Transformation of Dimetallacyclopentanes into β-Substituted Enamine-, Alkylidene-, and α,β-Unsaturated Acyl-Bridged Diiron Complexes

Abstract

Nucleophilic addition of primary amines RNH2 to Cβ of the diiron allenyl complex [Fe2(CO)6(μ-PPh2){μ−η1:η2-(H)CCCH2}] (1) affords the zwitterionic dimetallacyclopentane derivatives [Fe2(CO)6(μ-PPh2){μ−η1:η1-H2C−C(NHR)−CH2}] (R = Pri (2a), CH2Ph (2b), C6H11 (2c), Prn (2d)). In refluxing toluene, 2a − d rearrange to give the nitrogen-coordinated organodiiron α,β-unsaturated amines [Fe2(CO)5(μ-PPh2){μ−η1(C):η2(C):η1(N)-(H)CCCH3(NHR)}] (R = Pri (3a), CH2Ph (3b), C6H11 (3c), Prn (3d)) via loss of carbon monoxide, a 1,3-hydrogen migration, and coordination of the β-amino group. The bridging hydrocarbon in 3a − d adopts a highly unusual exo conformation with respect to the phosphido ligand. Compounds 3a − d are the first diiron β-substituted enamine complexes to be isolated and structurally characterized. Such compounds have previously been proposed as short-lived intermediates which readily isomerize to their alkylidene-bridged counterparts. Complexes 3a and 3c react with trimethyl phosphite to afford the stable alkylidene valence isomers [Fe2(CO)5{P(OMe)3}(μ-PPh2){μ-CHC(CH3)(NHR)}] (R = Pri (4a), C6H11 (4c)). In contrast, in refluxing toluene, 2a − b react with trimethyl phosphite to give the α,β-unsaturated bridging acyl complexes [Fe2(CO)5{P(OMe)3}(μ-PPh2){μ-OC−CHCMe(NHR)}] (R = Pri (5a), CH2Ph (5b)), which probably result from the formal insertion of carbon monoxide into an alkylidene intermediate. Single-crystal X-ray analyses of 5a and 5b showed the position of substitution to be the iron atom σ-bonded to the acyl oxygen, trans to the phosphido bridge in 5a and trans to the iron−iron bond in 5b. Variable-temperature 31P{1H} and 31P-31P 2D exchange NMR studies have been used to examine an exchange process that equilibrates two isomers of 4a and 5b, which differ in the position of the trimethyl phosphite. At low temperature, 4a exists as a 4:1 mixture of these two substitutional isomers, while at room temperature they are in fast exchange, on the NMR time scale. The free energy of activation associated with this exchange (ΔG ⧧ 273 = 9.5 kcal mol-1, 4a; ΔG ⧧ 273 = 10.4 kcal mol-1, 5b) is fully consistent with a restricted trigonal twist at the phosphite-substituted iron center. This exchange process also appears to be common to the alkylidene derivative 5b, which has similar 31P{1H} line broadening characteristics and free energy of activation. Full structural details of compounds 2a, 3a, 4c, 5a, and 5b are reported.

CC Bond Formation in Diiron Complexes

“…Conversely, bridging allenyl can undergo nucleophilic addition to both central and terminal allenyl carbons. Doherty also found interesting nucleophile–carbonyl–allenyl coupling involving alkyllithium reagents,94a amines94f, 95 and alcohols 96. One example illustrating a nucleophilic attack at CO with migration to the allenyl α‐C is shown in Scheme .…”

Zwitterionic Organometallates

“…[77] The negative charge can also reside on (and polarize) substituted Fe 2 (CO) 5 P cores as in the C β -phosphonio-phosphane complex 48 [78] (Scheme 21) or in related C α -phosphonio-phosphane complexes. [79] Related µ-σ,π-allenyl precursors generate similar phosphonium complexes, [80] but can also lead to a new type of mesoionic µ-η 2 -propylene complex 49, [81] which isomerizes to the non-dipolar complex 50 (Scheme 22).…”

Regioselective Addition of tert-BuN⋮C to the α Carbon Atom of the Allenyl Ligand in [Fe2(CO)6(μ-PPh2){μ-η1:η2α,β-(H)CαCβCγH2}]: Formation of [Fe2(CO)6(μ-PPh2){μ-η1:η1-(tert-BuN⋮C)CCCH3}] and [Fe2(CO)6(μ-PPh2)(μ-η1:η2-{tert-BuNHC(O)CHDoherty1, Hogarth2, Waugh3 et al. 1999OrganometallicsSelf Cite20280View full textAdd to dashboardCite

“…While examples of nucleophilic addition to the central carbon atom of mono- and binuclear σ−η-coordinated allenyl ligands are commonplace 2,5 reports of addition to the terminal carbon atoms, C α and C γ , remain relatively rare . In comparison to the large body of literature on the reactivity of protic nitrogen, , oxygen, , and phosphorus 9 nucleophiles toward either σ- or σ−η-bonded allenyl complexes, their reaction chemistry with soft neutral carbon nucleophiles such as isonitriles is less well developed, somewhat surprising considering that the unsaturated organic product of allenyl−isonitrile coupling could offer immense potential for further functionalization. In addition to direct nucleophilic addition to the coordinated hydrocarbon, isocyanides can also react via ligand addition/substitution or migratory insertion pathways.…”