print pageprinter friendly
Reports: UR151819-UR1: Phosphorus-Hydrogen Activation Using Alkynylmetal Complexes: New Methodology for the Preparation of Metallopolymers

Robert A. Stockland, Bucknell University

During year 2 of this project, our efforts were focused on devising a synthetic strategy for the addition of diarylphosphinic acids to alkynes using the single-component catalysts we discovered in year 1 of the project, as well as developing an efficient synthesis of new metallopolymers. During the first year of this project, we discovered that arylgold compounds were effective single component catalysts for the addition of phenols to internal alkynes.1 No acids or silver salts were needed to promote the reaction.2  During year 2, we have extended this discovery to include phosphinic acids.  After some experimentation with arylgold compounds, we found that a simple arylgold species bearing a bulky Buchwald dialkylbiarylphosphine ligand was an effective catalyst for this addition reaction.  In contrast with our hydrophenoxylation chemistry, we found that internal alkynes were unreactive towards diphenylphosphinic acid.  In contrast, terminal alkynes bearing a range of functional groups were successfully functionalized using this approach (Table 1).  The regioselectivity of the addition reaction was found to be dependent upon the electron withdrawing nature of the substituent on the aromatic ring. 

 Table 1. Gold catalyzed addition of diphenylphosphinic acid to alkynes.a

 

The development of an efficient synthesis of metallopolymers through P-H activation reactions was also investigated over the past year.  While our initial efforts in this area were focused on using alkynylgold compounds as building blocks for the addition polymerization, we discovered that diaryldigold compounds were much more active towards the polymerization reactions.  We also found that the amounts of unidentified side-products were significantly less when these monomers were used. The secondary products could have been due to reactions between the substrates and the free alkyne generated as a result of the P-H activation.  Altering the gold containing monomer such that an aryl group is eliminated instead of the terminal alkyne significantly reduced these side reactions.  Building on this observation, we synthesized a range of diphosphine ligated diaryldigold compounds using a microwave assisted arylation reaction that we recently reported.3  The isolated yields were moderate, an all the compounds were isolated a white solids (Scheme 1). 

Scheme 1: Preparation of diphosphine ligated diaryldigold building blocks.

 

With the new monomers in hand, they were screened for activity towards the addition polymerization reaction with bisphosphite ligands (Scheme 2).  After some experimentation, we discovered that carrying out the polymerization reactions at room temperature under bulk conditions or near solvent free conditions (minimal amounts of solvent) generated mostly oligomers.  Increasing the amount of solvent resulted in no reaction.  For the polymerizations carried out under bulk or near solvent free conditions, increasing the temperature resulted in an intractable mixture of products.  Analysis of the NMR spectra of these reaction mixtures suggested a range of thermolysis reactions had occurred.  In contrast, increasing the temperature of the polymerizations carried out in benzene as the solvent generated only the polymers with no evidence of secondary reactions.  In addition to the use of diphosphine ligands bearing methylene spacers, we also screened an acetylene bridged species.  The polymerization appeared to be successful; however, the polymer was extremely insoluble in all common solvents including dmso (Scheme 2, polymer 6).

Scheme 2: Synthesis of metallopolymers through P-H activation reactions.

During this second phase of the project, I was fortunate to have seven very talented undergraduates working on this chemistry (two were directly funded by this PRF award).   Returning veterans from year 1 included Erica Miller (BS Chemistry '16), Kevin Garcia (BS Chemistry '16), Rosa Ciccarelli (BS Chemistry '16), and Erin Holahan (BS Cell Biology/Biochemistry '14).  Joining the project this past year were Rachel Bergin (BS Chemistry '18), Stephanie Casino (BS Neuroscience '16), and Tyler Bogaczyk (BS Neuroscience '15).  4 of these students traveled to the 248th National ACS meeting in August to present three posters on their work: ORG681:  Microwave assisted synthesis of single component gold catalysts for organic transformations, Erin C. Holahan, Erica J. Miller, Kevin J. Garcia, Rosa M. Ciccarelli, and Robert A. Stockland Jr. ORG676: Gold catalyzed addition of phenols to unactivated internal alkenes under silver and acid free conditions, Erica J. Miller, Marcia E. Richard, Daniel V. Fraccica, Kevin J. Garcia, Rosa M. Ciccarelli, Erin C. Holahan, Victoria L. Resh, Aakash Shah, Peter M. Findeis, Robert A. Stockland Jr.  CHED348: Generation of metallopolymers through P-H activation, Kevin J. Garcia, Rosa M. Ciccarelli, Stephanie L. Casino, and Robert A. Stockland Jr. Summary: During the second phase of the project, we extended the scope of our single-component gold hydroelementation catalysts to the addition of diphenylphosphinic acid to alkynes.  After some experimentation, we devised a system that would effectively promote the addition reaction under solvent free conditions without the addition of acidic or silver promoters.  Furthermore, we defined the steric and electronic limits of this addition reaction and found that the regioselectivity correlated with the electron withdrawing nature of the organic substituent attached to the terminal alkyne. The conditions for the successful synthesis of several metallopolymers through P-H activation reactions were also defined, and a range of arylgold compounds were successfully prepared and screened for activity towards the addition polymerization reaction.  Both solution and bulk polymerizations were investigated and the solution polymerizations were found to generate higher amounts of polymeric materials.  We also discovered that the physical properties of the polymers were dependent upon the diphosphine linker.  During phase three of the project, we will correlate the molecular weight and polydispersity of the polymers with reaction conditions. References

1)      Richard, M. E.; Fraccica, D. V.; Garcia, K. J.; Miller, E. J.; Ciccarelli, R. M.; Holahan, E. C.; Resh, V. L.; Shah, A.; Findeis, P. M.; Stockland Jr., R. A.  Beilstein J. Org. Chem. 2013, 9, 2002 

2)      Typically, the catalytically active gold species is generated by the treatment of a chlorogold species with silver salts or addition of powerful acids to organogold compounds. a) Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chemie Int. Ed. 1998, 37, 1415.  Hashmi, A. S. Chem. Rev. 2007, 107, 3180.

3)      Lenker, H. K.; Gray, T. G.; Stockland Jr., R. A. Dalton Trans. 2012, 41, 13274-13276