Dioxolenium salts

Block or graft copolymem can be obtained by cationic polymerization of THF with macromolecular initiators. The recommended groups for the initiation are the dioxolenium cation, the acyl cation and the super acid ester, each of which can be introduced into the backbone polymer by reaction with silver salts of strong acids. Introduction of the dioxolenium group into polystyrene was carried out by the following route32 ... 

King and All butt (60, 62) have described the stereoselective hydrolysis of the dioxolenium ion 98 derived from a trans-decalin. Mild acid hydrolysis of 98 afforded almost entirely the axial ester product 99 with only a trace (<0.5%) of the equatorial ester 100. A similar result was obtained by using mild basic conditions. The authors further established by equilibration studies that the equatorial ester 100 is the most stable isomer, demonstrating that the formation of the axial isomer 99 is subject to kinetic rather than thermodynamic control. Essentially identical results were produced with two other dioxolenium salts derived from steroids. 

According to the present theory, these reactions can be explained in the following manner. Since dioxolenium ions are essentially planar (8, 9), the chair form of ring B of salt 98 must be distorted towards a half-chair. Hydration of 98 with stereoelectronic control must take place from the a face to give the half-chair hemi-orthoester 102 (Fig. 8), because the steric hindrance between the incoming water molecule and ring B must inhibit... 

Secondly, it was observed by the same authors (61) that the reaction of bicyclic 1,3-dioxolenium salt 162 with sodium hydrosulfide gave a mixture of the two isomeric hydroxy thionoacetates 164 and 165 in a 1.5 1 ratio. 

Confirming the above results, heating the carboxylic acid salts 384 and 385 (X=amidinium ion) for 1 min at their melting points (165° and 195°C respectively) gave a high yield of the 5-E, 9-E isomeric macrolide 386. Note again that, in the dioxolenium intermediate 387, the o — COO bond becomes anti-periplanar to the C9 - 0 bond and consequently the decarboxylation produces an E double-bond. 

Ozone oxidation of the trans-decal in di of benzylidene 133 has been carried out (71). Under kinetically controlled conditions, it produces the axial benzoate 134 in preference to the more stable equatorial benzoate 135. Similar results were obtained with an analogous case derived from cholestane-2e,3e-diol. These results are essentially identical to those obtained by King and Allbutt (60, 62) in their study on the hydrolysis of dioxolane orthoesters and dioxolenium salts (cf. p. 82), and can therefore be explained in the same manner. These results further confirm that the oxidation of acetals by ozone produces an intermediate which behaves like the hemiorthoester tetrahedral intermediate which is formed in the hydrolysis of orthoesters. 

Triphenyl-1,3-dithiole (233) (Rj = R2 = R3 = C6H5) has been converted into the corresponding salt by means of trityl perchlorate.24 On the other hand, the 4,5-dicyano derivative (236) [which was first prepared by Bahr et al.li8 from carbon disulfide and ammonium cyanide by spontaneous desulfurization of the salts of cyanodithio-formic acid (235), followed by ring closure with diiodomethane] is stable towards various oxidizing agents. The ease of oxidation of 233 to 234 is probably dependent on the same structural factors which govern the preparation of dioxolenium salts from 1,3-dioxolanes.149... 

Another hydride transfer has been observed when dioxolane 25 was reacted with 14c to give the dioxolenium salt 26. A methoxyl-anion, however, is transferred when 14c is reacted with 1 -methoxy-1 -methyldioxolane (27) to give 57 % of 1 -methyldioxo-lenium-tetrafluoroborate 29 and 4-methoxy-2,4,6-tris-(4 -methoxyphenyl)cyclo-2,5-hexadiene-one-1 (28)205 ... 

Oxocarbenium and carboxonium ions have an oxygen atom in the exposition to the carbon atom formally bearing the positive charge. Oxocarbenium salts are usually generated in situ [19], although some dioxolenium... 

Initiators such as triethyloxonium salts, oxycarbenium salts (e.g., C6H5CO + A ), or dioxolenium salts initiate the polymerization by direct... 

Dioxolenium salts initiate polyn izatkin of THF with rate constants comparable to that of projagation and provide a class of convenient initiators that can be prepared with a variety of anions, since the starting triphenylmethylium salts are available with SbFj, AsFj, CIOJ, BFJ, and PFg anions. A laboratory-scale preparatk>n, identification, properties and H-NMR spectra of unsubstituted diox-olenium salts are described in Refs. 71 and 72. 

It has been also shown that the H-NMR S values of dioxolenium salts differ for salts with different anions and drift downfield for the free dioxolenium cation ... 

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

Recently, an attempt was made to study the polymerization of various substituted DXL via the 1,3-dioxolenium initiation. However, the% monomers e substituted DXL salts failing to induce the polymerization of the parent monomers ... 

Reactions of 1,3-dioxolenium salts with THF and DXL were studied in a similar way. It was found from the NMR spectra that initiation involves attack at C-4 of the salt and opening of the 1,3-dioxolenium ring with the formation of formic acid derivatives (cf. discussion of the ambi-dent reactmty. p. 21) ... 

Rate constants of cationation do not markedly differ with varying structure of the nucleophile indeed, cationation of THF, oxepane and DXL with 1,3-dioxolenium salts with AsFg anions are equal to 0.9 10 , 1.1 10 and 0.23 10 (in mole 1 s , at 0 °C in CH3NO2), respectively. The closeness of these values indicates, that the bond breaking in the cation is much more advanced in the transition state than the bond making. 

Preparation of Macromolecular Dioxolenium Salts. Living polystyrene prepared by the polymerization of styrene in THF with a-methylstyrene tetramer dianion reacted with a 2.1-molar amount of ethylene oxide for three hours at room temperature a 6.6-molar amount of adipoyl chloride was added, and the mixture was stirred for 20 horns a 20-molar amount of ethylenebromohydrin was added. This mixture was stirred for 44 hours. The bromoethylated polystyrene was precipitated in excess methanol and freeze-dried from benzene in a vacuum system. A 1-nitropropane solution of polystyrene dioxolenium salt was prepared by reaction of bromoethylated polystyrene with silver perchlorate in 1-nitropropane. Silver bromide was removed from the reaction mixture by filtration. Molecular weight of the product was measured by a vapor-pressure osmometer it was 1910 for living polystyrene and 5190 for the bromoethylated polystyrene. Bromine analysis of the bromoethylated polystyrene showed 67.9% of the calculated value. 

Polytetramethylene glycol of molecular weight 2000 was freeze-dried from benzene in a vacuum. The bromoethylated prepolymer was obtained by reaction of the glycol, first with adipoyl chloride, then with ethylene bromohydrin (Table II) and precipitation in water and n-hexane. Nitromethane solution of the poly-THF dioxolenium salt was prepared after filtration of silver bromide from the reaction product of bromoethylated poly-THF with silver perchlorate in nitromethane. 

Polymerization of THF by Polystyrene Dioxolenium Salt. Polystyrene with terminal dioxolenium salt groups was prepared by the... 

The polymerization of THF with polystyrene dioxolenium salt was carried out in 1-nitropropane at 0°C. Table I shows the increase in molecular weight with conversion. The resulting copolymer was a... 

The polymerization of THF with poly-THF dioxolenium salt was carried out in nitromethane at 0°C. The results are shown in Table III. The agreement of the observed Mn with the calculated value from conversion, assuming the formation of living polymer was fairly good. The number of ester groups per polymer chain analyzed from IR spectra (7) remains four, as shown in Table III the values are similar to those in Table II. These results substantiate the formation of macromolecular dioxolenium salt at both ends of PTHF. 

Polymerization of Cyclic Ethers and Formats by Poly-THF Dioxolenium Salt. The polymerization of cyclic ethers and formals by PTHF-dioxolenium salt was carried out to clarify the presence of termination or transfer reactions. The results are shown in Table IV. In the polymerization of 3,3-bischloromethyloxetane (BCMO), block copolymer soluble in chloroform and having the expected molecular weight was formed the homopolymer of BCMO insoluble in chloroform was not observed. The block copolymer showed crystalline bands of BCMO at 700, 860, and 890 cm 1, suggesting the formation of ABA block. 

The polymerization of BCMO by dioxolenium salt seems to involve the initiation by bonding and the living nature of the propagation without chain transfer. However, attempted polymerization of BCMO by low concentrations of initiator (10 5M) stopped at low conversion of BCMO after several days, showing the importance of the termination reaction (6). 

Table IV. Polymerization of Cyclic Ethers and Formats by PTHF-dioxolenium Salt...

Polymerization of cyclic ethers by macromolecular dioxolenium salt yielded block copolymers. No termination or chain transfer was observed with THF, but some termination with BCMO and considerable chain transfer with OBH occurred. Cyclic formals such as TEX or DOL do not form block copolymers, showing that the initiation mechanism is not via bonding. 

A more detailed discussion will be presented in another chapter. l,3-Dioxolan-2-ylium (Dioxolenium) Salts. Triphenylmethyliura salts react with dioxolane by hydride transfer to form the corresponding dioxolenium salts (33). which react with nucleophiles exclusively by addition. 

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