Polar addition electrophilic

Only a few polar additions to azepines have been reported of which the most common are the electrophilic additions to the 10,11-bond of 5//-dibenz[ >,/]azepine these have been reviewed (74CRV101). The N-acetyl derivative adds fluoroxypentafluoroethane and the adduct on treatment with sodium hydroxide eliminates HF to yield lV-acetyl-10-perfluoroethoxy-5//-dibenz[6,/]azepine (80JOC4122). 

There is a further aspect of polar additions to alkenes that we should consider, namely, that electrophilic reagents form loose complexes with the 77 electrons of the double bonds of alkenes prior to reaction by addition. Complexes of this type are called charge-transfer complexes (or ir complexes). Formation of a complex between iodine and cyclohexene is demonstrated by the fact that iodine dissolves in cyclohexene to give a brown solution, whereas its solutions in cyclohexane are violet. The brown solution of iodine in cyclohexene slowly fades as addition occurs to give colorless trims-1,2-diiodocyclohexane. 

Haloamines and other precursors to aziridines can be generated by various polar additions . Three important groups of polar processes leading to aziridines are shown in Scheme 22. In the aza-Darzens route , the imine acts as an electrophile at carbon and later as a nucleophile at nitrogen, while the -haloenolate acts initially as a nucleophile at carbon and later as an electrophile at the same carbon. The roles of the two components are reversed for the polar aziridination route, which is related to the epoxidation reaction. In the -haloenone route, the 1,2-dihalide or -haloenone acts formally as a bis-electrophile while the amine acts as a bis-nucleophile. 

With CF3C=CCF3 or olefins with an internal double bond such as cis- or fraras-F-2-pentene, F-cyclobutene, or l,2-dichloro-l,2-difluoro-ethylene, or with a geminally disubstituted olefin such as 1,1-dichloro-2,2-difluoroethylene, only fully fluorinated products were obtained, likely through a nucleophilic fluorination mechanism. This fact, plus the necessity for a Lewis acid catalyst for addition to proceed, is evidence for an electrophilic addition mechanism. Others have suggested polar addition of BF3 to the olefin with RfBF2 as a reactive intermediate (294). 

The first step of the Friedel-Crafts alkylation is the coordination of the Lewis acid to the alkylating agent (e.g., alkyl halide) to give a polar addition complex. The extent of polarization in this complex depends on the branching of the alkyl group and almost total dissociation is observed in the case of tertiary and benzylic compounds. The rate determining step is the formation of the -complex by the reaction of the initial complex (electrophile) and the aromatic ring this step disrupts the aromaticity of the substrate. In the last step of the mechanism a proton is lost and the aromaticity is reestablished. 

Electrophilic Addition This can be considered as an acid—base reaction, where the reagent acts as an acid, whether a protic one (e.g., hydriodic acid or iodine monochloride) or a Lewis acid (e.g., molecular iodine, which can be polarized by electrophilic solvents or catalysts), and the double-bond acts as abase (Argentini, 1982). 

The reactions of HTIB with alkenes (Scheme 3.73) can be rationalized by a polar addition-substitution mechanism similar to the one shown in Scheme 3.70. The first step in this mechanism involves electrophilic flnfi-addition of the reagent to the double bond and the second step is nucleophilic substitution of the iodonium fragment by tosylate anion with inversion of configuration. Such a polar mechanism also explains the skeletal rearrangements in the reactions of HTIB with polycyclic alkenes [227], the participation of external nucleophiles [228] and the intramolecular participation of a nucleophilic functional group with the formation of lactones and other cyclic products [229-231]. An analogous reactivity pattern is also typical of [hydroxy(methanesulfonyloxy)iodo]benzene [232] and other [hydroxy(organosulfonyloxy)iodo]arenes. 

Although the vast majority of stepwise polar additions to ortho-benzyne involve nucleophilic attack on the aryne, electrophilic attack is also possible provided that the aryne is generated by a method that does not involve strongly basic conditions. Few such additions are synthetically useful, with the exception of the formation of 1,2-dihalobenzenes by reactions of ortho-benzynes with halogens, although alternative mechanisms initiated by nucleophilic attack of halide may be envisaged. Radical reactions of ortHo-benzyne, on the other hand, are extremely rare. 

The Chemistry of Vision Addition and Substitution Reactions Compared Polar Addition Reactions Addition of Unsymmetric Reagents to Unsymmetric Alkenes Markovnikov s Rule Mechanism of Electrophilic Addition to Alkenes Markovnikov s Rule Explained Reaction Equilibrium What Makes a Reaction Go ... 

Let us now consider the mechanism of polar addition to a carbon-carbon double bond, specifically the addition of acids to alkenes. The carbon-carbon double bond, because of its pi electrons, is a nucleophile. The proton (H ) is the attacking electrophile. As the proton approaches the pi bond, the two pi electrons are used to form a sigma bond between the proton and one of the two carbon atoms. Because this bond uses both pi electrons, the other carbon acquires a positive charge, producing a carbocation. 

The spectrum of mechanisms governing the reactions of electrophilic addition to multiple bonds is quite broad. The solvent, the polarity of electrophilic agent, the type and conformation of substituents at a multiple bond and its polarity all have a substantial effect on kinetics and stereochemistry of the reactions in question [1-3]. The most general and important features of various mechanisms are summarized in the following scheme ... 

In Summary a-Hydroxyketones are available from addition of masked acyl anions to aldehydes and ketones. The conversion of aldehydes into the anions of the corresponding 1,3-dithiacyclohexanes (1,3-dithianes) illustrates the method of reverse polarization. The electrophilic carbon changes into a nucleophilic center, thereby allowing addition to an aldehyde or ketone carbonyl group. Thiazolium ions catalyze the dimerization of aldehydes, again through the transformation of the carbonyl carbon into a nucleophilic atom. 

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