Pentadienyl systems

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. 

For a [1, 5] shift of hydrogen or deuterium the pentadienyl system must accommodate the six electrons, four from already two n bonds and one each obtained during homolytic... 

It is important that the five carbon atoms making up the 77-pentadienyl system have the cisoid arrangement. Thus fails to abstract a hydride... 

The trans isomer 76 was found to be more stable than the cis form 75 by 6 kcalmol"1. Since the experimental 13C NMR shift values exclude the possibility of a closed form, it would appear that Warner and Winstein prepared 76 rather than 75. Cremer and coworkers also pointed out that agreement between the calculated and experimental 13C NMR spectrum can only be obtained by re-assigning the shifts assigned to C(l) and C(2)183. When this is done, the chemical shifts resemble those of 77 rather than the homoaromatic models 11 or 71. This conclusion is supported by the calculated geometry of 76 which suggests the existence of an almost isolated double bond that does not interact with the pentadienyl system (Scheme 32)l83. 

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,... 

In keeping with the advantages of examining identity reactions, a number of identity proton transfers involving aromatic systems were subjected to ab initio calculations. The first study involved the highly aromatic benzene and cyclo-pentadienyl systems, Equations (33a) and (34a).141... 

The carbolithiation of six- to nine-membered 3-methylenecycloalka-l,4-dienes has been investigated and shown to be an exceptionally facile and general process.69 Primary, secondary, and tertiary organolithium reagents may be employed for carbolithiation of cyclic trienes with uniform efficiency, generating cyclic pentadienyl carbanions. The six-electron pentadienyl systems display unique reactivity as a function of ring size. 

Ernst et al. have described similarly structured Cp2Ta-pentadienyl systems,20 and Bazan et al. have synthesized a neutral [(.v-/ram-r 4-butadiene)... 

The two p orbitals of ethylene are described as being conjugated with each other in making the % bond. To make longer conjugated systems we add one p orbital at a time to the % bond to make successively the allyl system, butadiene, the pentadienyl system and so on. We continue to separate completely the a framework (using the 2s, 2px and 2py orbitals on carbon with the Is orbitals on hydrogen) from the % system made up from the 2pz orbitals. 

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... 

Complexes based on the indenyl-fluorenyl,576,725,785,786 heteroindenyl-fluorenyl,787 and indenyl-dithienocyclo-pentadienyl systems show improved catalytic performance, producing PPs with often higher molecular... 

The uniformity of the phenyl (benzyl)-cyclopentene ratio throughout all polymer fractions makes it difficult to accept a mechanism based solely on random attack by phenyl radicals on a growing fulvene polymer. The initial synthesis of a monomeric unit comprised of the benzylcyclo-pentadienyl system with subsequent diene type polymerization is consistent with all our observations. The phenyl radical produced in the discharge should collide with the nearest benzene molecule which will be excited to a relatively high vibrational energy level. The following reaction sequences are suggested for the phenyl radical ... 

The interaction of atomic orbitals giving rise to molecular orbitals is the simplest type of conjugation. Thus in ethylene the two p orbitals can be described as being conjugated with each other to make the n bond. The simplest extension to make longer conjugated systems is to add one p orbital at a time to the n bond to make successively the n components of the allyl system with three carbon atoms, of butadiene with four, of the pentadienyl system with five, and so on. Hiickel theory applies, because in each case we separate completely the n system from the a framework, and we can continue to use the electron-in-the-box model. 

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. 

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