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

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 MgBu2n 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-Bo(n) group. Related alkyl complexes (R = Me, Et, Pr-n, Pr-i, 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 H-1 NMR spectroscopy. Those where R = CH2CH2X (X = alkyl or phenyl) have characteristic alpha-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' and the structure of LMg(OBu')(THF) is reported and compared with that of the known compound L'Mg(OBu')(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-Bu-n group relative to both the L' and L ligand. With PhCHO, Ph2CO and cyclohexanone both LMgBun(THF) and L'MgBun(THF) react via beta-hydrogen atom transfer to generate the appropriate alkoxide with the elimination of 1-butene. Similarly L-lactide reacts by beta-H transfer to give poly-L-Iactide (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 epsilon-caprolactone also is initiated by beta-H transfer and is about ten times faster than for lactide; k(p)(LA) = 10.7 M-1 s(-1) vs. k(p)(CL) = 110 M-1 s(-1). Interestingly, the presence of lactide completely suppresses the polymerization of epsilon-caprolactone.