Hydride abstraction from cycloheptatriene

The highly crowded tris( 1 -naphthyl)methyl cation 150 and tris(2-naphthyl)methyl cation 151 were prepared and used to abstract hydride ion from cycloheptatriene to generate tropylium ion.363 Hydride abstraction, however, could be performed only with the less crowded cation 150. 

The generation of the parent benzocyclopropenylium and cyclopropa[6]naphthalenylium ions via hydride abstraction from benzocyclopropene (3a) and 1 //-cyclopropa[(j]naphthalene (3b)," respectively, has been attempted with triphenylmethyl fluoroborate, but only small quantities of the corresponding aromatic aldehydes 4 were isolated after quenching the mixture with water. Aldehyde formation is ascribed to attack of water on the intermediate cycloproparenyl cation. Benzocyclopropene is ca. 5 times less reactive towards triphenylmethyl fluoroborate than cycloheptatriene. 

Dauben and Honnen (36) first showed that triphenylmethyl fiuoroborate abstracts hydride ion from (cycloheptatriene)Mo(CO)3 to give w-tropylium-molybdenum tricarbonyl fiuoroborate... 

When cycloheptatriene is treated with a reagent that can abstract a hydride ion, it is converted to the cycloheptatrienyl (or tropylium) cation. The loss of a hydride ion from cycloheptatriene occurs with unexpected ease, and the cycloheptatrienyl cation is found to be unusually stable. The NMR spectrum of the cycloheptatrienyl cation indicates that all seven hydrogen atoms are equivalent. If we look closely at Fig. 14.12, we see how we can account for these observations. 

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. 

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

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.,... 

Cycloheptatriene reacts with platinum(IV) bromide in acetic acid to produce a mixture of tropylium hcxabromoplatinate (C7H7)2PtBra and (l,3,5-cycloheptatriene)m-dibromoplatinum(II) in a ratio of about 1 2 (359). It has been suggested (359) that the initial step involves hydride ion abstraction from the olefin and a concomitant... 

Although Able et al. (2, 8) had originally set out to prepare 7r-cyclo-heptatrienyl complexes of metals, the cycloheptatriene complexes they actually obtained served as key intermediates in forming the former complexes. In 1958 Dauben and Honnen (61) reported that cycloheptatriene-molybdenum tricarbonyl reacted with triphenylmethyl tetrafluoroborate in methylene chloride solution with abstraction of hydride ion from the molybdenum complex. The reaction products, obtained in nearly quantitative yields, were triphenylmethane and the 7r-cycloheptatrienyl complex [(7r-C7H7)Mo(CO)3]+BF4 . 

Phenyl-1,3-dithiolium or 1,3-benzodithiolium cations abstract hydride from cycloheptatriene, giving the thermodynamically more stable tropylium salt and 4-phenyl-l,3-dithiole or 1,3-benzodithiole. ... 

Brounts RHAM, Buck HM (1983) Hydride abstraction reactions from cycloheptatriene and 1-carbamoylcycloheptatriene. Effect on the CONH2 group orientation on the reactivity difference of the exo and... 

Recent work by several groups has shown that the dienyl-Fe(CO)3 exists as a stable group present in several related cationic species. Thus Dauben and Bertelli (27) have prepared cycloheptadienyl-iron tricarbonyl fluoroborate (LXXII) from cycloheptadiene-Fe(CO)3 (XXXIV) and triphenylmethyl fluoroborate. The inherent stability of the dienyl-Fe(CO)3 grouping is again seen in the reaction of cycloheptatriene-iron tricarbonyl (XXXIII) with the triphenylmethyl cation. Hydride abstraction to give the cycloheptatrienyl-Fe(CO)3 cation does not occur but rather the triphenylmethyl cation adds to the free olefinic bond to produce the cation (LXXIII)... 

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