Pentadienyl system anion

Under the basic reaction conditions, the carboxylate ion is formed from benzoic acid. Sodium then transfers a single electron to the aromatic system to produce a pentadienyl radical anion, 5-28. Protonation of 5-28 gives the radical, 5-29, which will be reduced to the corresponding anion, 5-30, by another sodium atom. Finally, 5-30 is protonated, giving the product. 

The symmetries of the MOs of conjugated tt systems with odd numbers of atoms also alternate. The allyl system has three MOs i/fQ (symmetric), tj/i (antisymmetric), and 1//2 (symmetric). In the allyl cation, is the HOMO and i/ri is the LUMO, whereas in the allyl anion, i/fj is the HOMO and ifc is the LUMO. The pentadienyl system has five MOs. In the pentadienyl cation, i/fi (antisymmetric) is the HOMO and ip2 (symmetric) is the LUMO, while in the pentadienyl anion, ip2 is the HOMO and 1/ 3 (antisymmetric) is the LUMO. 

There are also complexes containing polyhapto ligands with a more extended TT-system than that in T -allyl or -benzyl groups. Pentadienyl systems are the next higher anionic homolog of an allyl system. Pentadienyl systems are often formed by the addition of protons to, or removal of hydrides from, transition metal polyene complexes. Alternatively, pentadienyl groups have been generated by the addition of... 

Sketch the molecular orbitals for the pentadienyl system in order of ascending energy (see Figures 14-2 and 14-7). Indicate how many electrons are present, and in which orbitals, for (a) the radical (b) the cation (c) the anion (see Figures 14-3 and 14-7). Draw all reasonable resonance forms for any one of these three species. 

It has been found that unbranched conjugated cations, anions and free radicals possess odd number of carbon atoms. Two such simplest systems are allylic systems and 2, 4-pentadienyl systems. One important characterstic of... 

In another example, Shibasaki and co-workers applied an asymmetric Heck reaction/anion-capture cascade process to the total synthesis of marine sesquiterpenes capnellenols 14 and capnellene 17 (Schemes 13.5 and 13.6). The cascade involved the use of prochiral cyclo-pentadienyl systems for the generation of bicyclic 71-allyl-palladium species, which were then trapped by a suitable nucleophile. Initial studies in the total synthesis of capnellenols described the trapping of the 71-allylpalladium... 

The TT-electron system of the addition intermediate is isoelectronic with that of a pentadienyl anion. 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

Non-planarity is the result of the dominance of the destabilizing interactions of the sulfur lone pair and tt- occupied MOs of the pentadienyl anion over the stabilizing interaction of that lone pair and the LUMO of the anion fragment. In fact thiabenzene is antiaromatic in a planar configuration. Pyramidalization reduces the antiaromaticity induced by the sulfur. Although no X-ray data are available on the parent system, kinetic data have been obtained supporting a minimum barrier to inversion at the pyramidal sulfur of a 2-thianaphthalene of 99.1 kJ mol-1 (75JA2718). The formulation of the system as a cyclic ylide is supported by the chemical reactivity of the compounds as related in the reactivity section below. 

Electrocyclic closure of both pentadienyl cation and anion have been observed. Cations generated by protonation of dienones close in the predicted conrotatory manner as shown in Equation 12.55.99 The pentadienyl anion, a six-electron system, should close in the disrotatory sense a clear example is the rapid isomerization illustrated in Equation 12.56.100 Photochemical cyclization of pentadienyl cations has been observed Equation 12.57 shows an example in a cyclic system.101 The ready thermal reversion, which should be conrotatory and therefore difficult in the bicyclic system, may possibly occur by a stepwise path.102... 

Fig. 4.2 illustrates the first few members of the series of equilibria of conjugated ions. In cations, they are the equilibria between the allyl 4.11 and the cyclopropyl cation 4.12, the pentadienyl 4.13 and the cyclopentenyl cation 4.14, and the heptatrienyl 4.15 and cycloheptadienyl cation 4.16, In anions, they are between the allyl 4.17 and the cyclopropyl anion 4.18, the pentadienyl 4.19 and the cyclopentenyl anion 4.20, and the heptatrienyl 4.21 and cycloheptadienyl anion 4.22. There are heteroatom-containing analogues, with nitrogen and oxygen lone pairs rather than a carbanion centre, and the systems can again have substituents and fused rings. 

The possibility of rearrangement in pentadienyl anions must be borne in mind when they are employed synthetically. When 1- or 5-alkyl groups are present, intramolecular 1,6-sigmatropic hydrogen shifts are possible and the stereochemistry follows Woodward-Hoffmann rules, being thermally antara-facial but photochemically suprafacial. Bates, for example, showed that the same equilibrium mixture of isomers results at 40°C from the deprotonation of either 5-methyl-1,4-hexadiene or 2-methyl-1,4-hexadiene (79). The tendency is to form isomers with fewer alkyl groups in the 1,3, and 5 positions of the delocalized system (50). 

The X-ray crystal structure of LXXa confirms the presence of isolated cations and anions in the solid state. In the cation, the pentamethylcyclo-pentadienyl n system is symmetrically pentahapto bonded to the tin atom (see LXX in Fig. 14). The tin-ring centroid distance is considerably shorter than in decamethylstannocene. The methyl groups are bent away from the plane of the cyclopentadienyl ring. 

Initially, Hine (1966a) related PLM to the London-Eyring-Polanyi approach to three-center reactions. Since this method cannot easily be applied to most systems of interest, approximate methods have to be used. One of these minimizes bond motions and bond distortions. The geometric changes in pentadienyl or cyclohexadienyl anions brought about by protonation (Bates et al., 1967) in equation (192) are assumed to be roughly parallel to changes in bond order (BO). For deuteriation on the end carbons, 27(BO)2 = 2(2/3)2 + 2(l/3)2 = 10/9 for deuteriation... 

In addition to the fact that hydrogen/deuterium exchange reactions can be helpful to probe ion structures as will be shown later, they can also reveal mechanistic details such as the site of reaction within ions. For example, the pentadienyl anion exchanges four protons rapidly, demonstrating, as shown in (12), that proton addition occurs more rapidly at the ends of the conjugated system than in the middle (Stewart et al., 1977 DePuy et al., 1978a). 

PHOTOCHROMIC SYSTEMS BASED ON PENTADIENYL ANIONS WITH TWO HETEROATOMS... 

Substitution of two carbons in position 2,3 of the pentadienyl anion (see Section 6.1) creates a type 2,3 system. Such molecules are the dihydropyrazolo-pyridines (DHPP) 42.50 51 The 1,3 dipolar cycloaddition reaction, which is so effective as a method for synthesizing five-membered rings, can also be used to prepare spiroheterocyclic compounds of the 2,3 type. If a suitable bromo-substituted dibenzopentafulvene is reacted with pyridine the pyrazolo-pyridine 42 is formed in 30-90% yields (Scheme 18). 

C-Substituted Allyl Anions—Pentadienyl Anions. Allyl anions with C-substituents also suffer both a attack and y attack, as illustrated by the reactions of the open-chain C-substituted anions 4.41, and 4.42. Simple predictions based on the 7i orbitals suggest that the C-substituted system should be equally reactive at the a and y carbons, since mixing any amount of i/>3 of the pentadienyl anion... 

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