Toggle Main Menu Toggle Search

Open Access padlockePrints

Regioselective addition of tris(dialkylamino) phosphines to [Fe2(CO)6(μ-PPh2){μ-η 1:η2-(H)C=C=CH2}]: Novel P-C coupling reactions and unusual hydrocarbyl rearrangements

Lookup NU author(s): Dr Simon Doherty, Mark Waugh, Thomas Scanlan, Dr Mark Elsegood, Emeritus Professor Bill Clegg

Downloads

Full text for this publication is not currently held within this repository. Alternative links are provided below where available.


Abstract

Nucleophilic addition of tris(dialkylamino) phosphines, P(NR2)3 (R = Me or Et, nPr), to [Fe2(CO)6(μ-PPh2){μ-η 1:η2-(H)Cα=Cβ=C γH2}] (1) affords the dimetallacyclopentene derivatives [Fe2(CO)6(μ-PPh2)(μ-η 1:η1-HC=C{P(NR2)3}CH 2)] (R = Me, 2a; R = Et, 2b; R = nPr, 2c) or a mixture of the vinylidene- and dimetallacyclobutene-bridged complexes [Fe2(CO)6(μ-PPh2)-(μ-η 1-C=C(CH3){P(NMe2)3})] (3a) and [Fe2(CO)6(μ-PPh2)(μ-η 1:η1-(CH3)C=C{P(NMe2) 3})] (4a), respectively, depending upon the reaction conditions. For instance, addition of P(NR2)3 to an ether solution of [Fe2(CO)6(μ-PPh2){μ-η 1:η2-(H)Cα=Cβ=C γH2}] gave the dimetallacyclopentenes 2a-c, whereas pretreatment of a solution of the allenyl starting material with HBF4 prior to the addition of P(NR2)3 gave the vinylidene- and dimetallacyclobutene-bridged products, which co-crystallized as a 67:33 mixture, as determined by single-crystal X-ray crystallography and 1H NMR spectroscopy. We have subsequently shown that the σ-η-allenyl complex [Fe2(CO)6(μ-PPh2){μ-η 1:η2-(H)Cα=Cβ=C γH2}] undergoes a clean and quantitative acid-promoted rearrangement to the σ-η-acetylide-bridged isomer [Fe2(CO)6(μ-PPh2){μ-η1: η2-C≡CH3}] (5). 1H NMR and deuterium labeling studies suggest that this isomerization occurs via initial protonation at Cγ to afford a kinetic intermediate which rapidly rearranges to its thermodynamically more stable propyne-bridged counterpart followed by deprotonation. Clearly, the vinylidene and dimetallacyclobutene products isolated from the reaction between 1 and tris(dialkylamino) phosphine in the presence of acid arise from nucleophilic addition to the α- and β-carbon atoms of the acetylide bridge in [Fe2(CO)6(μ-PPh2){μ-η 1:η2-C≡CCH3}], and not from nucleophilic addition followed by hydrogen migration. In refluxing toluene, the dimetallacyclopentenes [Fe2(CO)6(μ-PPh2)(μ-η 1:η1-HC=C{P(NR2)3}CH 2)J slowly decarbonylate to give [Fe2(CO)5(μ-PPh2)(μ-η 1:η3-C(H)C{P(NR2)3}CH 2)] (R = Me, 6a; R = Et, 6b; R = nPr, 6c) bridged by a σ-η3-coordinated vinyl carbene. In the case of R = Et and nPr a competing isomerization also affords the highly unusual zwitterionic α-phosphoniumalkoxide-functionalized σ-σ-alkenyl complex [Fe2(CO)5(μ-PPh2){μ-η 1:η2-{P(NR2)3}C(O)CHC= CH2}] (R = Et, 7b; R = nPr, 7c), via a P(NR2)3-carbonyl-allenyl coupling sequence. In contrast, isomerization of dimetallacyclopentene [Fe2(CO)6(μ-PPh2)(μ-η 1:η1-HC=C{PPh3}-CH2)] (8) to its σ-η-alkeny] counterpart [Fe2(CO)5(μ-PPh2){μ-η 1:η2-PPh3C(O)CHC=CH2}] (9) is essentially complete within 1 h at room temperature with no evidence for the formation of the corresponding vinyl carbene. Thermolysis of a toluene solution of 8 in the presence of excess P(NEt2)3 results in exclusive formation of 7b, whereas at room temperature phosphine substitution affords 2b, via PPh3-P(NEt2)3 exchange. The isomerization of 8 to 9 and 2b,c to 7b,c appears to involve a dissociative equilibrium between the kinetic regioisomeric intermediate dimetallacyclopentene and 1, nucleophilic attack of phosphine at a carbonyl ligand of 1 to give a zwitterionic acylate intermediate, followed by acyl-allenyl coupling to afford the thermodynamically favored zwitterionic σ-η-alkenyl derivative. Qualitatively, the rate of isomerization increases as the steric bulk of the phosphine increases, in the order P(NMe2)3 < P(NEt2)3 ≈ P(NnPr2)3 < PPh3. The single-crystal X-ray structures of 2a, 3a, 4a, 6b, 7b, 8, and 9 are reported.


Publication metadata

Author(s): Doherty S, Waugh M, Scanlan TH, Elsegood MRJ, Clegg W

Publication type: Article

Publication status: Published

Journal: Organometallics

Year: 1999

Volume: 18

Issue: 4

Pages: 679-696

Print publication date: 26/01/1999

ISSN (print): 0276-7333

ISSN (electronic): 1520-6041

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/om980757n

DOI: 10.1021/om980757n


Altmetrics

Altmetrics provided by Altmetric


Actions

Find at Newcastle University icon    Link to this publication


Share