Radical reactions Lewis acid catalysis

If so, one may expect products to result from chemical bond formation between the cation-radical-anion-radical pair, which are both paramagnetic and of opposite charge. In the latter route, there is a precedent for the formation of dioxetane intermediates of stable olefin cation radicals [51], as in the characterization by Nelsen and coworkers of a dioxetane cation radical from adamantylidene cation radical [52]. If a dioxetane is formed, either in neutral form or as a cation radical, the Ti02 surface can function in an additional role, that is, as a Lewis acid catalyst, to induce decomposition of the dioxetane. Since no chemiluminescence could be observed in these reactions, apparently Lewis acid catalysis provides a nonradiative route for cleavage of this high-energy intermediate. That Ti02 can indeed function in this way can be demonstrated by independent synthesis of the dioxetane derived from 1,1-diphenylethylene, which does indeed decompose to benzophenone when it is stirred in the dark on titanium dioxide. 

The catalytic role of Mg + in Scheme 12 is ascribed to the 1 1 and 1 2 complex formation of Q and Mg +, which results in an increase in kobs with an increase in [Mg +], exhibiting first- and second-order dependences on [Mg ], respectively (Figure 7). This contrasts with the Lewis acid catalysis in conventional concerted Diels-Alder reactions, in which the catalyst is believed to activate dienophiles (not the radical anions) by coordination to the acidic metal center [208-212]. However, the exact catalytic mechanism of numerous Lewis acid-catalyzed Diels-Alder reactions [208-212] has yet to be clarified, including a possible contribution of Lewis-acid catalyzed electron transfer step. 

Simple a,/3-unsaturated aldehydes, ketones, and esters participate preferentially in inverse electron demand (LUMOdlcne controlled) Diels-Alder reactions with electron-rich, strained, or simple olefinic and acetylenic dienophiles.3 5 The thermal reaction conditions for promoting the [4 + 2] cycloadditions of simple 1-oxabutadienes (R = H > alkyl, aryl > OR), cf. Eq. (1), are relatively harsh (150-250°C), and the reactions are characterized by competitive a,/3-unsaturated carbonyl compound dimerization or polymerization. Usual experimental techniques employed to compensate for poor conversions include the addition of radical inhibitors to the reaction mixture and the use of excess 1-oxabutadiene for promoting the [4 + 2] cycloaddition. Recent efforts have demonstrated that Lewis acid catalysis and pressure-promoted reaction conditions28-30 may be used successfully to conduct the [4 + 2] cycloaddition under mild thermal conditions (25-100°C). 

Many chemical reactions involve a catalyst. A very general definition of a catalyst is a substance that makes a reaction path available with a lower energy of activation. Strictly speaking, a catalyst is not consumed by the reaction, but organic chemists frequently speak of acid-catalyzed or base-catalyzed mechanisms that do lead to overall consumption of the acid or base. Better phrases under these circumstances would be acid promoted or base promoted. Catalysts can also be described as electrophilic or nucleophilic, depending on the catalyst s electronic nature. Catalysis by Lewis acids and Lewis bases can be classified as electrophilic and nucleophilic, respectively. In free-radical reactions, the initiator often plays a key role. An initiator is a substance that can easily generate radical intermediates. Radical reactions often occur by chain mechanisms, and the role of the initiator is to provide the free radicals that start the chain reaction. In this section we discuss some fundamental examples of catalysis with emphasis on proton transfer (Brpnsted acid/base) and Lewis acid catalysis. 

Adsorption of Terminal Olefins on H Si(m) Since 1995 when Linford etal. first reported the fabrication methods of long alkyl chains (>10 carbons) [6], the reaction between H Si(lll) and terminally double-bonded olefins (1-alkenes) has been accepted as a practical method of organic deposition. The adlayer of long alkyl chains is robust and convenient to stabilize Si surfaces and to make modifications over the adsorbates. The deposition reaction was activated by the coexistence of a radical initiator in liquid phase [6], by heating [6], by light irradiation [46-48], or by using Lewis-acid catalysis. Use of diluted olefins in an inert solvent (such as saturated alkanes) improves the quality of the alkyl adlayer [49]. 

Electrochemical functionalization methods 3 Electrochemical reaction 6 Photochemical functionalization methods 4 Photochemical reaction 6 Silicon-carbon bond formation 1 Thermal functionalization methods 3 Thermal-Grignard and organolithium reagents 5 Thermal-heat reaction 3 Thermal-hydride abstraction 5 Thermal-Lewis acid catalysis 4 Thermal-radical initiators 3 Thermal-transition metal catalysis 4... 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Intramolecular radical cyclization of an aryl bromide and an alkyne can be used to produce dihydroquinolines (Equation 57) <1998TL2965>. An analogous reaction setup utilizes a Lewis acid-catalyzed novel one-pot domino pathway using silver catalysis in high regioselectivity (Scheme 26) <2005OL2675>. Three mole equivalents of the alkyne are used with the final cyclization step arising from alkynic addition. 

Electrophilic reagents are Lewis acids (- acid-base theories). Electrophilic catalysis is catalysis by Lewis acids. The term electrophilic is also used to designate the apparent polar character of certain -> radicals, as inferred from their higher relative reactivities with reaction sites of higher -> electron density. See also -> electrophilicity. Ref. [i] Muller P (1994) Pure Appl Chem 66 1077... 

However there are several major hurdles. The most common catalysts are based on acid catalysis with Bronsted or Lewis acid sites these sites promote the formation of propylene rather than ethylene as is witnessed by conventional FCC operations. Ethylene is promoted by free radical processes. Catalysis of free radical reactions is rare, but not unknown . One route is to take a conventional acid catalysis and to neutraUse the acid sites with alkaline metals (magnesium, calcium) or phosphorus or a mixture of such. This can generate a further problem, in that the catalyst promotes the formation of carbon (coke) and hydrogen which are thermodynamically favoured at the reaction temperatures. 

What gets the reaction going Sometimes nothing happens until a catalyst is added. Does the reaction require acidic or basic catalysis Is a weak acid or base capable of catalyzing the reaction Is a proton source, like trace water, required Do Lewis acids catalyze the reaction Do free radical inhibitors stop the reaction (see Chapter 11) Does the reaction require a free radical initiator or light For example, the addition of water to alkenes fails to go without an acid catalyst. Any postulated mechanism must explain the need for an acid catalyst. 

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