then owing to the following O—H bond break and 0=0 bond formation this electron is transferred to the hydrogen atom with simultaneous hydride-ion transfer. The whole sequence of electron transfer in BRC is implemented without high-energy consumption. 

Silole rings have been constructed by treating 3,3 -dibromo-2,2 -bithienyl with BuLi and reacting the dilithio derivative so obtained with a dialkyldichlorosilane <2006JA9034>. The aryl-aryl bond formation by electron-transfer oxidation of Lipshutz cuprates has been utilized for the construction of macrocycles involving thiophene rings <2006JOC6110>. 

Ru(Tp)(Ph)(NCMe)CO] reacts with the electron-rich olefins, ethyl vinyl sulfide and 2,3-dihydrofuran, yielding [Ru(Tp)(CO)( x-SEt)]2 and [Ru(Tp)(CO)(NCMe)(C=CCH2CH2OH)] through a transformation that involves stoichiometric C-S and C-H/C-O bond cleavage, respectively.326 [Ru(Tp)(Me)(CO)(NCMe)] reacts with pyrrole forming [Ru(Tp)(CO) K2- V, V-(H)N=C(Me)(NC4H3) ]. Mechanistic studies indicate that the most likely reaction pathway involves metal-mediated N-H activation of pyrrole to form [Ru(Tp)(CO)(A/-pyrrolyl)(NCMe)], followed by C-C bond formation and proton transfer (Fig. 2.71).327... 

Spin Conservation. In elementary chemical acts, such as bond cleavage, bond formation, or electron transfer, magnitude and direction of spin are conserved. This means that both the expectation value of total electron spin (operator S2) and that of its z projection (operator Sz) remain constant the same holds for the nuclear spin states. Hence, for example, fragmentation of a molecule M in a singlet state S>, which is characterized by zero total spin and zero z projection of spin, into two particles 1 and 2 yields either two singlet (i.e., diamagnetic) species, or two radicals, the spins of which are paired in a specific manner such that spin conservation is fulfilled (in a way, they are... 

The rate-determining formation of Cr(IV) is followed by the rapid oxidation of the hydroxyl center, the C-C bond scission, and electron transfer to Co(III) as in Scheme 1. 

Finally, associated with the bond formation, an electron transfer takes place between the proton and the surface oxygens, which modifies the charges of both species. This charge modification has to be taken into account in the estimation of the electrostatic energy. 

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck<77,78) has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of this interaction could vary from case to case. At one extreme this interaction could involve a-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction ... 

Diels-Alder cycloaddition reactions of electron-poor dienophiles to electron-rich dienes, which are generally carried out thermally, afford widespread applications for C—C bond formation. On the basis of their electronic properties, numerous dienes can be characterized as electron donors and dienophiles as electron acceptors. Despite the early suggestions by Woodward,206 the donor/ acceptor association and electron-transfer paradigm are usually not considered as the simplest mechanistic formulation for the Diels-Alder reaction. However, the examples of cycloaddition reactions described below will show that photoirradiation of various D/A pairs leads to efficient cycloaddition reactions via electron-transfer activation. 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

A CIEEL approach can also be used to explain chemiexcitation in luminol chemilumi-nescence . Two possibilities arise (i) an electron transfer from the amino group to the peroxidic moiety in the antiaromatic peroxide 33, resulting in bond cleavage followed by intramolecular back-electron transfer and formation of excited 3-aminophthalate (Scheme 24) (ii) the equilibrium between the peroxycarboxylic aldehyde 34, formed after elimination of nitrogen, and the cyclic peroxy semiacetal 35 is shifted in the direction of 35, as the result of an electron transfer from the amino group to the cyclic peroxide moiety, followed by 0—0 bond cleavage . Back-electron transfer would result in chemiexcitation (Scheme 25). 

Before we review the methods used to determine surface acidity, we wish to define the type of acidity that should be measured. An acid is an electron-pair acceptor. In our opinion, the term acid should be limited to this definition rather than broadening the term to include oxidizing agents as well. We agree with Flockhart and Pink (10) who suggest a clear distinction be made between Lewis acid-Lewis base reactions (which involve coordinate bond formation) and oxidation-reduction reactions (which involve complete transfer of one or more electrons). 

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