“…These activation parameters can be compared with those of cyclopropanation reactions by transition-metal alkylidene species. For examples, the reaction of the tungsten complex W(CO) 5 (CHC 6 H 4 -OMe- p ) with vinyl acetate has Δ H ⧧ = 10 kcal/mol and Δ S ⧧ = −34 cal/mol/K, that of Warren’s copper alkylidene [(nacnac)Cu(CPh 2 )] with styrene has Δ H ⧧ = 10 kcal/mol and Δ S ⧧ = −32 cal/mol/K, and the copper(I) triflate-catalyzed cyclopropanation of 1-hexene by ethyl diazoacetate has Δ H ⧧ = 19 kcal/mol and Δ S ⧧ = −9 cal/mol/K …”
Section: Resultsmentioning
confidence: 99%
“…These activation parameters can be compared with those of cyclopropanation reactions by transition-metal alkylidene species. For examples, the reaction of the tungsten complex W(CO) 5 (CHC 6 H 4 -OMe-p) with vinyl acetate has ΔH ⧧ = 10 kcal/mol and ΔS ⧧ = −34 cal/mol/K, 27 that of Warren's copper alkylidene [(nacnac)Cu(CPh 2 )] with styrene has ΔH ⧧ = 10 kcal/mol and ΔS ⧧ = −32 cal/mol/K, 28 and the copper(I) triflate-catalyzed cyclopropanation of 1-hexene by ethyl diazoacetate has ΔH ⧧ = 19 kcal/mol and ΔS ⧧ = −9 cal/ mol/K. 29 In order to probe the nature of the reactive species in the cyclopropanation reaction, the relative rates of the cyclopropanation of a series of para-substituted styrenes by the ylide complex 2 were measured by competition experiments (Scheme 3).…”
Transition-metal alkylidenes are important reactive organometallic intermediates, and our current knowledge on them has been mainly restricted to those with closed-shell electronic configurations. In this study, we present an exploration on open-shell iron alkylidenes with a weak-field tripodal amido-phosphine-amido ligand. We found that a high-spin (amido-phosphine-amido)iron(II) complex can react with (p-tolyl)CN to afford a high-spin (amido-ylide-amido)iron(II) complex, 2, which could transfer its alkylidene moiety to a variety of alkenes, either the electron-rich or electron-deficient ones, to form cyclopropane derivatives. The reaction of 2 with cis-β-deuterio-styrene gave deuterated cyclopropane derivatives with partial loss of the stereochemical integrity with respect to the cis-styrene. Kinetic study on the cyclopropanation reaction of 2 with 4-fluoro-styrene disclosed the activation parameters of ΔH = 23 ± 1 kcal/mol and ΔS = -20 ± 3 cal/mol/K, which are comparable to those of the cyclopropanation reactions involving transition-metal alkylidenes. However, the cyclopropanation of para-substituted styrenes by 2 shows a nonlinear Hammett plot of log(k/k) vs σ. By introduction of a radical parameter, a linear plot of log(k/k) vs 0.59σ + 0.55σ was obtained, which suggests the "nucleophilic" radical nature of the transition state of the cyclopropanation step. In corroboration with the experimental observations, density functional theory calculation on the reaction of 2 with styrene suggests the involvement of an open-shell (amido-phosphine-amido)iron alkylidene intermediate that is higher in energy than its (amido-ylide-amido)iron(II) precursor and an "outer-sphere" radical-type mechanism for the cyclopropanation step. The negative charge distribution on the alkylidene carbon atoms of the open-shell states (S = 2 and 1) explains the high activity of the cyclopropanation reaction toward electron-deficient alkenes. The study demonstrates the unique activity of open-shell iron alkylidene species beyond its closed-shell analogues, thus pointing out their potential synthetic usage in catalysis.
“…These activation parameters can be compared with those of cyclopropanation reactions by transition-metal alkylidene species. For examples, the reaction of the tungsten complex W(CO) 5 (CHC 6 H 4 -OMe- p ) with vinyl acetate has Δ H ⧧ = 10 kcal/mol and Δ S ⧧ = −34 cal/mol/K, that of Warren’s copper alkylidene [(nacnac)Cu(CPh 2 )] with styrene has Δ H ⧧ = 10 kcal/mol and Δ S ⧧ = −32 cal/mol/K, and the copper(I) triflate-catalyzed cyclopropanation of 1-hexene by ethyl diazoacetate has Δ H ⧧ = 19 kcal/mol and Δ S ⧧ = −9 cal/mol/K …”
Section: Resultsmentioning
confidence: 99%
“…These activation parameters can be compared with those of cyclopropanation reactions by transition-metal alkylidene species. For examples, the reaction of the tungsten complex W(CO) 5 (CHC 6 H 4 -OMe-p) with vinyl acetate has ΔH ⧧ = 10 kcal/mol and ΔS ⧧ = −34 cal/mol/K, 27 that of Warren's copper alkylidene [(nacnac)Cu(CPh 2 )] with styrene has ΔH ⧧ = 10 kcal/mol and ΔS ⧧ = −32 cal/mol/K, 28 and the copper(I) triflate-catalyzed cyclopropanation of 1-hexene by ethyl diazoacetate has ΔH ⧧ = 19 kcal/mol and ΔS ⧧ = −9 cal/ mol/K. 29 In order to probe the nature of the reactive species in the cyclopropanation reaction, the relative rates of the cyclopropanation of a series of para-substituted styrenes by the ylide complex 2 were measured by competition experiments (Scheme 3).…”
Transition-metal alkylidenes are important reactive organometallic intermediates, and our current knowledge on them has been mainly restricted to those with closed-shell electronic configurations. In this study, we present an exploration on open-shell iron alkylidenes with a weak-field tripodal amido-phosphine-amido ligand. We found that a high-spin (amido-phosphine-amido)iron(II) complex can react with (p-tolyl)CN to afford a high-spin (amido-ylide-amido)iron(II) complex, 2, which could transfer its alkylidene moiety to a variety of alkenes, either the electron-rich or electron-deficient ones, to form cyclopropane derivatives. The reaction of 2 with cis-β-deuterio-styrene gave deuterated cyclopropane derivatives with partial loss of the stereochemical integrity with respect to the cis-styrene. Kinetic study on the cyclopropanation reaction of 2 with 4-fluoro-styrene disclosed the activation parameters of ΔH = 23 ± 1 kcal/mol and ΔS = -20 ± 3 cal/mol/K, which are comparable to those of the cyclopropanation reactions involving transition-metal alkylidenes. However, the cyclopropanation of para-substituted styrenes by 2 shows a nonlinear Hammett plot of log(k/k) vs σ. By introduction of a radical parameter, a linear plot of log(k/k) vs 0.59σ + 0.55σ was obtained, which suggests the "nucleophilic" radical nature of the transition state of the cyclopropanation step. In corroboration with the experimental observations, density functional theory calculation on the reaction of 2 with styrene suggests the involvement of an open-shell (amido-phosphine-amido)iron alkylidene intermediate that is higher in energy than its (amido-ylide-amido)iron(II) precursor and an "outer-sphere" radical-type mechanism for the cyclopropanation step. The negative charge distribution on the alkylidene carbon atoms of the open-shell states (S = 2 and 1) explains the high activity of the cyclopropanation reaction toward electron-deficient alkenes. The study demonstrates the unique activity of open-shell iron alkylidene species beyond its closed-shell analogues, thus pointing out their potential synthetic usage in catalysis.
“…In the same way, cyclopentene and norbornene reacted with 17 to give the corresponding known , phenylcyclopropanes 26a and 26b as mixtures of isomers in, respectively, 55 and 27% yield (see the Experimental Section). Olefins such as cyclopentadiene and pentamethylcyclopentadiene, the reactivity of which has been found to be extremely high toward complex 3 even at −20 °C, led, at room temperature, to the same phenylcyclopropanes 27a,b in 30 and 40% yield.…”
Section: Resultsmentioning
confidence: 99%
“…A slightly modified method gave, later on, access to a series of diarylcarbene complexes of chromium and tungsten . Attempts to synthesize and isolate other simple metal(0) alkylidene complexes of this type either failed due to fast β-elimination reactions or led to unexpected dinuclear complexes (Scheme ). − However, a benzylidene complex, 3 , of W(0) (and later of Cr(0)) could be prepared in a two-step process by reacting successively (CO) 5 WC(OEt)Ph ( 1 ) with a complex hydride, KHB(OiPr) 3 , and an acid. − Due to its unstability, complex 3 could only be handled at low temperature (−80 to −20 °C). Its reaction with phosphines led however to stable phosphorus ylides 4a , b .…”
1,2- and 1,4-dihydropyridines react with alkoxycarbene complexes
of chromium and tungsten to give,
upon an unprecedented hydride transfer, alcohol elimination, and
pyridine fixation on the carbene carbon, a new
class of air-stable pyridinium ylide complexes. These
pyridine-protected alkylidene complexes of chromium(0)
and
tungsten(0) were fully characterized by X-ray crystallography.
In the case of
(CO)5WC(CH3)(OEt) (5a),
besides
the pyridinium ylide complex
(CO)5W-−C(H)(CH3)(pyridine)+
(7a), the dihydropyridinium complex
(CO)5W-−C(H)(CH3)(2,5-dihydropyridine)+
(8a) was also isolated. The intermediate tungstate
(CO)5W-−C(H)(CH3)(OEt)(CH3NC5H5)+ could be easily obtained
and characterized by using, as reducing agent,
N-methyldihydropyridine. Whereas
phenyl-substituted pyridinium complexes easily transferred the
benzylidene moiety to alkenes, alkyl-substituted
complexes appeared more reluctant to such a transfer: satisfactory
results were observed in the case of nucleophilic
olefins such as enol ethers. However, straightforward transfer of
the tungsten(0) alkylidene group took place, even
at room temperature, in the case of alkoxycarbene complexes tethered to
alkenes, giving access, upon intramolecular
cyclopropanation reactions, to polycyclic systems.
“…Naturally, such interference does not apply to benzylidene transfer reactions which can efficiently and stereoselectively be effected with (CO) 5 WdCH-C 6 H 4 R [17][18][19] or Cp(CO)(L)MdCH(C 6 H 4 R) + (M ) Fe, Ru; L ) CO, PPh 3 ; R ) p-H, p-F, p-Me, p-OMe). [20][21][22] Benzylidene transfer from (CO) 5 WdCHPh to the thiocarbonyl group in thiobenzophenones results in thiirane complexes.…”
Section: A [2+1] Metal-assisted Cycloaddition Reactionsmentioning
Table 3. Metal-Assisted [2+1] Cycloaddition: Ethylidene Transfer to Olefins from 3b Using Complexes Cp(CO)2FeCH(OCH3)CH3 (1) 9 and Cp(CO)2FeCH(SPh)CH3 (2) 10 as Precursors
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