Heteroscorpionate Rare-Earth Catalysts for the Hydroalkoxylation/Cyclization of Alkynyl Alcohols

The chiral, enantiopure bis­(pyrazol-1-yl)­methane-based
NNO-donor
scorpionate ligands in the form of the alcohol compounds bpzbeH (bpzbe
= 1,1-bis­(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide), bpzteH
(bpzte = 2,2-bis­(3,5-dimethylpyrazol-1-yl)-1-p-tolylethoxide),
and (R,R)-bpzmmH ((R,R)-bpzmm = (1R)-1-{(1R)-6,6-dimethylbicyclo­[3.1.1]-2-hepten-2-yl}-2,2-bis­(3,5-dimethylpyrazol-1-yl)­ethoxide)
have been used to obtain new NNO-heteroscorpionate yttrium and lutetium
complexes. The reactions of bpzbeH, bpzteH (racemic mixtures), and
(R,R)-bpzmmH (enantiopure compound)
with [M­{N­(SiHMe2)2}3(thf)2] (M = Y, Lu) in a 1:1 molar ratio in toluene afforded the mononuclear
bis­(silylamide) complexes [M­{N­(SiHMe2)2}2(κ3-NNO)­(thf)] (M = Y, Lu; 1–6), respectively. When the reaction was carried
out with a 2:1 molar ratio (ligand to metal precursor) or with excess
ligand, homoleptic complexes of the type [Y­(κ3-NNO)2(κ-O-NN)] (κ3-NNO = κ-O-NN =
bpzbe (7), bpzte (8)) were obtained. Compounds 1 and 3 were used as convenient starting materials
for the synthesis of the aryloxide and alkoxide yttrium compounds
[Y­(OR)2(κ3-bpzbe)] (OR = O-2,6-Me2C6H3 (9), O-CH2(2-CHC)­C6H4 (10)) and
[Y­(OR)2(κ3-bpzte)] (OR = O-2,6-Me2C6H3 (11), O-CH2(2-CHC)­C6H4 (12)). The
structures of the complexes were determined by spectroscopic methods,
and the X-ray crystal structures of 1, 2, and 7 were also established. Complexes 1–6 are efficient catalysts for the intramolecular
hydroalkoxylation of alkynyl alcohols and give TOF values up to 19.4
h–1 at 90 °C for (2-ethynylphenyl)­methanol
(13) by using the single enantiopure complex 5 as catalyst, producing exclusively the exo-methylene
products. The activation parameters ΔH⧧ = 19.93(0.2) kcal/mol, ΔS⧧ = −28.75(0.2)
eu, and Ea = 18.61(0.2) kcal/mol are consistent
with observations for other catalyst-mediated insertive hydroelementation
processes.