Catalytic Cyclotrimerization Pathway for Synthesis of Selaginpulvilins C and D: Scope and Limitations

A facile and unified approach to the main selaginpulvilin's framework was achieved by catalytic [2 + 2 + 2]-cyclotrimerization of a triyne with monosubtituted alkynes. The reaction proceeded with high "ortho" selectivity by using Wilkinson's catalyst (RhCl(PPh₃)₃) under ambient conditions with reasonable yields. The scope of the reaction with respect to the alkyne as well as the catalytic system was evaluated. The formal total modular syntheses of selaginpulvilin C and D were accomplished by transformation of the cyclotrimerization's products.

Hexadehydro-Diels-Alder Reaction: Benzyne Generation via Cycloisomerization of Tethered Triynes

The hexadehydro-Diels-Alder (HDDA) reaction is the thermal cyclization of an alkyne and a 1,3-diyne to generate a benzyne intermediate. This is then rapidly trapped, in situ, by a variety of species to yield highly functionalized benzenoid products. In contrast to nearly all other methods of aryne generation, no other reagents are required to produce an HDDA benzyne. The versatile and customizable nature of the process has attracted much attention due not only to its synthetic potential but also because of the fundamental mechanistic insights the studies often afford. The authors have attempted to provide here a comprehensive compilation of publications appearing by mid-2020 that describe experimental results of HDDA reactions.

A Traceless Tether Strategy for Achieving Formal Intermolecular Hexadehydro-Diels-Alder Reactions

A synthetic strategy formally equivalent to an intermolecular hexadehydro-Diels-Alder (HDDA) reaction is described. Sulfur-based linkers were designed and constructed by joining terminal alkynes or diynes using alkyne thiolate chemistry. The resulting tetraynes and triynes successfully underwent HDDA cyclization and benzyne trapping. Linker removal by reductive desulfurization was uneventful. The strategy was also found suitable for the tetradehydro-Diels-Alder (TDDA) reaction.

Methodology and applications of the hexadehydro-Diels–Alder (HDDA) reaction

Hexadehydro-Diels–Alder (HDDA) reactions between alkynes and 1,3-diynes readily generate highly reactive and synthetically useful arynes.

The pentadehydro-Diels-Alder reaction

In the classic Diels–Alder (DA) [4+2] cycloaddition reaction¹, the overall degree of unsaturation of the 4π (diene) and 2π (dienophile) pairs of reactants dictates the oxidation state of the newly formed six-membered carbocycle. For example, in the classic DA reaction, butadiene and ethylene combine to produce cyclohexene. More recent developments include variants in which the hydrogen atom count in the reactant pair and in the resulting product is reduced by², for example, four in the tetradehydro-DA (TDDA) and by six in the hexadehydro-DA (HDDA^3,4,5,6,7) reactions. Any oxidation state higher than tetradehydro leads to the production of a reactive intermediate that is more highly oxidized than benzene. This significantly increases the power of the overall process because trapping of the benzyne intermediate^8,9 can be used to increase the structural complexity of the final product in a controllable and versatile manner. In this manuscript, we report an unprecedented net 4π+2π cycloaddition reaction that generates a different, highly reactive intermediate known as an α,3-dehydrotoluene. This species is at the same oxidation state as a benzyne. Like benzynes, α,3-dehydrotoluenes can be captured by various trapping agents to produce structurally diverse products that are complementary to those arising from the HDDA process. We call this new cycloisomerization reaction a pentadehydro-Diels–Alder (PDDA) reaction—a nomenclature chosen for chemical taxonomic rather than mechanistic reasons. In addition to alkynes, nitriles (RC≡N), although non-participants in aza-HDDA reactions, readily function as the 2π-component in PDDA cyclizations to produce, via trapping of the α,3-(5-aza)dehydrotoluene intermediates, pyridine-containing products.

Hydrohalogenative aromatization of multiynes promoted by ruthenium alkylidene complexes

A new functionalization method of arynes promoted by a novel catalytic role of the Grubbs-type ruthenium alkylidene complex is described.

Rates of hexadehydro-Diels-Alder (HDDA) cyclizations: impact of the linker structure

The rates of the hexadehydro-Diels-Alder (HDDA) reaction of substrates containing, minimally, a 1,3,8-triyne subunit are reported. Several series of related substrates, differing in the nature of the three-atom tether that links the 1,3-diyne and diynophile, were examined. Seemingly small changes in substrate structure result in large differences in cyclization rate, spanning more than 8 orders of magnitude. The reactivity trends revealed by these studies should prove useful in guiding substrate design and choice of reaction conditions in future applications.

Formal hydrogenation of arynes with silyl Cβ–H bonds as an active hydride source

In stark contrast to the effective 1°, 2°, and 3° C–H bond insertion of alkyl groups tethered to arynes, the 2° and 3° C–H bonds on the β-carbon of silyl groups show high tendency for hydride transfer rather than C–H insertion, whereas the corresponding 1° C–H bonds exclusively undergo C–H insertion.

Regioselectivity in the nucleophile trapping of arynes: the electronic and steric effects of nucleophiles and substituents

The regioselectivity in nucleophile trapping is investigated with arynes generated directly from bis-1,3-diynes. The regioselectivity is profoundly influenced by not only the nature of nucleophiles but also the substituents on the arynes, which is the consequence of both the unfavorable steric interaction between the incoming nucleophile and the nearby substituent and the inherent electronic bias induced by different substituents on the arynes.

Synthesis of complex benzenoids via the intermediate generation of o-benzynes through the hexadehydro-Diels-Alder reaction

The hexadehydro-Diels-Alder (HDDA) cascade enables the synthesis of complex benzenoid products with various substitution patterns through aryne intermediates. The first stage of this cascade involves the generation of a highly reactive ortho-benzyne intermediate by a net [4+2] cycloisomerization of a triyne substrate. The benzyne can be rapidly 'trapped' either intramolecularly or intermolecularly with myriad nucleophilic or π-bond-donating reactants. As a representative example of a general procedure for synthesizing highly substituted benzenoids, this protocol describes the synthesis of a typical triyne substrate and its use as the reactant in an HDDA cascade to form a phthalide. The synthetic procedure detailed herein (four chemical reactions) takes 16-20 h of active effort over a period of several days for the preparation of the triyne precursor and ∼2 h of active effort over a 3-d period for the generation and trapping of the benzyne and isolation of the phthalide product.

The hexadehydro-Diels-Alder reaction

o-Benzynes (arynes) are among the most versatile of all reactive (short-lived) intermediates in organic chemistry. These species can be trapped to give products that are valuable from the perspective of both fine (pharmaceuticals) and commodity (agrochemicals, dyes, polymers, etc.) chemicals. Here we show a fundamentally new strategy that unites a de novo generation of benzynes, through the title hexadehydro-Diels–Alder (HDDA) reaction, with their in situ elaboration into structurally complex benzenoid products. In the HDDA reaction a 1,3-diyne is engaged in a [4+2] cycloisomerization with a third (pendant) alkyne–the diynophile–to produce the highly reactive benzyne intermediate. The metal- and reagent-free reaction conditions for this simple, thermal transformation are notable. The subsequent and highly efficient trapping reactions increase the power of the overall process. Finally, we provide examples of how this de novo benzyne generation approach allows new modes of intrinsic reactivity to be revealed.