Other Electrophilic Reactions

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

Taking into account the close relationship to pyridines one would expect 2-pyridones to express similar type of reactivities, but in fact they are quite different. 2-Pyridones are much less basic than pyridines (pKa 0.8 and 5.2, respectively) and have more in common with electron-rich aromatics. They undergo halogenations (a. Scheme 10) [67] and other electrophilic reactions like Vilsmeier formylation (b. Scheme 10) [68,69] and Mannich reactions quite easily [70,71], with the 3 and 5 positions being favored. N-unsubstituted 2-pyridones are acidic and can be deprotonated (pJCa 11) and alkylated at nitrogen as well as oxygen, depending on the electrophile and the reaction conditions [24-26], and they have also been shown to react in Mitsonobu reactions (c. Scheme 10) [27]. 

Cr + reacts with H atoms, generated by pulse radiolysis or uv flash photolysis, to give Cr(H20)5H, A = 1.5 X 10 M s . The product may be regarded as the first member of a series of compounds of general formulae Cr(H20)5R which arise from reaction of Cr " with RCl in anaerobic acid solution. It is the most reactive member of the series towards acidolysis (e.g. A (CrH )/fc(Cr(CH3) ) = 2 X 10 ) and in other electrophilic reactions. ... 

In their thoughtful 1983 review, Nielsen and coworkers noted that particles of diesel soot or wood smoke can absorb significant amounts of water. Thus, they suggested that the most plausible mechanism(s) for nitration (and possibly other electrophilic reactions) of particle-associated PAHs in ambient air may involve reactions both in a liquid film and on solid surfaces and that fundamental laboratory studies of the rates, products, and mechanisms of PAHs in polar solvents would be atmospherically relevant for reactions in the liquid films. Based on this, they proposed a classification scheme for the reactivities of key PAHs in electrophilic reactions, which was subsequently described in detail (Nielsen, 1984). 

The equimolar mixture of bromine trifluoride and bromine can be employed for the bro-mofluorination of fluorine-free alkcncs such as (Z)- and( )-l,2-dichloroethene, methyl acrylate and its a-substituted derivatives.1 The reactions are carried out in l,l,2-trichoro-l,2,2-tri-fluoroethane (CFC-113) using a 25-35 % excess of bromine trifluoridc/ bromine. The direction of the bromine monofluoride" addition to multiple bonds in the esters, much as in other electrophilic reactions, depends on the electronic nature of the substituent R. 

Few other electrophilic reactions are known. The trimethyl-3/f-pyrazoIe 105 is oxidized by Af-bromoacetamide (NBA) to the bromohydrin 168 and cyclized further to the epoxide 169 by base (Scheme 63).165 Peracid oxidation of 53 gives the mono ALoxide 75 regioselectively (Scheme 28).108... 

The almost exclusive ring closure at the a-position carries over to other electrophilic reactions of substituted 3 -aminopyridine derivatives. For example, the condensation of 3-aminopyridine with ethoxy-methylenemalonic ester (EMME) in dilute solution in boiling Dowtherm A gave the 1,5-naphthyridine 190.425,426 The only exception to date is that of 3-aminopyridine A -oxide which, as expected,... 

Little is known of the actual mechanism. A mode of reaction is possible, in which the oxygen atom at the top of the ozone molecule with a formal positive charge (p. 230) reacts with an electron pair, not localized in a bond but on one carbon atom, and in which the ozone therefore reacts by an electrophilic mechanism (Wibaut, Sixma and Kampschmidt). However, in order to explain the differences between the reaction course for ozonization and for other electrophilic reactions, e.g., bromination and nitration with pyrene, these authors assume also an interaction of one of the other oxygen atoms with the adjacent carbon atom. The net result is, however, about the same as that predicted by the bond localization hypothesis. 

It should therefore be no surprise that the nitration of methoxyben-zene is easier and faster than that of benzene and yields essentially only the 1,2- and 1,4-isomers (in almost equal amounts). Less than 1% of 3-nitroanisole is formed. Other electrophilic reactions follow this pattern. 

Alkylation Formation of Optically Active 4-Methyi-cyclohexylideneaikanes. Alkenylcopper reagents can be alkylated by treatment with alkyl halides and other electrophiles. Reaction of (S)-4-methylcyclohexylidenemethylcopper (enantiomer of the title reagent) with lodomethane gives (+)-(S)-4-(methylcyclohexylidene)ethane with 98% retention of configuration (eq 3). Alkylation of (R)-4-methylcyclohexylidenemethylcopper with methyl iodide and other alkyl halides is expected to show similar stereospecificity. 

Other electrophilic reactions of cyclopropanols have been reviewed . 

The same solvent system was shown to be useful in other electrophilic reactions of iodine. Thus the reaction of cyclohexene with sodium azide and iodine in this medium results in the trans-iodo azide (95% yield). 

Reactions with other Electrophiles. Reaction with trifluoroacetic acid leads to the slow formation of a ring-opened product trifluoroacetic anhydride provides a his(trifluoroacetoxy)cyclopropene [110], Triethyloxonium tetrafluoroborate ethylates the oxygen atom, giving an ethoxycyclopropenium salt [110] ... 

Alkylation and other electrophilic reactions of vinyl sulphides and selenides following of-metallation include a number of useful synthetic procedures. Addition of an alkyl-lithium to phenyl vinyl selenide gives the cc-lithio- -alkylated selenide, which, on further alkylation and oxidative deselenation, gives the 1,2-disubstituted ethylene (H2C=CHSePh - R CHjCHLiSePh - R CH2CHR SePh - R CH=CHR ), thus permitting the vinyl selenide to be... 

Reaction with Other Electrophiles. Reaction occurs between (1) and epoxides to yieid )/-hydroxysilanes. Michael addition is observed with nitroaikenes the sUyl group can then promote a Nef reaction to afford -siiyi ketones (eq 10). ... 

EDTA which, therefore, should only be used when necessary for the resolution. Anion-exchange column packing materials are particularly sensitive to destruction, because their ftinctional groups are sensitive to oxidation and other electrophilic reactions. 

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