Ruthenium-Mediated Cycloaromatization of Acyclic Enediynes and Dienynes at Ambient Temperature

Abstract

The ruthenium(II) cation, [Cp*Ru(NCMe)3]OTf (4), triggers the Bergman cycloaromatization of acyclic endiynes at room temperature in THF solvent. Treatment of 1,2-di(1-alkynynyl)cyclopentenes (13-Me, alkynyl = propynyl; 13-Prn, alkynyl = pentynyl; 13-Bui, alkynyl = 4-methyl-pent-1-ynyl) with 4 in THF solvent at room temperature gives rise to the ruthenium arene complexes: [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-5,6-dialkyl-1H-indene}]OTf (15-Me, alkyl = methyl, 64% yield; 15-Prn, alkyl = n-propyl, 73% yield; 15-Bui, alkyl = 4-methyl-1-pentynyl, 88% yield). In a similar fashion, the room-temperature reaction of 4 with 1-ethynyl-2-(1-propynyl)cyclopentene (11) and [2-(1-propynyl)-1-cyclopenten-1-yl]trimethylsilane (14) leads to the formation of [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-5-methyl-1H-indene}]OTf (12, 92% yield) and [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-6-methyl-1H-inden-5-yl)trimethylsilane}]OTf (16, 77% yield), respectively. The bis(TMS)-substituted enediyne (1-cyclopentene-1,2-diyldi-2,1-ethynediyl)bis(trimethylsilane) (9-TMS) and 4 underwent reaction at 100 degrees C to give [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-1H-inden-5-yl)trimethylsilane}]OTf (10, 69% yield). Deuterium-labeling studies rule out a mechanism that involves a ruthenium-vinylidene intermediate, and provide support for the involvement of a p-benzyne intermediate. In a similar fashion, complex 4 is shown to trigger the cycloaromatization of the conjugated dienyne, 1-ethenyl-2-(1-pentynyl)cyclopentene (19), at room temperature in chloroform-d1 solvent to give [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-5-(1-propyl)-1H-indene}]OTf (20, 96% yield), with no deuterium enrichment. In the absence of ruthenium the thermal cyclization reactions of unsubstituted acyclic enediynes (Bergman cycloaromatization) and acyclic conjugated dienynes (Hopf cyclization) typically require elevated temperatures (150-250 degrees C). Complexes 10 and 15-Prn were characterized structurally by X-ray crystallography.

10.1007/s11176-008-2012-z

“…O'Connor et al [118] showed that stable enediynes 205 undergo cycloaromatization at 23oC in the presence of catalyst 207 in THF to give ruthenium naphthalene complexes 206 in good yields (Scheme 60). O'Connor et al [118] showed that stable enediynes 205 undergo cycloaromatization at 23oC in the presence of catalyst 207 in THF to give ruthenium naphthalene complexes 206 in good yields (Scheme 60).…”

Geometric and Electronic Control of Thermal Bergman Cyclization

“…102,103 In fact, only recently have the electron withdrawing effects at the alkyne termini (i.e. lowering of the cyclization transition state energy) 85 and vinyl (i.e.…”

Synthesis of Unique Extended π Structures by Pt‐Mediated Benzannulation of Nickel(II) Tetraalkynylporphyrins

“…Analogously, OConnor et al have also shown ambient temperature enediyne activation through p complexation of the alkynes with Ru II or Fe II . [13] Although chemically facile, both methods necessitate either specific alkyne functionalization or unfavorable conditions to remove the activating metal fragment, neither of which are congruent with the efficient synthesis of extended porphyrin constructs.…”