Structural and Synthetic Insights on Oxidative Homocouplings of Alkynes Mediated by Alkali-Metal Manganates.

Exploiting bimetallic cooperation alkali-metal manganate (II) complexes can efficiently promote oxidative homocoupling of terminal alkynes furnishing an array of conjugated 1,3-diynes. The influence of the alkali-metal on these C-C bond forming processes has been studied by preparing and structurally characterizing the alkali-metal tetra(alkyl) manganates [(TMEDA)2 Na2 Mn(CH2 SiMe3 )4 ] and [(PMDETA)2 K2 Mn(CH2 SiMe3 )4 ]. Reactivity studies using phenylacetylene as a model substrate have revealed that for the homocoupling to take place initial metalation of the alkyne is required. In this regard, the lack of basicity of neutral Mn(CH2 SiMe3 )2 precludes the formation of the diyne. Contrastingly, the tetra(alkyl) alkali-metal manganates behave as polybasic reagents, being able to easily deprotonate phenylacetylene yielding [{(THF)4 Na2 Mn(C≡CPh)4 }∞ ] and [(THF)4 Li2 Mn(C≡CPh)4 ]. Controlled exposure of [{(THF)4 Na2 Mn(C≡CPh)4 }∞ ] and [(THF)4 Li2 Mn(C≡CPh)4 ] to dry air confirmed their intermediary in formation of 1,4-diphenyl-1,3-butadiyne in excellent yields. While the Na/Mn(II) partnership proved to be the most efficient in stoichiometric transformations, under catalytic regimes, the combination of MC≡CAr (M= Li, Na) and MnCl2 (6 mol %) only works for lithium, most likely due to the degradation of alkynylsodiums under the aerobic reaction conditions.

Exploiting Coordination Effects for the Regioselective Zincation of Diazines Using TMPZnX⋠LiX (X=Cl, Br).

A new method for regioselective zincations of challenging N-heterocyclic substrates such as pyrimidines and pyridazine was reported using bimetallic bases TMPZnX⋅LiX (TMP=2,2,6,6-tetramethylpiperidyl; X=Cl, Br). Reactions occurred under mild conditions (25-70 °C, using 1.75 equivalents of base without additives), furnishing 2-zincated pyrimidines and 3-zincated pyridazine, which were then trapped with a variety of electrophiles. Contrasting with other s-block metalating systems, which lack selectivity in their reactions even when operating at low temperatures, these mixed Li/Zn bases enabled unprecedented regioselectivities that cannot be replicated by either LiTMP nor Zn(TMP)2 on their own. Spectroscopic and structural interrogations of organometallic intermediates involved in these reactions have shed light on the complex constitution of reaction mixtures and the origins of their special reactivities.

Metallation of sensitive fluoroarenes using a potassium TMP-zincate supported by a silyl(bis)amido ligand.

Combining a bulky bis(amide) and a reactive one-coordinate TMP (2,2,6,6-tetramethylpiperidide) ligand, a new mixed K/Zn heteroleptic base has been developed for regioselective zincation of fluoroarenes. This special ligand set allows for trapping and structural authentication of the first intermediates of direct Zn-H exchange of fluoroarenes obtained via deprotonative metallation, providing mechanistic insights of the processes involved.

Tandem Mn-I Exchange and Homocoupling Processes Mediated by a Synergistically Operative Lithium Manganate.

Pairing lithium and manganese(II) to form lithium manganate [Li2 Mn(CH2 SiMe3 )4 ] enables the efficient direct Mn-I exchange of aryliodides, affording transient (aryl)lithium manganate intermediates which in turn undergo spontaneous C-C homocoupling at room temperature to furnish symmetrical (bis)aryls in good yields under mild reaction conditions. The combination of EPR with X-ray crystallographic studies has revealed the mixed Li/Mn constitution of the organometallic intermediates involved in these reactions, including the homocoupling step which had previously been thought to occur via a single-metal Mn aryl species. These studies show Li and Mn working together in a synergistic manner to facilitate both the Mn-I exchange and the C-C bond-forming steps. Both steps are carefully synchronized, with the concomitant generation of the alkyliodide ICH2 SiMe3 during the Mn-I exchange being essential to the aryl homocoupling process, wherein it serves as an in situ generated oxidant.