Excited states diphenylacetylene

The reaction of diphenylacetylene with benzothiophene allowed the isolation of low quantity of the first cycloadduct 90, but this reaction occurs with a mechanism different from that of propiolate considering that the excited states of diphenylacetylene is probably involved (69TL4791 71JOC3755). 

Amines, too, undergo photoaddition to alkynes. From diphenylacetylene and a secondary amine are obtained an enamine (which is hydrolysed during work-up) and products which arise by further reaction of stilbene, the photoreduction product of diphenylacetylene (equation 28). The products can be rationalized in terms of an initial hydrogen transfer to the excited state of the alkyne the fact that the N—H... 

The mechanism of this reaction is thought to involve the singlet excited state of the naphthalene, which forms an e.xciplex with the alkyne. Earlier it had been suggested that the reaction involved diphenylacetylene excitation, in view of the fact that dimethyl acetylenedicarboxylate gives different products (equation 70) and in the latter case the alkyne is definitely not excited first. However, selective excitation... 

INDO calculations on the potential curves of ethylene give very poor agreement with ab initio results.9 49>51 53 Reports have appeared on the geometries of excited states of small polyenes,54 the low-lying electronic states of the ethynyl free radical,55 the vibronic structure of the tt - tt transition in c/s-stilbene,56 the singlet-state geometries of diphenylacetylene,57 and the dissociation products of the HN2+ ion in its ground and excited states.58... 

Photochemistry.— The photochemical addition of dimethyl acetylene-dicarboxylate, methyl propiolate, or methyl phenylpropiolate to benzo[h]-thiophen and its 2- and 3-methyl and 2,3-dimethyl derivatives leads to cyclobutene derivatives of unexpected structures. From benzo[ >]-thiophen and dimethyl acetylenedicarboxylate (376) is obtained. Methyl propiolate adds in a direction opposite to that of methyl phenylpropiolate, suggesting that the excited state of benzo[h]thiophen is highly polarized. The mechanism of the addition is discussed, and several mechanistic alternatives are suggested. Only with diphenylacetylene, which reacts slowly, can the normal addition product (377) be obtained. The... 

A mechanism is proposed that involves the phototochemical activation of a diplatinum //-alkynylidene precursor complex to give a long-lived excited state that is quenched by diphenylacetylene through [2+2] cycloaddition or insertion reactions. The failure to observe emission from this complex, however, makes it difficult to verify these proposals. ... 

Similarly, irradiation of some selected 5-phenyl-substituted oxadiazoles 3 at 300 or 254 nm in methanol in the presence of a triplet sensitizer (diphenylacetylene or phenylacetylene) produces an excited triplet state species which collapses in a ring-closure reaction involving the C5-phenyl moiety and gives 2-substituted quinazolin-4(3//)-ones 4. 54.3.55... 

The 1,2-cycloaddition reaction can take place in an intramolecular manner (equation 63), although in this example the initial excitation involves the aromatic group . A reaction of a different type is thought to be involved in the first stage of the formation of azulene or naphthalene photodimers from diphenylacetylene (equation 64), though here it is claimed that an intermediate benzocyclobutadiene species has been detected . The intermediate isomer of diphenylacetylene is formed via the triplet state and is relatively long-lived at — 10 °C. The major dimers formed are 1,2,3-triphenylazulene and 1,2,3-triphenylnaphthalene hexaphenylbenzene and octaphenylcubane are also produced . 

---