Mechanistic spectrum

As in the El-E2-ElcB mechanistic spectrum, Bredt s rule applies and if a double bond is present, a conjugated system will be preferred, if sterically possible. Apart from these considerations, the following statements can be made for Ei eliminations ... 

Keywords Cycloadditions, Chemical orbital theory. Donor-acceptor interaction. Electron delocalization band. Electron transfer band, Erontier orbital. Mechanistic spectrum, NAD(P)H reactions. Orbital amplitude. Orbital interaction. Orbital phase. Pseudoexcitation band. Quasi-intermediate, Reactivity, Selectivity, Singlet oxygen. Surface reactions... 

With the power of the donors and acceptors, changes occur in the important frontier orbital interactions (Scheme 2) and in the mechanism of chemical reactions. The continuous change forms a mechanistic spectrum composed of the delocalization band to pseudoexcitation band to the electron transfer band. 

Photochemical [2h-2] cycloadditions of olefins occur with retention of configuration according to the Woodward-Hoffmarm rule [6,7], These are excited-state reactions in the delocalization band of the mechanistic spectrum. A striking example of the symmetry-allowed reaction was observed when the neat cis- and tran -butenes were irradiated (delocahzation band in Scheme 3) [8],... 

Thermal dimerization of ethylene to cyclobutane is forbidden by orbital symmetry (Sect 3.5 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). The activation barrier is high E =44 kcal mof ) [9]. Cyclobutane cannot be prepared on a preparative scale by the dimerization of ethylenes despite a favorable reaction enthalpy (AH = -19 kcal mol" ). Thermal reactions between alkenes usually proceed via diradical intermediates [10-12]. The process of the diradical formation is the most favored by the HOMO-LUMO interaction (Scheme 25b in chapter Elements of a Chemical Orbital Theory ). The intervention of the diradical intermediates impfies loss of stereochemical integrity. This is a characteric feature of the thermal reactions between alkenes in the delocalization band of the mechanistic spectrum. 

A part of the mechanistic spectrum is supported by the [2+2] cycloaddition reactions between the unsaturated carbon bonds and carbonyl compounds [24]. 

Scheme 6 Mechanistic spectrum of [2+2] cycloaddition of carbonyl compounds with alkenes and atkynes...

According to the Woodward-Hoffmann rule [6, 7], conjugate polyenes with 4n and 4n+2 n electrons undergo cychzations in conrotatory and disrotatory fashions under the thermal conditions, respectively. Recently, novel cycloisomerizations were found to be catalyzed by Lewis acid and to afford bicychc products [39] as photochemical reactions do [40]. The new finding supports the mechanistic spectrum of chemical reactions. 

The mechanistic spectrum shed new light on a familiar textbook example of organic reactions, i.e., electrophilic aromatic substitution (Scheme 9). 

Scheme 9 Mechanistic spectrum of electrophilic aromatic substitutions...

The theory of the mechanistic spectrum generally snggests that photochemical reactions between donors and acceptors in the delocalization band could be similar to thermal reactions between strong donors and acceptors in the pseudoexcitation band. This is fnrther snpported by the reactions of indoles with electron-accepting... 

Scheme 10 Mechanistic spectrum of the reactions of indoles with unsaturated acceptors...

A pair of reactions of 1,4-dihydropyridines with electron-accepting alkenes (Scheme 31) shows experimental evidence for the mechanistic spectrum between the pseudoexcitation and transfer bands. Acrylonitrile undergoes an ene reaction [143] (Scheme 31a). This is a reaction in the pseudoexcitation band. A stronger acceptor, alkylidene- and arylmethylydenemalonitriles are reduced [144] (Scheme 31b). This is a reaction in the transfer band, where a hydride equivalent shifts without bond formation between the ti bonds of the donors and acceptors. 

Scheme 31 a,b A mechanistic spectrum of NADH reactions (a) pseudoexcitation band (b) transfer band... 

There are other reactions apart from NADH reduction (Sect 4.1) where the hydride equivalent shifts between electron donors and acceptors without bond formation between the n bonds. The hydride equivalent transfer must be reactions in the transfer band. In fact, a photochemical reaction between donors and acceptors is similar to thermal reactions between strong donors and acceptors. This further supports the mechanistic spectrum (Scheme 32). 

The theory of interaction between a pair of orbitals, (J) and (Scheme la) is well established (Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume) and snccessfnlly applied to nnderstanding and designing molecnles and reactions (Chapter A Mechanistic Spectrum of Chemical Reactions by Inagaki in this volnme). Here, we describe a theory of the interaction of three orbitals, (j), (J), and (j), (Scheme lb). The interactions include indirect interactions of... 

Hydrolyses of alkyl halides and arenesulfonates have long been known to be micelle-inhibited (Gani et al., 1973 Lapinte and Viout, 1973, 1979) but now k+/k < 1, except for hydrolysis of methyl benzenesulfonate which involves extensive bond making in the transition state (Al-Lohedan et al., 1982b Bunton and Ljunggren, 1984). Thus values of k+/k < 1 seem to be characteristic of hydrolyses in the SN1-SN2 mechanistic spectrum which involve considerable bond breaking in the transition state, and k+/k is very low for hydrolyses of diphenylmethyl halides where the transition state has considerable carbocation character (Table 8). 

According to this picture, it is possible to restore the notion of a mechanistic spectrum of ET and Sfg2 processes that the sole consideration of energetic factors did not permit. Within this framework, there is no real difference between the notions of comptetition between the two mechanisms... 

With this exception we can see that the impact of the configuration mixing model on nucleophilic substitution reactions, which constitute the most widely studied organic reaction, is indeed extensive. The model readily rationalizes much available experimental data, relates the entire mechanistic spectrum within a single framework, challenges some fundamental precepts of physical organic chemistry and enables one to make reactivity predictions about reactions yet to be investigated. For such a simple, qualitative theory, this is no mean achievement. 

Fig. 26 Energy diagrams showing the mechanistic spectrum of elimination reactions (a) concerted E2, (b) concerted E2 with El character, (c) stepwise El, (d) concerted E2 with ElcB character, (e) stepwise ElcB. (f) concerted E2C and (g) stepwise E2C-I...

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