Oxonium cyclic

Typical Lewis acids like BF3 and SbCls coordinate with oxirane oxygen to give (presumably) a cyclic oxonium ion (41) which reacts further (Scheme 28) (64HC 19-1)446, B-67MI50505). 

These give the products expected from electrophilic attack on oxygen by the electrophilic reagent atom, followed by nucleophilic opening of the cyclic oxonium ion (e.g. 42 Scheme 29) <64HC(19-1)436). 

Like the un-ionized hydroxyl group, an alkoxy group is a weak nucleophile. Nevertheless, it can operate as a neighboring nucleophile. For example, solvolysis of the isomeric p-bromobenzenesulfonate esters 6 and 7 leads to identical prxKluct nuxtures, suggesting the involvement of a common intermediate. This can be explained by involvement of the cyclic oxonium icai which would result from intramolecular participation. ... 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

The remarkably high molecular weights of copolymers of dioxolan and styrene, which were achieved by Yamashita, are also easily intelligible in terms of the insertion mechanisms shown in schemes (D) and (E) the fact that cationically produced polystyrenes have low molecular weights is mainly due to proton transfer to monomer from the P-carbon at the growing end of a chain. If there is no growing end but a cyclic oxonium ion, this reaction cannot occur, and thus the principal chainbreaking reaction is frustrated. 

The fragment ions at m/z 149, [CgHsOs], and 167, [C8H704], are especially prominent in the El spectra of phthalates. The formation of the [CgHsOs]" ion has initially been attributed to a McLafferty rearrangement followed by loss of an alk-oxy radical and final stabilization to a cyclic oxonium ion. [104] However, it has been revealed that four other pathways in total lead to its formation excluding the above one. [105,106] The two most prominent fragmentation pathways are ... 

In the initial step of the polymerization, a cyclic oxonium ion is formed by transfer of an alkyl group from the initiator to the cyclic ether. Propagation occurs by SN2 attack of a monomer molecule at a ring a-methylene position of the cyclic tertiary oxonium ion, followed by opening of the oxonium ring and formation of a new cyclic oxonium ion. 

THF can be polymerized only with cationic initiators, for example, boron trifluoride or antimony pentachloride. The initial step consists of the formation of a cyclic oxonium ion one of two activated methylene groups in the a-position to the oxonium ion is then attacked by a monomer molecule in an S 2-reaction, resulting in the opening of the ring. Further chain growth proceeds again via tertiary oxonium ions and not, as formerly assumed, via free carbonium ions ... 

The substitution of a CH unit in benzene by 0+ (the oxonia group) gives rise to the pyrylium cation (3). Since this ring still possesses 6 ir-electrons, it may be expected to exhibit aromatic properties. As the oxygen is primarily tricovalent, the pyrylium ring may be formally regarded as a cyclic oxonium ion. However its enhanced stability relative to aliphatic and alicyclic oxonium salts is doubtless due to its aromatic nature. 

Certain cyclic ethers which will not homopolymerize will copolymerize with THF (25, 52). These cyclic ethers are stable five and six-membered ring compounds such as 2-methyltetrahydrofuran (2-MeTHF), and 1,4-dioxane (DOX). It is probable that 4-phenyl-l,3-dioxane (PhDOX), tetrahydropyran (THP), and 4-methyl-l,3-dioxolane (MDOL) which do not homopolymerize but which have been reported to copolymerize with BCMO (107, 108) would also copolymerize with THF. In the copolymerizations a correlation was again found between the basicity of the attacking ether and its reactivity with the cyclic oxonium ion. The... 

In a more recent study (104), similar results were obtained with the bicy-clic thiol hemiacetal 75 (Fig. 13). Cyclization of 75 under kinetically controlled conditions gave the ci monothioacetal 76 the trans isomer 77 was not observed. Thus the attack of an SH group on a cyclic oxonium ion must also take place preferentially with stereoelectronic control. 

We should also expect stereoelectronic control when the hydroxyl group is replaced by another nucleophile in the reaction with cyclic oxonium ions. A recent report (110) shows that hydride transfer to cyclic oxonium ion is subject to stereoelectronic control. Tricyclic spiroketal 140 (Fig. 19) undergoes an acid-catalyzed oxidation-reduction reaction to give the equatorial bicyclic aldehyde 147 stereospecifically. Similarly, spiroketals 148 and M9 gave the corresponding equatorial bicyclic ketone 150. 

Conformation of acetals Conformation of mono and dithioacetals Conformation of 1,3-oxazines and 1,3-diazines Formation and hydrolysis of the acetal function Hydride transfer to cyclic oxonium ion Oxidation of the C —H bond in acetals REFERENCES... 

The oxygen atom in jl can be replaced by sulfur, and the same prediction is maintained. We have already seen that this is indeed the case for cyclic oxonium and sulfonium ions (Chapter 2). It is essentially for the same reason that lactones, thionolactones (Chapter 3) as well as lactams and their derivatives (Chapter 4) behave in exactly the same manner. Indeed, an axial attack on 12 (X=0, 5, or MR) leads to an intermediate having a chair conformation (12 13) while an equatorial attack necessarily leads to the less favorable boat conformation (12 14). 

The reaction of nucleophiles with the conformationally rigid piperidinium ion lj>, like that with cyclic oxonium ions, can also be controlled by stereoelectronic effects. On that basis, the addition of a nucleophile on the upper face of Jjj must lead to the boat-like intermediate whereas that from the lower face must lead to the chair-like intermediate V7. The transition state leading to must be less favorable than that leading to Yl and product V7 should therefore be favored. 

Remote hydroxylation. Dehalogenation of ot-bromo ketosteroids by AgSbF6 generates by a series of hydride shifts cyclic oxonium salts, which are converted on hydrolysis into hydroxylated steroids. 

There have been several distonic superelectrophiles described in the literature that are oxonium-centered dications. For example, a series of diols were solvated in FSOsH-SbFs-SC and the dicationic species were persistent at —80°C.77 Upon warming to 25°C, the bis-oxonium ions undergo rearrangement to the more stable carboxonium monocations (eq 77). Barring a concerted mechanism, the transformations are thought to involve the carbo-oxonium dications (i.e., 225) and concomitant hydride shifts. Interestingly, 2,5-hexanediol ionizes in superacid, and with warming the cyclic oxonium ion (229) is formed (eq 78). 

Although not the best catalyst, AgSbF6 led to the rearranged product in 50% yield. This cascade reaction probably started with the well-known cyclization of the ketone to the alkyne on silver coordination, giving a cyclic oxonium intermediate that rearranged to furanone via an alkyl 1,2-migration (Scheme 3.54).82... 

From a mechanistic perspective, the reaction is similar to those described above. Silver-catalyzed cyclization of the ketone to the allene gave a cyclic oxonium intermediate. A [1,2]- or [1,5]-alkyl shift modified the sigma skeleton leading to an alkylsilver intermediate, which on elimination gave a trisubstituted furan. 

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