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Mechanisms of Cyclopalladation Reactions in Acetic Acid: Not So Simple One‐Pot Processes
Author(s) -
Gómez Montserrat,
Granell Jaume,
Martinez Manuel
Publication year - 2000
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(200001)2000:1<217::aid-ejic217>3.0.co;2-q
Subject(s) - chemistry , imine , lability , palladium , hydrolysis , medicinal chemistry , acetic acid , monomer , derivative (finance) , covalent bond , acetic anhydride , stereochemistry , catalysis , organic chemistry , polymer , financial economics , economics
The processes operating during the synthetic cyclopalladation reactions of imines in acetic acid have been studied from a kinetico‐mechanistic point of view. These reactions include a fast initial coordination to the palladium through the N‐donor atom of the imine, followed by the proper C–H bond activation to produce the acetato bridged dimeric species. At this point, the lability of the bridging acetato groups, the hydrolysis of the C–Pd bonds, and/or the hydrolysis of C=N exo bonds contribute to the generation of dark red polynuclear compounds. The processes occurring after the C–H activation have been followed kinetically, both from palladium acetate plus imine, and the synthetically pure isolated acetato dimers as starting materials. The kinetic and activation parameters have been found identical within experimental error whatever the starting material was ( k 323 = 1.5 × 10 –4 s –1 ; Δ H # = 51 kJ mol –1 ; Δ S # = –163 JK –1 mol –1 Δ V # = +19 cm 3 mol –1 for the 4‐ClC 6 H 4 –CH=N–CH 2 –C 6 H 5 imine derivative 1a ). Acidolysis of C–Pd bonds has been found to occur in these polynuclear species. When alternative monomeric C benzylic –Pd bond‐containing complexes are possible follow ups of the reactions produce them as final dead‐end complexes ( k 323 = 2.2 × 10 –5 s –1 ; Δ H # = 61 kJ mol –1 ; Δ S # = JK –1 mol –1 Δ V # ≈ 0 cm 3 cm –1 for the [2,4,6‐(CH 3 ) 3 ]C 6 H 2 –CH=N–CH 2 ‐[2‐(CH 3 ]C 6 [H 4 ] imine derivative 3d ). The same study has been carried out with primary amines in order to check the validity of the data if C=N bond hydrolysis is taking place in the imine derivatives with exo C=N bonds. For complexes with similar type of metallacycles, the results agree reasonably well with the proposed mechanism [ k 323 = 1.2·10 –4 s –1 , Δ H # = 46 kJ·mol –1 , Δ S # = –180 J·K –1 mol –1 , Δ V # = –16 cm 3 ·mol –1 for the polynuclear formation of the C 6 H 5 –CH 2 –NH 2 derivative 4e ; k 323 = 3.0·10 –4 s –1 , Δ H # = 55 kJ·mol –1 , Δ S ‡ = –147 J·K –1 mol –1 , Δ V # = –24 cm 3 ·mol –1 for the polynuclear formation of the C 6 H 5 –CH 2 –NCH–2,3,6‐(CH 3 ) 3 C 6 H 2 derivative 2c ].