Oxidative demetalation of Fischer alkoxy carbene complexes with stoichiometric pyridine N-oxide and NaBH4-promoted demetalation of Fischer iminocarbene complexes with sulfur and selenium

An expedient approach to ferrocenyl thioamides via Fischer carbenes

“…Table 3 summarizes the evaluation of the scope in demetalation reaction of amino ferrocenyl carbene complexes 11-15(a-c) to afford the novel ferrocenyl thioamides 16-20 (Scheme 6). In contrast with the results obtained by Yu and co-workers [35] with (1) (1) and Cg (2) Fischer imine carbenes, we obtain the un-coordinated ferrocenyl thioamides and apparently the presence of the ferrocenyl moiety increases the reactivity of carbene toward demetalation. Additionally, we observe that carbene precursors which contain chromium as metal are the most accessible, even though molybdenum precursors react in a short time, they conduce to side-products and tungsten carbene complexes need long reaction times.…”

“…However, the isolation of the desired product in some cases can be difficult and the use of aromatic solvents as benzene or toluene is necessary. Recently, a new demetalation protocol appears to be an efficient method to obtain thioureas, selenoureas [34], thioamides and selenoamides [35], by the use of a combination of elemental sulfur or selenium and metallic hydrides (LiAlH 4 , NaBH 4 ). Taking in account this strategy, we used the mixture S 8 /NaBH 4 as the demetalating agent, thus the O-ethyl ferrocenecarbothioate 6 was obtained in similar yield from 1a-c (Table 1, entries 5, 6 and 7), but only few minutes and room temperature were required to carry out this transformation.…”

Multiple reactivities of dithranol towards 1-alkynyl Fischer carbene complexes (CO)5MC(OEt)CCPh (M=Cr, W) – Efficient chemical synthesis of aromatic polyketides

“…With our previously developed methodologies for demetalation of Fischer carbene complexes [19,20], complexes 4-6 were efficiently converted to their corresponding esters, i.e., 7, 9, and 12, with pyridine-N-oxide (PNO) (Scheme 3). Complexes 4 were demetalated to 7 in 95% yields with 1.0 equiv.…”

1-Lithio-1,3-dienes as useful building blocks in organic synthesis

“…We found that oxidative demetalation of 7a and 7b with I 2 provided 3,4,5,6-tetrapropyl-2H-pyran-2-one 8 in excellent yield (Scheme 6, a) [17]. The reaction of 7a and 7b with elemental sulfur in the presence of NaBH 4 selectively generated pyran-thione complexes 9a and 9b in quantitative yields by insertion of sulfur into the M=C (M = Cr, W) bonds, respectively (Scheme 6, b) [18]. Compound 7b remained inert toward hydrolysis with diluted HCl, however, with 12 N HCl 7b was hydrolyzed to yield 2H-pyran 10 in 80 % isolated yield (Scheme 6, c), which was in marked contrast with the alkoxy carbene complexes that afforded aldehydes after acidolysis [18].…”

“…The reaction of 7a and 7b with elemental sulfur in the presence of NaBH 4 selectively generated pyran-thione complexes 9a and 9b in quantitative yields by insertion of sulfur into the M=C (M = Cr, W) bonds, respectively (Scheme 6, b) [18]. Compound 7b remained inert toward hydrolysis with diluted HCl, however, with 12 N HCl 7b was hydrolyzed to yield 2H-pyran 10 in 80 % isolated yield (Scheme 6, c), which was in marked contrast with the alkoxy carbene complexes that afforded aldehydes after acidolysis [18]. More interestingly, it was found that 7b underwent a novel reductive demetalation reaction with LiAlH 4 to give 3-cyclopentenone 11 involving contraction of six-to five-membered ring (Scheme 6, d) [20].…”