Strained cationic heterocycles and other novel phosphaalkene-derived species

• Main Author: Bates, Joshua Isaac
• Language: English
• Published: University of British Columbia 2011
• Online Access: http://hdl.handle.net/2429/34561

This thesis outlines the results from three projects undertaken as part of my Ph.D. studies, with Chapter 1 serving as a general introduction and Chapter 5 serving to summarize the thesis. Chapter 2 details a Lewis acid-mediated methodology for preparing phosphaalkenes from silyl phosphines [RP(SiMe₃)₂; R = alkyl, aryl, silyl] and aldehydes or ketones. The scope of this methodology was explored and phosphaalkenes tBuP=CHtBu (1), AdP=CHtBu (2), MesP=CHtBu (3) and MesP=CPh₂ (4) were prepared on preparative scales. For phosphaalkene 1, this reduced its synthesis from 11 weeks to less than one hour. Additionally, AlCl₃ and GaCl₃ adducts of phosphaalkenes 1 and 2 were synthesized and characterized by X-ray crystallography. In Chapter 3, the reactions of phosphaalkenes 1 and 2 with potential cationic initiators are discussed. For both phosphaalkenes, treatment with substoichiometric HOTf affords rare diphosphiranium cations. Mechanistic studies reveal that this process proceeds via phosphenium triflate intermediates. Unexpectedly, treatment with the related MeOTf affords diphosphetanium cations via methylenephosphonium intermediates. Additionally, it was found that the diphosphetanium cation formed from phosphaalkene 1 would react with two additional equivalents of MeOTf to afford an unprecedented dicationic diphosphetanium. Finally, Chapter 4 describes the abnormal reaction of IMes, a N-heterocyclic carbene (NHC), with phosphaalkenes to afford novel 4-phosphino-2-carbenes. Interestingly, DFT calculations of plausible reaction intermediates suggest the reactions proceed via free abnormal NHCs (aNHCs). The phosphino-functionalized NHC (5), derived from the reaction of IMes with MesP=CPh₂, was used to study the coordination properties of this novel class of ligands. Treating carbene 5 with substoichiometric (tht)AuCl (0.5 equiv) affords a biscarbene complex, indicating that AuCl is preferentially coordinated by the carbene functionality. P-coordination of AuCl occurs when carbene 5 is treated with additional equivalents of AuCl, confirming the bifunctional nature of this ligand. Additionally, rhodium and iridium complexes of the type (NHC)M(CO) ₂Cl (M = Rh, Ir) were prepared and CO stretching frequencies of these complexes suggest that carbene 5 has similar donor properties as IMes.