Photochemical Double 5-exoCyclization of Alkenyl-Substituted Dithienylacetylenes: Efficient Synthesis of Diarylated Dithienofulvalenes

Thiophene-Fused 1,4-Diazapentalene: A Stable C=N-Containing π-Conjugated System with Restored Antiaromaticity

A thiophene-fused 1,4-diazapentalene (TAP) was rationally designed and synthesized as a C=N-containing 4n π-electron system that exhibits restored antiaromaticity impaired by the doping with C=N bonds. X-ray crystallographic analysis and quantum chemical calculations revealed that the annulation of thiophene rings with the 1,4-diazapentalene moiety resulted in a much higher antiaromaticity than the pristine 1,4-diazapentalene. These effects can be ascribed to the reduced bond alternation of the eight-membered-ring periphery caused by stabilization of the less-stable bond-shifted resonance structure upon increasing the degree of substitution of imine moieties. Consequently, TAP underwent facile hydrogenation even under mild conditions because of its pronounced antiaromaticity and the high aromaticity of the corresponding hydrogenated product H₂ -TAP. In addition, the electrophilic C=N moieties in TAP led to the formation of a dense π-stacked structure. These results highlight the effect of partial replacement of C=C bonds with C=N bonds in antiaromatic π-electron systems.

1,3-Dien-5-ynes: Versatile Building Blocks for the Synthesis of Carbo- and Heterocycles

1,3-Dien-5-ynes have been extensively used as starting materials for the synthesis of a wide number of different carbo- and heterocycles. The aim of this review is to give an overview of their utility in organic synthesis, highlighting the variety of compounds that can be directly accessed from single reactions over these systems. Thus, cycloaromatization processes are initially commented, followed by reactions directed toward the syntheses of five-membered rings, other carbocycles and, finally, heterocycles. The diverse methodologies that have been developed for the synthesis of each of these types of compounds from 1,3-dien-5-ynes are presented, emphasizing the influence of the reaction conditions and the use of additional reagents in the outcome of the transformations.

The Missing C1-C5 Cycloaromatization Reaction: Triplet State Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes

The last missing example of the four archetypical cycloaromatizations of enediynes and enynes was discovered by combining a twisted alkene excited state with a new self-terminating path for intramolecular conversion of diradicals into closed-shell products. Photoexcitation of aromatic enynes to a twisted alkene triplet state creates a unique stereoelectronic situation, which is facilitated by the relief of excited state antiaromaticity of the benzene ring. This enables the usually unfavorable 5-endo-trig cyclization and merges it with 5-exo-dig closure. The 1,4-diradical product of the C1-C5 cyclization undergoes internal H atom transfer that is coupled with the fragmentation of an exocyclic C-C bond. This sequence provides efficient access to benzofulvenes from enynes and expands the utility of self-terminating aromatizing enyne cascades to photochemical reactions. The key feature of this self-terminating reaction is that, despite the involvement of radical species in the key cyclization step, no external radical sources or quenchers are needed to provide the products. In these cascades, both radical centers are formed transiently and converted to the closed-shell products via intramolecular H-transfer and C-C bond fragmentation. Furthermore, incorporating C-C bond cleavage into the photochemical self-terminating cyclizations of enynes opens a new way for the use of alkenes as alkyne equivalents in organic synthesis.