Communications
Table 1: Catalyst screening of tantalum amido complexes.
Entry
Catalyst
Conditions
Conversion [%][a]
1
2
3
4
5
6
7
1[b]
2
2
3
1308C, 1 week[c,d,e]
1308C, 24 h[c,e,f]
1108C, 68 h[c,f]
1108C, 63 h[c,f]
1108C, 77 h[c,f]
1108C[c,f]
71
84
69
96 (92)
85
4
[Ta(NMe2)5]
[Ta(NMe2)5]
n.r.
89 (80)
1308C, 67 h[c,f]
[a] Yield of isolated product given in brackets; conversion was estimated
by 1H NMR spectroscopy. [b] 10 mol%; [c] [N-methylaniline]=1m.
[d] N-methylaniline/1-octene 1:1.05. [e] [D8]toluene as solvent.
[f] N-methylaniline/1-octene 1:1.5. n.r.=no reaction.
Scheme 2. Synthesis and ORTEP diagram of solid state molecular
structure of 1. Thermal ellipsoids depicted at 50% probability. All
hydrogen atoms and most amidate ligand atoms omitted for clarity.
Selected bond lengths [ꢀ] and angles [8]: Ta1–N4 1.921(4), Ta1–N3
1.970(5), Ta1–C38 2.178(5), N4–C29 1.451(7), N4–C38 1.424(6), N3–
C28 1.459(9), N3–C27 1.463(9), Ta1–N1 2.179(4), Ta1–N2 2.245(4),
Ta1–O1 2.214(4), Ta1–O2 2.132(3); N4-Ta1-C38 40.04(17), Ta1-N4-C38
79.7(3), Ta1-N4-C29 155.8(4), Ta1-N3-C27 124.1(5), Ta1-N3-C28
123.0(4), N1-Ta1-O1 59.35(14), N2-Ta1-O2 59.34(12).
Scheme 3. Synthesis of mono(amidate)–tantalum tetrakis(dimethyl-
amido) complexes with variable steric bulk.
aniline with 1-octene shows that mono(amidate)–tantalum
complexes (Table 1, entries 2–5) allow reactions at a lower
temperature (1108C) compared to the parent bis(amidate)
compound 1. Whereas most mono(amidate)–tantalum com-
plexes show reactivity at 1108C, [Ta(NMe2)5] requires at least
1308C to effect product formation (Table 1, entries 6 and 7).
The observed reactivity trends suggest that steric bulk is
required to favor the b-hydrogen abstraction reaction, yet too
much steric bulk appears to inhibit olefin insertion. This
mechanistic interpretation would account for the ready
formation of metallaaziridine 1, as well as its overall reduced
catalytic activity. Furthermore, this is consistent with the
empirical observation that increased alkene loading (e.g.,
N-methylaniline/vinylcyclohexane (see below) 1:2.4 versus
1:1.2) results in improved relative rates of reaction in side-by-
side NMR tube experiments. These early results show that
sterically bulky mono(amidate)–tantalum complex 3 is the
most efficient among the tested precatalysts and a more
complete exploration of its substrate scope is presented
herein (Table 2 and Table 3).
In alkene substrate scope investigations terminal alkenes
having steric bulk, such as vinylcyclohexane, and easily
isomerized allylbenzene form the corresponding a-alkylation
products 6 and 7 in high product yields upon isolation.
Norbornene, as an activated olefin, is efficiently converted
into the corresponding product 8 with unprecedented diaste-
reoselectivity (> 20:1) at 1108C. Protected alcohols can be
used to access silyl protected amino alcohol derivative 9.
Exposing 1,7-octadiene to the standard conditions while using
1.9 equivalents of N-methylaniline results in the synthesis of
pure yellow crystalline solid in 84% yield, and diagnostic
signals in the 1H NMR spectrum for the diastereotopic
metallaaziridine CH2 protons are clearly observed as doublets
at d = 2.34 and 2.49 ppm (2JH,H = 3.5 Hz), whereas the N-
methyl of the metallaaziridine and the methyl signal of the
dimethylamido ligand resonate at d = 3.16 and 3.07 ppm,
respectively. The
X-ray crystallographic analysis of C1 symmetric 1 reveals a
cyclometalated N-methylenemethanamide fragment as
À
depicted in Scheme 2 with a H2C N bond length within the
À
range of lengths typical of C N single bond. Most impor-
tantly, complex 1 has been found to be a competent
precatalyst for the alkylation of N-methylaniline with
1-octene at 1308C to give the known product 5 (Table 1,
entry 1). Although spontaneous, stoichiometric tantallaazir-
idine formation has been previously disclosed,[10] such com-
plexes have never been reported to be catalytically active.
Previous to this work, in catalytically competent systems, such
tantallaaziridine complexes had not been observed in either
solution phase or solid state, although their formation has
been proposed to be rate limiting in the catalytic cycle.[4a]
A variety of mono(amidate)–tantalum complexes can be
easily prepared at ambient temperature in hexanes to give
crystalline complexes 2, 3, and 4, which have varying degrees
of steric bulk (Scheme3). Most importantly, a screen of
complexes 1–4 as catalysts for the a-alkylation of N-methyl-
8362
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 8361 –8365