Tropylium derivatives

Cations (61),129 (62),130 and (63)131 were obtained as perchlorate or BF4- salts. The cations were characterized by NMR and X-ray methods reduction potentials and pXR+ values were also determined. Density functional calculations were performed on the tropylium derivative (64), a number of substituted analogues, and a number of neutral systems.132 The objective was to assess the utility of NICS (nuclear independent chemical shift) as a probe for detecting/sensing variation in aromaticity due to transannular tx-tt interactions. A dicyanoheptafulvene has been prepared, with X-ray crystallographic analysis showing a significant contribution from the dipolar resonance contributor (65).133 Addition of acid results in the tropylium... 

Since both BF3 or SbFs form adducts with SO2, the rate acceleration [20,000 times for W(tj -CsH5)(Me)(CO)3] can be explained by an increase of the electrophilic character of sulfur in the presence of the Lewis acid. Insertion of the chalcogen dioxides EO2 has been reported with the tropylium derivative of molybdenum(O) Mo(>7 -C7H7)(R)(C0)2 ... 

The 1,2-dithiolylium and 1,3-dithiolylium ions (115) and (116) are iso-rr-electronic with the tropylium ion, from which they may be formally derived by replacing double bonds by sulfur atoms. Various calculations and structural data demonstrate that the rings are stabilized by tt-electron delocalization. 

Substituted dibenzo[6,/]thiepins can be generated from thioxanthene derivatives by the rearrangement of carbocation 1. Compared with other possible cations, the tropylium ion type 1C is favored because of its resonance energy. Depending on the reaction conditions, the thiepin cation can react to give thiepins by loss of a proton, or by trapping a nucleophile, followed by elimination. 

Unlike such unstable intermediates, the first, rare example of reversible dissociation of a carbon-carbon a bond into a stable carbocation and carbanion was reported for a nitro-dicyano compound (20) prepared from trimethyl- and triphenyl-cyclopropenylium tetrafluoroborate ([4" ]BF4 and [5 JBFJ) with the potassium salt of p-substituted-phenylmalononitrile anions (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985). Other ionically dissociative malononitrile derivatives have been prepared from such carbocations as the tropylium [S ] (Arnett and Troughton, 1983) and the tris(p-methoxyphenyl)methylium [93 j (Arnett and Troughton, 1983) ions. 

In spite of their high pAR+ values, the methyl cations have less negative reduction potentials as compared to those of the cyclic cations e.g., -1.12 V for the tropylium ion 8+ and -2.20 V for cyclopropenium ion 9+ (8). Presence of tert-butyl groups in cations 2b-d+ increased the reversibility of both reduction and oxidation waves. The most negative reduction potential in the dimethylamino derivative 20+ reflects its high electrochemical stability. 

The culmination of evidence supporting the homoaromaticity of homo-tropylium ions was provided by an X-ray crystallographic study on the simple derivative [16] (Childs et al., 1982). 

The tropol derivative (e.g., 229) was not isolated. Tropylium salt only in solution. 

Some unexpected reactions are observed during the interaction of cyclo-hepta[c]thien-6-one (231) or its 1,3-dibromo derivative and organometallic compounds (74YZ1429) These reactions are halogen-metal interconversion and subsequent dimerization, partial debromination, and simultaneous 1,2- and 1,4-addition of phenyllithium onto tropone 231 to yield tropylium salts 232 and 233, respectively (Scheme 56). 

In the presence of water the tropylium ion (50) has been found to undergo polar cycloaddition of the 4 + 2 type. Ito and Itoh found that cyclopentadiene reacted with the tropylium ion to afford a 90% yield of a complex mixture of alcohols, all of which could be derived from the allylic cation (51). 

Other li acetylides Li-C=C-R with R = hexyl [21] or benzylether dendrons [22, 23] (up to the fourth generation) have also been attached to (Figure 3.3), and various different electrophiles have been used to complete the reaction with the intermediate li-fuUeride (Scheme 3.2 and Figure 3.3). Besides the protonation, alkyl-, benzyl-, cycloheptatrienyl-, benzoyl- or vinylether-derivatives or formaldehyde and dichloro-acetylene were used as electrophiles [12,20]. Most of these electrophiles are attached to the anion in the expected C-2 position. The 1,4-adducts are available by quenching the anion with the tropylium cation or benzoyl chloride [12]. The fuUerene anion can be stabilized by introduction of benzylether dendrons. The lifetimes of the anions change with the size of the dendrons [22]. 

A growing body of work deals with CpV(cycloheptatrienyl) complexes and derivatives, which are formally V(0), if the cycloheptatrienyl (tropylium) ligand is considered to have a +1 charge [11-13]. The parent complex has a facile reversible oxidation (—0.17 V) in TlTF but it cannot... 

At present, quite a number of derivatives of analogues of the tropylium cation containing two sulfur atoms are known. Both 1,2-85 and 1,3-dithiolium cations86,87 (69 and 70, respectively) have been synthesized recently the possible existence of the cation 70 had been... 

Equally interesting is the situation in the second class of compounds studied (analogues of non-alternant hydrocarbons), which is best divided into two sub-groups analogues of the tropylium ion and analogues of azulene. The empirical correlation of experimental and theoretical excitation energies studied requires a further subdivision into compounds with one heteroatom (e.g. thiopyrylium ion) and two heteroatoms, either adjacent (e.g., 1,2-dithiolium ion) or non-adjacent (e.g., 1,3-dithiolium ion). Experimental and theoretical data are presented in Table VII. Table VIII summarizes data for the derivatives of dithiolia. Figure 15 shows the absorption curves of 1-benzo-... 

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