Initiating groups alkyl

Haloall lation. Haloalkyl groups can be introduced directiy by processes similar to Friedel-Crafts alkylation into aromatic and, to some extent, ahphatic compounds. Because halo alkylations involve bi- or polyfunctional alkylating agents, they must be performed under conditions that promote the initial halo alkylation but not, to any substantial degree, subsequent further alkylations with the initially formed haloalkylated products. 

Dibromoethane normally reacts with activated methylene groups to produce cyclopropyl derivatives [e.g. 25, 27], but not with 1,3-diphenylpropanone. Unlike the corresponding reaction of 1,3-dibromopropane with the ketone to form 2,6-diphenylcyclohexanone, 1,2-dibromoethane produces 2-benzylidene-3-phenyl-tetrahydrofuran and the isomeric 2-benzyl-3-phenyl-4,5-dihydrofuran via initial C-alkylation followed by ring closure onto the carbonyl oxygen atom (Scheme 6.2) [28],... 

Semitelechelic HPMA polymers were synthesized by free radical polymerization of HPMA using 2,2 -azobis(isobutyronitrile) (AIBN) as the initiator and alkyl mercaptans as chain transfer agents. Alkyl mercaptans with different functional groups, namely, 2-mercaptoethylamine, 3-mercapto-propionic acid, 3-mercaptopropionic hydrazide, and methyl 3-mercapto-propionate, were used as the chain transfer agents ST HPMA polymers... 

Substituted olefins that are capable of forming secondary or tertiary carbo-nium ion intermediates polymerize well by cationic initiation, but are polymerized with difficulty or not at all free radically. In general, vinyl or /-alkenes that contain electron donating groups (alkyl, ether, etc) polymerize well via a carbo-cationic mechanism. 

In the coordination polymerisation of olefins, the active site of the catalyst usually contains an alkyl group as the metal substituent forming with the metal an Mt-C active bond of the a type. The polymerisation consists in the insertion of the coordinated monomer into this bond with the regeneration of a metal-carbon bond of the same character [5], The initiation and propagation steps in the coordination polymerisation of olefins in the presence of catalysts containing an ethyl initiating group bound to the metal atom are as follows ... 

Fig. 2.23. Alkylating agents with structural elements that can act as neighboring groups in SN reactions. (In the example listed in parentheses, the neighboring group participation initiates an alkylation of the aromatic compound, in which an external nucleophile is not participating at all.)...

It has been proposed [62] that the polymerization of isobutene initiated by alkyl acetates/BCl, may occur via a pseudocationic pathway [112], in which the acetate end group is continuously activated by Lewis acids to provide an insertion of the monomer without the formation of carbocations ... 

One rather interesting aspect of the Friedel-Crafts reaction is that trialkylation of benzene often leads to a symmetrical 1,3,5-isomer rather than a 1,2,4-derivative as would be expected from the ortho-para orientation of the alkyl group introduced initially. Since alkylation by the Friedel-Crafts method has been shown to be a reversible reaction,68 this anomalous orientation has been explained on the basis that both alkylation and dealkylation occur readily in the ortho and para positions. Simultaneously, however, some alkylation occurs in the meta position but no dealkylation68 If the reaction mixture is allowed to stand in contact with aluminum chloride for some length of time, then principally meta trialkyl derivatives will be formed ... 

Several innovations have significantly extended the scope and synthetic utility of the classical Michaelis-Becker phosphonoacetate preparation. Eor example, the coupling of the Michaelis-Becker and Homer-Wadsworth-Emmons reactions for the synthesis of a-substituted acrylic acids represents a useful modification. According to Scheme 8.10, Michaelis-Becker alkylation of a dialkyl phosphite with a haloacetic acid in the presence of 3 eq of a base (one to neutralize the carboxyl group, one to form the phosphite conjugate base, and one to deprotonate the initially formed alkylation product) leads to the phosphoryl-stabilized anion directly. Treatment of the anion... 

Figure 8.6 illustrates, for example, the complicated equilibria [19] in initiation by alkyl halides which are widely used as initiators in combination with coinitiator such as aluminum alkyl halides or aluminum halide Lewis acids. Each carbocation can initiate polymerization or remove an alkyl (ethyl) group from the counterion to produce a saturated hydrocarbon, REt, and a more acidic Lewis acid. The propagating cation can also terminate by the same process to produce ethyl-capped polymers and new Lewis acids. Thus, even though the coinitiator used is diethylaluminum chloride there may be major contributions to the polymerization from ethylaluminum dichloride or aluminum chloride. 

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