| Summary: | 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.
|
|---|
