Tropylidenes

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

In the chromium complex (34) of a cycloheptafuran the chromium is bonded only to carbon atoms of the boat-shaped tropylidene ring, not to the furan carbon atoms, and the planar furan ring has little influence on the dimensions of that ring. In contrast, there is a... 

The comparisons in Tables IX and XIX (e.g., of compounds 51 and 221) reveal the strong low-field shifts (especially of the seven-membered-ring protons) of about 2 ppm that accompany the transition from troponoids to tropylium compounds. In the synthetic sequence from tropones via tro-pylidenes to tropylium salts (e.g., from 327 to 329, Scheme 84), even larger low-field shifts are noted between the methine protons of (less aromatic) tropylidenes like 328 and those of tropylium salts (74T3765 86CL1925). 

Although tropylium salts usually show bathochromic shifts with respect to the corresponding troponoids (compare Table XXI with Tables XIV and XVI, e.g., substances 221 and 51), they show even higher shifts with respect to the precursor tropylidenes (e.g., from 320 up to 634 nm for furan 333 Table XXI). 

Cyclobutatropylium iron carbonyl complex 285 shows C=0 bands at 2095, 2049, and 2032 cm"1, compared to 2044 and 1980 cm"1 for the precursor tropylidene mixture 284 (78AJC1607). This shift to higher wave numbers is a result of increased back-bonding from iron to the cyclobutadiene ligand in the charged complex. 

Certain syntheses of tropylium salts starting from tropylidenes (Section II,B,l,a Scheme 53) are characterized by the formation of more stable cations at the cost of less stable ones they enable one to judge relative stabilities (73ACS2257). 

Pyridotropone 401 (Scheme 107) is converted to tropylidene 402 by the Wolff-Kishner reduction (72JMC750), which, however, fails in the case of enol ether 398b (85JHC1205). 

Another example of bromotropone contraction is derived from Nozoe s benzotropazine chemistry [Scheme 146 89H(29)1459]. Condensation of tropone 552 with further o-aminothiophenol (117b) gave benzenoid compound 554 instead of the expected tris(benzazino)tropylidene. Presumably, this condensation takes place via initial cyclization (to benzothiazine), then cyclocondensation (to spirobenzothiazoline), followed by tautomerization (to norcaradiene 553), ring contraction, and dehydrogenation. 

The addition of the hydride ion and of carbon nucleophiles was thoroughly investigated by El tsov and co-workers (68ZOR1096 69ZOR2072 70ZOR2126). Again, tropylidenes substituted at the 4- and 6-positions (e.g., 593 and 594) were obtained. 

Besides complex hydrides, other hydride-ion donors were used 1,3-dimethyl-2-phenyl-, l,3-dimethyl-5-nitro-, and 1,3-dimethylbenzimidazo-line l,2-dimethyI-3-phenyl- and 1,2-dimethylindazoline or suitable tropylidenes. 

Imidazolium salt 343 adds malononitrile at C-6 after spontaneous dehydrogenation, imidazoheptafulvene is isolated (65CPB810). Likewise, di-methylaniline attacks oxazolotropylium salt 23a (Scheme 161) preferentially at C-6 to give tropylidene 601 in addition to a small amount of the 4-substituted isomer (67CPB627). 

Benzofurotropylium salts (cf. Section II,C,3,a) that bear a hydroxyl group at C-3 (e.g., 611) are deprotonated to yield mesomeric furo-p-benzoquinonetropides or tropylidene quinones like 613 [67BGJ1480 71JCS(P1)2399]. 

Abstraction of hydride ions from tropylidenes by nitrosyl or trityl tetra-fluoborates leads to 2-imino-3-arylthiazolotropylium salts like 640a,b [94H(38)2691] and to cycloheptaazaazulenium salt 643 or (unstable) 14-77-electronic pyrrolotropylium salt 647 [Scheme 173 94JCS(P1)2579]. 

Thus, tropylidene 644 and tropones 645 and 646 are converted into N-protonated or /V-methylated species 648-650 tropone 646 can even be bis-protonated at the nitrogen to give 651. 

Additions of carbenes to perfluorobenzene have been reported [182, 183] and tropylidene structures for the products have been proposed (Figure 9.77). 

Elimination of difluorocarbene from a o-complex 9.79A has been proposed, although the fate of the rest of the molecule is not clear, and the addition could involve either an insertion reaction (a) or formation of a tropylidene (b) (Figure 9.79). 

The presence of the methoxycarbonyl substituent at C-7 affects the reaction course of the cycloaddition of diethyl diazenedicarboxylate to the cycloheptatriene moiety, since the ene product is not produced. A mixture of tropylidene- and norcaradiene-type adducts 8 and 9 is obtained, whose ratio is dependent on the temperature13. 4-Phenyl-3//-l,2,4-triazole-3,5(4//)-dione, however, affords the norcaradiene-type adducts 10 exclusively with 7-substituted cyclo-heptatrienes14"19. 

The cycloaddition of 4-phenyl-3//-l,2,4-triazole-3,5(4//)-dione to a [2,6]spirocyclohepta-triene. which cannot equilibrate with the norcaradiene-type valence tautomer, affords two diastereomeric tropylidene-type adducts23. 

An examination has been made of substituent effects on the product distribution of tropylidene- and norcaradiene-derived endoperoxides arising on singlet oxygenation of 7-aryl- and 7-alkylcyclohepta-l,3,5-trienes. The ratio... 

According to Eq. (11), Confalone has presented a new method of generation of azomethine ylides through the condensation of N-substituted ot-amino esters with aldehydes (83JOC2994 84JA7175). Thus, 5-formylmethyldibenzo-[a.d]tropylidene or o-(allyloxy)benzaldehyde is heated with ethyl sarcosinate or methyl prolinate under reflux in toluene. The water formed is continuously driven off with the aid of a Dean-Stark trap. The ester-stabilized azomethine ylide 77 or 78 quantitatively generated is trapped in an intramolecular fashion. 

It has been known for many years that substituents can change the position of the tropylidene-norcaradiene (4 5) equilibrium (equation 4). While the parent compound... 

Another type of reaction allowed us to study the philicity of silylene 4. As we have shown earlier, 4 adds smoothly to a variety of alkynes giving way to the silacyclopropene framework (Eq. 4) [10], Varying the / ara-substituents of diphenylacetylene offers us the possibility to tune the electron density of the triple bond, and we now studied the rates of the reaction of 3 with diphenylacetylenes lOa-c. The absolute reaction rates of these three first-order reactions are identical (Ai = 6.3 0.2 10 4s-l) this result is in accordance with a mechanism, in which the formation of silylene 4 from cyclotrisilane 3 is the rate determining step [6], However, the relative reaction rates of the addition of 4 to the triple bond of lOa-c, which were determined by competition experiments, turned out to differ appreciably from each other. Electron withdrawing substituents favor the addition of 4 to the alkyne, whereas electron donating substituents, such as a methyl group, slow down the reaction rate. As shown in Fig. 5, the relative reactions rates correlate well with the [Pg.62]

Ahyd// was measured in acetic acid and corrected for Aso // of tropone in acetic acid using Pd on BaS04 catalyst. The reaction product was cycloheptanone. The resonance stabilization by the carbonyl group is only slightly less than that of the methylene group in heptafulvene. Compare tropylidene and heptafulvene. 

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