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Volumn 129, Issue 6, 2007, Pages 1776-1783

TiIV-mediated reactions between primary amines and secondary carboxamides: Amidine formation versus transamidation

Author keywords

[No Author keywords available]

Indexed keywords

CATALYSIS; COMPLEXATION; REACTION KINETICS; STOICHIOMETRY; TITANIUM COMPOUNDS;

EID: 33846985315     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja0650293     Document Type: Article
Times cited : (76)

References (62)
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    • Equilibrium-controlled manipulation of other functional groups is the subject of significant current interest. For reviews, see: (a) Lehn, J.-M. Chem, Eur. J. 1999, 5, 2455-2463
    • Equilibrium-controlled manipulation of other functional groups is the subject of significant current interest. For reviews, see: (a) Lehn, J.-M. Chem. - Eur. J. 1999, 5, 2455-2463.
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    • Most precedents consist of intramolecular reactions. See, for example: (a) Çalimsiz, S.; Lipton, M. A. J. Org. Chem. 2005, 70, 6218-6221.
    • Most precedents consist of intramolecular reactions. See, for example: (a) Çalimsiz, S.; Lipton, M. A. J. Org. Chem. 2005, 70, 6218-6221.
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    • For intermolecular precedents, see: a
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    • (1952) J. Polym. Sci , vol.8 , pp. 395-407
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    • 4 catalyzes the hydroamination of alkynes and alkenes, reactions that probably proceed via imidotitanium intermediates. For leading references, see: (a) Shi, Y.; Ciszewski, J. T.; Odom, A. L. Organometallics 2001, 20, 3967-3969.
    • 4 catalyzes the hydroamination of alkynes and alkenes, reactions that probably proceed via imidotitanium intermediates. For leading references, see: (a) Shi, Y.; Ciszewski, J. T.; Odom, A. L. Organometallics 2001, 20, 3967-3969.
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    • In light of these results, it is noteworthy that Schafer and co-workers recently reported the first example of Ti and Zr imido complexes bearing ancillary amidate ligands. The lack of amidine formation from these complexes, even at elevated temperatures (up to 140°C), presumably reflects the presence of the extremely bulky 2,6-diisopropylphenyl substituents on the amidate nitrogen atoms. See: Thomson, R. K.; Bexrud, J. A.; Schafer, L. L. Organometallics 2006, 25, 4069-4071.
    • In light of these results, it is noteworthy that Schafer and co-workers recently reported the first example of Ti and Zr imido complexes bearing ancillary amidate ligands. The lack of amidine formation from these complexes, even at elevated temperatures (up to 140°C), presumably reflects the presence of the extremely bulky 2,6-diisopropylphenyl substituents on the amidate nitrogen atoms. See: Thomson, R. K.; Bexrud, J. A.; Schafer, L. L. Organometallics 2006, 25, 4069-4071.
  • 38
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    • As noted by a reviewer, we observed catalyst-dependent chemoselectivity in our initial discovery of transamidation (ref 6) - namely, alkylamide substrates are more reactive with Al catalysts, whereas arylamides are more reactive with Ti complexes. The present study focuses exclusively on Ti chemistry and, therefore, does not address the origin of transamidation chemoselectivity. The latter issue will be the focus of future work.
    • As noted by a reviewer, we observed catalyst-dependent chemoselectivity in our initial discovery of transamidation (ref 6) - namely, alkylamide substrates are more reactive with Al catalysts, whereas arylamides are more reactive with Ti complexes. The present study focuses exclusively on Ti chemistry and, therefore, does not address the origin of transamidation chemoselectivity. The latter issue will be the focus of future work.
  • 44
    • 1242272178 scopus 로고    scopus 로고
    • For similar structural parameters observed in κ1-amidate structures of aluminum and lithium, see a
    • 1-amidate structures of aluminum and lithium, see (a) Peng, Y.; Bai, G.; Fan, H.; Vidovic, D.; Roesky, H. W.; Magull, J. Inorg. Chem. 2004, 43, 1217-1219.
    • (2004) Inorg. Chem , vol.43 , pp. 1217-1219
    • Peng, Y.1    Bai, G.2    Fan, H.3    Vidovic, D.4    Roesky, H.W.5    Magull, J.6
  • 46
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    • After these reactions were complete, a small amount of water was added to quench the Ti catalyst, and the organic layer was analyzed by gas chromatography. No evidence for the presence of free Cp*H ligand was found. We conclude from this result that a [Cp*Ti]-based complex accounts for the observed transamidation, not a small amount of a Cp*-free Ti complex that forms under the reaction conditions.
    • After these reactions were complete, a small amount of water was added to quench the Ti catalyst, and the organic layer was analyzed by gas chromatography. No evidence for the presence of free Cp*H ligand was found. We conclude from this result that a [Cp*Ti]-based complex accounts for the observed transamidation, not a small amount of a Cp*-free Ti complex that forms under the reaction conditions.
  • 49
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    • Further support for (amidate(titanium species under catalytic conditions is obtained from kinetic studies. Both Ti- and Al-catalyzed transamidation reactions exhibit a zero-order rate dependence on [carboxamide], consistent with the proposal that the amide substrate is complexed to the catalytic metal center prior to the rate-determining step (ref. 7b for detailed discussion).
    • Further support for (amidate(titanium species under catalytic conditions is obtained from kinetic studies. Both Ti- and Al-catalyzed transamidation reactions exhibit a zero-order rate dependence on [carboxamide], consistent with the proposal that the amide substrate is complexed to the catalytic metal center prior to the rate-determining step (ref. 7b for detailed discussion).
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