Cation pentadienyl

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. 

Fig. 10.1. TT-Molecular orbitals and energy levels for the pentadienyl cation.

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. 

While most synthetic examples of this cyclization have involved protonation of divinyl ketones to give 3-hydroxy-1,4-pentadienyl cations, theoretical studies suggest that the cyclization would occur even more readily with alternative substituents at C-3. °... 

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. 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

Part of the reason for this enhanced reactivity is the stabilization that accrues from hyperconjugation of the C-Sn bond with the LUMO of the pentadienyl cation, which is much more powerful than C-H hyperconjugation. This is illustrated above in formula 2 where it is presented in terms of the solvation of a stannonium ion by an arene.149... 

Tius and co-workers investigated a number of cationic cyclopentannelations of allenyl ethers [113] and found that 1-lithio-l-alkoxyallenes 180 react with a,/3-unsatu-rated carbonyl compounds 181 leading to highly functionalized cyclopentenones 182 (Scheme 8.44). The primary products are a-allenyl ketones 183, which form pentadienyl cations 184 by protonation. The latter undergo a thermally allowed 4jt-conrotatory ring closure to intermediates 185, which with elimination of R1 finally lead to the expected products 182 (Scheme 8.45). 

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. 

In Scheme 1, the radical cations of the linear hexadienes and some cyclic isomers are contrasted. The heats of formation, AHr, as determined from the heats of formation of the species involved, as well as the heats of formation of the isomeric radical cations themselves clearly reveal the favourable stability of the cyclic isomers and/or fragment ions. Thus, instead of the linear pentadienyl cation (3), the cyclopenten-3-yl cation (2) is eventually formed during the loss of a methyl radical from ionized 1,3-hexadiene (1). Since 1,2-H+ shifts usually have low energy requirements (5-12 kcalmol-1), interconversion of the linear isomers, e.g., 4, and subsequent formation of the cyclic isomers, in particular of the ionized methylcyclopentenes 5 and 6, can take place easily on the level of the... 

The oxidative electrochemistry of phenols has been developed elegantly [51, 52]. The key intermediate is often the pentadienyl cation (96) that is formed after the loss of two electrons and one proton from (95) (Scheme 22). It can be intercepted by alkenes, at the terminal carbons of the pentadienyl array, to achieve a [5 + 2] cycloaddition (e.g. (96) to 97), or by a nucleophilic solvent such as methanol, leading to a conjugated diene (99). The... 

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) ... 

The pentadienyl cation 126 thus formed undergoes an electrocyclic ring closure that leads, after hydrolysis, to the annulated ketone 127 (Scheme 16) . 

Figure 11.3. SHMO frontier orbitals and total energies for amino-substituted pentadienyl cations.

X is NR R2. The substituent is converted to a Z substituent via the low-lying a orbital, and the ring is deactivated toward further electrophilic attack. The ortho and para channels lead to products. The interaction diagram for an X -substituted pentadienyl cation, substituted in the 1-, 2-, and 3-positions, as models of the transition states for the ortho, meta, and para channels, are too complex to draw simple conclusions. The HOMO and LUMO of the three pentadienyl cations with an amino substituent are shown in Figure 11.3. Notice that the LUMO of each is suitable to activate the C—H bond at the saturated site toward abstraction by the base. Curiously, the meta cation has the lowest LUMO and should most readily eliminate the proton. The stabilities of the transition states should be in the order of the Hiickel n energies. These are 6a — 8.7621/ , 6a — 8.499 / , and 6a — 8.718 / , respectively. Thus the ortho and para channels are favored over the meta channel, and the ortho route is slightly preferred over the para route. Experimentally, para substitution products are often the major ones in spite of there being two ortho pathways. The predominance of para products is usually attributed to steric effects. 

Vinylsilanes (8, 491 492) attylic alcohols (9, 340). Details are available for conversion of a ketone to a vinylsilane in which the C—Si bond has replaced the C -O group (cncsilylation). The reaction affords the less substituted vinylsilane in the case of unsymmetrical ketones. The paper includes details for use of vinylsilanes for cyclopcntenone annelalion by l riedel-Crafts acylation with acryloyl chlorides and subsequent cyclization of pentadienyl cations (9, 498-499).1... 

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. 

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