Addition and Cyclization Reactions in the Thermal Conversion of Hydrocarbons with an Enyne Structure, 5. High-Temperature Ring Closures of 1,3-Hexadien-5-ynes to Naphthalenes – Competing Reactions via Isoaromatics, Alkenylidene Carbenes, and Vinyl-type Radicals

Thermal Rearrangement of Azulenes to Naphthalenes: A Deeper Insight into the Mechanisms

The thermal rearrangement of azulene to naphthalene has been the subject of several experimental and computational studies. Here, we reexamine the proposed mechanisms at the DFT level. The use of different functionals showed that the HF-exchange contribution significantly affects reaction energies and barrier heights. Accordingly, all proposed pathways were investigated with the optimal method, M06-2X/6-311+G(d,p), which confirms the norcaradiene-vinylidene mechanism (A) as the dominant unimolecular route (E_a ≈ 76 kcal/mol) able to account for the major products of pyrolyses using ¹³C- or substituent-labeled azulenes. Moreover, a facile vinylidene-acetylene interconversion will scramble the terminal carbon atoms in the vinylidene. Several other potential intramolecular reaction mechanisms (B-E) are ruled out because of higher activation energies (>84 kcal/mol) and failure to reproduce the results obtained with substituted and ¹³C-labeled azulenes and benzazulenes. These experimental results also demonstrate that the proposed free radical or H atom-induced intermolecular methylene walk and spiran mechanisms cannot be major contributors, especially under flash vacuum pyrolysis conditions.

Nanographene and Graphene Nanoribbon Synthesis via Alkyne Benzannulations

The extension of π-conjugation of polycyclic aromatic hydrocarbons (PAHs) via alkyne benzannulation reactions has become an increasingly utilized tool over the past few years. This short review will highlight recent work of alkyne benzannulations in the context of large nanographene as well as graphene nanoribbon synthesis along with a brief discussion of the interesting physical properties these molecules display.

Diene-yne cyclisation reactions of 1-ethynyl-2-vinyl-3,4-dihydronaphthalenes and 1-ethynyl-2-vinylnaphthalenes

The reaction of 1-ethynyl-2-vinyl-3,4-dihydronaphthalenes and 1-ethynyl-2-vinylnaphthalenes over RuCl2(p-cymene) PPh3 leads to 9,10-dihydrophenanthrenes and phenanthrenes in those cases where the ethynyl group in the substrates carries a terminal proton. When 1-phenylethynyl-2-vinyl-3,4-dihydronaphthalenes or 1-phenylethynyl-2-vinylnaphthalenes are reacted over Pt(PPh3)4, 1-methylene-1H-benz[e]-4,5-dihydroindenes and 1-methylene-1H-benz[e]indenes are formed.

Photocyclization of 2-vinyldiphenylacetylenes and behavior of the isonaphthalene intermediates

The conformation, electronic structure, spectroscopy, and unimolecular photoisomerization of 2-vinyldiphenylacetylene and two derivatives have been investigated. 2-Vinyldiphenylacetylene exists predominantly in a planar anti conformation. Introduction of an alpha-methyl substituent results in increased phenyl-vinyl dihedral angles for both syn and anti conformers, whereas a cyclic analog is constrained to a syn conformation with a large phenyl-vinyl dihedral angle. All three molecules undergo photocyclization to yield unstable cyclic allene (isonaphthalene) intermediates which undergo further reactions leading to stable products. Both the photocyclization process and behavior of the allene intermediate are dependent upon ground state conformation. The photophysical behavior of the 2-vinyl derivative, namely its short singlet lifetime and low fluorescence quantum yield, is similar to that of diphenylacetylene. It also has a low quantum yield for photocyclization. The 2-isopropenyl derivative and conformationally locked cyclic analog have relatively long singlet lifetimes and large quantum yields for fluorescence and cyclization. The difference in excited state behavior of the planar 2-vinylacetylene and its non-planar analogs is attributed to the effect of the phenyl-vinyl dihedral angle on the barriers for activated decay of the linear singlet state. However, the behavior of the 2-isopropenyl derivative does not appear to be dependent upon ground state conformation (synvs.anti). The cyclic allene intermediates undergo sequential protonation-deprotonation in methanol solution to yield stable products. The 2-vinyl derivative yields only the fully aromatized 2-phenylnaphthalene. However, the 2-isopropenyl and cyclic derivatives yield mixtures of fully and partially aromatized products. Preferential formation of the partially aromatized products is attributed to a stereoelectronic effect on the deprotonation step. In diethyl ether solution only the fully aromatized product is formed via a free radical mechanism.

The cyclization of parent and cyclic hexa-1,3-dien-5-ynes--a combined theoretical and experimental study

The thermal cycloisomerization of both parent and benzannelated hexa-1,3-dien-5-yne, as well as of carbocyclic 1,3-dien-5-ynes (ring size 7-14), was investigated by using pure density functional theory (DFT) of Becke, Lee, Yang, and Parr (BLYP) in connection with the 6-31G* basis set and the Brueckner doubles coupled-cluster approach [BCCD(T)] with the cc-pVDZ basis set for the parent system. The initial cyclization product is the allenic cyclohexa-1,2,4-triene (isobenzene), while the respective biradical is the transition structure for the enantiomerization of the two allenes. Two consecutive [1,2]-H shifts further transform isobenzene to benzene. For the benzannelated system, the energetics are quite similar and the reaction path is the same with one exception: the intermediate biradical is not a transition state but a minimum which is energetically below isonaphthalene. The cyclization of the carbocyclic 1,3-dien-5-ynes, which follows the same reaction path as the parent system, clearly depends on the ring size. Like the cyclic enediynes, the dienynes were found to cyclize to products with reduced ring strain. This is not possible for the 7- and 8-membered dienynes, as their cyclization products are also highly strained. For 9- to 11-membered carbocycles, all intermediates, transition states, and products lie energetically below the parent system; this indicates a reduced cyclization temperature. All other rings (12- to 14-membered) have higher barriers. Exploratory kinetic experiments on the recently prepared 10- to 14-membered 1,3-dien-5-ynes rings show this tendency, and 10- and 11-membered rings indeed cyclize at lower temperatures.