Allene bond stability

Allen, K.W., Greenwood, L. and Wake, W.C., The stability of adhesive bonding between silicone mbber and alumina for neural prostheses. J. Adhes., 16(1), 61-76 (1983). 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

In the reaction of 1 with alkynes possessing electron-withdrawing substituents, the corresponding silacyclopropene derivatives 66 and 67 are formed, as described in Scheme 23.29 An unexpected pathway was observed in the reaction with the electron-poor hexafluorobutyne(2) the X-ray characterized heterocycle 68 was most likely obtained by nucleophilic attack of 1 at the triple bond. A subsequent shift of a fluorine atom from carbon to silicon creates an allene-type molecule which was stabilized by a [2 + 2] cycloaddition process involving a double bond from the pentamethylcyclopentadienyl unit, as described in Scheme 24.33... 

We start with a discussion of allene (propadiene), the simplest diene of all. Its gas phase enthalpy of formation is 190.5 1.2 kJmol-1. We wish to compare this quantity with that of related monoenes. The first comparison addresses the relative stability of one and two double bonds in a 3-carbon chain. Conceptually, this may be expressed as the enthalpy of the formal reaction 9... 

Using standard references and protocol, we find the three reactions are respectively endothermic by ca 2, 8 and 6 kJmol-1, or ca 2, 4 and 3 kJmol-1 once one remembers to divide by 2 the last two numbers because the allene is dialkylated. So doing, from equations 10 and 11 we find an average ca 3 kJmol-1 (per alkyl group) lessened stability for alkylated allenes than the correspondingly alkylated alkenes. This is a small difference that fits most naturally in the study of substituted cumulenes such as ketenes and ketenimines, i.e. not in this chapter. But it is also a guideline for the understanding of polyenes with more cumulated double bonds. 

Analogously, the trienynoate 92 reacted in a 1,10-addition to give the 3,5,7,8-tetra-enoate 93 and the even higher unsaturated allene 95 was obtained from the Michael acceptor 94 containing four double bonds between the triple bond and the acceptor substituent (Scheme 2.33). In the latter case, however, the yield was only 26% this is presumably due to the reduced thermal stability of the starting material and/or the addition product (the 1,12-adduct 95 was the only isolable reaction product, apart from polymeric compounds) [57]. 

In this section, compounds are described that differ from 6 by the replacement of one or more methylene groups by heteroatoms or heteroatom groups. Quantum-chemical calculations on such species have not been carried out. However, on the basis of the results discussed in the above sections and depicted in Schemes 6.42 and 6.64, there is no doubt that all reactive intermediates under consideration are genuine allenes. After all, the tether across the allene subunit is larger in the corresponding compounds in this section than in 3<52-lH-pyridine (179), 3<52-pyran (180) and 3<52-thiopyran (299) owing to the absence of a double bond. Thus, compared with these models, the allene structures in this section suffer from less strain and are hence stabilized relative to their zwitterionic states. 

NHCs are often compared to phosphines and theoretical studies by Frenking et al. [211-213] showed that the substitution of the phosphines in 89 for two NHCs leads to compounds with a carbon(O) atom that is formally stabilized by two NHC C donor-acceptor bonds. The first carbodicarbene 90 was prepared by Bertrand et al. who selected benzimidazolin-2-ylidenes as carbene donors and the deprotonation of the conjugated acid of an allene as the synthetic strategy (Fig. 29)... 

Recoil UC atoms have been produced by nuclear transformations and allowed to react with ethylene.15 Both Q1/)) and C(3P) atoms are formed, and both add to the double bond and insert into the vinylic C—H bond. The resulting hot singlet adducts relax primarily to allene and methylacetylene, whereas the hot triplet adducts decompose to acetylene or are stabilized as carbenes, which mainly add to more ethylene to yield various C5 products. 

The values presented in Table 2 depict the variation of C = C re-bond strength with increasing fluorination and are consistent with the differences in reactivity associated with differing degrees of fluorination.3 Fluorination also destabilizes allenes and acetylenes.20 22 Similarly, per-fluoroalkyl groups destabilize C = C bonds. Note, however, that perfluoroalkyl groups can lend kinetic stabilization to strained molecules.3... 

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