Regioselective addition of tris(dialkylamino) phosphines to [Fe-2(CO)(6)(mu-PPh2){mu-eta(1):eta(2)-(H)C=C=CH2}]: Novel P-C coupling reactions and unusual hydrocarbyl rearrangements

Nucleophilic addition of tris(dialkylamino) phosphines, P(NR2)(3) (R = Me o r Et, Pr-n), to [Fe-2(CO)(6)(mu-PPh2){mu-eta(1):eta(2)-(H)C-alpha=C-beta=C gamma H-2}] (1) affords the dimetallacyclopentene derivatives [Fe-2(CO)(6)( mu-PPh2)(mu-eta(1):eta(1)-HC=C{P(NR2)(3)}CH2)] (R = Me, 2a; R = Et, 2b; R = Pr-n, 2c) or a mixture of the vinylidene- and dimetallacyclobutene-bridged complexes [Fe-2(CO)(6)(mu-PPh2)-(mu-eta(1)-C=C(CH3){P(NMe2)(3)})] (3a) and [Fe-2(CO)(6)(mu-PPh2)(mu-eta(1):eta(1)-(CH3)C=C{P(NMe2)(3)})] (4a), respec tively, depending upon the reaction conditions. For instance, addition of P (NR2)(3) to an ether solution of [Fe-2(CO)(6)(mu-PPh2){mu-eta(1):eta(2)-(H) C-alpha=C-beta=CgammaH2}] gave the dimetallacyclopentenes 2a-c, whereas pre treatment of a solution of the allenyl starting material with HBF4 prior to the addition of P(NR2)(3) gave the vinylidene- and dimetallacyclobutene-br idged products, which co-crystallized as a 67:33 mixture, as determined by single-crystal X-ray crystallography and H-1 NMR spectroscopy. We have subs equently shown that the sigma-eta-allenyl complex [Fe-2(CO)(6)(mu-PPh2){mu- eta(1):eta(2)-(H)C-alpha=C-beta=CgammaH2}] undergoes a clean and quantitati ve acid-promoted rearrangement to the sigma-eta-acetylide-bridged isomer [F e-2(CO)(6)(mu-PPh2){mu-eta(1): eta(2)-C=CH3}] (5). H-1 NMR and deuterium la beling studies suggest that this isomerization occurs via initial protonati on at C-gamma to afford a kinetic intermediate which rapidly rearranges to its thermodynamically more stable propyne-bridged counterpart followed by d eprotonation. Clearly, the vinylidene and dimetallacyclobutene products iso lated from the reaction between 1 and tris(dialkylamino) phosphine in the p resence of acid arise from nucleophilic addition to the alpha- and beta-car bon atoms of the acetylide bridge in [Fe-2(CO)(6)(mu-PPh2){mu-eta(1):eta(2) -C=CCH3}], and not from nucleophilic addition followed by hydrogen migratio n. In refluxing toluene, the dimetallacyclopentenes [Fe-2(CO)(6)(mu-PPh2)(m u-eta(1):eta(1)-HC=C{P(NR2)(3)}CH2)] slowly decarbonylate to give [Fe-2(CO) (5)(mu-PPh2)(mu-eta(1):eta(3)-C(H)C{P(NR2)(3)}CH2)] (R = Me, 6a; R = Et, 6b ; R = Pr-n, 6c) bridged by a sigma-eta(3)-coordinated vinyl carbene. In the case of R = Et and Pr-n a competing isomerization also affords the highly unusual zwitterionic alpha-phosphonium-alkoxide-functionalized sigma-sigma- alkenyl complex [Fe-2(CO)(5)(mu-PPh2){mu-eta(1):eta(2)-{P(NR2)(3)}C(O)CHC=C H2}] (R = Et, 7b; R = Pr-n, 7c), via a P(NR2)(3)-carbonyl-allenyl coupling sequence. In contrast, isomerization of dimetallacyclopentene [Fe-2(CO)(6)(mu-PPh2)(m u-eta(1):eta(1)-HC=C{PPh3} CH2)] (8) to its sigma-eta-alkenyl counterpart [ Fe-2(CO)(5)(mu-PPh2){mu-eta(1):eta(2)-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(N Et2)(3) exchange. The isomerization of 8 to 9 and 2b,c to 7b,c appears to i nvolve a dissociative equilibrium between the kinetic regioisomeric interme diate dimetallacyclopentene and 1, nucleophilic attack of phosphine at a ca rbonyl ligand of 1 to give a zwitterionic acylate intermediate, followed by acyl-allenyl coupling to afford the thermodynamically favored zwitterionic sigma-eta-alkenyl derivative. Qualitatively, the rate of isomerization inc reases as the steric bulk of the phosphine increases, in the order P(NMe2)( 3) < P(NEt2)(3) approximate to P((NPr2)-Pr-n)(3) < PPh3. The single-crystal X-ray structures of 2a, 3a, 4a, 6b, 7b, 8, and 9 are reported.