Chiral multidentate oxazoline ligands based on cyclophosphazene cores: synthesis, characterization and complexation studies.

Dheeraj Kumar, Jatinder Singh, Anil J Elias

Index: Dalton Trans. 43(37) , 13899-912, (2014)

Full Text: HTML

Abstract

Chiral oxazoline based bi and hexadentate ligands built on cyclophosphazene cores have been synthesized and characterized. (NPPh2)2[NP(m-OC6H4C(O)OCH3)2] (1) was prepared by the reaction of gem-(NPPh2)2(NPCl2) with methyl-3-hydroxy benzoate in the presence of Cs2CO3. Compound 1 was converted to the dicarboxylic acid (NPPh2)2[NP(m-OC6H4C(O)OH)2] (2) by base promoted hydrolysis with KO(t-Bu). The dicarboxylic acid 2 on reaction with oxalyl chloride followed by (S)-(+)-2-amino-3-methyl-1-butanol, triethylamine and mesyl chloride was converted to the C2-symmetric phosphazene based chiral bisoxazoline ligand (NPPh2)2[NP{m-OC6H4(4-iPr-2-Ox)}2] (3) (Ox = oxazolinyl). A similar C2-symmetric bisoxazoline derivative having an oxazoline group attached to the para position of the phenyl ring was also synthesized starting from (NPPh2)2[NP(p-OC6H4C(O)OCH3)2] (4) which was first converted to the dicarboxylic acid (NPPh2)2[NP(p-OC6H4C(O)OH)2] (5) and finally to (NPPh2)2[NP{p-OC6H4(4-iPr-2-Ox)}2] (6) and (NPPh2)2[NP{p-OC6H4(4-Ph-2-Ox)}2] (7) under similar reaction conditions. Reaction of 6 with Pd(OAc)2 in acetic acid at room temperature and with PdCl2(C6H5CN)2 in refluxing benzene resulted in chiral palladium complexes Pd(OAc)2(NPPh2)2[NP{p-OC6H4(4-iPr-2-Ox)}2] (8) and PdCl2(NPPh2)2[NP{p-OC6H4(4-iPr-2-Ox)}2] (9), respectively. The utility of these palladium complexes as chiral catalysts for the asymmetric rearrangement of trichloroacetimidates to trichloroacetamides has been explored. The hexa(methylbenzoate) derivative of cyclophosphazene [PN(OC6H4COOCH3)2]3 (10) on treatment with KO(t-Bu) and H2O gave the hexacarboxylic acid derivative [PN(OC6H4COOH)2]3 (11), which on treatment with oxalyl chloride followed by (S)-(+)-2-amino-3-methyl-1-butanol/(S)-(+)-2-phenylglycinol, triethylamine and mesyl chloride was converted to the C3-symmetric cyclophosphazene based chiral hexaoxazoline ligands [PN{OC6H4(4-iPr-2-Ox)}2]3 (12) and [PN{OC6H4(4-Ph-2-Ox)}2]3 (13). The bis(phebox) derivative of the cyclophosphazene was prepared starting from (NPPh2)2[NP{OC6H3(COOCH3)2}2] (14), by the reaction of gem-Ph4P3N3Cl2 with dimethyl 5-hydroxyisophthalate in the presence of Cs2CO3. Compound 14 was converted to the tetracarboxylic acid (NPPh2)2[NP{OC6H3(COOH)2}2] (15) by base promoted hydrolysis with KO(t-Bu). The tetracarboxylic acid 15 on reaction with oxalyl chloride followed by (S)-(+)-2-amino-3-methyl-1-butanol/(S)-(+)-2-phenylglycinol, triethylamine and mesyl chloride was converted to the bis(phebox) substituted tetraphenylcyclophosphazene derivatives (NPPh2)2[NP{OC6H3(4-iPr-2-Ox)2}2] (16)/(NPPh2)2[NP{OC6H3(4-Ph-2-Ox)2}2] (17). A similar tetra(phebox) derivative was synthesized from (NPPh2)[NP{OC6H3(COOCH3)2}2]2 (18) which was first converted to (NPPh2)[NP{OC6H3(COOH)2}2]2 (19) and further converted to the tetra(phebox) derivative (NPPh2) [NP{OC6H3(4-Ph-2-Ox)2}2]2 (20). All new compounds were characterized by IR, NMR [(1)H, (13)C{(1)H} and (31)P{(1)H}] and HRMS studies. Compounds 1, 2, 4, 5, 7, 14 and 18 have also been structurally characterized.