Pentadienyl system cation

If the transition state resembles the intermediate n-complex, the structure involved is a substituted cyclohexadienyl cation. The electrophile has localized one pair of electrons to form the new a bond. The Hiickel orbitals are those shown for the pentadienyl system in Fig. 10.1. A substituent can stabilize the cation by electron donation. The LUMO is 1/13. This orbital has its highest coefficients at carbons 1, 3, and 5 of the pentadienyl system. These are the positions which are ortho and para to the position occupied by the electrophile. Electron-donor substituents at the 2- and 4-positions will stabilize the system much less because of the nodes at these carbons in the LUMO. 

The analogous cycloheptadienyl complex (5 equation 1) was similarly prepared by Dauben and Ber-telli,4 but the acyclic pentadienyl systems were a little more difficult to obtain. The triphenylmethyl cation does not remove hydride from tricarbonyl(frans-pentadiene)iron (6 equation 2). The corresponding c/s-pentadiene complex (7 equation 3) cannot be prepared directly from the diene and an iron carbonyl,... 

The nature of the diene formed in the interaction of a second F with the C6p7 is determined by the LUMO of the latter. The K system of the cyclohexadienyl cation is essentially that of a pentadienyl entity. The v orbitals of an idealized pentadienyl system, simply represented, are... 

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. 

These differences in the rates are due to essential differences in the structure of the MCRs of these cations. For example, the hindrances to 1,2-shifts in cyclobutenyl cations relative to those in benzenonium ones are likely to be connected with these two types of ions being topologically opposite the latter are antihomoaromatic systems in the ground state (true, the overlap between the ends of the pentadienyl system and, hence, the extent of antihomoaromaticity seems to be very small. 

Nucleophilic addition of primary and secondary alcohols to an acyclic l-butyl-( 7 -pentadienyl)iron cation was found to proceed exclusively at the unsubstituted C5 position. Here, a one-pot reaction system has been developed for in situ generation of the acyclic ( 7 -pentadienyl)iron cation followed by etherification. 

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 the pentadienyl cationic complexes described so far the five sp hybridized carbon atoms are necessarily held in a cis configuration. This restriction of configuration does not hold in the case of acyclic pentadienyl systems. Nevertheless in each of the acyclic pentadienyl-Fe(CO)3 cations so far prepared the arrangement of carbon atoms is cis. Reactions which might be expected to yield the trans configuration either fail to produce a stable cationic system or proceed to yield the cis isomer. 

In order for a substitution to occur, a n-complex must be formed. The term a-complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the hydrogen that is displaced. As the term implies, a a bond is formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a-complex is a four-7t-electron delocalized system that is electronically equivalent to a pentadienyl cation (Fig. 10.1). There is no longer cyclic conjugation. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive chargex)f the cation is located at the positions ortho and para to the site of substitution. 

An example of preferred conrotatory cyclization of four-7c-electron pentadienyl cation systems can be found in the acid-catalyzed cyclization of the dienone 12, which proceeds through the 3-hydroxypentadienyl cation 13. The stereochemistry is that expected for a conrotatory process. 

Mass spectral data on l-(arylsulfonyl)-l//-azepines have been amassed,73 and the fragmentation patterns of several 1-acyl-1//-azepines elucidated.61 For the latter systems, the base peaks correspond to the azatropylium cation (m/z 92). Loss of hydrogen cyanide to yield the cyclo-pentadienyl cation (m/z 65) has also been noted. 

Participation by cumulative double bonds and vinyl cation intermediates has been suggested in the solvolyses of a number of homoallenyl systems. Hanack and Haffner (87) reported the solvolyses of 3,4-pentadienyl jS-naphthalene sulfonate, 110 (R = H) under a variety of conditions. The products... 

The w-pentadienyl cation system can also be generated by perchloric acid treatment of complexed alcohols and an interesting rearrangement of a primary to a secondary alcohol can thus be achieved (39) ... 

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. 

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... 

Dienones, which because of the electron deficiency induced at the carbonyl carbon may be regarded as analogs of pentadienyl cations, are known to close photochemically.103 These closures occur readily in cyclic systems where the geometry requires the disrotatory mode, but the proposed intermediate product, formally a 1,3 diradical, is ordinarily stabilized through rearrangement.104 Woodward and co-workers have demonstrated the predicted photochemical... 

This chapter includes those transition metal-pentadienyl cationic complexes that are quite stable, can be stored and handled easily, and are therefore useful as stoichiometric intermediates for organic synthesis. The dienyliron systems, which are readily available and inexpensive, have dominated this area of chemistry, and will occupy the larger part of the discussion. The chemistry of more expensive and less easily prepared dienylmetal complexes, such as those of manganese and cobalt, will be dealt with at the end of the chapter. 

Another rare kind of 6-electron ionic cycloaddition is that between a pentadienyl cation and an alkene. A telling example is the key step 2.66 — 2.67 in a synthesis of gymnomitrol 2.68, where the nature of the pericyclic step is heavily disguised, but all the more remarkable for that. Ionization of the acetal gives the cationic quinone system 2.66. That this is a pentadienyl cation can be seen in the drawing of a canonical structure on the left, with the components of the pericyclic cycloaddition emphasized in bold. Intramolecular [4+2] cycloaddition takes place, with the pentadienyl cation as the 4-electron component and the cyclopentene as the 2-electron component. Th is reaction is an excellent example of how a reaction can become embedded in so much framework that its pericyclic nature is obscured. 

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. 

Coordination to the ketone makes it more of a carbocation, and hence the conjugated system more of a pentadienyl cation 4.90. The cyclization takes place to give the cyclopentenyl cation 4.91, which loses the silyl group and picks up a proton to give the ketone 4.92. The relative stereochemistry at C-l and C-5 has the two hydrogen atoms trans, proving that the cyclization has... 

An electrocyclic reaction is the formation of a new o-bond across the ends of a conjugated 7T-system or the reverse. They thus lead to the creation or destruction of one a-bond. Hexatrienes 1 can cyclise to six-membered rings 2 in a disrotatory fashion but we shall be more interested in versions of the conrotatory cyclisation of pentadienyl cations 3 to give cyclopentenyl cations 4. The different stereochemistry results from the different number of rt-electrons involved.1... 

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. 

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