Electrophiles ether transfer reactions

An example of the first type of study is the cationic pol erization of alkenes and heterocyclic monomers in the presence of 2-alWlfurans. As discussed above, electrophilic substitution at C5 is quite facile with these compounds and one can therefore prepare monofunctional oligomers bearing a furanic end-group. By a judicious choice of experimental conditions this transfer reaction will predominate over all other chain-breaking events and virtually all the chains will have the same terminal structure, i.e. a 5-oligomer-2-al lfuran. Structure 32 illustrates this principle with isobutyl vinyl ether oligomers capped by 2-methylfuran ... 

The first results of anionic polymerization (the polymerization of 1,3-butadiene and isoprene induced by sodium and potassium) appeared in the literature in the early twentieth century.168,169 It was not until the pioneering work of Ziegler170 and Szwarc,171 however, that the real nature of the reaction was understood. Styrene derivatives and conjugated dienes are the most suitable unsaturated hydrocarbons for anionic polymerization. They are sufficiently electrophilic toward carbanionic centers and able to form stable carbanions on initiation. Simple alkenes (ethylene, propylene) do not undergo anionic polymerization and form only oligomers. Initiation is achieved by nucleophilic addition of organometallic compounds or via electron transfer reactions. Hydrocarbons (cylohexane, benzene) and ethers (diethyl ether, THF) are usually applied as the solvent in anionic polymerizations. 

Treatment of a,a-dicyanoalkyl phenyl selenide (14) with vinyl ether initiated by AIBN under benzene refluxing conditions generates methyl ketone (15) (eq. 4.8). An electrophilic a,a-dicyanoalkyl radical is first formed, and then it adds to vinyl ether, followed by hydrolysis. Diethyl 3-iodoalkylphosphonate (17) can be formed through AIBN-initiated addition reaction of diethyl 1-iodomethylphosphonate (16) to alkene (eq. 4.9). This is an atom-transfer reaction. Both reactions (eqs. 4.8 and 4.9) do not require Bu3SnH. 

Electrophilic a-amination reactions were also included in organocascade sequences. Jorgensen developed a formation of hydro) - and amino-esters in combination by aminocatalysis using diaiylprolinol silyl ether and a redox reaction followed by acyl transfer to the corresponding esters using NHC catalysis. This useful method did not require an inert atmosphere or anhydrous conditions to form the corresponding products in excellent enantioselectivities and high yield. 

Finally, the reaction of 19b with potassium fluoride in the presence of a crown-ether phase-transfer agent118 to yield the sulfonyl fluoride 67 and diphenylacetylene119 belongs to the same category in which a nucleophile (F in this case) attacks the electrophilic sulfur of the sulfone group (equation 19). 

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

Three major topics of research which are based on phase transfer catalyzed reactions will be presented with examples. These refer to the synthesis of functional polymers containing functional groups (i.e., cyclic imino ethers) sensitive both to electrophilic and nucleophilic reagents a novel method for the preparation of regular, segmented, ABA triblock and (A-B)n alternating block copolymers, and the development of a novel class of main chain thermotropic liquid-crystalline polymers, i.e., polyethers. 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

Lithium Enolates. The control of mixed aldol additions between aldehydes and ketones that present several possible sites for enolization is a challenging problem. Such reactions are normally carried out by complete conversion of the carbonyl compound that is to serve as the nucleophile to an enolate, silyl enol ether, or imine anion. The reactive nucleophile is then allowed to react with the second reaction component. As long as the addition step is faster than proton transfer, or other mechanisms of interconversion of the nucleophilic and electrophilic components, the adduct will have the desired... 

Because the cellulose ether alkoxide is present entirely in the aqueous phase, the rate-limiting step may be the partitioning (phase transport) of the hydrophobic electrophile across the interface from the organic to aqueous phase. If the reaction rate is controlled by diffusion of the electrophile across the interface, then one would expect a correlation between water solubility of the hydrophobe and its alkylation efficiency. The fact that the actual alkylation reaction is probably occurring in the aqueous phase (or at the interface) yet the electrophile itself is principally soluble in the organic phase has important mechanistic ramifications. This type of synthetic problem, in which one reactant is water soluble and the other organic soluble, should be amenable to the techniques of phase transfer catalysis (PTC) to yield significant improvements in the alkylation efficiency. 

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