Ruthenium‐Catalyzed Synthesis of Alkylidenecyclobutenes via Head‐to‐Head Dimerization of Propargylic Alcohols and Cyclobutadiene–Ruthenium Intermediates

Abstract

The reaction of propargylic alcohols with carboxylic acid, or phenol derivatives, in the presence of the precatalyst [RuCl(cod)(C5Me5)] leads selectively to a variety of alkylidenecyclobutenes through head-to-head dimerization of propargylic alcohol. The first step is the formation of a cyclobutadiene-ruthenium intermediate resulting from the head-to-head coupling of two molecules of propargylic alcohol. On protonation with strong acids (HPF6, HBF4) dehydration of the cyclobutadiene complex leads to formation of an alkylidenecyclobutenyl-ruthenium complex. The X-ray structure of one such complex, [RuCl(C5Me5)(eta4-R'CCH--CH--C=CR2)] (R'=cyclohexen-1-yl, CR2 = cyclohexylidene) has been determined. Carboxylate is added at the less substituted carbon of the cyclic allylic ligand. DFT/B3 LYP calculations confirm that the intermediate arising from head-to-head coupling of alkyne to the RuClCp* species yields the cyclobutadiene-ruthenium complex more easily with propargylic alcohol than with acetylene.

Ruthenium-Catalyzed Alkyne trans-Hydrometalation: Mechanistic Insights and Preparative Implications

“…Although adduct formation is fast and clean according to NMR, our early attempts at isolating the resulting π-complexes had met with failure; as mentioned above, only downstream [2+2] cycloadducts such as 3 had been obtained. , It was only after considerable experimentation that our efforts were finally rewarded with success. Specifically, single crystals suitable for X-ray diffraction could be grown from the representative complexes 4 – 6 comprising a propargyl alcohol or a propargylic sulfonamide, respectively (Figure and Figures S2 and S3 in the Supporting Information).…”

Transition metals in organic synthesis: Highlights for the year 2005

“…Ruthenium catalyzed cyclization of 1-yne-ols in the presence of an acid to give alkylidenecyclobutenes (Eq. (310)) [1407]. Ruthenium catalyzed cycloisomerizations of 1,5-enynes in the presence of water to form cyclopentanones (Eq.…”

Fulvene–Ruthenium and Cp–Ruthenium Complexes via [2 + 2 + 1] Cyclotrimerization of Phenylacetylene with [RuCl(Tp)(1,5-cod)]

“…In ruthenium(II) systems, reactions involving terminal alkynes usually give either η 6 -benzene, cyclobutene, vinylidene, or alkylidenecyclobutadiene derivatives, with dien-ynes to give cyclohexenes, with diynes together with enynes and diazoalkanes to give alkenyl­bicyclo­[3.1.0]­hexanes, 2,5-disubstituted biscarbene ruthenacycles, as well as other uncommon carbocycles. − In addition, the mechanisms proposed for these organic transformations are derived from the successful isolation of ruthena­cyclo­pentatrienes, η 4 -cyclobutadienes, or η 6 -benzene complexes. , The commonly employed ruthenium(II) precursors (via the pseudo -14-electron species CpRuCl, and CpRu + ) for which these transformations typically occur: [Ru(Cp′)­(1,5-cod)­(L)] (Cp′ = C 5 H 5 , C 5 Me 5 ; L = Cl, Br), [Ru(Cp′)­(CH 3 CN) 2 ­(L)] + (L = CH 3 CN, PR 3 , AsR 3 , SbR 3 , CO), and [Ru(η 5 -C 5 H 4 CH 2 CH 2 -κ 1 P-PPh 2 )­(CH 3 CN) 2 ] + . , These reactive organometallic catalysts have been employed in numerous organic transformations, for which a variety of important organometallic intermediates have been reported to date. , Tris(pyrazolyl)­hydridoborato (Tp) and cyclopentadienyl (Cp) ligands have often been compared as these classes of 6-electron donor ligands exhibit the same charge, although they differ in steric and electronic properties …”