25 research outputs found
Asymmetric Intra- and Intermolecular Hydroamination Catalyzed by 3,3′-Bis(trisarylsilyl)- and 3,3′-Bis(arylalkylsilyl)-Substituted Binaphtholate Rare-Earth-Metal Complexes
The series of novel 3,3′-bisÂ(trisarylsilyl)-
and 3,3′-bisÂ(arylalkylsilyl)-substituted
binaphtholate rare-earth-metal complexes 2a–i (SiR3 = SiÂ(o-biphenylene)ÂPh
(a), SiCyPh2 (b), Si-t-BuPh2 (c), SiÂ(i-Pr)3 (d), SiCy2Ph (e), SiÂ(2-tolyl)ÂPh2 (f), SiÂ(4-t-Bu-C6H4)3 (g), SiÂ(4-MeO-C6H4)ÂPh2 (h), SiBnPh2 (i)) have been prepared via arene elimination
from [LnÂ(o-C6H4CH2NMe2)3] (Ln = Y, Lu) and the corresponding
3,3′-bisÂ(silyl)-substituted binaphthol. The complexes exhibit
high catalytic activity in the hydroamination/cyclization of aminoalkenes,
with activities exceeding 1000 h–1 for (R)-2f-Ln, (R)-2g-Ln, and (R)-2h-Ln in the cyclization of 2,2-diphenylpent-4-enylamine
(3a) at 25 °C, while the rigid dibenzosilole-substituted
complexes (R)-2a-Ln and
the triisopropylsilyl-substituted complexes (R)-2d-Ln exhibited the lowest activity in the range
of 150–270 h–1. Catalysts (R)-2b-Lu, (R)-2c-Lu, (R)-2f-Lu, and (R)-2i-Lu provide
the highest selectivities for the majority of the substrates, while
the yttrium congeners are usually less selective. The highest enantioselectivities
of 96% ee were observed using (R)-2a-Lu and (R)-2c-Lu in the cyclization of (4E)-2,2,5-triphenylpent-4-enylamine
(9). The reactions show apparently zero-order rate dependence
on substrate concentration and first-order rate dependence on catalyst
concentration, with some reactions exhibiting a slightly accelerated
rate at high conversion due to a shift in the equilibrium between
a less active, higher coordinate catalyst species in favor of a more
active, lower coordinate species as a result of weaker binding of
the hydroamination product in comparison to the aminoalkene substrate.
The shift in equilibrium from the higher to the lower coordinate species
is also entropically favored at elevated temperatures, which results
in an unusual increase in selectivity in the cyclization of 2,2-dimethylpent-4-enylamine
(3d), presumably due to a higher selectivity of the lower
coordinate catalyst species. All binaphtholate yttrium complexes,
except (R)-2a-Y, are catalytically
active in the intermolecular hydroamination of benzylamines with terminal
alkenes. The highest selectivity of 66% ee was observed for the reaction
of benzylamine with 4-phenyl-1-butene using (R)-2h-Y at 110 °C