Pentadienyle anion

New photochromic dyes with electrocycHc reactions have been proposed on the basis of 1,5-electtocycHzation of heterogenous pentadienyl—anions (124). StiH newer are investigations into the photocycHzation of 2,4,6-tri-isoptopylbenzophenones for vinyl polymers ia the glassy state (133). 

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. 

In contrast to pentadienyl anions, heptatrienyl anions have been found to cyclize readily to cycloheptadienyl anions. The transformation of heptatrienyl anion to cyclo-heptadienyl anion proceeds with a half-life of 13 min at — 13°C. The Woodward-Hoffinann rules predict that this would be a conrotatory closure. ... 

Unsymmetrically substituted pentadienyl anions populate six planar conformations, which are in equilibration13 a 18. The energy barrier for a torsion in the potassium compound (R = primary alkyl) was estimated to be approximately 35 keal/mol for the 1,2-bond and 15 keal/mol for the 2,3- and 3,4-bonds. The barriers are much lower in the lithium compound. Not only the rate, but also the position of the equilibrium is greatly influenced by the cation from trapping experiments18 it was concluded that the exo-VJ anion is most stable for lithium and the exo-U form for potassium. 

In the 1930 s HiickeP proposed, on the basis of molecular-orbital calculations, a theoretical criterion for aromaticity of cyclic polyenes, known as Hiickers rule, which states that cyclic polyenes should be aromatic if, and only if, they contain 4n- -2 Jt-electrons. At that time only two of such cyclic polyenes were known benzene and cyclo-pentadienyl anion, each having six rc-electrons and satisfying Huckel s rule. Since then, the validity of Hiickel s rule had not been challenged... 

The structure of the products is determined by the site of protonation of the radical anion intermediate formed after the first electron transfer step. In general, ERG substituents favor protonation at the ortho position, whereas EWGs favor protonation at the para position.215 Addition of a second electron gives a pentadienyl anion, which is protonated at the center carbon. As a result, 2,5-dihydro products are formed with alkyl or alkoxy substituents and 1,4-products are formed from EWG substituents. The preference for protonation of the central carbon of the pentadienyl anion is believed to be the result of the greater 1,2 and 4,5 bond order and a higher concentration of negative charge at C(3).216 The reduction of methoxybenzenes is of importance in the synthesis of cyclohexenones via hydrolysis of the intermediate enol ethers. 

The unsubstituted pentadienyl anion also appears generally to be in the fully extended form, the so-called W-structure (Figure 4) examples are pentadienyllithium in THF73... 

Classical aromatics like the electron-rich, cyclobutadiene dianion A or cydo-pentadienyl anion B and electron-precise hydrocarbons (e.g., benzene C, Figure 3.2-1) have pure n multicenter bonds and therefore are generally not regarded as clusters. 

The HOMO of the pentadienyl cation is j/, which is antisymmetric, so a conrotatory ring closure occurs, consistent with the four electrons involved in this reaction. The HOMO of the pentadienyl anion is /2, which is symmetric, so a disrotatory ring closure occurs, consistent with the six electrons involved in this reaction. 

From their analysis of the conformational energies of pentadienyl anion and the penta-dienyl metal compounds, Pratt and Streitwieser in 2000 pointed out that the stabilization of the planar forms of the organometallic structures results from both conjugation and electrostatic attraction between the negative carbons and the alkali metal cations. To determine the relative magnitude of these effects, the reaction energies were determined for hypothetical reaction, shown in Scheme 1 where M represents any alkali metal. 

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. 

Many odd AHs contain even AH fragments on which the NBMO completely vanishes, i.e., the coefficients are zero on both the starred and unstarred sets of atoms. These regions to which the NBMO does not extend are referred to as inactive segments. For the purpose of this review we will describe those odd AHs that contain one or more inactive segments as reducible odd alternant hydrocarbons (ROAH). Examples of ROAH anions are the 3-phenylbenzyl anion (10), which contains an inactive phenyl group (11), and the 2,4(1,8-naphthalenediyl)pentadienyl anion (12), which contains an inactive napthalenediyl group (13). 

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. 

In Table 4.77 a small selection of carbanionic species has been compiled [502-510]. The carbanionic carbon shift of methyllithium is — 16.6 ppm (Table 4.71) in comparison to - 23.1 ppm, which is predicted for an sp2 carbon containing two electrons in a p orbital, following the empirical carbon-13 shift to charge density correlation [76, 507]. Carbanion carbon shifts become progressively more positive with increasing delocalization of the negative charge by resonance (mesomeric) effects, as shown for allyl and pentadienyl anions [503-505] in Table 4.77. 

The bonding between the metal and the cyclopentadiene rings involves the it electrons of the two rings, all carbons being equally bonded to the central ferrous ion. The latter, in accepting a share of 12 tt electrons from two cyclo-pentadienyl anions, achieves the 18 outer-shell electron configuration1 of the inert gas, krypton. Analysis of the structure of crystalline ferrocene shows... 

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... 

A ring-opening has also been seen, when the relief of strain in a cyclopropane makes it thermodynamically favourable—the cyclopentenyl anion 4.100 opens to the pentadienyl anion 4.101. This reaction had no option but to be disrotatory with the two hydrogen atoms moving outwards 4,98, since a trans double bond is impossible in the 6-membered ring. 

Intramolecular hydroamination of cyclohexa-2,5-dienes has afforded the corresponding bicyclic allylic amines with high selectivity (Scheme 13).80 The reaction does not proceed through a direct hydroamination of one of the diastereotopic alkenes but more likely involves a diastereoselective protonation of a pentadienyl anion, followed by addition of a lithium amide across the double bond of the resulting 1,3-diene and a highly regioselective protonation of the final allylic anion. 

Protonation of pentadienyle anion gives rise to 1,4-pentadiene, instead of the expected conjugated 1,3-pentadiene. Is it really so surprising, when we look at the HOMO coefficients and the charge distribution (STO-3G calculations) ... 

---