“…A less common annulation is the intramolecular (6 + 2) cycloaddition between non-activated alkynes and a cycloheptatriene. These cycloadditions were previously reported in the context of a stoichiometric chromium(0) activation of the triene unit [ 64 ]. The use of AuCl 3 or PtCl 2 rendered the reaction catalytic [ 65 ].…”
SummaryIn the last years there have been extraordinary advances in the development of gold-catalyzed cycloaddition processes. In this review we will summarize some of the most remarkable examples, and present the mechanistic rational underlying the transformations.
“…A less common annulation is the intramolecular (6 + 2) cycloaddition between non-activated alkynes and a cycloheptatriene. These cycloadditions were previously reported in the context of a stoichiometric chromium(0) activation of the triene unit [ 64 ]. The use of AuCl 3 or PtCl 2 rendered the reaction catalytic [ 65 ].…”
SummaryIn the last years there have been extraordinary advances in the development of gold-catalyzed cycloaddition processes. In this review we will summarize some of the most remarkable examples, and present the mechanistic rational underlying the transformations.
“…With terminal alkynes, the chromium(0)-promoted cycloadditions are difficult to arrest at the 1:1 alkyne−CHT cycloadduct stage; instead, tetra- and pentacyclic 2:1 alkyne−CHT adducts are produced (Scheme ) . These multicomponent reactions are believed to occur through a stepwise [6 + 2] cycloaddition− homo [6 + 2] cycloaddition sequence. , To date, chromium(0)-promoted [6 + 2] cycloadditions with alkynes remain confined to internal alkynes. 1 Cycloadditions of [(η 6 -CHT)Cr(CO) 3 ] and Alkynes …”
[reaction: see text] The CoI2(dppe)/Zn/ZnI2 system effectively catalyzes the [6 + 2] cycloaddition of cycloheptatriene with terminal alkynes to afford 7-alkyl-bicyclo[4.2.1]nona-2,4,7-trienes in fair to excellent yields. The catalyst proved to be tolerant toward functional groups such as ketone, sulfone, ester, ketal, ether, alcohol, imide, and nitrile. The enantioselective cycloaddition of CHT and phenylacetylene with BINOL phosphoramidite ligand 4 afforded the cycloadduct 3a (R = Ph) in 91% yield and 74% ee.
“…The cycloisomerization proceeded with excellent chemoselectivity, whereas heating at 80 °C in toluene in the absence of catalyst resulted in the recovery of unchanged 1 a (Table 1, entry 1). To the best of our knowledge, intramolecular [6+2] cycloaddition reactions between alkynes and CHT have been only been achieved by using tricarbonyl(η 6 ‐7‐ exo ‐alkynyl‐1,3,5‐cycloheptatriene)chromium(0) complexes at high temperatures (140–170 °C) 9. In comparison, the present cycloisomerization is performed catalytically and under mild conditions (room temperature), and affords higher yields without preliminary coordination of the triene to the metal.…”
Transition-metal-catalyzed cycloisomerization reactions of enynes have attracted considerable attention because of their high synthetic potential for the construction of useful carbo-and heterocyclic structural features.[1] Important advances in this timely area of research have been achieved using late-transition metals (for example, Ru, Pt, and Au) as p-acid catalysts.[2] In this context, novel modes of connection between alkenes and alkynes still remain an attractive field of investigation, although the chemoselectivity of the reactions often depends on the structural patterns of the substrates. The cycloisomerization of polyene-ynes takes advantage of additional unsaturated bond(s) to trap transient reactive intermediates, thus allowing the elaboration of complex polycycles. For example, the gold-catalyzed cycloisomerization of dienynes affords Diels-Alder adducts, [3] whereas cyclohexadienyl alkynes generate tetracycles. [4,5] In the particular case of aryl-tethered alkynes, the cycloisomerization with various electrophilic metal catalysts afforded Friedel-Crafts-type compounds, but these reactions generally required electronrich arenes.[6] As part of our recent studies on the cobaltcatalyzed intermolecular [6+2] cycloaddition of cycloheptatriene (CHT) with alkynes, [7] we were intrigued by the behavior of CHT tethered to alkynes in cycloisomerization reactions involving p-acid catalysts. In these reactions, nucleophilic attack of CHT on an electrophilic metal-coordinated alkyne B is expected to generate a pentadienyl cation C which may evolve through ring closure to produce [2+2], [4+2], and/or [6+2] adducts (Scheme 1).Herein, we report the platinum-catalyzed cycloisomerization of 1-(pent-4-ynyl)-1,3,5-cycloheptatrienes, in which connections are made between the carbon termini of the triene and the acetylenic carbon atoms, to afford tricyclotrienes F, similar to a formal intramolecular [6+2] cycloaddition.Exposure of triene-yne 1 a to catalytic amounts of PtCl 2 (5 mol %) in toluene at room temperature resulted in a complete and clean conversion into a single adduct 2 a [8] in 94 % yield (Table 1, entry 10). The cycloisomerization proceeded with excellent chemoselectivity, whereas heating at 80 8C in toluene in the absence of catalyst resulted in the recovery of unchanged 1 a (Table 1, entry 1). To the best of our knowledge, intramolecular [6+2] cycloaddition reactions between alkynes and CHT have been only been achieved by using tricarbonyl(h 6 -7-exo-alkynyl-1,3,5-cycloheptatriene)-chromium(0) complexes at high temperatures (140-170 8C).[9] In comparison, the present cycloisomerization is performed catalytically and under mild conditions (room temperature), and affords higher yields without preliminary coordination of the triene to the metal. A catalyst loading as little as 1 mol % can be used, but the reaction time increases
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
