Initiator dicationic

As mentioned earlier, metal complexation not only allows isolation of the QM derivatives but can also dramatically modify their reactivity patterns.29o-QMs are important intermediates in numerous synthetic and biological processes, in which the exocyclic carbon exhibits an electrophilic character.30-33 In contrast, a metal-stabilized o-QM can react as a base or nucleophile (Scheme 3.16).29 For instance, protonation of the Ir-T 4-QM complex 24 by one equivalent of HBF4 gave the initial oxo-dienyl complex 25, while in the presence of an excess of acid the dicationic complex 26 was obtained. Reaction of 24 with I2 led to the formation of new oxo-dienyl complex 27, instead of the expected oxidation of the complex and elimination of the free o-QM. Such reactivity of the exocyclic methylene group can be compared with the reactivity of electron-rich enol acetates or enol silyl ethers, which undergo electrophilic iodination.34... 

The discussion can be restricted to the first and second reduction processes that are of particular interest in this context. The shift of the bipyridinium-based process is in agreement with the catenane coconformation in which the bipyridinium unit is located inside the cavity of the macrocyclic polyether (Fig. 13.33a) because of the CT interactions established with both the electron donor units of the macrocycle, its reduction is more difficult than in the free tetracationic cyclophane. The shift of the trans-1,2-bis(4-pyridinium)ethylene-based reduction indicates that, once the bipyridinium unit is reduced, the CT interaction that stabilize the initial coconformation are destroyed and, thereby, the tetracationic cyclophane circumrotates through the cavity of the macrocyclic polyether moving the tra ,v-bis(pyridinium)ethylene unit inside, as shown by comparison of its reduction potential with that of a catenane model compound.19 The original equilibrium between the two coconformations associated with catenane 384+ is restored upon oxidation of both units back to their dicationic states. 

Nonactivated C—H bonds in imines can be selectively monofluorinated in HF-SbF5 in the presence of CC14 to yield the corresponding fluoroketones when the reaction mixture is quenched with HF-pyridine534 (Scheme 5.54). The transformation is initiated by hydride abstraction with CC13+ from the most reactive carbon farthest from the functional group and involves dicationic intermediates 134 and 135. 

Several types of onium dications have been studied in which a single acyl cationic center has been part of 1,3-dicationic superelectrophiles. For example, pyruvic acid has been studied in FSOsH-SbFs solution at low temperatures.34 Initially the diprotonated species is observed in equilibrium with some of the monocation (eq 55). 

Although no detailed mechanism for the conversion was proposed, the initial step likely involves the formation of the superelectrophilic, dipro-tonated species (213), which reacts with benzene by electrophilic attack. There is some 1,3-dicationic character in 213, however it is understood that the positive charge is delocalized throughout the naphthalene ring-system. Evidence for the dicationic species comes from cryoscopic... 

Initial ionization gives an ammonium-carboxonium dication, which then produces the ammonium-carbenium dication (230). A variety of dicationic electrophiles like 230 have been proposed. 

Molecules with two or more cationic active centres are very useful for certain macromolecular syntheses. The first dicationic initiator was used in our laboratory [251-253]. It is formed by hydrolysis and condensation of dimethyldichlorosilane in the presence of a strong acid, e. g. HC104. The condensation is an equilibrium process. When its volatile products are removed by evacuation, a siloxonium dication is finally formed. 

It appears that a combination of polymeric ions with difunctional initiators should be of some advantage over the transformation processes described above [241]. We have found that the combination of a macroanion with a dicationic initiator and of a macrocation with a dianionic initiator proceeds with high efficiency [242]. Macroions can therefore be transformed by difunctional initiators... 

Yamashita has used dioxolenium slats, prepared differently, as initiators for the polymerization of THF Ttese salts yield dicationically growing chains whose... 

Anhydrides of strong i x)tonic acids provide a group of initiators able to give dicationically terminated macromolecules °° The anhydride of trifluoromethane-sulfonic acid (triflic anhydride) initiates the polymerization of THF in this way both reactions, with rate constants ki and kai, are faster than the formation of the alkyltetrahydrofuranium cation with the corresponding triflic acid ester ... 

Neither the first nor the second step in Scheme (42) has been directly observed, but it has conclusively been shown by studies of the number of the end groups in poly-THF initiated with triflic anhydride that one molecule of the anhydride gives two growing ends of the cationic and/or ester structure in one macxomolecufe. These structures are interconvertible. Recently, Smith separated and characterized two intmnediate products tetramethylene-bis-triflate, resulting from the attack of the anion on the o-methylene carbon atom in salt 42a, and the corresponding dicationic trimer... 

The large concentrations of initiator may lead to the precipitation of a portion of the macroions this was indeed observed when polymerization performed in CCI4 was rechecked The precipitation of the low molecular macroions has also recently been reported for dicationic species The precipitated macroions become undetectable by NMR but can still be reactive in propa tion. Obviously, this will result in the wrong ccmclusion that all of the observed growth is due to macroesters and will give a value for k larger than the real oi e. 

In Nolte and Drenth s nickel catalyzed system, the polymerization was believed to be initiated by a nucleophilic attack by the alcohol used as a solvent or the halide on the starting complex on the coordinated isocyanides. Successful asymmetric polymerization was achieved using a dicationic tetrakis(isocyanide)nickel(II) complex 46 with enantiopure primary amines, which served as a chiral nucleophile in the initial step (Scheme 35) [58, 59]. In a typical experiment, a catalyst prepared from (f-BuNC)4Ni(II)(C104)2 (46a) (1 mol%) and an optically active amine (1 mol%), was used for polymerization of isocyanides with, or without a solvent, such as n-hexane, in... 

The mechanistic difference is explained by the different nucleo-philicities of the counteranions Cl and Br. Dicationical ly terminated macromolecules can be obtained by the initiation with anhydrides of strong protonic acids such as trifluoromethanesul-fonic anhydride (53. 54). 

This is in contrast to the already discussed polymerization of cyclic ethers and acetals, where two growing species are formed, yielding dicationically growing macromolecules. The amide group, which could be a potential site for the second growing species, is unreactive. As shown in Table 10.1 the majority of polymers were prepared with Lewis acid initiators, most frequently with the BF3 OEt2 complex. 

Kucera et al. reported the synthesis of the dicationically growing polyTHF from the disilicenium cation 119>. The exact structure of this initiator prepared from dimethyl-... 

THF polymerization was initiated with triethyloxonium tetrafluoroborate. BF anions are known to decompose during THF polymerization, producing dead polyTHF carrying an alkyl fluoride end-group. Another reason for the formation of polyTHF may be chain transfer to polymer during THF polymerization, yielding dead blocks and dicationically living chains ... 

To prevent the formation of dead chains, initiators giving bidirectional growth have to be used [e.g. (CF3S02)20] then the chains will possess cations on both ends. This method has recently been used by Goethals and Tezuka for preparing dicationically living poly(t-butyl aziridine)124),... 

In the first stage, DXP was polymerized with (CF3S02)20 initiator in CH2C12 solvent at —78 °C. After the completion of DXP polymerization, a sample was taken for DPn determination of polyDXP and then a solution of DME was added. The system was kept at —78 °C for several hours and then the polymerization was terminated by sodium methanolate. The precipitated polymer was isolated, dried and characterized (yield, DPh, NMR). Results of typical experiments are given in Table 15.10. The molecular weights were close to the values calculated by assuming that each dicationic living polyDXP initiated DME polymerization. 

Table 15.10. Polymerization of 1,2-dimethoxyethylene (DME) initiated with living dicationic poly-1,3-dioxepane (DXP) -78 °C, CH2C12...

Other bifunctional initiators are the di-l,3-dioxolan-2-ylium cation 127) dicationic living polysiloxane 119) and ditetrahydrofuranium cations 128). 

Dianionic living polystyrene reacts in 95 % yield with dicationic silicenium initiator at —10 °C, transforming anionic to cationic growing centers143). The latter species were used as to initiate the polymerization of THF to form multiblock copolymers (cf. Sect. 15.3.1.2.3) ... 

In the face of these confusing results, Stanley and co-workers decided to initiate an extensive series of in situ spectroscopic studies to understand what was occurring under hydroformylation conditions with the neutral and dicationic precursor complexes rac-1 and rac-4, why they give such dramatically different catalytic results, in marked contrast to mononuclear hydroformylation catalyst. In a recent book on catalysis by di- and polynuclear metal complexes, edited by R. D. Adams and F. A. Cotton, George G. Stanley gives a detailed account of this fascinating story which reads as exciting as a detective novel. [12]... 

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