Aluminium alkyl complexes supported by imino-phosphanamide ligand as precursors for catalytic guanylation reactions of carbodiimides

• Published in: RSC advances Vol. 12; no. 8; pp. 451 - 459
• Main Authors: Karmakar, Himadri, Anga, Srinivas, Panda, Tarun K, Chandrasekhar, Vadapalli
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 03-02-2022; The Royal Society of Chemistry
• Subjects: Aluminum; Ambient temperature; Amines; Carbodiimides; Catalysts; Chemical synthesis; Chemistry; Coordination compounds; Dipping; Functional groups; Guanidines; Ligands; Molecular structure; Precursors; Single crystals; Substrates; Toluene
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/d2ra00242f

Herein, we report the synthesis, characterisation, and application of three aluminium alkyl complexes, [κ
2
-{NHI
R
P(Ph)NDipp}AlMe
2
] (R = Dipp (
2a
), Mes (
2b
);
t
Bu (
2c
), Dipp = 2,6-diisopropylphenyl, Mes = mesityl, and
t
Bu =
tert
-butyl), supported by unsymmetrical imino-phosphanamide [NHI
R
P(Ph)NDipp]
−
[R = Dipp (
1a
), Mes (
1b
),
t
Bu (
1c
)] ligands as molecular precursors for the catalytic synthesis of guanidines using carbodiimides and primary amines. All the imino-phosphanamide ligands
1a
,
1b
and
1c
were prepared in good yield from the corresponding N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenylphosphanamine, PhP(Cl)NHDipp. The aluminium alkyl complexes
2a
,
2b
and
2c
were obtained in good yield upon completion of the reaction between trimethyl aluminium and the protic ligands
1a
,
1b
and
1c
in a 1 : 1 molar ratio in toluene
via
the elimination of methane, respectively. The molecular structures of the protic ligands
1b
and
1c
and the aluminium complexes
2a
,
2b
and
2c
were established
via
single-crystal X-ray diffraction analysis. Complexes
2a
,
2b
and
2c
were tested as pre-catalysts for the hydroamination/guanylation reaction of carbodiimides with aryl amines to afford guanidines at ambient temperature. All the aluminium complexes exhibited a high conversion with 1.5 mol% catalyst loading and broad substrate scope with a wide functional group tolerance during the guanylation reaction. We also proposed the most plausible mechanism, involving the formation of catalytically active three-coordinate Al species as the active pre-catalyst.
Three aluminium alkyl complexes, [κ
2
-{Im
R
NP(Ph)NDipp}AlMe
2
] (
2a-2c
), supported by unsymmetrical imino-phosphanamide were synthesised and utilised as competent precatalysts for the hydroamination of carbodiimides under ambient conditions.