Pentadienyl system bonding

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... 

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 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 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. 

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. 

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... 

Consideration of the feasibility of these shifts as concerted processes, i.e. via cyclic transition states, requires as usual a consideration of the symmetry of the orbitals involved. A model related to the transition state can be constructed by the device of assuming that the C—H a bond that is migrating can be broken down into a hydrogen Is orbital and a carbon 2p orbital. For the case where x = 1 in (36), the T.S. can then be considered as being made up from a pentadienyl radical (38), with a hydrogen atom (one electron in a Is orbital) migrating between the terminal carbon atoms of its Site system (i.e. a 6e system overall is involved) ... 

There are several polymer supported transition metal hydrofomylation catalysts (42 ). Most are attached by phosphine ligation and suffer fron catalyst leaching. There are no n5-cyclo-pentadienyl half sandwich systems despite the potentially, clearly advantageous presence of the relatively strong Cp-metal bond (43,MO. Resin 5 was used in the following brief study in which the potential of polystyrene-supported CpCo(C0)2 to function as a hydrofomylation catalyst was tested. 

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. 

The allyl group can be extended to create a pentadienyl group, a five-carbon system with two double bonds, C U—CHCH=CH—CH,. Its many modes of bonding (t7J, tj3. i)1) have led to a rich chemistry, including the synthesis of mctallabenzene complexes which are thought to be aromatic [Pg.346]

Such specific ligand systems as catenanes [968], calixarenes [969], fullerenes [970a,b], and cyclodextrines [971], and functionalized dendrimers [972a-d] are of great interest. Thus, on the basis of fullerenes C60 and C80 were obtained metal-cyclo-pentadienyl and metal-carbonylcyclopentadienyl compounds with rj1 - and r 5-coordination bonds [970b]. 

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 molecular structures of three >/3-pentadienyl complexes having W, S, and U configurations, respectively, are shown in Fig. 5. Apart from showing the short uncoordinated C=C bond of the vinyl system a common feature is the unsymmetrical bonding of the fj3-allyl group with the longest M—C bond to the atom which bears the vinyl substituent (149). 

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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