Functionalized aminotroponiminate zinc complexes as catalysts for the intramolecular hydroamination of alkenes

Article abstract of DOI:10.1021/om901012f

Substituted aminotroponimines, {R-ATI(iPr)₂}H (R = Br, I, PhN₂, NO₂, PhS, PhSe, PhTe, 3,5-(CF₃) ₂C₆H₃S, PhS(O)), bearing different functional groups in the 5-position were prepar

Full text of DOI:10.1021/om901012f

Organometallics 2010, 29, 2637–2645 2637
DOI: 10.1021/om901012f
Functionalized Aminotroponiminate Zinc Complexes as Catalysts for
the Intramolecular Hydroamination of Alkenes
§
§
†
Maximilian Dochnahl,†,‡ Karolin Lohnwitz, Anja Luhl, Jens-Wolfgang Pissarek,
€
€
Mustafa Biyikal,† Peter W. Roesky,*,§ and Siegfried Blechert*,†
†
€
Institut fu€r Chemie, Technische Universitat Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany, and
§Institut fu€r Anorganische Chemie, Karlsruher Institut fu€r Technologie (KIT), Engesserstrasse 15,
76131 Karlsruhe, Germany. Present address: BASF SE, Global Research Crop Protection,
67056 Ludwigshafen, Germany
‡
Received November 23, 2009
Substituted aminotroponimines, {R-ATI(iPr)2}H(R=Br, I, PhN2, NO2, PhS, PhSe, PhTe, 3,5-(CF3)2-
C6H3S, PhS(O)), bearing different functional groups in the 5-position were prepared. Reaction of
these compounds with ZnMe2 in toluene at 0 °C delivered the corresponding methyl zinc complexes
[{R-ATI(iPr)2}ZnMe] in high yields. The solid state structures of four selected examples were estab-
lished via single-crystal X-ray diffraction analysis. In all compounds the zinc atoms are coordinated
in a trigonal-planar fashion. All complexes were investigated as catalysts in the intramolecular
hydroamination of non-activated alkenes. The attachment of electron-withdrawing groups in the
5-position such as nitro-or sulfoxide decreased the activity of the corresponding zinc catalysts.
In contrast, donor substituents such as a thioether moiety at the backbone of the ligand increased
the stability of the chelate and also rendered the zinc atom more reactive. On the other hand, the
5-brominated compound is very labile under the catalytic conditions used. In order to study the
potential of an additional beneficial effect from the steric environment around the zinc atom on both
reactivity and stability of the corresponding complexes, a prototypic ligand bearing a 5-phenylsulfanyl
substituent at the backbone and two cyclohexyl substituents at the nitrogen atoms, {PhS-ATI(Cy)2}H,
and its corresponding zinc complex, [{PhS-ATI(Cy)2}ZnMe], were synthesized.
Introduction
and alkynes.6,7 The formed zinc catalyst shows, besides its
good catalytic activity and selectivity, high tolerance toward
polar functional groups and high stability toward air and
moisture.8-14 These advantages make zinc complexes an
attractive alternative to the previously used catalysts for
Aminotroponiminates ((R2)ATIs) are a well established
class of ligands, which were introduced into coordination
chemistry in the 1960s by researchers from DuPont.1-4 Like
1,4-diazabutadiene, {(R)2ATI}- can form five-membered
metallacycles upon coordination to a metal atom. Today it
is well established that aminotroponimines can act as ligands
for a wide range of main group elements and transition and
f-block metals.5 The corresponding deprotonated aminotro-
poniminates usually act as bidentate, monoanionic ligands
containing a 10 π-electron backbone (Scheme 1). In most of
the reports only simple {(R)2ATI}- systems were used, in
which the R group usually is methyl or isopropyl.
€
(8) Meyer, N.; Lohnwitz, K.; Zulys, A.; Roesky, P. W.; Dochnahl,
M.; Blechert, S. Organometallics 2006, 25, 3730–3734.
(9) Alex, K.; Tillack, A.; Schwarz, N.; Beller, M. ChemSusChem
2008, 1, 333–338.
(10) Horrillo-Martinez, P.; Hultzsch, K. C. Tetrahedron Lett. 2009,
50, 2054–2056.
(11) Okuma, K.; Seto, J.-I.; Sakaguchi, K.-I.; Ozaki, S.; Nagahora,
N.; Shioji, K. Tetrahedron Lett. 2009, 50, 2943–2945.
(12) Prior, A. M.; Robinson, R. S. Tetrahedron Lett. 2008, 49, 411–414.
(13) Yin, Y.; Ma, W.; Chai, Z.; Zhao, G. J. Org. Chem. 2007, 72,
5731–5736.
Recently, we have shown that N-isopropyl-2-(isopropyl-
amino)troponiminate zinc methyl, [{H-ATI(iPr)2}ZnMe] (I), is
a highly active precatalyst for the hydroamination of alkenes
€
(14) Biyikal, M.; Lohnwitz, K.; Roesky, P. W.; Blechert, S. Synlett
2008, 3106–3110.
(15) Aillaud, I.; Collin, J.; Hannedouche, J.; Schulz, E. Dalton Trans.
2007, 5105–18.
*Corresponding authors. E-mail: roesky@kit.edu; blechert@chem.
tu-berlin.de.
(1) Brasen, W. R.; Holmquist, H. E.; Benson, R. E. J. Am. Chem. Soc.
1960, 82, 995–956.
(2) Brasen, W. R.; Holmquist, H. E.; Benson, R. E. J. Am. Chem. Soc.
1961, 83, 3125–3135.
(16) Andrea, T.; Eisen, M. S. Chem. Soc. Rev. 2008, 37, 550–567.
(17) Beller, M.; Tillack, A.; Seayad, J. In Transition Metal Organic
Synthesis, 2nd ed.; VCH: Weinheim, 2004; Vol. 2, pp 403-414.
(18) Brunet, J.-J.; Neibecker, D. In Catalytic Heterofunctionalization;
VCH: Weinheim, 2001; Vol. 1, pp 91-141.
(19) Bytschkov, I.; Doye, S. Eur. J. Org. Chem. 2003, 935–946.
(20) Doye, S. Synlett 2004, 1653–1672.
(3) Benson, R. E. J. Am. Chem. Soc. 1960, 82, 5948–9.
(4) Phillips, W. D.; Benson, R. E. J. Chem. Phys. 1960, 33, 607–608.
(5) Roesky, P. W. Chem. Soc. Rev. 2000, 29, 335–345.
(21) Hii, K. K. Pure Appl. Chem. 2006, 78, 341–349.
(22) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673–686.
(23) Hultzsch, K. C. Org. Biomol. Chem. 2005, 3, 1819–1824.
(24) Lee, A. V.; Schafer, L. L. Eur. J. Inorg. Chem. 2007, 2243–2255.
(25) Mei, L. Lett. Org. Chem. 2008, 5, 174–190.
€
(6) Zulys, A.; Dochnahl, M.; Hollmann, D.; Lohnwitz, K.; Herrmann,
J.-S.; Roesky, P. W.; Blechert, S. Angew. Chem., Int. Ed. 2005, 44, 7794–
7798.
€
€
€
(7) Lohnwitz, K.; Molski, M. J.; Luhl, A.; Roesky, P. W.; Dochnahl,
(26) Muller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M.
M.; Blechert, S. Eur. J. Inorg. Chem. 2009, 1369–1375.
Chem. Rev. 2008, 108, 3795–3892.
r
2010 American Chemical Society
Published on Web 05/20/2010
pubs.acs.org/Organometallics

2638 Organometallics, Vol. 29, No. 12, 2010
Dochnahl et al.
Scheme 1
4 metals,19,70-90 the platinum metals (Rh,91-97 Ir,98,99
Pd,100-107 Pt108-113), nickel,114-116 copper,117-121 silver,122
and gold.31,123-128
(65) Zi, G.;Wang, Q.;Xiang, L.;Song, H. Dalton Trans. 2008, 5930–5944.
(66) Kim, H.; Livinghouse, T.; Shim, J. H.; Lee, S. G.; Lee, P. H. Adv.
Synth. Catal. 2006, 348, 701–704.
(67) Kim, J. Y.; Livinghouse, T. Org. Lett. 2005, 7, 4391–4393.
(68) Kim, Y. K.; Livinghouse, T. Angew. Chem., Int. Ed. 2002, 41,
3645–3647.
(69) Kim, Y. K.; Livinghouse, T.; Horino, Y. J. Am. Chem. Soc. 2003,
125, 9560–9561.
(70) Ackermann, L.; Bergman, R. G.; Loy, R. N. J. Am. Chem. Soc.
the hydroamination reaction15-32 mainly based on lithium,33
the alkaline earth metals,34-39 the lanthanides,40-69 group
(27) Odom, A. L. Dalton Trans. 2005, 225–233.
2003, 125, 11956–11963.
(71) Ackermann, L.; Sandmann, R.; Kaspar, L. T. Org. Lett. 2009,
11, 2031–2034.
(72) Cao, C.; Li, Y.; Shi, Y.; Odom, A. L. Chem. Commun. 2004,
2002–2003.
(73) Heutling, A.; Pohlki, F.; Bytschkov, I.; Doye, S. Angew. Chem.,
(28) Pohlki, F.; Doye, S. Chem. Soc. Rev. 2003, 32, 104–114.
(29) Seayad, J.; Tillack, A.; Hartung, C. G.; Beller, M. Adv. Synth.
Catal. 2002, 344, 795–813.
(30) Severin, R.; Doye, S. Chem. Soc. Rev. 2007, 36, 1407–1420.
(31) Widenhoefer, R. A.; Han, X. Eur. J. Org. Chem. 2006, 4555–4563.
€
(32) Roesky, P. W.; Muller, T. E. Angew. Chem., Int. Ed. 2003, 42,
Int. Ed. 2005, 44, 2951–2954.
2708–2710.
(33) Martinez, P. H.; Hultzsch, K. C.; Hampel, F. Chem. Commun.
2006, 2221–2223.
(74) Kaspar, L. T.; Fingerhut, B.; Ackermann, L. Angew. Chem., Int.
Ed. 2005, 44, 5972–5974.
(75) Li, C.; Thomson, R. K.; Gillon, B.; Patrick, B. O.; Schafer, L. L.
(34) Arrowsmith, M.; Hill, M. S.; Kociok-Kohn, G. Organometallics
2009, 28, 1730–1738.
(35) Barrett, A. G. M.; Boorman, T. C.; Crimmin, M. R.; Hill, M. S.;
Kociok-Kohn, G.; Procopiou, P. A. Chem. Commun. 2008, 5206–5208.
(36) Barrett, A. G. M.; Casely, I. J.; Crimmin, M. R.; Hill, M. S.;
Lachs, J. R.; Mahon, M. F.; Procopiou, P. A. Inorg. Chem. 2009, 48,
4445–4453.
Chem. Commun. 2003, 2462–2463.
(76) Pohlki, F.; Doye, S. Angew. Chem., Int. Ed. 2001, 40, 2305–2308.
(77) Tillack, A.; Castro, I. G.; Hartung, C. G.; Beller, M. Angew.
Chem., Int. Ed. 2002, 41, 2541–2543.
(78) Tillack, A.; Jiao, H.; Castro, I. G.; Hartung, C. G.; Beller, M.
Chem.-Eur. J. 2004, 10, 2409–2420.
(79) Vujkovic, N.; Ward, B. D.; Maisse-Francois, A.; Wadepohl, H.;
Mountford, P.; Gade, L. H. Organometallics 2007, 26, 5522–5534.
(37) Crimmin, M. R.; Casely, I. J.; Hill, M. S. J. Am. Chem. Soc. 2005,
127, 2042–2043.
€
(80) Weitershaus, K.; Ward, B. D.; Kubiak, R.; Muller, C.; Wade-
(38) Datta, S.; Roesky, P. W.; Blechert, S. Organometallics 2007, 26,
4392–4394.
(39) Lachs, J. R.; Barrett, A. G. M.; Crimmin, M. R.; Kociok-Kohn,
G.; Hill, M. S.; Mahon, M. F.; Procopiou, P. A. Eur. J. Inorg. Chem.
2008, 4173–4179.
pohl, H.; Doye, S.; Gade, L. H. Dalton Trans. 2009, 4586–4602.
(81) Zhang, Z.; Leitch, D. C.; Lu, M.; Patrick, B. O.; Schafer, L. L.
Chem.-Eur. J. 2007, 13, 2012–2022.
(82) Ackermann, L.; Kaspar, L. T.; Althammer, A. Org. Biomol.
Chem. 2007, 5, 1975–1978.
(40) Arredondo, V. M.; McDonald, F. E.; Marks, T. J. J. Am. Chem.
Soc. 1998, 120, 4871–4872.
(83) Kim, H.; Lee, P. H.; Livinghouse, T. Chem. Commun. 2005,
5205–5207.
(41) Arredondo, V. M.; Tian, S.; McDonald, F. E.; Marks, T. J.
J. Am. Chem. Soc. 1999, 121, 3633–3639.
(42) Collin, J.; Daran, J.-C.; Jacquet, O.; Schulz, E.; Trifonov, A.
(84) Kim, H.;Livinghouse, T.;Lee,P. H.Tetrahedron 2008,64, 2525–2529.
(85) Knight, P. D.; Munslow, I.; O’Shaughnessy, P. N.; Scott, P.
Chem. Commun. 2004, 894–895.
Chem.-Eur. J. 2005, 11, 3455–3462.
(43) Collin, J.; Daran, J.-C.; Schulz, E.; Trifonov, A. Chem. Commun.
2003, 3048–3049.
(86) Lee, A. V.; Schafer, L. L. Organometallics 2006, 25, 5249–5254.
(87) Smolensky, E.; Kapon, M.; Eisen, M. S. Organometallics 2007,
26, 4510–4527.
(44) Gagne, M. R.; Brard, L.; Conticello, V. P.; Giardello, M. A.;
Stern, C. L.; Marks, T. J. Organometallics 1992, 11, 2003–2005.
(45) Gribkov, D. V.; Hultzsch, K. C. Angew. Chem., Int. Ed. 2004, 43,
5542–5546.
(46) Gribkov, D. V.; Hultzsch, K. C.; Hampel, F. Chem.-Eur. J.
2003, 9, 4796–4810.
(88) Thomson, R. K.; Bexrud, J. A.; Schafer, L. L. Organometallics
2006, 25, 4069–4071.
(89) Watson, D. A.; Chiu, M.; Bergman, R. G. Organometallics 2006,
25, 4731–4733.
(90) Wood, M. C.; Leitch, D. C.; Yeung, C. S.; Kozak, J. A.; Schafer,
L. L. Angew. Chem., Int. Ed. 2007, 46, 354–358.
(91) Baudequin, C.; Brunet, J.-J.; Rodriguez-Zubiri, M. Organome-
tallics 2007, 26, 5264–5266.
(47) Heck, R.; Schulz, E.; Collin, J.; Carpentier, J.-F. J. Mol. Catal.
A: Chem. 2007, 268, 163–168.
(48) Hong, S.; Marks, T. J. J. Am. Chem. Soc. 2002, 124, 7886–7887.
(49) Hong, S.; Tian, S.; Metz, M. V.; Marks, T. J. J. Am. Chem. Soc.
2003, 125, 14768–14783.
(50) Hultzsch, K. C.; Gribkov, D. V.; Hampel, F. J. Organomet.
Chem. 2005, 690, 4441–4452.
(92) Burling, S.; Field, L. D.; Messerle, B. A.; Turner, P. Organome-
tallics 2004, 23, 1714–1721.
(93) Field, L. D.; Messerle, B. A.; Vuong, K. Q.; Turner, P. Dalton
Trans. 2009, 3599–3614.
(94) Field, L. D.; Messerle, B. A.; Vuong, K. Q.; Turner, P.; Failes, T.
(51) Ki, Y.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 9295–9306.
(52) Lu, E.; Gan, W.; Chen, Y. Organometallics 2009, 28, 2318–2324.
(53) Molander, G. A.; Dowdy, E. D. J. Org. Chem. 1999, 64, 6515–6517.
(54) Molander, G. A.; Hasegawa, H. Heterocycles 2004, 64, 467–474.
(55) Motta, A.; Fragala, I. L.; Marks, T. J. Organometallics 2006, 25,
5533–5539.
(56) O’Shaughnessy, P. N.; Knight, P. D.; Morton, C.; Gillespie, K.
M.; Scott, P. Chem. Commun. 2003, 1770–1771.
(57) Panda, T. K.; Hrib, C. G.; Jones, P. G.; Jenter, J.; Roesky, P. W.;
Tamm, M. Eur. J. Inorg. Chem. 2008, 4270–4279.
(58) Panda, T. K.; Zulys, A.; Gamer, M. T.; Roesky, P. W.
J. Organomet. Chem. 2005, 690, 5078–5089.
Organometallics 2007, 26, 2058–2069.
(95) Fukumoto, Y.; Asai, H.; Shimizu, M.; Chatani, N. J. Am. Chem.
Soc. 2007, 129, 13792–13793.
(96) Liu, Z.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 1570–1571.
(97) Takemiya,A.;Hartwig,J.F.J. Am. Chem. Soc. 2006,128,6042–6043.
(98) Field, L. D.; Messerle, B. A.; Wren, S. L. Organometallics 2003,
22, 4393–4395.
(99) Lai, R.-Y.; Surekha, K.; Hayashi, A.; Ozawa, F.; Liu, Y.-H.;
Peng, S.-M.; Liu, S.-T. Organometallics 2007, 26, 1062–1068.
(100) Cochran, B. M.; Michael, F. E. J. Am. Chem. Soc. 2008, 130,
2786–2792.
(101) Li, K.; Hii, K. K. Chem. Commun. 2003, 1132–1133.
(102) Lutete, L. M.; Kadota, I.; Yamamoto, Y. J. Am. Chem. Soc.
2004, 126, 1622–1623.
(59) Panda, T. K.; Zulys, A.; Gamer, M. T.; Roesky, P. W. Organo-
metallics 2005, 24, 2197–2202.
(60) Riegert, D.; Collin, J.; Meddour, A.; Schulz, E.; Trifonov, A.
J. Org. Chem. 2006, 71, 2514–2517.
(61) Ryu, J.-S.; Marks, T. J.; McDonald, F. E. Org. Lett. 2001, 3,
3091–3094.
(62) Tian, S.; Arredondo, V. M.; Stern, C. L.; Marks, T. J. Organo-
(103) Michael, F. E.; Cochran, B. M. J. Am. Chem. Soc. 2006, 128,
4246–4247.
(104) Qiu, S.; Wei, Y.; Liu, G. Chem.-Eur. J. 2009, 15, 2751–2754.
(105) Sakai, N.; Ridder, A.; Hartwig, J. F. J. Am. Chem. Soc. 2006,
128, 8134–8135.
metallics 1999, 18, 2568–2570.
(63) Wang, Q.; Xiang, L.; Song, H.; Zi, G. J. Organomet. Chem. 2009,
694, 691–696.
(106) Shaffer, A. R.; Schmidt, J. A. R. Organometallics 2008, 27,
1259–1266.
(107) Utsunomiya, M.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125,
14286–14287.
(64) Yu, X.; Marks, T. J. Organometallics 2007, 26, 365–376.

Article
In our ongoing research on zinc-catalyzed hydroamina-
Organometallics, Vol. 29, No. 12, 2010 2639
5-position of the ligand. The new complexes were studied as
precatalysts for the intramolecular hydroamination reaction
in terms of their activity and stability. We were also inter-
ested to learn if a combination of steric modifications of the
alkyl groups R of the {(R)2ATI}- systems and electronic
modifications in the 5-position has an additive effect.
tion we also described the preparation of zinc complexes
with sterically and electronically modified ATI ligands and
studied the influence of structural modifications on their
catalytic activity.129,130 In this context we first investigated
the influence of steric modifications of the substituents R of
the {(R)2ATI}- systems.129,130 We were able to show the
major influence of the steric environment around the zinc
atom on both reactivity and stability of the corresponding
complexes. Complexes bearing ligands with cyclic alkyl
groups, especially cyclohexyl, showed superior activity in a
number of selected reactions with functionalized aminoalk-
enes. In a second project we aimed to increase the activity of
aminotroponiminate zinc complexes by changing the elec-
tronic properties of the ligand. We could demonstrate that
the electronically modified zinc complex 5-phenylsulfanyl-
N-isopropyl-2-(isopropylamino)troponiminate zinc methyl,
[{PhS-ATI(iPr)2}ZnMe], has an increased reactivity in the
intramolecular hydroamination reaction of non-activated
alkenes compared to the parent, nonsubstituted complex.131
Motivated by these preliminary findings, we were inter-
ested in a broader study of the effect of the electronic
properties of the aminotroponiminate ligand on the zinc-
catalyzed intramolecular hydroamination reaction in more
detail. Hence, we herein describe the synthesis and charac-
terization of 10 aminotroponiminate zinc methyl complexes,
[{R-ATI(iPr)2}ZnMe], bearing different substituents in the
Experimental Section
General Procedures. All manipulations of air-sensitive mate-
rials were performed with the rigorous exclusion of oxygen and
moisture in flame-dried Schlenk-type glassware either on a dual
manifold Schlenk line, interfaced to a high-vacuum (10-4 Torr)
line, or in an argon-filled MBraun glovebox. Tetrahydrofuran
was predried over Na wire and distilled under nitrogen from Na/K
benzophenone ketyl prior to use. Hydrocarbon solvents (toluene
and n-pentane) were distilled under nitrogen from LiAlH4. All
solvents for vacuum line manipulations were stored in vacuo
over LiAlH4 in resealable flasks. Deuterated solvents were
obtained from Chemotrade Chemiehandelsgesellschaft mbH
(all g99 atom % D) or Euriso-Top GmbH (all g99 atom %
D) and were dried, degassed, and stored in vacuo over Na/K
alloy in resealable flasks. NMR spectra were recorded on a Jeol
JNM-LA 400 FT NMR or on a Bruker DRX 500 spectrometer.
Chemical shifts are referenced to internal solvent resonances
and are reported relative to tetramethylsilane. Mass spectra
were recorded at 70 eV on a Varian MAT 711. IR spectra were
obtained on a Shimadzu FTIR-8400s or on a Nicolet FT-IR 750
spectrometer. Elemental analyses were carried out with an Ele-
mentar Vario EL III. [PhNMe2H][B(C6F5)4] was purchased from
Strem. ZnMe2 was obtained from Sigma-Aldrich. (1-Allylcyclo-
hexylmethyl)benzylamine,109 benzyl-(2,2-diphenylpent-4-enyl)-
amine,109 (2,2-diphenylpent-4-enyl)furan-2-ylmethylamine,130
(2,2-diphenylpent-4-enyl)thiophen-2-ylmethylamine,130 H-ATI-
(iPr)2H,132 (4-bromo-7-isopropyliminocyclohepta-1,3,5-trien-
yl)isopropylamine, {Br-ATI(iPr)2}H, (1a),131 isopropyl-(7-iso-
propylimino-4-phenylsulfanylcyclohepta-1,3,5-trienyl)amine (5a),131
[{N-isopropyl-2-(isopropylamino)-5-phenylsulfanyltroponiminato}-
methylzinc] (5b),131 and N-cyclohexyl-2-(cyclohexylamino)-
troponimine129 were prepared according to literature procedures.
The synthesis of compounds 2a-10a is described in the Sup-
porting Information.
(108) Bender, C. F.; Hudson, W. B.; Widenhoefer, R. A. Organome-
tallics 2008, 27, 2356–2358.
(109) Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2005, 127,
1070–1071.
(110) Brunet, J.-J.; Chu, N. C.; Diallo, O. Organometallics 2005, 24,
3104–3110.
(111) Brunet, J.-J.; Chu, N.-C.; Rodriguez-Zubiri, M. Eur. J. Inorg.
Chem. 2007, 4711–4722.
(112) Karshtedt, D.; Bell, A. T.; Tilley, T. D. J. Am. Chem. Soc. 2005,
127, 12640–12646.
(113) Reithofer, M. R.; Galanski, M.; Arion, V. B.; Keppler, B. K.
Chem. Commun. 2008, 1091–1093.
(114) Castonguay, A.; Spasyuk, D. M.; Madern, N.; Beauchamp,
A. L.; Zargarian, D. Organometallics 2009, 28, 2134–2141.
(115) Fadini, L.; Togni, A. Chem. Commun. 2003, 30–31.
(116) Fadini, L.; Togni, A. Tetrahedron: Asymmetry 2008, 19, 2555–
2562.
General Procedure for the Synthesis of Complexes 1b-10b. A
solution of the respective aminotroponimine in toluene was
slowly added to an excess solution of ZnMe2 in toluene at 0 °C.
The solution was slowly warmed to room temperature and
stirred for another 3 h. Then, the solution was filtered. After
evaporating the solvents the residue was washed with n-pentane
(2 ꢀ5 mL).
€
(117) Ackermann, L.; Barfusser, S.; Potukuchi, H. K. Adv. Synth.
Catal. 2009, 351, 1064–1072.
(118) Munro-Leighton, C.; Delp, S. A.; Blue, E. D.; Gunnoe, T. B.
Organometallics 2007, 26, 1483–1493.
(119) Ohmiya, H.; Moriya, T.; Sawamura, M. Org. Lett. 2009, 11,
2145–2147.
(120) Taylor, J. G.; Whittall, N.; Hii, K. K. Org. Lett. 2006, 8, 3561–
3564.
(121) Zhou, L.; Bohle, D. S.; Jiang, H.-F.; Li, C.-J. Synlett 2009, 937–
940.
(122) Robinson, R. S.; Dovey, M. C.; Gravestock, D. Tetrahedron
Lett. 2004, 45, 6787–6789.
(123) Kang, J.-E.; Kim, H.-B.; Lee, J.-W.; Shin, S. Org. Lett. 2006, 8,
3537–3540.
5-Bromo-N-isopropyl-2-(isopropylamino)troponiminate Zinc
Methyl (1b). A solution of 1a (0.50 g, 1.77 mmol) in 25 mL of
toluene was slowly added to a solution of ZnMe2 (2.0 M in
toluene, 2 mL, 4.0 mmol) in 15 mL of toluene. Yield: 0.35 g
(55%). 1H NMR (C6D6, 400 MHz): δ (ppm) -0.04 (s, 3 H), 1.06
(d, J =6.3 Hz, 12 H), 3.58 (sept, J=6.3 Hz, 2 H), 6.11 (d, J=
13.4 Hz, 2 H), 7.19 (t, J=12.3 Hz, 2 H). 13C{1H} NMR (C6D6,
100 MHz): δ (ppm) -9.7, 24.0, 48.2, 109.9, 110.3, 136.4, 158.9.
MS (EI, 80 eV): m/z (%) 362 [Mþ] (58), 347 (83), 267 (69), 186
(50), 145 (100). Anal. Calcd for C14H21N2BrZn C: 46.37, H:
5.84, N: 7.73. Found: C: 46.48, H: 5.89, N: 7.53.
(124) Nishina, N.; Yamamoto, Y. Angew. Chem., Int. Ed. 2006, 45,
3314–3317.
(125) Nishina, N.; Yamamoto, Y. Synlett 2007, 1767–1770.
(126) Zhang, J.; Yang, C.-G.; He, C. J. Am. Chem. Soc. 2006, 128,
1798–1799.
5-Iodo-N-isopropyl-2-(isopropylamino)troponiminate Zinc Methyl
(2b). A solution of 2a (0.10 g, 0.3 mmol) in 5 mL of toluene was
slowly added to a solution of ZnMe2 (1.2 M in toluene, 0.3 mL,
(127) Zhang, Y.; Donahue, J. P.; Li, C.-J. Org. Lett. 2007, 9, 627–630.
(128) Zhang, Z.; Lee, S. D.; Widenhoefer, R. A. J. Am. Chem. Soc.
2009, 131, 5372–5373.
1
4.0 mmol) in 5 mL of toluene. Yield: 0.09 g (74%). H NMR
(C6D6, 400 MHz): δ (ppm) -0.04 (s, 3 H), 1.05 (d, J = 6.3 Hz,
12 H), 3.55 (sept, J=6.3 Hz, 2 H), 5.95 (d, J=12.1 Hz, 2 H), 7.36
€
(129) Dochnahl, M.; Lohnwitz, K.; Pissarek, J.-W.; Biyikal, M.;
€
Schulz, S. R.; Schon, S.; Meyer, N.; Roesky, P. W.; Blechert, S.
Chem.-Eur. J. 2007, 13, 6654–6666.
(130) Dochnahl, M.; Pissarek, J.-W.; Blechert, S.; Lohnwitz, K.;
€
Roesky, P. W. Chem. Commun. 2006, 3405–3407.
(131) Dochnahl, M.; Lohnwitz, K.; Pissarek, J.-W.; Roesky, P. W.;
Blechert, S. Dalton Trans. 2008, 2844–2848.
€
(132) Dias, H. V. R.; Jin, W.; Ratcliff, R. E. Inorg. Chem. 1995, 34,
6100–6105.

2640 Organometallics, Vol. 29, No. 12, 2010
Dochnahl et al.
(d, J= 12.2 Hz, 2 H). 13C{1H} NMR (C6D6, 100 MHz): δ (ppm)
-9.9, 24.0, 48.2, 111.34, 128.1, 142.52, 159.3. Anal. Calcd for C14H21-
N2IZn C: 41.05; H: 5.17; N: 6.84. Found C: 41.10; H: 5.56; N: 6.92.
5-Phenylazo-N-isopropyl-2-(isopropylamino)troponiminate Zinc
Methyl (3b). A solution of 3a (0.12 g, 0.4 mmol) in 5 mL of
toluene was slowly added to a solution of ZnMe2 (1.2 M in
toluene, 0.5 mL, 0.6 mmol) in 5 mL of toluene. Yield: 0.15 g
(99%). 1H NMR (C6D6, 400 MHz): δ (ppm) -0.06 (s, 3 H), 1.04
(d, J = 6.2 Hz, 12 H), 3.67 (sept, J = 6.3 Hz, 2 H), 6.54 (d, J =
12.2 Hz, 2 H), 7.12 (d, J=7.4 Hz, 1 H), 7.28 (t, J=7.8 Hz, 2 H),
8.18 (d, J = 7.6 Hz, 2 H), 8.25 (d, J = 12.1 Hz, 2 H). 13C{1H}
NMR (C6D6, 100 MHz): δ (ppm) -10.0, 23.9, 48.4, 109.9, 122.6,
129.1, 129.2, 131.0, 144.3, 153.6, 159.9. MS (EI, 80 eV): m/z (%)
386 [Mþ] (100), 371 (73), 307 (26), 291 (76), 281 (40). HRMS
C20H26N4Zn: calcd 386.14490, found 386.14516.
5-Nitro-N-isopropyl-2-(isopropylamino)troponiminate Zinc Methyl
(4b). A solution of 4a (0.25 g, 1.0 mmol) in 10 mL of toluene was
slowly added to a solution of ZnMe2 (1.2 M in toluene, 1.0 mL,
1.2 mmol) in 10 mL of toluene. Yield: 0.20 g (61%). 1H NMR
(C6D6, 400 MHz): δ (ppm) -0.02 (s, 3 H), 1.07 (d, J=6.3 Hz, 12
H), 3.59 (sept, J=6.1 Hz, 2 H), 6.00 (d, J=12.6 Hz, 2 H), 8.21
(d, J=12.5 Hz, 2 H). 13C{1H} NMR (C6D6, 100 MHz): δ (ppm)
-9.6, 23.7, 49.3, 107.3, 130.2, 136.4, 160.9. MS (EI, 80 eV): m/z
(%) 327 [Mþ] (100), 311 (57), 269 (11), 172 (26), 144 (34). HRMS
C14H21N3O2Zn: calcd 327.09253, found 327.09186.
5-Phenylselenyl-N-isopropyl-2-(isopropylamino)troponiminate
Zinc Methyl (6b). A solution of 6a (0.20 g, 0.7 mmol) in 10 mL of
toluene was slowly added to a solution of ZnMe2 (1.2 M in
toluene, 0.6 mL, 0.7 mmol) in 10 mL of toluene. Yield: 0.21 g
(85%). 1H NMR (C6D6, 400 MHz): δ (ppm) -0.03 (s, 3 H), 1.07
(d, J = 6.3 Hz, 12 H), 3.63 (sept, J = 6.3 Hz, 2 H), 6.28 (d, J =
11.8 Hz, 2 H), 6.91 (d, J=5.7 Hz, 1 H), 6.97 (t, J=7.6 Hz, 2 H),
7.45 (d, J = 7.1 Hz, 2 H), 7.54 (d, J = 12.1 Hz, 2 H). 13C{1H}
NMR (C6D6, 100 MHz): δ (ppm) -10.0, 24.3, 48.4, 111.0, 116.3,
126.3, 127.8, 128.2, 129.5, 130.5, 142.4, 159.9. Anal. Calcd for
C20H26N2SeZn: C: 54.75; H: 5.97; N: 6.38. Found: C: 54.40; H:
6.10; N: 6.30.
49.3, 108.4, 125.0, 127.3, 127.8, 190.0, 130.0, 160.2, 180.8. MS
(EI, 80 eV): m/z (%) 406 [Mþ] (64), 391 (18), 358 (26), 343 (32),
328 (100). HRMS C20H26N2OSZn: calcd 406.10573, found
406.10514. Anal. Calcd for C20H26N2OSZn: C: 58.89; H: 6.42;
N: 6.87. Found: C: 58.48; H: 6.55; N: 6.60.
5-Phenylsulfanyl-N-cyclohexyl-2-(cyclohexylamino)troponiminate
Zinc Methyl (10b). A solution of 10a (0.36 g, 0.9 mmol) in 10 mL
of toluene was slowly added to a solution of ZnMe2 (1.2 M in
toluene, 1.0 mL, 1.2 mmol) in 10 mL of toluene. Yield: 0.26 g
(61%). 1H NMR (C6D6, 400 MHz): δ (ppm) 0.04 (s, 3 H), 1.11
(m, 6 H), 1.39 (m, 4 H), 1.54 (d, J=13.2 Hz, 2 H), 1.62 (m, J=
13.1 Hz, 4 H), 1.79 (d, J=11.2 Hz, 4 H), 3.41 (m, 2 H), 6.48 (d,
J=12.1 Hz, 2 H), 6.88 (t, J=7.4 Hz, 1 H), 7.02 (t, J=7.4 Hz,
2 H), 7.33 (d, J = 7.3 Hz, 2 H), 7.42 (d, J = 12.1 Hz, 2 H).
13C{1H} NMR (C6D6, 100 MHz): δ (ppm) -9.8, 25.8, 26.1, 35.2,
57.4, 110.9, 125.4, 127.0, 127.5, 129.2, 130.7, 141.6, 160.0. MS
(EI, 80 eV): m/z (%) 470 [Mþ] (100), 455 (10), 392 (11), 238 (21).
HRMS C26H34N2SZn: calcd 470.1734, found 470.1742.
General Procedure for the Intramolecular Hydroamination of
Aminoalkenes on an NMR Tube Scale. A predried NMR tube
was charged with the aminoalkene (430 μmol). A solution of the
catalyst (11 μmol, 2.5 mol %) and [PhNMe2H][B(C6F5)4] (9 mg,
11 μmol, 2.5 mol %) in 0.5 mL of C6D6 was added under a
nitrogen atmosphere. The NMR tube was flame-sealed under
vacuum. The reaction mixture was then heated to 80 °C for the
stated duration of time. The reaction progress was monitored by
1H NMR. When the reaction was judged to be complete, the crude
reaction mixture was directly subjected to column chromatography
on silica. All products were analyzed by 1H and 13C NMR, IR,
MS, and HR-MS. The NMR yields were determined by com-
paring the integration of a well-resolved signal for the starting
material with a well-resolved signal for the heterocyclic product.
Ferrocene was used as internal standard for selected reactions
(see Table 2).
X-ray Crystallographic Studies of 1b, 2b, 6b, and 10b. A suit-
able crystal was covered in mineral oil (Aldrich) and mounted on
a glass fiber. The crystal was transferred directly to the -73 °C
cold stream of a STOE IPDS 2T diffractometer. Subsequent com-
putations were carried out on an Intel Pentium Core2Duo PC.
All structures were solved by the Patterson method (SHELXS-
97133). The remaining non-hydrogen atoms were located from
successive differences in Fourier map calculations. The refine-
ments were carried out by using full-matrix least-squares tech-
niques on F, minimizing the function (Fo - Fc)2, where the
weight is defined as 4Fo2/2(Fo2) and Fo and Fc are the observed
and calculated structure factor amplitudes using the program
SHELXL-97.133 Carbon-bound hydrogen atom positions were
calculated. The hydrogen atom contributions were calculated,
but not refined. The final values of refinement parameters are
given in the Supporting Information. The locations of the
largest peaks in the final difference Fourier map calculation as
well as the magnitude of the residual electron densities in each
case were of no chemical significance. Positional parameters, hydro-
gen atom parameters, thermal parameters, and bond distances
and angles have been deposited as Supporting Information.
5-Phenyltellanyl-N-isopropyl-2-(isopropylamino)troponiminate
Zinc Methyl (7b). A solution of 7a (0.35 g, 0.7 mmol) in 10 mL of
toluene was slowly added to a solution of ZnMe2 (1.2 M in
toluene, 1.5 mL, 1.8 mmol) in 5 mL of toluene. Yield: 0.30 g
1
(73%). H NMR (d8-THF 400 MHz): δ (ppm) -0.67 (s, 3 H),
1.24 (d, J=6.3 Hz, 12 H), 3.99 (sept, J=6.2 Hz, 2 H), 6.27 (d,
J=11.7 Hz, 2 H), 7.10 (m, 3 H), 7.51 (m, 2 H), 7.58 (d, J=11.8
Hz, 2 H). 13C{1H} NMR (d8-thf, 100 MHz): δ (ppm) -11.6, 24.2,
49.4, 97.5, 110.8, 119.0, 127.4, 129.9, 136.2, 147.8, 161.0. MS
(EI, 80 eV): m/z (%) 487 [Mþ] (54), 473 (29), 367 (47), 316 (21),
207 (38), 91 (100). Anal. Calcd for C20H26N2TeZn: C: 49.28; H:
5.38; N: 5.75. Found: C: 49.00; H: 5.58; N: 5.45.
5-(3,5-Bis(trifluoromethyl)phenylsulfanyl)-N-isopropyl-2-(iso-
propylamino)troponiminate Zinc Methyl (8b). A solution of 8a
(0.10 g, 0.7 mmol) in 5 mL of toluene was slowly added to a
solution of ZnMe2 (1.2 M in toluene, 1.0 mL, 1.2 mmol) in 5 mL
of toluene. Yield: 0.34 g (96%). 1H NMR (C6D6, 400 MHz): δ
(ppm) -0.06 (s, 3 H), 1.18 (d, J=6.3 Hz, 12 H), 3.70 (sept, J=
6.7 Hz, 2 H), 6.29 (d, J=11.9 Hz, 2 H), 7.28 (d, J=7.3 Hz, 2 H),
7.42 (s, 1 H), 7.61 (s, 2 H). 13C{1H} NMR (C6D6, 100 MHz): δ
(ppm) -10.0, 23.8, 48.4, 110.4, 113.9, 125.4, 127.6, 128.1, 132.3,
141.5, 146.0, 160.0. MS (EI, 80 eV): m/z (%) 526 [Mþ] (16), 448
(36), 433 (35), 405 (100), 389 (42). HRMS C22H24N2F6SZn:
calcd 526.08557, found 526.08490.
5-Benzenesulfinyl-N-isopropyl-2-(isopropylamino)troponiminate
Zinc Methyl (9b). A solution of 9a (0.24 g, 0.7 mmol) in 10 mL of
toluene was slowly added to a solution of ZnMe2 (1.2 M in
toluene, 1.0 mL, 1.2 mmol) in 5 mL of toluene. Yield: 0.29 g
(98%). 1H NMR (C6D6, 400 MHz): δ (ppm) 0.20 (s, 3 H), 1.40
(d, J=6.2 Hz, 6 H), 1.45 (d, J=6.2 Hz, 6 H), 3.78 (sept, J=6.5
Hz, 2 H), 5.96 (d, J=12.0 Hz, 2 H), 6.93 (t, J=7.3 Hz, 1 H), 7.01
(m, J=7.9 Hz, 2 H), 7.15 (d, J=7.2 Hz, 2H), 7.35 (d, J=7.2 Hz,
2 H). 13C{1H} NMR (C6D6, 100 MHz): δ (ppm) -10.5, 23.9,
Results and Discussion
Ligand Synthesis A. Tropolone and aminotroponimine are
electron-rich systems, which can undergo electrophilic aro-
matic substitution reactions.134,135 Whereas the reaction of
tropolone was reported to be unselective toward a bromi-
nation reaction,136 early literature reports suggest that the
(133) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–122.
(134) Sainsbury, M. In Aromatic Chemistry; Oxford University Press:
Oxford, 1992.
(135) Taylor, R. J. K. In Electrophilic Aromatic Substitution; Wiley:
Chichester, 1990.
(136) Pauson, P. L. Chem. Rev. 1955, 55, 9–136.

Article
Organometallics, Vol. 29, No. 12, 2010 2641
Scheme 2
Figure 1. Perspective ORTEP view of the molecular structure
of 1b. Thermal ellipsoids are drawn to encompass 50% proba-
bility. Hydrogen atoms are omitted for clarity. Shown is one of
the two independent molecules in the asymmetric unit. Selected
˚
distances [A] and angles [deg]: N1-Zn 1.971(6), N2-Zn 1.977(5),
Zn2-N3 1.967(6), Zn2-N4 1.974(6), Zn1-C14 1.941(7),
Zn2-C28 1.932(7); N1-Zn-C14 139.6(3), N2-Zn-C14
138.9(3), N1-Zn-N2 81.4(2), N3-Zn2-C28 141.0(3), N4-
Zn2-C28 137.8(3), N3-Zn2-N4 81.2(2).
Figure 2. Perspective ORTEP view of the molecular structure
of 2b. Thermal ellipsoids are drawn to encompass 50% proba-
bility. Shown is one of the two independent molecules in the
˚
asymmetric unit. Selected bond lengths [A] and angles [deg]:
N1-Zn 1.971(2), N2-Zn 1.977(2), Zn2-N3 1.978(5), Zn2-N4
1.963(5), Zn-C14 1.953(3), Zn2-C28 1.943(6); N1-Zn-N2
82.57(9), N1-Zn-C14 139.37(10), N2-Zn-C14 137.35(10);
N4-Zn2-C28 137.0(3), N3-Zn2-C28 140.9(3), N3-Zn2-N4
82.0(2).
Scheme 3
same type of reaction with aminotroponimine should be
much more selective.1,2 Halogenated aminotroponimines
then can be further reacted with nucleophiles to form other
derivatives.1 Since we aimed to increase the activity of amino-
troponiminate zinc complexes by changing the electronic
properties of the ligand, we used N-isopropyl-2-(isopropyl-
amino)troponimine, {H-ATI(iPr)2}H, as starting material to
have later the possibility to compare the new system with the
first-generation catalyst [H-ATI(iPr)2ZnMe] (I).6 At a later
stage of the project, we synthesized an additional aminotro-
ponimine ligand with N-cyclohexyl groups because we were
also interested to learn if a combination of steric modifications
of the alkyl groups R in the {(R)2ATI}- systems and electronic
modifications in the 5-position has an additive effect.129,130
We previously reported that the reaction of {H-ATI(iPr)2}H
with bromine at 0 °C selectively resulted in a bromination
in the 5-position (Scheme 2).131 The resulting product {Br-
ATI(iPr)2}H (1a) was isolated in 90% yield. The reaction was
Figure 3. Perspective ORTEP view of the molecular structure
of 6b. Thermal ellipsoids are drawn to encompass 50% proba-
bility. Selected bond lengths [A] and angles [deg]: N1-Zn
1.986(2), N2-Zn 1.992(2), Zn-C14 1.953(3); N1-Zn-N2
81.56(9), N1-Zn-C14 138.07(13), N2-Zn-C14 138.45(13).
˚
performed at a scale of up to 30 mmol. In contrast, reactions
with other electrophiles worked less smoothly. Thus, the
reaction of {H-ATI(iPr)2}H with iodine monochloride gave
the corresponding iodo species, {I-ATI(iPr)2}H (2a), in
about 43% yield. The azo coupling of {H-ATI(iPr)2}H with

2642 Organometallics, Vol. 29, No. 12, 2010
Table 1. Cyclization of (1-Allylcyclohexylmethyl)benzylaminea)
Dochnahl et al.
entry
catalyst
time [h]
conv [%]b
TOF [h-1
]
1
2
3
4
5
6
7
8
9
10
[{H-ATI(iPr)2}ZnMe] (I)
[{Br-ATI(iPr)2}ZnMe] (1b)
[{I-ATI(iPr)2}ZnMe] (2b)
[{PhN2-ATI(iPr)2}ZnMe] (3b)
[{NO2-ATI(iPr)2}ZnMe] (4b)
[{PhS-ATI(iPr)2}ZnMe] (5b)
[{PhSe-ATI(iPr)2}ZnMe] (6b)
[{PhTe-ATI(iPr)2}ZnMe] (7b)
[{(CF3)2C6H3S-ATI(iPr)2}ZnMe] (8b)
[{PhS(O)-ATI(iPr)2}ZnMe] (9b)
0.5
0.75
10
10
10
2
10
10
10
quant.
quant.
59
93
16
quant.
94
97
95
6
80129
53
2.4
3.7
0.6
20131
3.8
3.9
3.8
0.2
10
a Reaction conditions: 2.5 mol % of catalyst and 2.5 mol % of [PhNMe2H][B(C6F5)4] in 0.5 mL of C6D6 at 80 °C. b Determined by 1H NMR.
phenyl diazonium tetrafluoroborate137 resulted in deep red
{PhN2-ATI(iPr)2}H (3a) in 24% yield, and the reaction
stopped, which was usually the case after three hours. Under
these conditions the reaction exclusively led to the formation
of methylzinc complexes of general composition [{R-ATI-
(iPr)2}ZnMe] (R=Br (1b), I (2b), PhN2 (3b), NO2 (4b), PhSe
(6b),PhTe(7b),3,5-(CF3)2C6H3S(8b),PhS(O) (9b)) (Scheme3).
The 1H and 13C{1H} NMR spectra of all compounds showed
the expected set of signals. The chemical shift of the Zn-Me
groups was observed for all compounds in a characteristic
138
with NO2BF4 gave [NO2-ATI(iPr)2]H (4a) in 19% yield
(Scheme 2). With respect to the yields all reactions were not
optimized.
The subsequent nucleophilic aromatic substitution reac-
tions were performed by treatment of the bromo compound
1a with different nucleophiles. It has been reported before
that the nucleophilic displacement with thiophenol in the pre-
sence of potassium carbonate in DMF proceeded smoothly
and delivered the desired ligand {PhS-ATI(iPr)2}H (5a) in
quantitative yields from 1a.131 The analogous reactions with
in situ generated PhSeH139 and PhTeH140 gave the corres-
ponding species {PhSe-ATI(iPr)2}H (6a) and {PhTe-ATI-
(iPr)2}H (7a), respectively, in more than 90% yield. Obvi-
ously, the nucleophilic substitution reaction of 5a effectively
works with very soft nucleophiles. Thus, the reaction of 5a
with the soft nucleophile 3,5-bis(trifluormethyl)thiophenol
resulted in {3,5-{(CF3)2C6H3S-ATI(iPr)2}H (8a) in high
yield, whereas no reaction with the harder nucleophile 2,4-
dimethoxythiophenol was observed under the same condi-
tions. To tune the donor strength of the substituent in the
5-position, we finally oxidized the sulfur atom of compound
5a. Oxidation of 5a with mCPBA in CH2Cl2 at 0 °C resulted
in the sulfoxide {PhS(O)-ATI(iPr)2}H (9a) in good yield
(Scheme 2).
1
region. Thus, in the H NMR this signal was seen around
0 ppm, whereas in the 13C{1H} NMR this signal was found
between -9.6 and -10.0 ppm. This observation is in agree-
ment with the chemical shifts observed for compound I (δ 1H
0.00 ppm; 13C{1H} -9.89 ppm).141 The EI mass spectra show
the molecular ion peak [Mþ] for all compounds whose MS
data were acquired.
Furthermore, the structures of 1b, 2b, and 6b were con-
firmed by single-crystal X-ray diffraction in the solid state
(Figures 1-3). Data collection parameters and selected bond
lengths and angles are given in Table 1 and in the captions of
Figures 1-3. Compounds 1b and 2b are isostructural to each
other. Both crystallize in the monoclinic space group P21/c
with two independent molecules of the corresponding zinc
complex in the asymmetric unit. As observed for other amino-
troponiminate zinc compounds, the metal atoms of all three
complexes are coordinated in a trigonal-planar fashion.
˚
The Zn-N distances are between 1.967(6) and 1.992(2) A,
comparable to the previously published ATI zinc com-
Attempts to replace the bromine atom with less-stabilized
nucleophiles based on nitrogen and oxygen were all met with
failure.
pounds.6,129-131,142 The Zn-C distances are in the range
1.932(7) to 1.953(3) A. These distances are typical for zinc
˚
methyl bonds.6,129-131,142
Metal Complexes A. The previously reported complexes
[{H-ATI(iPr)2}ZnMe] (I) and [{PhS-ATI(iPr)2}ZnMe] (5b)
were prepared by the reaction of {(iPr)2ATI}H and [{PhS-
ATI(iPr)2}H, respectively, with ZnMe2 in high yields.131,141
In a similar synthetic procedure we reacted the neutral
aminotroponimines {R-ATI(iPr)2}H (1a-4a and 6a-9a)
with an excess of ZnMe2 in toluene at 0 °C. All reactions
were judged to be complete after the gas evolution had
Catalysis A. The above synthesized complexes were used as
precatalysts for the cyclization of (1-allylcyclohexylmethyl)-
benzylamine in order to study the influence of the electronic
modifications of compounds 1b-9b on the hydroamination
cyclization reaction. The reactions were performed under
previously established conditions. All reactions were carried
out in C6D6 at 80 °C with a low catalyst loading of 2.5 mol %.
The reactions were conducted with one equivalent (with
respect to the catalyst) of [PhNMe2H][B(C5F5)4] as a cocata-
lyst. Theresults of thisscreening, reactiontimes and yields, are
showninTable1 andFigure 4. Obviously, the results obtained
with the different complexes differ significantly, and the
catalysts can be subdivided into different groups of reactivity.
(137) Dunker, M. F. W.; Starkey, E. B.; Jenkins, G. L. J. Am. Chem.
Soc. 1936, 58, 2308–2309.
(138) Olah, G. A. J. Org. Chem. 2005, 70, 2413–2429.
(139) Gysling, H. J.; Luss, H. R. Organometallics 1989, 8, 363–368.
(140) Detty, M. R.; Murray, B. J.; Smith, D. L.; Zumbulyadis, N.
J. Am. Chem. Soc. 1983, 105, 875–882.
(141) Herrmann, J.-S.; Luinstra, G. A.; Roesky, P. W. J. Organomet.
Chem. 2004, 689, 2720–2725.
(142) Meyer, N.; Roesky, P. W. Organometallics 2009, 28, 306–311.

Article
Organometallics, Vol. 29, No. 12, 2010 2643
Table 2. Comparison of the Reactivity of the New Catalysts
Established Systemsa
Figure 4. Cyclization of (1-allylcyclohexylmethyl)benzylamine
using compounds I and 1b-9b.
Using compounds 4b and 9b as catalysts (entries 5 and 10)
resulted in very low conversions of less than 20%. The iodo-
substituted complex gave about 60% conversion, whereas all
other complexes catalyzed the reaction to at least 93% con-
version to the desired product. The highest turnover frequen-
cies (TOF) were observed with the first-generation catalysts I
(entry 1) as well as with the bromo (1b, entry 2) and the
phenylsulfanyl compounds (5b, entry 6). Interestingly the
brominated compound 1b showed a high initial activity.
However, a detailed kinetic investigation revealed that this
compound was very labile under the used reaction conditions,
leading to a rapidly decreasing catalytic activity. Apparently
some rapid decomposition of the catalyst took place. As a
result of this decomposition process, the measured results in
the catalysis tests lack sufficient reproducibility. To get more
insight into the decomposition process, the reaction mixture
with complex 1b was analyzed after aqueous workup. It could
be shown by NMR and MS that the bromine atom had been
replaced by a hydrogen atom. Next the ligand precursor 1a
was heated with the amine 11 for 20 h at 120 °C. In contrast to
earlier observations by Schafer et al. no reaction and thus no
nucleophilic substitution by the substrate was oberserved.143
Heating complex 1b without [PhNMe2H][B(C6F5)4] for 16 h
at 80 °C did not result in any decomposition of the catalyst.
Thereforewe suggest thatundercatalyticconditionsthe active
zinc species reacts in an intermolecular fashion with the
bromine atom of the ligand.
These results led us to the following conclusions: (i) the
substituent in the 5-position has a significant influence on the
reaction rate. The effects are much greater than the influence
of different groups at the nitrogen atoms of the aminotro-
poniminate ligands. (ii) The incorporation of electron accep-
tors such as nitro or sulfoxide groups in the 5-position
decreased the activity of the corresponding zinc catalyst.
This is most impressively seen by a comparison of com-
pounds 5b and 9b. A simple oxidation of the sulfur atom has
a dramatic influence on the activity. The TOF is decreased by
about 2 orders of magnitude.
The low activity of catalysts bearing electron-accepting
groups can be rationalized by the Lewis basic nature of these
substituents. Accordingly, they may intermolecularly coor-
dinate to the zinc atom of another complex and thus block
the vacant coordination sites for incoming substrate mole-
cules. To support this assumption, the catalytic conversion
of substrate 11 with complex I as catalyst was repeated in the
a Reagents and conditions: substrate(430 μmol), catalyst (2.5 mol %),
1
[PhNMe2H][B(C6F5)4] (2.5 mol %), C6D6, 80 °C. b Determined by H
NMR. c Isolated yield. d The reaction was performed with 0.8 mol %
[PhNMe2H][B(C6F5)4]. e The reaction was additionally performed with
ferrocene as an internal standard.
(143) Bexrud, J. A.; Li, C.; Schafer, L. L. Organometallics 2007, 26,
6366–6372.

2644 Organometallics, Vol. 29, No. 12, 2010
Dochnahl et al.
Scheme 4
Figure 5. Perspective ORTEP view of the molecular structure
of 10b. Thermal ellipsoids are drawn to encompass 50% proba-
presence of catalytic amounts of DMSO to mimic the sulfinyl
group in compound 9b and in the presence of catalytic
amounts of nitrobenzene to mimic the nitro group in com-
pound 4b, respectively. No catalytic conversion was observed
in the presence of DMSO, whereas in the presence of nitro-
benzene a significantly lower reaction rate was observed.
The effects of the electron-donating substituents are less
obvious. Electron-donating substituents increase the electron
density of the ligand and thus decrease the Lewis acidity of the
metal. We suggest that the numbers of substrate molecules,
which are coordinated to the metal center, are thus decreased. It
was shown earlier that in hydroamination cyclization catalysis
the cyclization of the substrate is facilitated by a sterically less
hindered metal center.144,145 Thus, a low-coordinated zinc
species should be more reactive than a higher coordinated one.
Ligand Synthesis and Metal Complex B. In the second
stage of our investigation, we wanted to use aminotroponi-
mines with other N-alkyl groups as starting material for the
functionalization in the 5-position. Since we have shown that
complexes with ligands bearing cyclohexyl groups at the
nitrogen atoms showed superior activities in a number of
selected reactions with functionalized aminoalkenes,129,130
we used N-cyclohexyl-(2-cyclohexylamino)troponimine, {H-
ATI(Cy)2}H, as starting material. In a procedure analogous
to the synthesis of compounds 1a and 5a the cyclohexyl
derivatives 4-bromocyclohexyl-(7-cyclohexyliminocyclohepta-
1,3,5-trienyl)amine, {Br-ATI(Cy)2}H, and cyclohexyl-(7-cyclo-
hexylimino-4-phenylsulfanylcyclohepta-1,3,5-trienyl)amine
{PhS-ATI(Cy)2}H (10a) were synthesized (Scheme 4). High
yields were obtained in both steps (Scheme 4). The zinc com-
pound [{PhS-ATI(Cy)2}ZnMe] (10b) was obtained as a yellow
crystalline, analytically pure solid, which was characterized
by 1H and 13C NMR spectroscopy. The NMR spectra showed
the expected set of signals with the characteristic low-field-
shifted signal of the Zn-Me group and were observed for
˚
bility. Selected bond lengths [A] and angles [deg]: N1-Zn
1.982(4), N2-Zn 1.979(4), Zn-C26 1.935(5); N1-Zn-N2
82.2(2), N1-Zn-C26 137.5(2), N2-Zn-C26 139.5(2).
Catalysis B. To have a better understanding of the different
influences of both steric and electronic modifications of sub-
stituents attached to the aminotroponimine on the catalytic
activity of the corresponding zinc complex in the intramole-
cular hydroamination reaction of non-activated terminal
aminoolefins, we screened different classes of compounds
(Table 2). As reference systems we used the first-generation
catalyst I and the cyclohexyl complex [N-cyclohexyl-
2-(cyclohexylamino)troponiminato] zinc methyl, [{(Cy)2ATI}-
ZnMe] (II), which was reported earlier to be a very active
system. In addition we employed those compounds that had
previously displayed the highest activities (see Table 1), i.e.,
compounds 1b, 5b, and 6b. However, as already mentioned
the application of 1b is of limited use because of its rapid
decomposition under the reaction conditions. Finally, we
also tested compound 10b as catalyst to study if a combina-
tion of steric and electronic modifications of the ligand had
an additive effect. The results of this screening are shown in
Table 2. The reactions were carried out at 80 °C in benzene-d6
with a catalyst loading of 2.5 mol % and 2.5 mol % of
[PhNMe2H][B(C6F5)4]. Ferrocene was used as internal stan-
dard for selected examples. The isolated yields were around
90% or more in all the investigated cases. The reaction times
greatly varied depending on both the substrate and the
catalyst tested. As already mentioned, the bromine-substi-
tuted catalyst 1b (entries 3, 9, and 15) decomposed rapidly.
Thus, a significant amount of time was needed to reach
quantitative conversions. The phenylsulfanyl complex 5b
(entries 4, 10, 16, and 21) was superior to the nonsubstituted
first-generation catalyst I (entries 1, 7, 13, and 19) except for
substrate 13. The reactions with complexes 5b and II (entries
2, 8, 14, and 20) showed that they displayed comparable
activities for substrates 11, 12, and 14. Replacement of the
sulfur atom in compound 5b by a selenium atom in com-
pound 6b had a detrimental effect on the catalytic activity
(entries 5, 11, and 17), as the observed reaction times for
compound 6b are much higher. The logical combination of
both modifications that had previously been shown to be bene-
ficial, i.e., a phenylsulfanyl at the 5-position and cyclohexyl
substituents at the nitrogen atoms, was realized in compound
10b. The observed catalytic results (entries 6, 12, and 18) did
not serve as a proof-of-concept for such additive effects.
Instead of the expected decrease of the reaction times
compared to the first-generation catalysts I (entries 1, 7,
13, and 19), a significant increase was observed. Obviously
each effect, the increase of steric bulk of the ligand and the
1
all compounds in a characteristic region (δ H 0.04 ppm;
13C{1H} -9.8 ppm).
Furthermore, the structure of compound 10b was con-
firmed by single-crystal X-ray diffraction in the solid state
(Figures 5). Data collection parameters and selected bond
lengths and angles are given in Table 1 and in the captions of
Figure 5. Compound 5 crystallizes in the monoclinic space
group P21/c with four molecules in the unit cell. As observed
for other compounds 1b, 2b, and 6b, the zinc atom is
coordinated in a trigonal-planar fashion showing Zn-N
˚
˚
bond distances of N1-Zn 1.982(4) A and N2-Zn 1.979(4) A
˚
and a Zn-C bond distance of 1.935(5) A.
(144) Gagne, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc.
1992, 114, 275–294.
(145) Tobisch, S. J. Am. Chem. Soc. 2005, 127, 11979–11988.

Article
Organometallics, Vol. 29, No. 12, 2010 2645
increase of electron density of the metal center, separately
increases the activity of the catalysts. We suggest that each
effect lowers the coordination number on the zinc atom and
thus facilitates the cyclization reaction, but no additive
effects were observed.
bearing ATI ligands with cyclohexylamino groups gave
superior activities in a number of selected reactions with
functionalized aminoalkenes, we synthesized the corres-
ponding complex bearing 5-phenylsulfanyl and cyclohexyl
groups. An additional effect of both beneficial modifica-
tions on the catalytic activity of the zinc complexes was not
observed. As an interpretation of these results, we suggest
that each effect, the increase of the steric bulk of the ligand
and the increase of electron density, lowers the coordina-
tion number of the zinc atom and thus increases the cata-
lytic activity. On the other hand, it is not surprising that
both effects do not sum up.
Summary
In summary, we have prepared 10 substituted aminotro-
ponimines that have different functional groups in the
5-position. We studied the complexation of these com-
pounds to zinc and investigated the catalytic activity of
the corresponding zinc complexes. We have demonstrated
that electronic modifications of the ligand have a high
impact on the catalytic activity of the zinc complexes. The
attachment of electron-withdrawing groups such as nitro or
sulfoxide groups in the 5-position significantly decreased
the activity of the corresponding zinc catalyst. In contrast,
donor substituents such as a thioether moiety at the back-
bone of the ligand, which increased the electron density at
the nitrogen and the zinc atoms, also rendered the zinc atom
more reactive. On the other hand, the brominated com-
pound is very labile under the used catalytic conditions.
Since it had previously been shown that zinc complexes
Acknowledgment. This work was supported by the
Cluster of Excellence “Unifying Concepts in Catalysis”
€
coordinated by the Technische Universitat Berlin, by the
Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie.
Supporting Information Available: Experimental details of the
synthesis of compounds 2a-10a and X-ray crystallographic
files in CIF format for the structure determinations of 1b, 2b,
6b, and 10b are available free of charge via the Internet at http://
pubs.acs.org.