Mechanistic Studies on Organogold Complexes. Toward the Development of Novel Homogeneous Catalysts for C-O, C-N, and C-C Bond Formations

Reports: GB3 48223-GB3: Mechanistic Studies on Organogold Complexes. Toward the Development of Novel Homogeneous Catalysts for C-O, C-N, and C-C Bond Formations

Christine Hahn, University of Texas of the Permian Basin

The object of this research project was the synthesis of organogold complexes and the study of their catalytic activities in alkene functionalization reactions. This project has been expanded to the study of platinum complexes and the electrophilic activation of alkynes. During the initial studies the synthesis of a gold(III) alkene complex of the proposed type [AuL(R₂C=CR₂)]ⁿ⁺ using various tridentate ligands L with N, C, O, S, and P donor functions failed. Only the already known Au^III complex [Au(terpy)Cl]Cl₂ (terpy = 2,2′:6′,2′′-terpyridine) was found to be reasonable stable in solution to conduct reactivity studies. However, no alkene complex was accessible either. Since the [Au(terpy)Cl]²⁺ complex contains a strongly electrophilic Au center, it was thought to be interesting to investigate C-H bond activation of alkynes. [Au(terpy)Cl]Cl₂ was reacted with phenylacetylene in the presence of either AgBF₄ or NaBF₄ and gave a light sensitive and thermally unstable compound. The IR spectrum showed a characteristic band at 2021 cm^-1 for the coordinated C≡C bond suggesting the formation of [Au(terpy)(C≡CPh](BF₄)₂. The synthesis of an analogous complex with HC≡C^tBu was attempted, but no defined product could be obtained.

During the previous report period the synthesis of cationic gold(I) alkene complexes containing a bidentate ligand was explored. Using the diphos ligand (diphos = 1,2-bis(diphenylphosphino)benzene) the gold(I) complex [AuCl{Ph₂PC₆H₄P(O)Ph₂}] was obtained which was characterized by NMR spectroscopy and X-Ray crystal structure analysis. The complex has the structure L-Au-Cl (L = phosphine) which is characteristic for gold complexes catalyzing the hydration of alkynes. Therefore the new gold(I) complex was tested as catalysts for this reaction using AgBF₄ and HBF₄ as co-catalysts. With 10% catalyst loading the hydration of phenylacetylene was practically complete forming acetophenone within 24 h at room temperature. Similar results were obtained with other alkynes (1-hexyne, 2-hexyne), while diphenylacetylene gave somewhat lower conversion. The terminal alkynes gave selectively Markovnikov addition products, whereas the regioselectivity of the hydration of 2-hexyne was rather moderate.

In contrast to the new gold(I) complex, the dicationic platinum(II) complex [Pt(PNP)(CH₂=CH₂)](BF₄)₂ (PNP = 2,6-bis(diphenylphosphinomethyl)pyridine did not catalyze the hydration of alkynes but promoted the stoichiometric addition of water at -78°C. This seems to be the lowest temperature reported for water addition to alkynes. The reaction has been further studied using various alkynes. The alkynes reacted rapidly with the dicationic platinum(II) ethylene complex by ligand substitution to give [Pt(PNP)(RC≡CR)](BF₄)₂ which was suggested by ³¹P NMR spectroscopy. So far no dicationic platinum alkyne complex could be isolated. With propyne a for platinum unusual anti-Markovnikov product [Pt(PNP)(COCH₂CH₃)]BF₄ was formed together with the Markovnikov product in a ration of 4:1. In contrast to propyne other terminal alkynes such as HC≡C^tBu and HC≡CPh gave the corresponding acetylide complexes [Pt(PNP)(C≡CR)]BF₄ by C-H bond activation in the presence of water.

Furthermore it was interesting to study C-C bond formation reactions by nucleophilic addition of arenes to the in situ generated [Pt(PNP)(RC≡CR)](BF₄)₂. Similar as found by other groups studying the hydroarylation of alkynes, only sufficiently electrophilic arenes added to the coordinated C-C triple bond. No addition was observed with benzene, toluene, anisole, o-xylene, and p-xylene, whereas m-xylene gave a moderate yield of the corresponding addition product [Pt(PNP)E-(CH=CHAr)]BF₄ (mixture of two regioisomers). Mesitylene and pentamethylbenzene gave practically complete addition to the coordinated acetylene.

The alkynes are highly activated in the dicationic platinum(II) complexes adding very readily weakly basic nucleophiles such as water and arenes. However, the required Pt-C bond cleavage step was not observed under the given conditions (room temperature and atmospheric pressure) to afford catalytic C-O and C-C bond formation reactions. Therefore it was thought to undertake structural modifications at the PNP ligand to change the electronic properties in the Pt(PNP) complex fragment. The Milstein group reported very recently the synthesis of [Pt(^t^BuPNP)Cl]Cl (^t^BuPNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine). Exchanging the phenyl for tert-butyl groups may increase somewhat the electron density at the platinum center which could help to facilitate the product releasing step. First the synthesis of an analogous dicationic ethylene complex [Pt(^t^BuPNP)(CH₂=CH₂)](BF₄)₂ was attempted by reaction of [Pt(^t^BuPNP)I]I with two equivalents of AgBF₄ in the presence of an excess of ethylene. However, only the outersphere iodide was exchanged by the tetrafluoroborate. The abstraction of the iodo ligand was found to be very sluggish and succeeded only partially by refluxing [Pt(^t^BuPNP)I]I and silver tetrafluoroborate in CH₃CN forming the corresponding dicationic acetonitrile complex [Pt(^t^BuPNP)(CH₃CN)](BF₄)₂ in 15% yield.

Overall, despite of many difficulties and dead ends in exploring the proposed gold chemistry, a gold(I) complex could be identified as catalyst for the hydration of alkynes. This gold(I) complex may be used to study other types of alkyne and alkene functionalization reactions. The PRF grant also gave the opportunity to develop the chemistry of dicationic platinum alkyne complexes. Although they could not be isolated, they show a very high reactivity toward weakly basic nucleophiles. In further studies useful catalytic C-C, C-O, and C-N bond formation reactions may be developed with dicationic platinum(II) complexes.

The results reported are being summarized in manuscripts for publication in suitable journals. Four posters were presented at Regional and National ACS Meetings. Two further poster presentations have been submitted for the 66^th SW/SE Regional ACS Meeting. The research activities have had a considerable impact to the scientific education of the undergraduate students. The involvement of undergraduate students in research and presentation of their results at meetings gave them the opportunity for their professional development including the exchange of experiences with faculty and students of other universities and research fields. It gave them deeper appreciation of the professional world.