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Subtle differences in how pyridine is introduced can direct mechanochemical reactivity by enabling bond activation and favoring one reaction pathway over another. 

Metal complexes with t-Bu-substituted allyl ligands are relatively rare, especially compared to their conceptually similar trimethylsilyl-substituted analogs. The scarcity partially stems from the few general synthetic entry points for the t-Bu versions. This situation was studied through a modified synthesis for the allyl ligand itself and by forming several mono(allyl)nickel derivatives. After 2,2,6,6-tetramethyl-4-hepten-3-one was converted to the related 5-bromo-2,2,6,6-tetramethylhept-3-ene (A2tBr), a mixture of Ni(COD)2 and A2tBr in the presence of a neutral donor ligand such as MeCN was found to produce the dark red dimeric π-allyl complex [{A2tNiBr}2]. Both NMR and X-ray crystallographic data confirmed that the t-Bu substituents are in a syn, syn-conformation, like that in the previously described [{A′NiBr}2] (A′ = 1,3-(TMS)2C3H3) complex. [{A2tNiBr}2] will form adducts with neutral donors such as PPh3 and IMes (IMes = 1,3-dimesitylimidazol-2-ylidene), but the resulting [A2tNi(PPh3)Br] complex is not as stable as its trimethylsilyl analog. The [A2tNi(IMes)Br] complex crystallizes from hexanes as a monomer, with an η3-coordinated [A2t] ligand, and in contrast to the starting arrangement in [{A2tNiBr}2], the t-Bu groups on the A2t ligand are in a syn, anti-relationship. This structure is paralleled in the trimethylsilyl analog [A′Ni(IMes)Br]. DFT calculations were used to compare the structures of t-Bu- and related trimethylsilyl-substituted complexes. 

Layered solids (AsI₃, SbCl₃) can serve as templates for diastereomer formation, and mechanochemical variables can shift their ratios. 

Solution-based reactions are a staple of synthetic chemistry—but what happens mechanochemically, when thereisno solvent? 

Owing to the strength of the C–F bond, the ‘direct’ preparation of Grignard reagents, i.e., the interaction of elemental magnesium with an organic halide, typically in an ethereal solvent, fails for bulk magnesium and organofluorine compounds. Previously described mechanochemical methods for preparing Grignard reagents have involved ball milling powdered magnesium with organochlorines or bromines. Activation of the C–F bond through a similar route is also possible, however. For example, milling 1- and 2-fluoronaphthalene with an excess of magnesium metal for 2 h, followed by treatment with FeCl3 and additional milling, produces the corresponding binaphthalenes, albeit in low yields (ca. 20%). The yields are independent of the particular isomer involved and are also comparable to the yields from corresponding the bromonaphthalenes. These results may reflect similar charges that reside on the α-carbon in the naphthalenes, as indicated by density functional theory calculations.