Alkynes frontier orbitals

Cycloaddition reactions also have important applications for acyclic chalcogen-nitrogen species. Extensive studies have been carried out on the cycloaddition chemistry of [NSa]" which, unlike [NOa]", undergoes quantitative, cycloaddition reactions with unsaturated molecules such as alkenes, alkynes and nitriles (Section 5.3.2). ° The frontier orbital interactions involved in the cycloaddition of [NSa]" and alkynes are illustrated in Fig. 4.13. The HOMO ( Tn) and LUMO ( r ) of the sulfur-nitrogen species are of the correct symmetry to interact with the LUMO (tt ) and HOMO (tt) of a typical alkyne, respectively. Although both... 

It is evident that the relative proportions of products from [2 + 2], [4 + 2] and ene reactions depend strongly on the polarity of the Si=C bond, on the polarization of the frontier orbitals and on steric factors both in the silenes and in the reactant. Therefore, we will first discuss cycloaddition reactions of silenes without separating them into subclasses like [2+2] or [2+4] cycloadditions, but we will concentrate on the cycloaddition behaviour of several classes of silenes towards alkenes, alkynes and dienes. We will then discuss some special cycloaddition reactions in subsequent sub-sections. 

Frontier Orbital Interactions in the Transition States of the Unknown One-Step Cycloadditions of Alkenes or Alkynes to Alkenes... 

Both combinations of alkyne n orbitals find filled dir orbital symmetry matches in these d4 complexes. Extended Huckel calculations on Mo(HC=CH)2(S2CNH2)2 indicate a large HOMO-LUMO gap of 1.62 eV. These octahedral complexes have proved to be quite robust and resist exchange and substitution reactions in accord with a substantial frontier orbital energy gap (153). 

Since perfluoroalkyl-substituted olefins and alkynes possess low-lying frontier orbitals, [4 + 2] cycloaddition reactions to oxazoles and thiazoles without strongly electron-donating substituents are unfavorable. On the other hand, five-membered heteroaromatic compounds possessing an electron-rich diene substructure, like furans, thiophenes, and pyrroles, should be able to add perfluoroalkyl-substituted olefins as well as alkynes in a normal Diels-Alder process. A reaction sequence consisting of a Diels-Alder reaction with perfluoroalkyl-substituted alkynes as dienophile, and a subsequent retro-Diels-Alder process of the cycloadduct initially formed, represents a preparatively valuable method for regioselective introduction of perfluoroalkyl groups into five-membered heteroaromatic systems. 

Figure 2 Frontier orbital interactions in the cycloaddition reaction of NS2+ with alkynes...

The concepts of hardness, softness, and of frontier orbitals, with which latter the Fukui function is closely coimected, have been severely criticized [103, 120]. It is also true that in some cases the results predicted using these methods can also be understood in terms of more traditional chemical concepts. Thus, in the alkyne-HNC reaction, resonance theory leads one to suspect that the zwitterion A, with the positive charge formally on the more substituted carbon, will be favored over B. Indeed, AMI, HF/6-31G and pBP/DN DFT calculations all show A to be of lower energy than B. 

Nucleophilic alkyl radicals, i.e. c-C6Hir or t-Bu% add to activated alkynes 3.0-5.2 times slower than the corresponding substituted alkenes, whereas nucleophiles having lone pairs of electrons attack alkynes markedly faster than alkenes. Application of Frontier Orbital theory indicates early and late transition states for radical nucleophiles and nonradical nucleophiles, respectively. ... 

Reviews have appeared on the use of the Wittig reaction in industrial practice, the Claisen rearrangement, synthetic applications of the retro-Diels-Alder reaction, organo-palladium intermediates for the alkylation and arylation of olefins, the Prins reaction to give 1,3-dienes, and intramolecular [4 + 2] (Diels-Alder) and [3 + 2] cycloadditions.An interesting discussion of the regiospecificity of the Diels-Alder reaction in terms of frontier orbital overlap favours the Woodward-Katz concept. Useful alkyne and polyene coupling reactions are described in reviews on the chemistry of vitamin the synthesis of insect sex... 

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