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

Abstract

Further investigations into the chemistry of the rhenacyclobutadiene complexes (CO) 4Re(η 2-C(R)C(CO 2Me)C(X)) ( 1: R=Me, X=OEt ( 1a), O(CH 2) 3CCH ( 1b), NEt 2 ( 1c); R=CHEt 2, 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 η 5-cyclopentadienylrhenium tricarbonyl complexes, 2. The symmetrical alkynes R ′CCR ′ (R ′=Ph, Me, CO 2Me) afford the pentasubstituted complexes (η 5-C 5(Me)(CO 2Me)(OEt)(Ph)(Ph))Re(CO) 3 ( 2d), (η 5-C 5(Me)(CO 2Me)(OEt)(Me)(Me))Re(CO) 3 ( 2e), (η 5-C 5(Me)(CO 2Me)(OEt)(CO 2Me)(CO 2Me))Re(CO) 3 ( 2f), and (η 5-C 5(Me)(CO 2Me)(NEt 2)(CO 2Me)(CO 2Me))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, (η 5-C 5(Me)(CO 2Me)(OEt)(Ph)H)Re(CO) 3 ( 2a)), or both, (η 5-C 5(Me)(CO 2Me) (OEt)(Ph)(Me))Re(CO) 3 ( 2b) and (η 5-C 5(Me)(CO 2Me)(OEt)(Me)(Ph))Re(CO) 3 ( 2c), (η 5-C 5(Ph)(CO 2Me)(OEt)(Ph)H)Re(CO) 3 ( 2g) and (η 5-C 5(Ph)(CO 2Me)(OEt)(H)(Ph))Re(CO) 3 ( 2h), of the possible products of [3 + 2] cycloaddition of alkyne to η 2-C(R)C(CO 2Me)C(X). Thermolysis of (CO) 4Re(η 2-C(Me)C(CO 2Me)C(O(CH 2) 3CCH)) ( 1b) containing a pendant alkynyl group proceeds to (η 5-C 5(Me)(CO 2Me)(O(CH 2) 3)H)Re(CO) 3 ( 2j), a η 5-cyclopentadienyl-dihydropyran fused-ring product. Competition experiments showed that each of PhCCH and MeO 2CCCCO 2Me 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(κ 2-OC(OMe)C(CHCR 2)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(κ 2-OC(OMe)C(CHCEt 2)C(OEt)) ( 4). A mechanism is proposed for these reactions. The sulfonium ylides Me 2SCHC(O)Ph and Me 2SC(CN) 2 (Me 2SCRR ′) react with 1a in acetonitrile at reflux temperature by nucleophilic addition of the ylide to the ReC(Me) carbon, loss of Me 2S, and rearrangement to a rhenafuran-type structure to yield (CO) 4Re(κ 2-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.