Chemistry of Fischer-type rhenacyclobutadiene complexes. I. Deprotonation, addition/substitution of nucleophilic reagents at α-carbon, and insertion of heteroatoms into rhenium–carbon bonds

The rhenacyclobutadienes (CO)4Re(η2- C(R)C(CO2Me)C(OR′)) (2) undergo a number of reactions that mirror those of Fischer alkoxycarbene complexes. Thus, (CO)4Re(η2-C(Me)C(CO2Me)C(OEt)) (2a) can be deprotonated by LDA, Na[OBu-t], or Na[CH(CO2Me)2] to give the ylide-like conjugate base [(CO)4Re(η2-C(CH2)C(CO2Me)C(OEt)]− (3), which was isolated as PPN(3). Li(3) undergoes deuteriation with DCl/D2O and alkylation with Et3OPF6 at ReCCH2, with the latter reaction affording (CO)4Re(η2-C(CH2Et)C(CO2Me)C(OEt)) (4). Repetition of the sequence deprotonation-ethylation on 4 generates (CO)4Re(η2-C(CHEt2)C(CO2Me)C(OEt)) (5). The nature of the alkoxy substituent in 2 can be varied by use of the rhenacyclobutenones Na[(CO)4Re(η2-C(R)C(CO2Me)C(O))] (Na(1)) in conjunction with AcCl and R′OH to produce a series of new complexes (R=Ph, R′=Et (2b); R=Me, R′=CH2CHCH2 (2c), (CH2)3CCH (2d), Me (2e)). Aminolysis of 2a with the primary and secondary amines PhNH2, HO(CH2)2NH, p-TolNH2, and Et2NH yields the aminorhenacyclobutadiene complexes (CO)4Re(η2-C(Me)C(CO2Me)C(NHR′ or NR′2)) (R2′=Et2 (6a); R′=Ph (6b), (CH2)2OH (6c), p-Tol (6d)). These complexes display lesser carbene-like character than their alkoxy counterparts 2, as evidenced by 1H and 13C NMR spectroscopic properties and lack of reactivity toward LDA by 6a. Reactions of each 2a and 6a with PPhMe2 at low temperature afford (CO)4Re(η2-C(Me)(PPhMe2)C(CO2Me)C(OEt)) (7) and (CO)3(PPhMe2)Re(η2-C(Me)C(CO2Me)C(NEt2)) (9), respectively, further in agreement with the more carbenoid nature of 2a than 6a. 7 undergoes conversion to (CO)3(PPhMe2)Re(η2-C(Me)C(CO2Me)C(OEt)) (8) upon heating. 2a reacts with each of (NH4)2[Ce(NO3)6], DMSO, EtNO2/Et3N, and Me3NO under various conditions to afford one or both of the oxygen atom insertion products into the ReC bonds, (CO)4Re(κ2-OC(Me)C(CO2Me)C(OEt)) (10) and (CO)4Re(κ2-C(Me)C(CO2Me)C(OEt)O) (11). In contrast, no reaction occurred between 2a and S8 on heating. However, 6a was converted to the NH insertion product (CO)4Re(κ2-NHC(Me)C(CO2Me)C(NEt2)) (12) by the action of H2NNH2 · H2O at 0 °C. All new compounds were characterized by a combination of elemental analysis, mass spectrometry, and IR and NMR spectroscopy.