Homoconjugated systems

Little is known about the extent to which heteroatoms can be incorporated into the conjugated chain of a homoconjugative system. In principle it should be possible to include heteroatoms with lone-pair electrons that can contribute their n-type electrons. Alternatively, replacement of the CH2+ group by BH2 or other groups with empty pir-orbitals should also lead to a retention of homoconjugation (see the next chapter3). 

Homoconjugation thus involves electron and bond delocalization (in the heuristic sense) in the homoconjugative system. 

Since energy is the most important molecular property for a chemist, the following discussion will focus on the energies of homoconjugative systems. Comparable arguments are valid for the other properties that depend on all electrons of the molecule. 

When vinylcyclopropane is used as the key reference compound, the only examples of significant bond (electron) delocalization energies are cyclic homoconjugative systems with potentially aromatic electron ensembles (compare compounds 27-38 in Table 2). For example, norcaradienes 33 and 34 possess small, but significant bond delocalization energies RE of about 3 kcal mol1 (Table 2). Similarly, the RE values of the cyclohepta-trienes 30,31 and 32 are between 4 and 6 kcal moT1. 

With these definitions, useful descriptions of non-planar rc-systems and potentially homoconjugated systems have been developed. The descriptions are particularly attractive to experimentalists because orbital overlap is accepted as a major contributing factor to bonding and it is easy to visualize. Although Haddon did not formally define homoaromaticity in his work63 one can use his threshold value of S to define homoaromaticity in the following way ... 

The remainder of this chapter is organized into a series of sections which examine the currently available results on a variety of homoconjugative systems. The review is selective in terms of its coverage with examples being chosen that illustrate the issues at hand. Readers are referred to other reviews cited earlier for more comprehensive but, in most cases, less detailed accounts14"22. 

A similar point has been made by Roth and coworkers who investigated potentially homoconjugated systems using a modified MM2 force field, MM2ERW. The latter contained as reference bonds not only C(sp )—C(sp ), C(sp )—C(sp ), etc. bonds needed for the description of polyenes, but also the C(sp )—C(cyclopropyl) single bond taken from vinylcyclopropane in order to adequately describe cyclopropyl-substituted compounds 17-26 listed in Table 1. With this extended set of reference bonds, Roth and coworkers were able to reproduce experimental heats of formation of polyenes and... 

One of the most used ways to gauge the extent of proton transfer is to plot appropriate chemical shifts (O H or C) vs. the difference in pXa values of the donor and the acceptor, or simply the pXa value of the phenol itself if the acceptor is the same for a series . Another parameter used is A14 = (5C-1 — 5C-lphenoi) (5C-4 — 5C-4phenoi) (phenol refers to the unsubstituted compound). The normal type of plot is seen in Figure 4a, but also a plot of the type of Figure 4b may be found. This is ascribed to a homoconjugate system (NO- H+ ON), e.g. in a system like 2,6-bis(diethylaminomethyl)phenol di-Af-oxide ° (see Section II.L.l). 

Electrophilic attack by trityl cation on olefin complexes cani also occur to abstract a hydride from the allylic position. This process converts a neutral olefin complex into a cationic allyl complex and has been conducted with complexes that are less strongly backbonding than the complexes described in the preceding two paragraphs. One example of such an allylic hydride abstraction from a rhenium-propene complex is shown in Equation 12.69. An early example to form a homoconjugated system is shown in Equation 12.70. ... 

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