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

摘要

Chiral oxazoline based bi and hexadentate ligands built on cyclophosphazene cores have been synthesized and characterized. (NPPh_2)_2[NP(m-OC_6H_4C(O)OCH_3)_2] (1) was prepared by the reaction of gem- (NPPh_2)_2(NPCl_2) with methyl-3-hydroxy benzoate in the presence of Cs_2CO_3. Compound 1 was converted to the dicarboxylic acid (NPPh_2)_2[NP(m-OC_6H_4C(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 C_2-symmetric phosphazene based chiral bisoxazoline ligand (NPPh_2)_2[NP{m-OC_6H_4(4-iPr-2-Ox)}_2] (3) (Ox = oxazolinyl). A similar C_2-symmetric bisoxazoline derivative having an oxazoline group attached to the para position of the phenyl ring was also synthesized starting from (NPPh_2)_2[NP(p-OC_6H_4C(O)OCH_3)_2] (4) which was first converted to the dicarboxylic acid (NPPh_2)_2[NP(p-OC_6H_4C(O)OH)_2] (5) and finally to (NPPh_2)_2[NP{p-OC_6H_4(4-iPr-2-Ox)}_2] (6) and (NPPh_2)_2[NP{p-OC_6H_4(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 PdCl_2(C_6H_5CN)_2 in refluxing benzene resulted in chiral palladium complexes Pd(OAc)_2(NPPh_2)_2[NP{p-OC_6H_4(4-iPr-2-Ox)}_2] (8) and PdCl_2(NPPh_2)_2[NP{p- OC_6H_4(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(OC_6H_4COOCH_3)_2]_3 (10) on treatment with KO(t- Bu) and H_2O gave the hexacarboxylic acid derivative [PN(OC_6H_4COOH)_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 C_3-symmetric cyclophosphazene based chiral hexaoxazoline ligands [PN{OC_6H_4(4-iPr-2-Ox)}_2]3 (12) and [PN{OC_6H_4(4-Ph-2-Ox)}_2]_3 (13). The bis(phebox) derivative of the cyclophosphazene was prepared starting from (NPPh_2)_2[NP{OC_6H_3(COOCH_3)_2}_2] (14), by the reaction of gem-Ph_4P_3N_3Cl_2 with dimethyl 5-hydroxyisophthalate in the presence of Cs_2CO_3. Compound 14 was converted to the tetracarboxylic acid (NPPh_2)_2[NP{OC_6H_3(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 (NPPh_2)_2[NP{OC_6H_3(4-iPr-2-Ox)_2}_2] (16)/ (NPPh_2)_2[NP{OC_6H_3(4-Ph-2-Ox)_2}_2] (17). A similar tetra(phebox) derivative was synthesized from (NPPh_2)- [NP{OC_6H_3(COOCH_3)_2}_2]_2 (18) which was first converted to (NPPh_2)[NP{OC_6H_3(COOH)_2}_2]_2 (19) and further converted to the tetra(phebox) derivative (NPPh_2) [NP{OC_6H_3(4-Ph-2-Ox)_2}_2]_2 (20). All new compounds were characterized by IR, NMR [~1H, ~(13)C{~1H} and ~(31)P{~1H}] and HRMS studies. Compounds 1, 2, 4, 5, 7, 14 and 18 have also been structurally characterized.