Acyclic Species

It may appear that nonconjugated, acyclic dienes are the simplest and least interesting of all the classes of compounds to be discussed in the current chapter. The reader may wish to ask of the current author who has written numerous earlier reviews on organic 

The simplest nonconjugated, acyclic diene is 1,4-pentadiene (1), with its enthalpy of formation of 105.6 kJmol-1. The obvious question is whether the two double bonds are truly independent. If they are, then the enthalpy of hydrogenation of one double bond as in (the identical) reactions 4a and 4b would be precisely one half of that of the hydrogenation of both as in reaction 5. 

We would likewise deduce that the formal reaction in equation 6 

We now turn to the isomeric hexadienes, of which three species qualify for consideration the 1,5- and the (Z)- and ( )-l,4- compounds, species 13, 14 and 15, respectively. If interaction between the two double bonds in 1,4-pentadiene is so small, we expect this as well for the 1,5-hexadiene. One test of this is to consider reaction 7 by analogy to reaction 6. 

To do so, one can take the enthalpy of formation of n -hexane from Pedley, and with the phase independence assumptions in Reference 7, employ the enthalpies of hydrogenation of 1-hexene and 1,5-hexadiene from References 11 and 12 respectively. Alternatively13, one can forget about the first quantity altogether and simply take the difference of the enthalpies of hydrogenation of the diene and twice that of the monoene. This reaction is endothermic by 1.1 1.8 kJ mol-1, a value statistically indistinguishable from the absence of any interolefin interaction in the diene. Relatedly, for the isomeric 1,4-hexadienes 14 and 15, equation 8 may be used. 

---

Benzene remains safely aromatic by this definition. After all, its enthalpy of formation is 82.6 kJ mol-1 while that of the reference acyclic species is 167.5 kJ mol-1, considerably higher. 

Figure 3.81 Geminal interactions in cyclopropane and propane, showing bent cyclopropane (a) bond

In contrast, reactions catalyzed by la were typically conducted with added [Ir (C0D)C1]2 to trap the K -phosphoramidite ligand after dissociation, and thereby, to leave the unsaturated active catalyst. Under these conductions, as much as half of the iridium in the system is present in an inactive acyclic species. In contrast, when ethylene adduct lb is used as the catalyst, all of the iridium belongs to the active metalacyclic species. Hartwig and coworkers have recently taken advantage of the increased availability of the active catalyst generated from lb to develop new allylic substitution reactions. These new processes include the reactions of carbamates, nitrogen heterocycles, and ammonia. 

Next, enantiopure silicon allylation reagent will be presented, which already inherits Lewis acidity. It is accepted that Lewis acidity of silicon, as well as its high tendency to expand valence shell, increases [59, 60] if it is tetravalent and incorporated into strained four- or five-membered ring systems (strain-release Lewis acidity) [61]. This corresponds to smaller energy gaps between sp and dsp orbitals of a strained system as compared to an acyclic species. 

Interestingly, two of the other species in Table 3 are nitrolates, i.e. ethers of a-nitrooximes, an otherwise thermochemically unprecedented class of compounds. We already have briefly discussed one, 3-nitroisoxazoline, and the second is 1-nitroacetaldehyde 0-(l,l-dinitroethyl)oxime (ONo-ld-dinitroethyl acetonitronate), MeC (NOala—O—N=C(N02)Me. The latter acyclic species is a derivative of 1,1-dinitroethanol—we know of the enthalpy of formation of no other a-nitroalcohol or derivative. Nonetheless, we may ask if the two calorimetric data are internally consistent. Consider the condensed phase reaction 47, which involves formal cleavage of the O — bond in the nitroisoxazoline by the C—H bond of the dinitromethane. It is assumed that the isoxazoline has the same strain energy as the archetypal 5-atom ring species cyclopentane and cyclopentene, ca 30 kJ mol . ... 

Lewis acids cause complete dissociation to an acyclic species. These two species represent extremes of behavior, and intermediate levels of enantioselectivity are also observed.71... 

Ring contraction by the non-least-motion path requires the intermediate formation of an acyclic species that is able to undergo structural changes that finally provide a new three-membered ring in which a formerly exo-cyclic atom has moved to a ring position. An explanation of this behavior is not always simple (discussed later) because unexpected products may... 

For condensed-phase reactions, Kirmse (1964) has used SS as a criterion of cyclic (singlet) versus acyclic (triplet) transition states and/or intermediates in these cases. When insertion is stereospecific with retention, a cyclic species is assumed when insertion is unselective the acyclic species is assumed. We can, in fact, associate the singlet with a one-step, stereospecific process. On the other hand, one or more products could be formed from a triplet intermediate. 

Cleavage of the tetrameric ring followed by cyclization of the phenylated acyclic species, Ph3P=N—PPhCl =(NPC12)2 —NMgBr, would give the ring-contracted derivative (22). 

---