Bonds electrophilic

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

Figure 2,40. Schematic of the two extreme conformations of adsorbed atomic oxygen on Ag covalently bonded electrophilic oxygen (a-) and ionically bonded oxygen (P-).98 Reprinted with permission from Academic Press.

The mechanism of this new reaction is shown in Scheme 14. Coordination of the diene to palladium(II) makes the diene double bond electrophilic enough to be attacked by the allylsilane. The attack by the allylsilane takes place on the face of the diene opposite to that of the palladium (anti). This is the first example of an anti attack by an allylsilane on a 7T-(olefin)metal complex. Benzoquinone (BQ)-induced anti attack by chloride ion produces the product 58. 

POCI3 It is not a Lewis acid it is a a bond electrophile at P. Because P9 is electrophilic and 01 is nucleophilic, the first step must be formation of 01-P4 bond. If this is true, the P-containing by-product has an 0-P bond. Make 01-P9, C2-C7. Break 01-C2, P9-C110. 

When a metal atom donates electron density to a bound ligand, usually by means of Ji-back bonding, electrophilic substitution reactions may be promoted. This is observed then usually with metals in low oxidation states and is therefore prevalent with organometallic complexes - and less with those of the Werner-type, where the metals are usually in higher oxidation states. Nevertheless there have been detailed studies of electrophilic substitution in metal complexes of P-diketones, 8-hydroxyquinolines and porphyrins. Usually the detailed course of the reaction is unaffected. It is often slower in the metal complexes than in the free ligand but more rapid than in the protonated form. 

Electrophiles are positively charged ions or neutral molecules that are deficient in electrons. Examples include H, NO and SO3. They are capable of accepting a pair of electrons from an electron pair donor to form a covalent bond. Electrophilic means electron-loving and so electrophiles will be attracted to and will attack species with a negative or partial negative charge. 

When the same substituents are at each end of the double or triple bond, it is called symmetrical. Unsymmetrical means different substituents are at each end of the double or triple bond. Electrophilic addition of unsymmetrical reagents to unsymmetrical double or triple bonds follows Markovnikov s rule. According to Markovnikov s rule, addition of unsymmetrical reagents, e.g. HX, H2O or ROH, to an unsymmetrical alkene proceeds in a way that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. The reaction is not stereoselective since it proceeds via a planar carbocation intermediate. However, when reaction proceeds via a cyclic carbocation intermediate, it produces regiospecific and stereospecific product (see below). A regioselective reaction is a reaction that can potentially yield two or more constitutional isomers, but actually produces only one isomer. A reaction in which one stereoisomer is formed predominantly is called a stereoselective reaction. 

Concepts Assumed (j— and ir-bonds. Polarisable bonds. Electrophile and nucleophile. 

In contrast to C02,189 S02,132 and the previously described substrates which insert into the metal-oxygen bond, electrophilic alkynes such as dimethyl acetylenedicarboxylate insert into the 0—0 bond (Figure 2).44,190... 

Many aspects of the characteristics of the double-bonded functional groups have been reviewed in an impressive series of treatises, published over a period of more than 30 years. These reviews cover mostly the physicochemical aspects of double bonds electrophilic additions to carbon-carbon double bonds1, directing and activating effects of doubly bonded groups2 etc. 

Platinum complexes (continued) with aryls, thallium adducts, 3, 399 with bis(alkynyl), NLO properties, 12, 125 with bisalkynyl copper complexes, 2, 182-186 with bis(3,5-dichloro-2,4,6-trifluorophenyl), 8, 483 and C-F bond activation, 1, 743 in C-H bond alkenylations, 10, 225 in C-H bond electrophilic activation studies, 1, 707 with chromium, 5, 312 with copper, 2, 168 cyclometallated, for OLEDs, 12, 145 in diyne carbometallations, 10, 351-352 in ene-yne metathesis, 11, 273 in enyne skeletal reorganization, 11, 289 heteronuclear Pt isocyanides, 8, 431 inside metallodendrimers, 12, 400 kinetic studies, 1, 531 on metallodendrimer surfaces, 12, 391 mononuclear Pt(II) isocyanides, 8, 428 mononuclear Pt(0) isocyanides, 8, 424 overview, 8, 405-444 d -cP oxidative addition, PHIP, 1, 436 polynuclear Pt isocyanides, 8, 431 polynuclear Pt(0) isocyanides, 8, 425 Pt(I) isocyanides, 8, 425 Pt(IV) isocyanides, 8, 430... 

Another instructive scenario may be found when considering the metalation of arenes. There are two distinct mechanisms for the metalation of aromatic C-H bonds - electrophilic substitution and concerted oxidative addition (Box2). The classical arene mercuration, known for more than a century, serves to illustrate the electrophilic pathway whereas the metal hydride-catalyzed deuterium labeling of arenes document the concerted oxidative addition mechanism [8, 17]. These two processes differ both in kinetic behavior and regioselectivity and thus we may appreciate the need to differentiate these two types of process. However, the choice of C-H bond activation to designate only one, the oxidative addition pathway, creates a similar linguistic paradox. Indeed, it is hard to argue that the C-H bond in the cationic cr-complex is not activated. 

The observed rc-face differentiation of the electrophilic animation process was rationalized by the authors [14b]. NMR Nuclear Overhauser experiments agree with the ( -configuration of the O-silyl ketene acetals 35 and with a j-yn-periplanar disposition of the C -OSi and C2-Ha bonds. Electrophiles E+ , such as Lewis acids-co-ordinated DTBAD, attack 35 preferentially from the less hindered C(a)-Si (back) face (Scheme 17). 

Electrophilic addition reaction (Section 11.1) A reaction that results in the addition of two groups, an electrophile and a nucleophile, to the carbons of a CC double or triple bond. Electrophilic aromatic substitution reaction (Section 17.1) A reaction in which an elec-ttophile is substituted for a hydrogen on an aromatic ring. 

In heterolytic cleavages of tin carbon bonds, electrophilic attack (by E) at carbon is usually the more influential, although nucleophilic assistance (by Nu) at tin can also play a role, and in some cases can become dominant (equation 37). 

The electron ejected from the monomer molecule attaches to the double bond of another monomer molecule, which leads to an anion-free radical. In essence, the irradiation produces cation, anion, and free radical, any of which can initiate the monomer unaffected by the irradiation. Which end of the initiator, i.e., cation, anion, or free radical, initiates the polymerization is dependent on the nature of the double bond (electrophilic or electrophobic) and the purity of the monomer [2]. In some monomers in extremely high purity, polymerization proceeds by all three polymerizations, i.e., cationic, anionic, and free radical, as depicted schematically in Figure 5.1. 

Hydrazoic acid adds readily to alkenes which are conjugated to powerful electron-withdrawing groups. Since such groups reduce the basicity of the carbon-carbon double bond, electrophilic attack by hydrazoic acid is more difficult, so that where reaction occurs it is likely to proceed by nucleophUic attack. The best known examples are the additions to a,/3-unsaturated carbonyl compounds and these are discussed subsequendy. 

It might be anticipated that when the azido substituent is conjugated to the olelinic bond, electrophilic addition (Adc ) reactions would be activated by the undeniably powerful +K effect and directed as shown in equation (3). An example, albeit an indirect one, where this effect may be inferred occurs in the reaction of ethoxyacetylene with... 

Some carbonium-ion- and carbanion-pro-ducing heterolyses of carbon-metal bonds Electrophilic substitutions at saturated carbon atoms... 

Ytterbium metal has been found to promote cross-coupling reactions between diaryl ketones and a variety of C—rr-bond electrophiles. The reactions reportedly occur by nucleo[Mlic addition of an ytterbium diaryl kettxie dianion to the electrophiles. The net result of these transformations is that the diaryl ketones have been converted by the ytterbium from an electrophilic species to a nucleophilic diarylcarbi-nol anion equivalent. Although this methodology probably will not be a general one from the point of view of the ketone (alkyl ketone dianions are, in general, energetically inaccessible), the procedure does have synthetic utility when nucleophilic incorporation of diarylcarbinols is desired. 

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