Synthesis and catalytic evaluation of chiral ferrocenyl p^p and p^n palladium(II) complexes
Chapter 2 outlines initial attempts made to synthesize analogues of the P,N-type chiral ligands (η5-C5H5)Fe(η5-C5H3-PPh2)C*H(OH)(3,5-R2pz) R = H SPNa and R = Me SPNb first prepared by Togni, and the P,P-type chiral ligands (η5-C5H5)Fe(η5-C5H3-PR2)C*H(OH)(PPh2) R = Ph SPPa, R = i-Pr SPPb of the Josiphos family. In these ligands, the methyl group on the stereogenic carbon is replaced with a hydroxyl group. The preparation of SPNa and SPNb included the use of the scaffolds (η5-C5H5)Fe(η5-C5H4)(CO)(3,5-R2pz) R = H 3a and R = Me 3b which were prepared from the reaction of ferrocenoyl chloride with appropriate pyrazolyl moieties. It was unfortunately discovered that neither 3a nor 3b could be reduced to the corresponding alcohol derivatives (η5-C5H5)Fe(η5-C5H4C*H(OH)(3,5-R2pz) R = H 3-OHa and R = Me 3-OHb which were the required intermediates towards the preparation of SPNa and SPNb. The preparation of SPPa and SPPb used the scaffold (η5-C5H5)Fe(η5-C5H4)(CO)(PPh2) 4 which was prepared similarly to ligands 3a and 3b using lithium diphenylphosphine. Disappointingly, scaffold 4 was obtained in yields less than 10%. Furthermore, it could also not be reduced to the required intermediate (η5-C5H5)Fe(η5-C5H4)C*H(OH)(PPh2) 4-OH as it was the case for 3a and 3b. The alternative scaffolds (η5-C5H5)Fe(η5-C5H4-COMe) 5 and (η5-C5H5)Fe(η5-C5H4-PPh2) 7 were then synthesized. Compound 5 could be reduced to (η5-C5H5)Fe(η5-C5H4) C*H(OH)(Me) 5-OH which was subsequently used to prepare the ligand intermediates (η5-C5H5)Fe(η5-C5H4)C*H(3,5-R2pz)(Me) where R = H 6a and R = Me 6b by a substitution reaction with appropriate pyrazolyl moieties. The lithiatiation of 6b followed by the reaction with chlorodiphenylphosphine yielded the chiral ligand (η5-C5H5)Fe(η5-C5H3-PPh2)C*H(3,5-Me2pz)(Me) LPNb. Friedel-Crafts acetylation of 7 with acetyl chloride afforded a heteroannular intermediate (η5-C5H4-PPh2)Fe(η5-C5H4-COMe) 8 instead of the desired homoannular intermediate. This intermediate could be reduced to (η5-C5H4-PPh2)Fe(η5-C5H4)C*H(OH)(Me) 8-OH which was then used as a starting material in the attempts to synthesize heteroannulated analogues of the alternative P,N and P,P-type ligands proposed previously. Decomposed products were obtained when substitutions with pyrazolyl and diphenylphosphino moieties were attempted. Palladium(II) complexes of the ligands 6a, 6b and LPNb were then prepared using PdCl2(NCMe)2 as the metal precursor, while the one for 7 was prepared using PdCl2 as the metal precursor. In Chapter 3, the prepared complexes [PdCl2{(η5-C5H5)Fe(η5-C5H4)C*H(3,5-R2pz)(Me)}2] R = H CNa and R = Me CNb, [PdCl2(η5-C5H5)Fe(η5-C5H3- PPh2)C*H(3,5-Me2pz)(Me)] CPNb and [PdCl2{(η5-C5H5)Fe(η5-C5H4-PPh2)}2] CP1 were catalytically evaluated in a Suzuki-Miyaura coupling reaction of phenylboronic acid with iodobenzene to obtain biphenyl as the product. Interestingly, CNb could catalyse this reaction to give yields of at least 50% at 30 °C. However, the best yields were obtained when the temperature is doubled, using 2 M sodium hydroxide as the base in tetrahydrofuran. From the tested complexes, CNa and CP1 gave maximum conversions of over 90%, although the former achieved these conversions in half the time.
chiral ferrocenyl --palladium (II) --complexes
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