Evidence for the formation of a metal alkyl intermediate in the zinc mediated intramolecular hydroamination

Article abstract of DOI:10.1039/c3cc45491f

A diketiminato zinc amide complex, LZnNMe₂ (1) [L = CH{(CMe)₂(2,6-ⁱPr₂C₆H ₃N)₂}], was prepared and investigated for the catalytic hydroamination of 2,2-dimethylpent-4-en-1-amine. The reaction with the amino olefin resulted in a transamination reaction and the subsequent insertion of the olefin moiety into the metal-amide bond. However, the reaction stopped at this point providing access to the metal alkyl intermediate, LZnR (R = NH-CH ₂-CPh₂-CH₂-CH-) (2). The isolation of this primary insertion product further strengthens the widely accepted mechanism for the intramolecular hydroamination. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc45491f

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Evidence for the formation of a metal alkyl
intermediate in the zinc mediated intramolecular
hydroamination†
Cite this: Chem. Commun., 2013,
49, 9672
Received 19th July 2013,
Accepted 16th August 2013
Sankaranarayana Pillai Sarish, Dirk Schaffner, Yu Sun and Werner R. Thiel*
DOI: 10.1039/c3cc45491f
www.rsc.org/chemcomm
A diketiminato zinc amide complex, LZnNMe2 (1) [L = CH{(CMe)2-
Therefore we decided to replace the calcium centre by zinc
(2,6-iPr2C6H3N)2}], was prepared and investigated for the which shows some similarities to the lighter alkaline earth
catalytic hydroamination of 2,2-dimethylpent-4-en-1-amine. metals but has a higher tolerance towards functional groups as
The reaction with the amino olefin resulted in
a trans- well as to air and moisture. In contrast to its heavier congeners
amination reaction and the subsequent insertion of the olefin cadmium and mercury, zinc is non-toxic and the applied zinc
moiety into the metal–amide bond. However, the reaction stopped catalyst was expected to be less reactive than the corresponding
at this point providing access to the metal alkyl intermediate, LZnR magnesium counterpart.5 b-Diketiminate (L) coordinated zinc
complexes have already been used as catalysts in hydroamination
before, but under more demanding conditions and a co-catalyst
was required for adequate activity.6 So it seemed to be worth
investigating the potential of a more reactive zinc amide complex
in the aforementioned reaction without any co-catalyst. We,
herein, describe the isolation of a molecular hydrocarbon soluble
zinc alkyl intermediate by the reaction of the b-diketiminato zinc
dimethyl amide, LZnNMe2(1), with an aminoalkene.
Compound 1 was obtained by the conversion of LH with one
equivalent of KN(SiMe3)2, ZnCl2 and LiNMe2 in THF at room
temperature.7 It is soluble without decomposition in a number
of non-protic organic solvents and was characterized by mass
spectrometry, NMR spectroscopy (1H, and 13C), X-ray single
crystal structure, and elemental analysis.‡ The 1H spectrum of 1
shows singlets at 2.36 ppm and 4.89 ppm for the NMe2 and
g-CH protons respectively. The molecular ion peak corresponding
to 1 was not found in its EI mass spectrum, only fragment ions
were observed. Colourless crystals suitable for an X-ray structural
analysis were obtained when a concentrated solution of 1 was
kept at 15 1C for one day in toluene. The structural analysis of 1
(Fig. 1) confirms the presence of a six-membered C3N2Zn ring.
The zinc atom is found in a distorted trigonal planar coordination
environment bound to two nitrogen atoms of the ligand L and
one nitrogen atom of the NMe2 unit. The Zn–N (1.9365av Å) bonds
within the C3N2Zn ring are longer than the Zn–NMe2 bond
(1.8385 Å) and all Zn–N bond distances are found to be smaller
than those in the related amide complex, LZnNiPr2.8
(R = NH–CH2–CPh2–CH2–CH–) (2). The isolation of this primary
insertion product further strengthens the widely accepted mecha-
nism for the intramolecular hydroamination.
The formal addition of a N–H bond over a C–C double or triple
bond (known as hydroamination) is an atom-efficient and
thermodynamically feasible route to produce nitrogen-containing
molecules from inexpensive precursors.1 However, the kinetic
barrier for this addition reaction is quite high and therefore
catalytic pathways are required. In 2005, Hill et al. showed that
the b-diketiminato calcium complex LCaN(SiMe3)2(thf) (L =
CH{(CMe)2(2,6-iPr2C6H3N)2}) is an excellent catalyst/pre-catalyst
for hydroamination reactions.2 They also compared the reac-
tivity of LCaN(SiMe3)2(thf) with its magnesium analogue,
LMgN(SiMe3)2(thf), and investigated the mechanistic pathway
of the cyclization reaction.3 The proposed mechanism is similar to
the one postulated for the lanthanide mediated hydroamination.4
The reactivity pattern of these complexes is mainly governed by
s-bond metathesis and the insertion of alkenes into polar M–X
(X = C, N) bonds, both of which may be altered by the choice of the
metal and the supporting ligand. Nevertheless the insertion
product, the highly reactive primary alkyl intermediate, has not
been isolated and the existence of such a species has solely been
confirmed by deuterium labelling studies.3
¨
Technische Universitat Kaiserslautern, Fachbereich Chemie, Kaiserslautern,
¨
Erwin-Schrodinger-Str. 54, 67663 Kaiserslautern, Germany.
The preparation of 1 prompted us to test its activity in the
hydroamination of unactivated primary amino olefins. An NMR
scale reaction between a catalytic amount (20 mol%) of 1 and
the substrate, 2,2-dimethylpent-4-en-1-amine,9 resulted in complete
protonolysis of the dimethylamido ligand and the formation of a
E-mail: thiel@chemie.uni-kl.de; Fax: +49 631 2054676
† Electronic supplementary information (ESI) available: Synthetic, and spectral
data for compounds 1 and 2. X-ray crystallographic details for compounds 1 and 2.
CCDC 943711 and 943712. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3cc45491f
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Fig. 1 Molecular structure of compound 1 in the solid state; thermal ellipsoids
are shown at the 50% probability level; selected bond lengths (Å) and bond
angles (1): Zn(1)–N(1) 1.9380(12), Zn(1)–N(2) 1.9350(12), Zn(1)–N(3) 1.8385(15);
N(1)–Zn(1)–N(2) 98.31(6), N(1)–Zn(1)–N(3) 130.54(6), N(2)–Zn(1)–N(3) 131.12(6).
Hydrogen atoms are omitted for clarity.
Scheme 2 Proposed mechanistic pathway for the formation of zinc alkyl
complex 2.
olefin substrate.4b,13 For compound 1 the experiment is consistent
with the insertion of the olefin into the zinc–nitrogen bond. Only a
primary alkylzinc intermediate is formed by this regioselective
amide/olefin addition. The reaction stops at this point, a s-bond
metathesis of 2 and another equivalent of the aminoalkene is not
Scheme 1 Formation of the b-diketiminato zinc alkyl complex 2.
new compound indicated by the presence of additional resonances observed.
in the NMR spectra. Accordingly a preparative 1 : 1 reaction of 1 and
The zinc alkyl complex 2 is a colorless solid, being soluble in
the amino olefin led to complete conversion of the aminoalkene THF, toluene, and benzene. It was characterized by 1H NMR
and almost quantitative formation of the zinc alkyl insertion spectroscopy, ESI mass spectrometry, and X-ray single-crystal
product 2.10
structure analysis. The 1H NMR spectrum exhibits the N–H and
In both cases the expected 1-methyl-4,40-diphenylpyrrolidine g-CH protons of 2 resonating at 4.97 and 0.76 ppm respectively.
is not formed. However, the conversion stops at the primary The ESI-mass spectrum of 2 reveals the base peak at 718.5
insertion product 2 (Scheme 1). Astonishingly, even intra- ([M + H]+) in high intensity.
molecular protonolysis is not observed for compound 2. We
The molecular structure of 2 was further confirmed by single
assign this to the inability of the substrate to protonolyse the crystal X-ray diffraction analysis (Fig. 2), demonstrating the
Zn–C bond of 2. This is confirmed by the stability of the monomeric nature of this complex. Colourless crystals of 2
sterically even more crowded zinc amide LZnN(SiMe3)2,10 were obtained by keeping a toluene solution at r.t. for one day.
which does not react with the amine substrate at all, and can The structure reveals the presence of a six-membered C3N2Zn
be assigned to the almost perfect steric shielding of the critical ring and a five-membered C4N ring being connected by a
zinc-element bond by the bulky b-diketiminato ligand. Schulz methylene bridge. The coordination polyhedron around the
et al. previously described the sluggish reactivity of b-diketiminato zinc site exhibits the usually distorted trigonal planar geometry
Zn alkyl and hydrido complexes.11
with two nitrogen atoms of the b-diketiminato ligand, and the
A tentative mechanism for the formation of the zinc-coordinated carbon atom of the methylpyrrolidine ring. The sum of angles
cyclization product by reaction of the aminoalkene and 1 is shown around the zinc atom is 359.991. The Zn–N bond distances
in Scheme 2. First a rapid s-bond metathesis between the substrate (1.970av Å) in 2 are longer compared to those of 1 (1.9365(3)av Å).
and 1 takes place. For the second step, there are different The bond distance Zn–C [1.974(2) Å] is Zn–C distances found for
mechanistic ideas discussed in the literature. The most common LZn(Et)14 (1.963(5) Å) and LZn(Me) (1.941(3) Å).15
mechanism involves an intramolecular nucleophilic attack of the
In summary, a hitherto unknown zinc alkyl intermediate of
metal coordinated amide on the olefin via a highly ordered the alkene hydroamination could be isolated starting from a
metallacyclic transition state.12 An alternative pathway reported zinc dimethylamide complex. The zinc alkyl complex is found to
by Marks et al. and Sundermeyer et al. for organolanthanide be stable towards protonolysis by the substrate as well as against
mediated hydroamination involves a noninsertive mechanism in intramolecular protonolysis. This is a remarkable difference to
which cyclization occurs via the coordination of an extra amino alkaline earth metal counterparts. Hydroamination catalyzed by
c
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7 b-Diketiminatozinc dimethylamide 1: 4.18 g (10.0 mmol) of ligand
LH and 2.1 g (10.5 mmol) of KN(SiMe3)2 were dissolved in 20 mL of
thf and the mixture was stirred at r.t. for 5 h. It was then added to a
suspension of 1.36 g (10.0 mmol) of ZnCl2 in 10 mL of thf and
stirred at r.t. for another 5 h. Then 0.53 g (10.5 mmol) LiNMe2
dissolved in 20 mL of thf were added and the stirring was continued
for another 12 h. All the volatiles were removed under vacuum, the
residue was extracted with toluene (30 mL) and the solution was
filtered. After concentration of the filtrate, it was kept in a freezer at
ꢀ15 1C to obtain colourless crystals of 1 after 1 d. Yield: (3.84 g,
73%). mp.: 200–202 1C (dec.). 1H NMR (600 MHz, C6D6): d 7.11–7.10
3
(m, 6H, HAr), 4.89 (s, 1H, g-CH), 3.19 (sept, JHH = 6.9 Hz, 4H,
CH(CH3)2), 2.36 (s, 6H, NMe2), 1.66 (s, 6H, CH3), 1.36 (d, 12H,
CH(CH3)2), 1.18 (d, 12H, CH(CH3)2). 13C{1H} NMR (125.77 MHz,
C6D6): d 169.0, 145.1, 142.4, 126.9, 124.6, 96.0, 47.0, 29.1, 24.0, 22.0,
23.8. MS (70 eV): m/z (%): 403.32 (100) [L ꢀ Me]+. Anal. calc. for
C31H47N3Zn (527.13): C, 70.64; H, 8.99; N, 7.97. Found: C, 69.47; H,
8.96; N, 7.19%.
8 M. H. Chisholm, J. Gallucci and K. Phomphrai, Inorg. Chem., 2002,
41, 2785–2794.
9 S. Hong, S. Tian, M. V. Metz and T. J. Marks, J. Am. Chem. Soc., 2003,
125, 14768–14783.
Fig. 2 Molecular structure of compound 2 in the solid state; thermal ellipsoids
are shown at the 50% probability level; selected bond lengths (Å) and bond
angles (1): Zn(1)–N(1) 1.9811(17), Zn(1)–N(2) 1.9588(18), Zn(1)–C(6) 1.974(3),
C(6)–C(7) 1.542(4); N(1)–Zn(1)–N(2) 95.35(7), N(1)–Zn(1)–C(6) 124.41(8),
N(2)–Zn(1)–C(6) 140.24(8), Zn(1)–C(6)–C(7) 120.42(19). The disordering of the
five membered ring and all hydrogen atoms are omitted for clarity.
10 Synthesis of 2. A solution of 0.237 g (1.0 mmol) of 2,2-diphenylpent-
4-en-1-amine in 5 mL of toluene was added to a solution of 0.527 g
(1.0 mmol) of 1 in 4 mL of toluene at r.t. and stirred for 1 h. All
volatiles were removed under vacuum and the solid residue was
dissolved in toluene. The solution was concentrated and kept at
room temperature for 1 d whereby colorless crystals of 2 appeared.
Yield: (0.617 g, 86%). mp: 181–182 1C. 1H NMR (400 MHz, C6D6): d
3
7.18–7.00 (m, 16H, HAr), 4.97 (s, 1H, g-CH), 3.44 (dd, JH–H = 5 Hz,
1 with different substrates in the presence of a proton donor is
currently in progress in our laboratory.
S. P. S. thanks the Alexander von Humboldt foundation for a
post-doctoral research fellowship.
2JH–H = 10.3 Hz, 1H, CH2NH), 3.13 (m, 6H, CH(CH3)2, CH(CH2)NH
and CH2–NH), 1.67 (s, 6H, CH3), 1.45 (dd, 3JH–H = 1.9 Hz, 2JH–H = 10
3
2
Hz, 1H, CH2CH(CH2)NH), 1.38 (dd, JH–H = 5.5 Hz, JH–H = 11.7 Hz,
3
1H, CH2CH(CH2)NH), 1.25–1.22 (d, 6H, JH–H = 6.9 Hz, CH(CH3)2),
1.15–1.07 (3d, 18H, CH(CH3)2), 0.76 (br, m, 1H, NH), 0.56 (dd,
2
3JH–H = 5.6 Hz, JH–H = 12.6 Hz, 1H, CH2CH(CH2)NH), 0.31(dd,
Notes and references
2
3JH–H = 9.9 Hz, JH–H = 12.6 Hz, 1H, CH2CH(CH2)NH). 13C{1H}
NMR (100.6 MHz, C6D6): d = 168.1, 150.3, 149.5, 145.6, 142.2,
142.3, 128.5, 128.3, 126.7, 126.3, 126.1, 124.7, 124.6, 96.0, 59.3,
58.6, 58.1, 50.9, 29.1, 29.08, 25.0, 24.6, 24.3, 24.2, 24.1, 17.9. MS:
m/z (%): 718.5 (100) [M + H]+. Anal. calc. for C46H59N3Zn (719.36): C,
76.80; H, 8.27; N 5.84. Found: C, 76.06; H, 8.00; N, 5.89%.
‡ Crystal data for 1 and 2. 1: C31H47N3Zn, Mr = 527.09, monoclinic, space
group P21/c, a = 15.3901(3) Å, b = 9.7161(2) Å, c = 20.5004(4) Å, b =
93.737(2)1, V = 3058.94(11) Å3, Z = 4, rcalcd = 1.145 Mg mꢀ3, F(000) = 1136,
T = 150(2) K, m(Cu Ka) = 1.259 mmꢀ1, 11 229 reflections measured, 4883
independent (Rint = 0.0149). The final refinement converged to R1 = 0.0306
for I > 2s(I), wR2 = 0.0898 for all data. 2: C46H59N3Zn, Mr = 719.33,
monoclinic, space group P21/c, a = 22.8607(3) Å, b = 9.6928(1) Å,
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This journal is The Royal Society of Chemistry 2013