Pentalene

Attempts to prepare the antiaromatic compound pentalene have so far failed. Various transition metal complexes of pentalene are, however, easily prepared [for example, ruthenium carbonyl complexes, Eq. (167) (Brookes et at., 1973)] so that pentalene might be generated in situ from such complexes and trapped with various reagents (Knox and Stone, 1974). 

It has been postulated as an intermediate in the photochemical reaction of benzene with cyclobutene, but it has not been isolated in the free state. Its tri-carbonyliron complex, however, is readily isolated as a stable crystalline solid [Eq. (168) (Cotton and Deganello, 1972)] presenting the possibility of generating the free ligand in situ by oxidative removal of the tricarbonyliron group. 

---

A more general classification considers the phase of the total electronic wave function [13]. We have treated the case of cyclic polyenes in detail [28,48,49] and showed that for Hiickel systems the ground state may be considered as the combination of two Kekule structures. If the number of electron pairs in the system is odd, the ground state is the in-phase combination, and the system is aromatic. If the number of electron pairs is even (as in cyclobutadiene, pentalene, etc.), the ground state is the out-of-phase combination, and the system is antiaromatic. These ideas are in line with previous work on specific systems [40,50]. 

Mitex [2385-85-5] is l,2,3,4,5,5,6,7,8,9,10,10-dodecachloro-octahydro-l,3,4-metheno-2JT-cyclobuta-p,<7 -pentalene (37) (mp 485°C). The rat LD s are 306, 600 (oral) and >2000 (dermal) mg/kg. Mirex is extremely resistant to biodegradation and was once considered the perfect stomach poison iasecticide for use ia baits to control imported fire ants. However, even at doses of a few milligrams per 10 m it was found to bioaccumulate ia birds and fish and its registrations were canceled ia the United States ia 1976. 

Carbethoxy)-3-rnethyl-3,4,5,6-tetrahydrO 2H-pentalen-l-one (2). A solution of ethyl 2-ethytidene-3-(1-cyclopenten-1-yl)-3-oxo-propanoate 1 (210 mg. 1 mmol) in CH2CI2 (10 mL) was stirred with SnCU (780 mg, 3 mmol) lor 24 h at 20°C After quenching with water (50 mL), the organic layer was washed (N3HCO3), dned (UgS04) and punlied by preparative TLC to afford 62 5 mg of 2 (30%)... 

The predictions of relative stability obtained by the various approaches diverge more widely when nonbenzenoid systems are considered. The simple Hiickel method using total n delocalization energies relative to an isolated double-bond reference energy (a + fi) fails. This approach predicts stabilization of the same order of magnitude for such unstable systems as pentalene and fulvalene as it does for much more stable aromatics. The HMO, RE, and SCF-MO methods, which use polyene reference energies, do much better. All show drastically reduced stabilization for such systems and, in fact, indicate destabilization of systems such as butalene and pentalene (Scheme 9.2). 

In contrast to the significant resonance stabilization of azulene, pentalene and heptalene are indicated to be destabilized relative to a reference polyene ... 

Preparation of pentalene is followed by immediate dimerization. Low-temperature photolysis produces a new species believed to be pentalene, but the compound reverts to dimer at — 100°C. The matrix-isolated monomer has been characterized spectroscopically. The results are in accord with the predicted lack of stabilization. ... 

Heterocyclic [3,2-d]-fused pentalenes and their benzoannelated derivatives 97CLY547. 

The moleeular geometry and eleetronie strueture of 3,4-diaza-l,6,6aiC -trithia-pentalene 85a has been studied and eompared with the nitrogen-free l,6,6aiC -trithiapentalene [98PS35]. Whereas Hartree-Foek ealeulations prediet 85b and 85c to be valenee isomers, DFT and MP2 ealeulations prediet the minimum to be of C2u symmetry eorresponding to 85a (Seheme 56). 

Pentalene is a most elusive molecule and has never been isolated. The penta-lene dianion, however, is well known and quite stable. Explain. 

The addition of dimethyl acetylenedicarboxylate to 5-/er/-butyl-Ar,Ar,Ar, Ar -tetramethyl-2-aza-pentalene-1,3-diamine (20) is frontier orbital rather than charge controlled, and results initially in attack at the 3a-position to give, via a dipolar intermediate, tricycle 21, which undergoes valence isomerization to the cyclopent[c]azepine 22.107... 

This type of asymmetric conjugate addition of allylic sulfinyl carbanions to cyclopen-tenones has been applied successfully to total synthesis of some natural products. For example, enantiomerically pure (+ )-hirsutene (29) is prepared (via 28) using as a key step conjugate addition of an allylic sulfinyl carbanion to 2-methyl-2-cyclopentenone (equation 28)65, and (+ )-pentalene (31) is prepared using as a key step kinetically controlled conjugate addition of racemic crotyl sulfinyl carbanion to enantiomerically pure cyclopentenone 30 (equation 29) this kinetic resolution of the crotyl sulfoxide is followed by several chemical transformations leading to (+ )-pentalene (31)68. 

Simple resonance theory predicts that pentalene (48), azulene (49), and heptalene (50) should be aromatic, although no nonionic canonical form can have a double bond at the ring junction. Molecular orbital calculations show that azulene should be stable but not the other two, and this is borne out by experiment. Heptalene has been prepared but reacts readily with oxygen, acids, and bromine, is easily hydrogenated, and polymerizes on standing. Analysis of its NMR spectrum shows that it is... 

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. 

A theoretical explanation for such an anomalous phenomenon in certain nonalternant hydrocarbons has first been attempted, in case of pentalene, by Boer-Veenendaal and Boer followed by Boer-Veenen-daal et Snyder and Nakajima and Katagiri for other related nonalternant hydrocarbons. By making allowance for the effects of <7-bond compression, these authors have shown that a distorted structure resembling either of the two Kekule-type structures is actually energetically favored as compared with the apparently-full symmetrical one. 

Fig. 2. Correlation of the molecular orbitals of cyclooctatetraene with those of pentalene...

Inspection of Table 2 reveals that all those molecules that suffer a molecular-symmetry reduction in the ground state possess (E2 — E1) values considerably larger than the critical value, so that they should have a fully-symmetrical nuclear configuration in their first excited states. On the other hand, there are cases where a molecule has an ( , — Eg) value significantly higher than the critical value, but has a relatively smaller (Ej— i) value. The ( 2 i) value of the pentalene dianion (I ) is of the same order of magnitude as the critical value and those for the peri-condensed nonalternant hydrocarbon, XVII, the fulvalenes, XXI, XXII and XXIII, and the dianions, IVand VII are significantly smaller than the critical value ( 0.6eV). 

The orbital arrangement for pentalene shown in Fig. 2 serves to indicate how close the second excited state is to the first excited state when two more electrons are placed in the nonbonding orbital to form the dianion. The very small (E2 — E1) values for fulvalene and hepta-fulvalene are realized from the orbital arrangements shown in Fig. 4 in both molecules the two lowest excited states ( 3 and 211) have the same energy in the Huckel picture. 

As the starting geometries for iterative calculation, we take all the possible structures in which bond lengths are distorted so that the set of displacement vectors may form a basis of an irreducible representation of the full symmetry group of a molecule. For example, with pentalene (I), there are 3, 2, 2 and 2 distinct bond distortions belonging respectively to a, b2 and representations of point group D21,. 

Of the cata-condensed nonalternant hydrocarbons undergoing a pseudo Jahn-Teller distortion, pentalene (I) and heptalene (VII), having the largest value, are predicted to possess a strong bond alternation. This confirms the results of the previous theoretical investi-gations " and agrees with the available experimental facts ... 

---