Date on Master's Thesis/Doctoral Dissertation
Committee Co-Chair (if applicable)
Ligands (Biochemistry); Zinc
Achiral arenesulfonates ligands having a (NNN) or (NNO) side arm have been developed (Chapter 2) and their respective zinc ethyl complexes, NNNZnEt 3.2 and NNOZnEt 3.3, have been synthesized and characterized using spectroscopic methods (Chapter 3). NNNZnEt as well as NNOZnEt complexes are stable under nitrogen atmosphere. In the NNOZnEt 3.3 complex the protons associated with the pendant (dimethylamino)ethyl arm are broad indicating that a fluxional process is operative, most likely dissociation of the (dimethylamino)ethyl arm at 25 °C in CD2Cl2. The fluxional behavior present in the 1H NMR spectrum of 3.2 could be arrested by cooling the CD2Cl2 solution to -60 °C. In Chapter 3, the reactivity of 3.2 and 3.3 with alcohols was explored in detail. NNOZnEt was found to be unstable in the presence of alcohols. The NNNZnEt complex does not show any reactivity with weak acids (pKa = 16), such as ethanol, anthrylmethanol or anthrylethanol. Strong acids (pKa = 8-10) like phenols protonate the zinc ethyl bond to produce zinc phenoxide complexes with release of ethane. The rate of the reaction is directly proportional to the acid strength (pKa) of the alcohols. The NNNZnEt complex as well as its corresponding phenoxides show dynamic behavior at 25 °C. The dimethylaminoethyl arms in these complexes are hemilable as indicated by 1H, 13C, 31P and variable temperature NMR, i.e., the dynamic behavior of these complexes was demonstrated. The detail structure of the NNNZnEt was elucidated by examining the single X-crystal structure of analogous air stable NNNZnMe complex. Zinc phenoxides 3.10-3.12 were isolated in good yield by reaction of NNNZnEt complex and one equivalent of the corresponding solid phenols in CH2Cl2 under N2. Zinc phenoxides 3.10- 3.12 were characterized by 1H, 13C, and 31P and multi-dimensional NMR. Fluxional behavior was observed at 25 °C for the pendant (dimethylamino)ethyl arm in complexes 3.10-3.12 as also seen in zinc alkyl complexes 3.7 and 3.13. The dynamic behavior due to association and disassociation of the (dimethylamino) ethyl arm in complexes 3.10- 3.12 could be halted by cooling the CD2Cl2 solutions to -40 °C. Complexes 3.10-3.12 were stable in dry CD2Cl2 for several days at 25 °C and 12 hours at 45 °C. The catalytic activity of zinc phenols with lactide were tested on NMR scale reactions. The discrete zinc phenoxide as well as in situ generated zinc phenoxides 3.10-3.12 show low catalytic activity with dl-lactide. The low reactivity of zinc phenoxides with dl-lactide is probably due to the low electrophilicity of the zinc center. In the second part of this dissertation (Chapter 4), a series of tolyl/phenylethyl phosphinimines 4.1-4.8 were synthesized by Staudinger reaction between azide and various phosphines. These phosphinimines were characterized using 1H, 13C, 31P NMR. In general, the phosphininimine only ring substituted with an electron donating group, such as methoxy or dimethylanino, enhances the basicity of the phosphinimines. The correlation between the basicity of the phosphinimines and the downfield shift of the alcohol hydroxyl group when activated by phosphinimine was studied. The catalytic activity of these phosphinimines towards ring opening polymerization of lactide is correlated by alcohol hydroxyl proton shift when interacted with an alcohol (1:1) in nonhydrogen bonding solvent CDCl3 at 25 °C under N2 atmosphere. The phosphinimines 4.5 to 4.14 were tested for ROP of lactides and their correlation with the basicity estimated with alcohol activation was shown. These phosphinimines represent highly active catalysts for ROP under N2 at ambient temperature. The phosphinimine 4.7 reacts with lactide monomer even in absence of alcohol initiation with reasonable reaction rate. Phosphinimines initiate the ROP of lactide even in the absence of initiating alcohol. However, the addition of alcohol enhances the rate of the reaction. We anticipate that the ease of handling of these catalysts, combined with their high reactivity on the ROP of lactide will make them useful to the synthetic community.
Thapa, Rajesh, "Phosphinimine zinc complexes and neutral phosphinimines for ring opening polymerization of lactides." (2015). Electronic Theses and Dissertations. Paper 2243.