Heptafulvenes structures

Of the fundamental nonalternant hydrocarbons, only two prototypes were known about fifteen years ago azulene (XI, Fig. 5), the molecular structure of which was determined by Pfau and Plattner and fulvene (XIX) synthesized by Thiec and Wiemann. Early in the 1960 s many other interesting prototypes have come to be synthesized. Doering succeeded in synthesizing heptafulvene (XX) fulvalene (XXI) and heptafulvalene (XXIII). Prinzbach and Rosswog reported the synthesis of sesquifulvalene (XXII). Preparation of a condensed bicyclic nonalternant hydrocarbon, heptalene (VII), was reported by Dauben and Bertelli . On the other hand, its 5-membered analogue, pentalene (I), has remained, up to the present, unvanquished to many attempts made by synthetic chemists. Very recently, de Mayo and his associates have succeeded in synthesizing its closest derivative, 1-methylpentalene. It is added in this connection that dimethyl derivatives of condensed tricyclic nonaltemant hydrocarbons composed of 5- and 7-membered rings (XIV and XV), known as Hafner s hydrocarbons, were synthesized by Hafner and Schneider already in 1958. 

The cyanosubstituted diphenylcyclopropenyl heptafulvenylium cation 144 (R = CN) was recently synthesized by Kitahara110 from 8-cyano heptafulvene and ethoxy cyclopropenium cation 75 and its structure has been proven by X-ray analysis111. ... 

The 13C NMR data for pentafulvene (1) and heptafulvene (2) (Table 3) and for 6,6-dimethylpentafulvene (5) and sesquifulvalene (6), afford evidence of the extent to which polar structures of the types 5a and 6a contribute to the ground state. If the chemical shifts are analyzed on the basis of electron density, these hydrocarbons are to be considered as olefinic systems with only a small contribution (10% at most) from the polar structures 5a and 6a. 

J With enamines. l-Oxa-2-azulenones (as tethered heptafulvenes) with enamines undergo [8+2] cycloaddition as well (85HOU(5/2c)127, p. 218 98SL950). This way, in the case of heterocyclic enamines (such as 40), azuleno[l,2]-fused azoles and azines are formed (Schemes 13 and 14) The reactions of oxaazulenone 37 with the isomer mixtures of enamines 40a or 40b yield mixtures of isomeric dihydrothiophenes 41 and 42 that are dehydrogenated by 2,3-dichloro-5,6-dicyano-p-benzoqui-none (DDQ) to yield thiophenes 43 and 44, respectively (83CL1721). Whereas the latter [l,2-c]-fused compound is unstable because of its o-quinonoid structure, thiophenes 43a and b are demethoxycarbonylated to yield substances 45a and b. 

Heptafulvenes (XLII) are analogues of fulvenes, but in this case the exocyclic group would carry a negative charge if there were any contribution from a dipolar structure such as (XLIII) consequent upon development of tropylium ion character in the seven-membered ring. 

Because the five-membered ring is a substituted cyclopentadienide anion in some dipolar resonance structures, it might be expected that exocyclic groups that could strongly stabilize a positive charge might lead to a large contribution from dipolar structures and enhanced stability. The heptafulvene (8) and triafulvene (9)... 

CO) 3 and to the reaction of (tropone)Fe(CO)3 with CsHg. (Tropone)Fe(CO)s reacts with A -methyltriazolinedione by 1,5-addition to give (77), characterized crystallographically, while reaction of TCNE with (heptafulvene)Fe(CO)j complexes results in [8 + 2] addition to yield adducts of structure (78). TCNE undergoes 1,3-addition to the unco-ordinated double bond of both (1- and 2-acetyl-chpt)Fe(CO)3, whereas protonation occurs at the co-ordinated double bond of the 2-acetyl derivative. Full details of the 1 1 adduct formed between (chpt)Fe(CO)s and S02 have also been reported. ... 

Figure 6.1 Pentafulvene (fulvene) 1, triafulvene 2, heptafulvene 3 and their polar structures.

Figure 6.18 Heptafulvene and its polar structure 3, tropone 16, tropolone 447 and azulene 4.

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