Chemistry of Fischer-type rhenacyclobutadiene complexes. II. Reactions with alkynes and sulfonium ylides, and rearrangements induced by nitriles and pyridine

Further investigations into the chemistry of the rhenacyclobutadiene complexes (CO)4Re(η2-C(R)C(CO2Me)C(X)) (1: R=Me, X=OEt (1a), O(CH2)3CCH (1b), NEt2 (1c); R=CHEt2, X=OEt (1d); R=Ph, X=OEt (1e)) are reported. Reactions of 1 with alkynes at reflux temperature of toluene and at ambient temperature either under photochemical conditions or in the presence of PdO yield ring-substituted η5-cyclopentadienylrhenium tricarbonyl complexes, 2. The symmetrical alkynes R′CCR′ (R′=Ph, Me, CO2Me) afford the pentasubstituted complexes (η5-C5(Me)(CO2Me)(OEt)(Ph)(Ph))Re(CO)3 (2d), (η5-C5(Me)(CO2Me)(OEt)(Me)(Me))Re(CO)3 (2e), (η5-C5(Me)(CO2Me)(OEt)(CO2Me)(CO2Me))Re(CO)3 (2f), and (η5-C5(Me)(CO2Me)(NEt2)(CO2Me)(CO2Me))Re(CO)3 (2i) on reaction with the appropriate 1, whereas the unsymmetrical alkynes R′CCR″ (R′=Ph; R″=H, Me) give either only one, (η5-C5(Me)(CO2Me)(OEt)(Ph)H)Re(CO)3 (2a)), or both, (η5-C5(Me)(CO2Me) (OEt)(Ph)(Me))Re(CO)3 (2b) and (η5-C5(Me)(CO2Me)(OEt)(Me)(Ph))Re(CO)3 (2c), (η5-C5(Ph)(CO2Me)(OEt)(Ph)H)Re(CO)3 (2g) and (η5-C5(Ph)(CO2Me)(OEt)(H)(Ph))Re(CO)3 (2h), of the possible products of [3 + 2] cycloaddition of alkyne to η2-C(R)C(CO2Me)C(X). Thermolysis of (CO)4Re(η2-C(Me)C(CO2Me)C(O(CH2)3CCH)) (1b) containing a pendant alkynyl group proceeds to (η5-C5(Me)(CO2Me)(O(CH2)3)H)Re(CO)3 (2j), a η5-cyclopentadienyl-dihydropyran fused-ring product. Competition experiments showed that each of PhCCH and MeO2CCCCO2Me reacts faster than PhCCPh with 1a. The results with unsymmetrical alkynes are rationalized by steric properties of substituents at the CC and ReC bonds and by a preference of ReC(Me) over ReC(OEt) to undergo alkyne insertion. A mechanism is proposed that involves substitution of a trans CO by alkyne in 1, insertion of alkyne into ReC bond to give a rhenabenzene intermediate, and collapse of the latter to 2. Complexes 1a and 1d undergo rearrangement in MeCN at reflux temperature to give rhenafuran-like products, (CO)4Re(κ2-OC(OMe)C(CHCR2)C(OEt)) (R=H (3a) or Et (3b)). The reaction of 1d also proceeds in EtCN, PhCN, and t-BuCN at comparable temperature, but is slower (especially in t-BuCN) than in MeCN. In pyridine at reflux temperature, 1a undergoes a similar rearrangement, with CO substitution, to give (CO)3(py)Re(κ2-OC(OMe)C(CHCEt2)C(OEt)) (4). A mechanism is proposed for these reactions. The sulfonium ylides Me2SCHC(O)Ph and Me2SC(CN)2 (Me2SCRR′) react with 1a in acetonitrile at reflux temperature by nucleophilic addition of the ylide to the ReC(Me) carbon, loss of Me2S, and rearrangement to a rhenafuran-type structure to yield (CO)4Re(κ2-OC(OMe)C(C(Me)CRR′)C(OEt)) (R=H, R′=C(O)Ph (5a); R=R′CN (5b)). All new compounds were characterized by a combination of elemental analysis, mass spectrometry, and IR and NMR spectroscopy.

The rhenacyclobutadienes (CO)4Re(η2-C(R)C(CO2Me)C(X)) (1: R=Me, Ph; X=alkoxy, amido group) react with a variety of alkynes to give ring-substituted η5-cyclopentadienylrhenium tricarbonyl complexes. Complexes 1 also undergo rearrangement in nitriles and pyridine to yield rhenafuran-like products, and react with sulfonium ylides to afford similar 5-membered ring compounds.Figure optionsDownload as PowerPoint slide