Reductive Metallation of Dendralenes and Myrcene Using Dimagnesium(I) Compounds: A Facile Route to Unsaturated Organomagnesium Compounds.

The two-electron reduction of 2,3-dimethylbuta-1,3-diene (DMB) with ß-diketiminate and guanidinate substituted dimagnesium(I) compounds has given complexes in which two bidentate amido-magnesium fragments are bridged through the π-system of the DMB dianion, viz. [(LMg)2 (µ-DMB)] (L=Xyl Nacnac, [HC(MeCNXyl)2 ]- , Xyl=2,6-xylyl; or Priso=[(DipN)2 CNPri 2 ]- , Dip=2,6-diisopropylphenyl). Similar double reductions of [4]dendralene (4dend) have afforded the complexes, [(LMg)2 (µ-4dend)] (L=Ar Nacnac, Ar=Xyl or mesityl (Mes); or Priso) in which the 4dend dianion is π-coordinated to the bidentate amido-magnesium fragments. Treatment of several such complexes with THF leads to Z- to E-isomerization of the dendralene fragment, and formation of purely σ-bonded Mg-C interactions in the THF coordinated products [{(Ar Nacnac)(THF)Mg}2 (µ-4dend)] (Ar=Xyl, Mes or Dip). Reaction of myrcene (Myr) with [{(Xyl Nacnac)Mg}2 ] proceeds via reductive coupling of Myr to give a previously unknown acyclic, branched C20 tetra-olefin dianion complex [{(Xyl Nacnac)(THF)Mg}2 (µ-Myr)2 ]. Preliminary reactions of [(LMg)2 (µ-DMB)] with H2 and/or CO yielded a series of products, including novel magnesium hydride compounds, products derived from couplings of CO with the reduced DMB fragment (viz. magnesium dimethylcyclohexadienediolates), and one magnesium cyclopropanetriolate complex from the magnesium(I) induced coupling of DMB with H2 and CO.

Molybdenum carbonyl assisted reductive tetramerization of CO by activated magnesium(i) compounds: squarate dianion vs. metallo-ketene formation.

Reactions of a dimagnesium(i) compound, [{(DipNacnac)Mg}2] (DipNacnac = [HC(MeCNDip)2]-, Dip = 2,6-diisopropylphenyl), pre-activated by coordination with simple Lewis bases (4-dimethylaminopyridine, DMAP; or TMC, :C(MeNCMe)2), with 1 atmosphere of CO in the presence of one equivalent of Mo(CO)6 at room temperature, led to the reductive tetramerisation of the diatomic molecule. When the reactions were carried out at room temperature, there is an apparent competition between the formation of magnesium squarate, [{(DipNacnac)Mg}{cyclo-(κ4-C4O4)}{µ-Mg(DipNacnac)}]2, and magnesium metallo-ketene products, [{(DipNacnac)Mg}[µ-O[double bond, length as m-dash]CC{Mo(CO)5}C(O)CO2]{Mg(D)(DipNacnac)}], which are not inter-convertible. Repeating the reactions at 80 °C led to the selective formation of the magnesium squarate, implying that this is the thermodynamic product. In an analogous reaction, in which THF is the Lewis base, only the metallo-ketene complex, [{(DipNacnac)Mg}[µ-O[double bond, length as m-dash]CC{Mo(CO)5}C(O)CO2]{Mg(THF)(DipNacnac)}] is formed at room temperature, while a complex product mixture is obtained at elevated temperature. In contrast, treatment of a 1 : 1 mixture of the guanidinato magnesium(i) complex, [(Priso)Mg-Mg(Priso)] (Priso = [Pri2NC(NDip)2]-), and Mo(CO)6, with CO gas in a benzene/THF solution, gave a low yield of the squarate complex, [{(Priso)(THF)Mg}{cyclo-(κ4-C4O4)}{µ-Mg(THF)(Priso)}]2, at 80 °C. Computational analyses of the electronic structure of squarate and metallo-ketene product types corroborate the bonding proposed from experimental data, for the C4O4 fragments of these systems.

C-H Activation of Inert Arenes using a Photochemically Activated Guanidinato-Magnesium(I) Compound.

UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg}2 ] 3 (Priso=[(DipN)2 CNPri 2 ]- , Dip=2,6-diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic C-H bond of the solvent in all cases, and formation of aryl/hydride bridged magnesium(II) products, [{(Priso)Mg}2 (µ-H)(µ-Ar)] 4-9. In contrast to similar reactions reported for ß-diketiminate coordinated counterparts of 3, these C-H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p-NC5 H4 But (pyBut ), gave [(Priso)Mg(pyBut H)(pyBut )2 ] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(pyBut ⋅)(pyBut )2 ] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene C-H activations. One of these involves photochemical cleavage of the Mg-Mg bond of 3, generating magnesium(I) doublet radicals, (Priso)Mg⋅. These then doubly reduce the arene substrate to give "Birch-like" products, which subsequently rearrange via C-H activation of the arene. Circumstantial evidence for the photochemical generation of transient magnesium radical species includes the fact that irradiation of a cyclohexane solution of 3 leads to an intramolecular aliphatic C-H activation process and formation of an alkyl-bridged magnesium(II) species, [{Mg(µ-Priso-H )}2 ] 12. Furthermore, irradiation of a 1 : 1 mixture of 3 and the ß-diketiminato dimagnesium(I) compound, [{(Dip Nacnac)Mg}2 ] (Dip Nacnac=[HC(MeCNDip)2 ]- ), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)Mg-Mg(Dip Nacnac)] 13. Finally, the EPR spectrum (77 K) of a glassed solution of UV irradiated 3 is dominated by a broad featureless signal, indicating the presence of a doublet radical species.