Chemistry of magnesium alkyls supported by 1,5,9-trimesityldipyrromethene and 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene. A comparative study

Abstract

The compounds LMgBuⁿ(THF) and L′MgBuⁿ(THF) where L = 1,5,9-trimesityldipyrromethene and L′ = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene have been prepared from reactions between MgBuⁿ₂ and the protonated ligands LH and L′H, respectively. Single crystal X-ray crystallographic studies reveal that in each compound the Mg²⁺ ion is in a distorted tetrahedral environment and further that the ligand L is more sterically demanding with respect to access to the Mg–Buⁿ group. Related alkyl complexes (R = Me, Et, Prⁿ, Prⁱ, n-hexyl and CH₂CH₂Ph) were prepared from reactions involving LLi(THF) or L′Li(THF) and the appropriate RMgX (X = Cl or Br) and characterized by ¹H NMR spectroscopy. Those where R = CH₂CH₂X (X = alkyl or phenyl) have characteristic α-CH₂ proton resonances which are the part of an AA′XX′ pattern. Reactions with alcohols ROH (1 equiv.) give the kinetic products LMg(OR)(THF) and L′Mg(OR)(THF) which are isolable and kinetically persistent when R = a bulky group such as Bu^t and the structure of LMg(OBu^t)(THF) is reported and compared with that of the known compound L′Mg(OBu^t)(THF). With less bulky groups the compounds are labile toward ligand scrambling and the compound L′MgBuⁿ(THF) reacts with alcohols (2 equiv.) to give L′H and Mg(OR)₂. Reactions with amines and carbon dioxide are described which indicate the greater reactivity of the Mg–Buⁿ group relative to both the L′ and L ligand. With PhCHO, Ph₂CO and cyclohexanone both LMgBuⁿ(THF) and L′MgBuⁿ(THF) react via β-hydrogen atom transfer to generate the appropriate alkoxide with the elimination of 1-butene. Similarly L-lactide reacts by β-H transfer to give poly-L-lactide (P-L-LA) and 1-butene while rac-lactide yields atactic polylactide in toluene–dichloromethane solutions but in the presence of ≥10 equiv. of THF, heterotactic PLA is formed. The ring-opening polymerization of ε-caprolactone also is initiated by β-H transfer and is about ten times faster than for lactide; k_p(LA) = 10.7 M⁻¹ s⁻¹vs. k_p(CL) = 110 M⁻¹ s⁻¹. Interestingly, the presence of lactide completely suppresses the polymerization of ε-caprolactone.