Ethers hydride abstraction from

The chiral (V-camphanoyl iminium ion 7, prepared by hydride abstraction from 2-camphanoyl-l,2,3,4-tetrahydro-6,7-dimethoxyisoquinoline 6 (see Appendix) with triphenylcarbenium te-trafluoroborate, reacts with silyl enol ethers to give 1-substituted tetrahydroisoquinoline derivatives with reasonable diastereoselectivity, 0°. On addition of titanium(IV) chloride, prior to the addition of the silyl enol ether, the diastereoselectivity gradually rises to an optimum at 2.5 equivalents of the Lewis acid, but the yield drops by 20%. 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

Initiation with Tropylium Ion. When cycloheptatrienyl hexachlor-antimonate is used as initiator for tetrahydrofuran polymerization, the reactions are somewhat cleaner, and strong colors do not develop as readily as when the corresponding trityl salts are used (17). Rates of initiation are much lower, and the reaction is hardly noticeable at room temperature. However, at 50 °C. and above initiation is significant, and the polymerizations proceed almost to the expected theoretical conversion of monomer to polymer even when hexachlorantimonate is the anion (Table III). Therefore, the apparent low equilibrium conversion obtained with the rapidly initiating trityl salts is minimized in this case by the comparatively low rate of consumption of initiator. Once again GLC demonstrates clearly that the initiation reaction involves primarily hydride abstraction from the ether. 

The nitronium ion is also capable of acting as an oxidizing agent effecting hydride abstraction from a variety of functionalized alkanes. The oxidation of diarylmethyl methyl ethers is best illustrated involving pentacoordinate carbocations [Eq. (6.44)]. 

P,P] Vinylthionium ions such as 40.1 are potent Michael acceptors. Additions of silyl and stannyl enol ethers to vinylthionium ions such as 40.1 have been reported by Mukaiyama and co-workers (84-86). Two methods were employed for generation of the Michael acceptors elimination of ethyl sulfide from 40.2 under the action of a cationic trityl species (method A) and hydride abstraction from 40.3 and 40.4 by a trityl salt (method B) to give 40.1. Although both procedures use a strong Lewis acid to promote reaction, they differ substantially from the prior examples because an acceptor-acid... 

Chirality in the /V-acyl substituent can also induce stereoselectivity in the iminium addition process (equation 86). The chiral auxiliary in (115) appears to be one of the best chiral inductors and reacts with the silyl enol ether from acetophenone in 90% diastereoselectivity. It is argued that the A -acylim-inium intermediate in this reaction adopts the s-trans conformation. Similar methodology is used for the asymmetric synthesis of tetrahydroisoquinolines (equation 87). The reactive intermediate is generated through hydride abstraction from the amide by using the triphenylmethane cation. 

If triphenylmethyl chloride in ether is treated with sodium, a yellow colour is produced due to the presence of the anionic spiecies PhsC". Alternatively, if triphenylmethyl chloride is treated with silver perchlorate in a solvent such as THF, the triphenylmethyl cation is obtained. More conveniently, triphenylmethyl salts, PhsC X", can be obtained as orange-red crystalline solids from the action of the appropriate strong acid on triphenylcarbinol in ethanoic or propanoic anhydride solution. The perchlorate, fluoroborate and hexafluoro-phosphate salts are most commonly used for hydride ion abstraction from organic compounds (e.g. cycloheptatriene gives tropylium salts). The salts are rather easily hydrolysed to triphenylcarbinol. 

Iron pentacarbonyl and l-methoxy-l,4-cyclohexadiene react as shown by Birch and oo-workera, but in dibutyl ether this solvent has been found superior. The tricarbonyl(methoxy-l,3-cyclohexadiene)iron isomers undergo hydride abstraction with triphenylmethyl tetrafluoro-borate to form the dienyl salt mixture of which the 1-methoxy isomer is hydrolyzed by water to the cyclohexadienone complex. The 2-methoxy isomer can be recovered by precipitation as the hexafluoro-phosphate salt. By this method the 3-methyl-substituted dienone complex has also been prepared from l-methoxy-3-methylbenzene. The use of the conjugated 1-methoxy-1,3-cyclohexadiene in Part B led to no increase in yield or rate and resulted chiefly in another product of higher molecular weight. An alternative procedure for the dienone is to react tricarbonyl(l,4-dimethoxycyclohexadiene)iron with sulfuric acid. ... 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

Bawn, Bell, Fitzsimmons, and Ledwith (20) have suggested that transfer reaction in the bulk polymerization of THF must involve either hydride ion abstraction from the alpha methylene of THF or of tetra-methyleneoxy units in the polymer, or degradative oxonium ion formation with the ethereal oxygen atoms of polymers of the type discussed in Section IIID4. 

A simple example involves the reaction of the silyl ether 213, made from the corresponding 4-hydroxy-alkene by treatment with (bromomethyl)chloro-dimethylsilane, with tributyltin hydride and a radical initiator. Bromine abstraction and intramolecular cyclization with the double bond leads to the bicyclic 214, which upon oxidation with hydrogen peroxide gives the branched-chain 215 in an overall yield of 73% from the alcohol precursor of 213 (Scheme 21). When the sequence is conducted with the C-4 epimeric starting alcohol, the final product again has the hydroxymethyl group cis-related to the hydroxy group.217... 

In the initial phases of this effort a means for introducing appropriate functionality in the C-l/C-2 positions from an unsubstituted precursor such as 80 that would eventually lead to an aziridine ring in the final target compound(s) was needed. When considering the oxidation of simple hexahydro-8-oxo-1 //-pyrrolo[ 1,2-a] indoles using DDQ, two mechanistic pathways are possible leading to either of the carbocationic intermediates 81 or 82 via hydride abstraction at C-5 or C-1, respectively (Scheme 12).69 Proton loss from 81 would ultimately lead to the phenolic product 83 whereas nucleophilic trapping of intermediate 82 by alcohols would afford the C-l-substituted ethers 84. Oxidative transformations at... 

Initiation with Tropylium Ion. Tropylium hexachlorantimonate reacts with vinyl alkyl ethers in a manner very similar to the reactions of triphenylmethyl salts. Again, rapid initiation is followed by propagation without apparent termination. Termination can be demonstrated to be absent from experiments in which fresh samples of monomer are added to completed polymerizations, whereupon the measured reaction rates parallel those previously recorded (Table II). Molecular weights of the polymers from isobutyl vinyl ether are very similar to those obtained with triphenylmethyl salts as initiators and again give clear evidence for excessive monomer transfer. Gas chromatographic analysis of the reaction mixtures showed that cycloheptatriene (product of hydride abstraction) was not present which indicates clearly that initiation must arise via addition of the tropylium ion to the vinyl ether—i.e.,... 

The oxidation of ketones to enones via the reaction of their silyl enol ethers with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) has been suggested originally to proceed via allylic hydride abstraction [195-198]. A recent reinvestigation, however, [199] has established the intermediate formation of a substrate-quinone adduct 96 which was presumably formed from a geminate radical ion pair after electron transfer. Decomposition of the adduct then finally afforded the observed enone product 97. Recently, the critical role of solvent polarity in the formation of 97 from the PET reaction of 93 and chloranil has been identified by time-resolved spectroscopy [200]. 

The finding that enol ethers are more reactive to chloranil than are ketones [234] suggested that the ketone dehydrogenations proceed by hydride ion abstraction from enolic intermediates, and detailed mechanistic studies confirm this [233,... 

ConpBng. Japanese chemists have reported the synthesis of 8,8 -biheplafulvenyl (4) from the bromide (1). Treatment of (1) with magnesium in ether produced 1,2-di(3-cycloJicptatrienyl)cthane (2) in 85 % yield. Hydride abstraction with trityl fluoroborate (1,1256 1258 2. 454 this volume) in methylene chloride gave the salt (3). Treatment... 

The carbonium ion abstracts a tertiary hydrogen from the isoparaffin, yielding a hexyl ion which then abstracts a hydride ion from the unrearranged isoparaffin, saturates itself, and starts a new cycle. Carbonium ions are formed when unsaturated compounds are dissolved in proton acids or, in general, if the compound is sufficiently basic relative to the acid, i.e., alcohols, ethers, esters, acid anhydrides, alkyl-substituted aromatic compounds will form carbonium ions in sulfuric, hydrofluoric, and other acid solvents. 

Triphenylcarbenium salts (PhgC X") selectively oxidize secondary t-butyl or triph-enylmethyl (trityl) ethers derived from alcohols. The oxidation proceeds via initial hydride abstraction followed by loss of the group on oxygen. The secondary-overprimary selectivity results from preferential formation of an oxocarbenium ion intermediate at the secondary center (R2 C-OTr is formed faster than RH C-OTr). 

The perchlorate or tetrafluoroborate salt of the trityl cation abstracted the hydride ion from trisubstituted cyclopropenes 1 bearing a tertiary hydrogen to give the corresponding cyclopropenylium salts 2. The reaction was usually conducted in polar solvents, such as acetonitrile or dichloromethane, at room temperature under protection from moisture. Addition of a nonpolar solvent such as diethyl ether resulted in precipitation of the formed cyclopropenylium salts. " ... 

Condensation of the isothiouronium salt, obtained from the vinyl alcohol (413), with 2-methyl-cyclopentane-1,3-dione has led to a series of A-nor-3-oxa-steroids and similar condensation of the thia-analogue (408) with 2-methyl-cyclopentane-1,3-dione afforded the thia-oestrane (410), which was reduced catalytically to a mixture of 14a- and 14/S-H-dihydro-epimers. Reduction of the remaining conjugated 8,9-unsaturation was less easy than in the carbocyclic analogue metal-ammonia afforded thiols, and catalytic reduction was slow and led to a mixture of, presumably, 8a,9a- and 8, 9jS-epimers. The former isomer (416) which has its 9 -H favourably disposed for hydride abstraction with DDQ readily dehydrogenated to the 9(ll)-olefin which, on subsequent catalytic reduction, gave 6-thia-oestrone methyl ether. Condensation of the same thia-... 

Acetattzation. Diethyl acetals are formed from carbonyl compounds by functional group exchange with triethyl orthoformate (EtOH also present) under the influence of DDD." Benzylic activation. Hydride abstraction by DDQ from benzyl ethers, where the benzylic position is also activated by a nuclear substitutent (e.g., methoxy group), prepares such compounds to be attacked by nucleophiles. The reaction constitutes an important step in a synthesis of deoxyfrenolicin. ... 

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