Palladium-catalyzed Trimerization of Strained Cycloalkynes: Synthesis of Decacyclene

Abstract

Palladium-catalyzed cyclotrimerization was applied to three strained cycloalkynes. Pd(PPh 3 ) 4 and Pt(PPh 3 ) 4 -catalyzed cyclotrimerizations of cyclohexyne (2) afforded dodecahydrotriphenylene (3) in 64% and 62% yields, respectively, but subjecting cyclopentyne to the same conditions failed to afford isolable amounts of the cyclotrimer. Finally, decacyclene (15), a putative C 60 -fullerene precursor, was obtained in 23% yield by Pd 2 (dba) 3 -catalyzed cyclotrimerization of acenaphthyne (14) .

Palladium-Catalyzed Cycloaddition Reactions of Arynes

“…Some significant results of this seminal study are shown in Table 1.After some preliminary experiments in which nickel complexes were used as catalysts (entry 1), it was found that nucleophilic palladium(0) complexes efficiently promote the desired [2+2+2] cycloaddition of the strongly electrophilic substrate 1 (entries 2-6) [40]. At the same time, a systematic study of benzyne generation conditions showed that Kobayashi's method 118 E. Guitián et al [fluoride-induced decomposition of 2-(trimethylsilyl)phenyl trifluoromethanesulphonate, (55)], is the method of choice [41]. The best yield of triphenylene (83%) is obtained when 2-(trimethylsilyl)-phenyl triflate (55) is treated at room temperature with caesium fluoride in acetonitrile in the presence of 10 mol% of Pd(PPh 3 ) 4 (entry 3).…”

“…At the same time, a systematic study of benzyne generation conditions showed that Kobayashi's method 118 E. Guitián et al [fluoride-induced decomposition of 2-(trimethylsilyl)phenyl trifluoromethanesulphonate, (55)], is the method of choice [41]. The best yield of triphenylene (83%) is obtained when 2-(trimethylsilyl)-phenyl triflate (55) is treated at room temperature with caesium fluoride in acetonitrile in the presence of 10 mol% of Pd(PPh 3 ) 4 (entry 3). The combination of acetonitrile and caesium fluoride, which is only partially soluble in this solvent, results in a slow rate of benzyne generation and a correspondingly low concentration of this reactive species in the reaction medium, which is optimal for the catalytic transformation.…”

“…For example, cyclohexyne (96) undergoes [2+2+2] cycloaddition to form dodecahydrotriphenylene (97) in 65% yield, a reaction that can be promoted by either Pd(PPh 3 ) 4 or Pt(PPh 3 ) 4 with identical efficiency [55]. In this case the synthesis of the cycloalkyne precursor 95 requires a modified procedure, involving the conjugate addition of hydride to ketone 94 and trapping of the resulting enolate with N-phenyltriflimide (Scheme 21) [56].…”

“…It starts with the transformation of acenaphthylene (99) into bromoketone 100 by one-pot bromination-oxidation with bromine and DMSO. The transformation of 100 into 101 is accomplished by enolization with Hünig's base and trapping with Tf 2 O, followed by lithium-bromine exchange and silylation of the resulting carbanion with TMSCl.Attempts at generation and trimerization of the highly strained acenaphthyne 5 in the reaction conditions developed for arynes and cyclohexyne [CsF, Pd(PPh 3 ) 4 , acetonitrile, rt] afforded very low yields of cyclotrimer, but forced reaction conditions involving Bu 4 NF as fluoride source, Pd 2 (dba) 3 as catalyst and higher reaction temperature gave decacyclene (102) in acceptable yields (Scheme 22) [55]. Although the reaction is thought to involve cycloalkyne 5, an alternative mechanism involving the initial oxidative addition of the C-OTf bond to palladium cannot be ruled out, particularly considering the higher reaction temperatures at which this reaction takes place.…”

Pd(0)‐ and Pd(II)‐Catalyzed Reactions of Alkynes and Benzynes

“…They generated highly strained cyclohexyne (181) by the treatment of 2-trimethylsilylcyclohexenyl triflate (180) with CsF in the presence of Pd(PPh 3 ) 4 . Cyclotrimerization of cyclohexyne (181) occurred rapidly as soon as it was generated, and they obtained dodecahydrotriphenylene (182) in 62 % yield [55]. [4 + 2] addition of an enyne with an alkyne, offering useful synthetic methods of substituted benzenes in short steps [56].…”