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(entries 1 and 2). Intrigued by this phenomenon, the families
of complexes 4–7 (entries 5–8) and 8–10 (entries 9–11) were
also tested under rigorously identical reaction conditions. In
all cases, the trend (Mg !)Ca < Sr< Ba was obtained, that is,
irrelevant of the identity of the ligand, the catalytic activity
increased with the size of the metal. In agreement with the
proposed theoretical models[14] and experimental observa-
tions,[3d,14,15] the reaction was fully regioselective, as the anti-
Markovnikov product of addition to the alkene was always
exclusively formed. Note that for any given metal, maximal
activity was achieved with the ligand {L3}ꢀ, whereas the
lowest conversions were recorded with the phenolate {L2}ꢀ
ligand (compare entries 1, 6, and 9; 2, 7, and 10; 4, 8, and 11).
This data confirmed the superiority of the b-diketiminate over
other ligand frameworks as is often observed for a number of
reactions catalyzed by divalent metals. However, complexes
1–3 are more readily synthesized than 8–10 and displayed only
slightly lower efficiency. Thus, the most active catalyst in this
family, the Ba derivative 3, was selected for subsequent
investigations. The role of the identity of the amide moiety in
the catalyzed reaction was negligible, as control experiments
demonstrated that the activities of [{L3}CaN(SiMe3)2(THF)]
and [{L2}BaN(SiMe2H)2][16e] matched those of 8 and 7,
respectively.[21]
Figure 1. Representation of the molecular solid-state structure of
[{L1}BaN(SiMe3)2(THF)2] (3). Only the main sites are drawn for the
disordered THF molecule (O(101)). Hydrogen atoms omitted for
clarity. Selected bond lengths [ꢁ] and angles [8]: Ba(1)–N(51) 2.623(3),
Ba(1)–N(9) 2.677(2), Ba(1)–O(201) 2.766(2), Ba(1)–O(101) 2.810(3),
Ba(1)–N(1) 2.825(3), Ba(1)–Si(1) 3.676(1), Ba(1)–Si(2) 3.758(1);
Ba(1)-N(51)-Si(2) 119.78(14), Ba(1)-N(51)-Si(3) 114.85(15),
N(1)-Ba(1)-N(9) 65.99(8).
The molecular structure of the five-coordinated complex 3
was obtained and is depicted in Figure 1. The Ba atom is
located 1.14 ꢀ above the mean plane formed by the NCCCN
core, and accordingly the bite angle N(1)-Ba(1)-N(9) is very
narrow (65.998). The new, stable complexes [{L2}AeN-
(SiMe3)2(THF)n] [Ae = Mg (n = 0, 4); Sr (n = 1, 6)], which
incorporate the tetradentate amino-ether phenolate ligand
{L2}ꢀ,[18] were synthesized following procedures already
developed to obtain their Ca (n = 0, 5)[16d] and Ba (n = 0,
7)[16e] analogues. Inspired by the pioneering work by Anwan-
der and co-workers with rare-earth metals,[19] we have
recently shown that internal Ae···H–Si agostic interactions
help stabilize heteroleptic Ae complexes against Schlenk-type
equilibria.[16c–f] Exploiting this strategy that relies on the use of
the N(SiMe2H)2ꢀ amido group, we have now prepared cleanly
and in good yields (74–78%) the complexes [{L3}AeN-
(SiMe2H)2(THF)n] [Ae = Ca (n = 1, 8); Sr (n = 2, 9); Ba
(n = 2, 10)] bearing the ubiquitous b-diketiminate ligand
{L3}ꢀ. Until now, available synthetic heteroleptic Ae precur-
sors containing this ancillary ligand were confined to
[{L3}MgN(SiMe3)2] (and its alkyl/alkoxide derivatives)[20]
and [{L3}CaN(SiMe3)2(THF)].[9,10b,11] The Sr and Ba conge-
ners could be prepared but were prone to ligand scrambling,
which hampered the synthesis of pure compounds.[10b]
The presence of an electron-donating substituent on the
aromatic ring in vinyl arenes led to a marked decrease in
catalyst activity (entries 4, 19, and 20), which is consistent
with earlier results with Ae[3d,14] and rare-earth metals.[8] In
our case, the presence of a chlorine atom did not lead to
improved activity either (entry 21). The hydroamination of
styrene with n-hexylamine also occurred fairly rapidly
(entry 13), but the reaction was obviously sensitive to steric
factors (entry 12).
The fastest reaction rates were achieved with pyrrolidine,
as conversion of 50 equivalents was complete within 1 hour
(entry 14).
With
[styrene]/[pyrrolidine]/[3] = 500:500:1
(entry 15) and 1000:1000:1 (entry 18), 85 and 58% conver-
sions,respectively, were achieved within 2 hours with corre-
sponding turnover frequencies (TOFs) of 212 and 290 hꢀ1.
These values, which were achieved under mild reaction
conditions, exceed those reported to date for intermolecular
hydroamination reactions catalyzed by Ae,[3d,14,15] rare-
earth,[8] or even late-transition-metal[22] complexes by one to
two orders of magnitude. Notably, under identical reaction
conditions, the best[15] bis(amide)s [{Ae[N(SiMe3)2]2(THF)2}2]
(Ae = Ca, entry 16; Sr, entry 17), and homoleptic complexes
recently reported[23] displayed vastly lower reaction rates. The
selection of alkene was not restricted to vinyl arenes, as 3
catalyzed the reaction of isoprene and pyrrolidine with equal
competence (entries 22 and 23). Full conversion was observed
within 1 hour using 2 mol% of 3, and gratifyingly the
conversion reached 59% after 2 hours (TOF = 295 hꢀ1)
when as little as 0.1 mol% of 3 was employed. The reaction
was 1,4-regioselective, with anti-Markovnikov addition of
pyrrolidine occurring exclusively on the least encumbered
unsaturation to give 1-(3-methylbut-2-en-1-yl)pyrrolidine.
Kinetic studies of the hydroamination of styrene with
pyrrolidine catalyzed by 3 were performed under a broad
range of amine, styrene, and catalyst concentrations by using
The ability of the new heteroleptic complexes 1–3 to
catalyze the intermolecular hydrofunctionalization of acti-
vated alkenes was interrogated (Table 1). A moderate
catalyst loading of 2 mol% in neat substrate was typically
used at 608C. Much to our delight, 1–3 promoted the
hydroamination of styrene with benzylamine, but contrary
to expectations,[14,15] we found that the performance improved
substantially and regularly from Ca to Ba. Wherein the Ba
complex
3 achieved near-complete conversion within
18.5 hours (entry 4), the Ca (1) and Sr (2) complexes
converted 34 and 71%, respectively, of the substrates
2
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!
Liu, Bo
