Regioselective Addition of Tris(dialkylamino) Phosphines to [Fe₂(CO)₆(μ-PPh₂){μ-η¹:η²-(H)CCCH₂}]:  Novel P−C Coupling Reactions and Unusual Hydrocarbyl Rearrangements

Abstract

Nucleophilic addition of tris(dialkylamino) phosphines, P(NR2)3 (R = Me or Et, nPr), to [Fe2(CO)6(μ-PPh2){μ-η1:η2-(H)CαCβCγH2}] (1) affords the dimetallacyclopentene derivatives [Fe2(CO)6(μ-PPh2)(μ-η1:η1-HCC{P(NR2)3}CH2)] (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-η1-CC(CH3){P(NMe2)3})] (3a) and [Fe2(CO)6(μ-PPh2)(μ-η1: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-η1:η2-(H)CαCβCγH2}] 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-η1:η2-(H)CαCβCγH2}] 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-η1:η2-C⋮CCH3}], and not from nucleophilic addition followed by hydrogen migration. In refluxing toluene, the dimetallacyclopentenes [Fe2(CO)6(μ-PPh2)(μ-η1:η1-HCC{P(NR2)3}CH2)] slowly decarbonylate to give [Fe2(CO)5(μ-PPh2)(μ-η1:η3-C(H)C{P(NR2)3}CH2)] (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-phosphonium-alkoxide-functionalized σ−σ-alkenyl complex [Fe2(CO)5(μ-PPh2){μ-η1: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-η1:η1-HCC{PPh3}CH2)] (8) to its σ−η-alkenyl counterpart [Fe2(CO)5(μ-PPh2){μ-η1: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 acylat...

Synthesis and reaction chemistry of [(μ-SeR)(μ-σ,π-CCPh)]Fe2(CO)6

“…A very distinct signal at 5.0 ppm is observed, which is compared to free P(O i C 3 H 7 ) 3 shifted to higher field. A similar trend was observed for {(l-Ph 2 P)[l-g 1 :g 1 -(CH 3 )C@C-(P(NMe 2 ) 3 )]}Fe 2 (CO) 6 [7].…”

“…The short P(1 0 )-C(2 0 ) bond length of 1.807(18) Å , with the asymmetry in the vinylidene-iron connectivity suggests that the electron density is partially delocalised through the bonds between P(1 0 ), C(2 0 ), C(1 0 ) and Fe (1). The dihedral angle between the plane formed by P(1 0 ), C(2 0 ), C(1 0 ) and Fe (1), Fe(2), C(1 0 ) is 4.0°and the angle subtended by the metal atom at the l-C@CPh(P(O i C 3 H 7 ) 3 ) bridge of 85.7(8)°is characteristic for a binuclear vinylidene species [7,14,15]. The molecular structure shown in Fig.…”

“…The reaction chemistry of related thio-and phosphido-bridged acetylide diiron hexacarbonyl complexes was explored [1,[4][5][6][7]. For example, the reactions of structural type A molecules with diverse transition metal carbonyls, such as Fe(CO) 5 , Fe 2 (CO) 9 and Co 2 (CO) 8 , respectively, were used to good advantage for the preparation of higher nuclearity, homoand heterometallic acetylide clusters [1,3].…”

“…For example, the reactions of structural type A molecules with diverse transition metal carbonyls, such as Fe(CO) 5 , Fe 2 (CO) 9 and Co 2 (CO) 8 , respectively, were used to good advantage for the preparation of higher nuclearity, homoand heterometallic acetylide clusters [1,3]. These multimetallic species possess novel properties [1][2][3][4][5][6][7]. Included in these studies were also the reaction behaviour of molecules of type [(l-ER n )(l-r,p-C"CR 0 )]Fe 2 (CO) 6 (E = P, n = 2; E = S, n = 1) towards diverse Lewis-bases L (L = amines, phosphines and phosphites) which revealed a varied and exciting organometallic chemistry with particularly relevant applications to organic synthesis, whereby numerous examples of unusual reactivity patterns have been reported [7].…”

“…These multimetallic species possess novel properties [1][2][3][4][5][6][7]. Included in these studies were also the reaction behaviour of molecules of type [(l-ER n )(l-r,p-C"CR 0 )]Fe 2 (CO) 6 (E = P, n = 2; E = S, n = 1) towards diverse Lewis-bases L (L = amines, phosphines and phosphites) which revealed a varied and exciting organometallic chemistry with particularly relevant applications to organic synthesis, whereby numerous examples of unusual reactivity patterns have been reported [7].…”