Novel (N-heterocyclic carbene)-palladium(0) complexes as catalysts in element-element bond additions to unsaturated moieties

• Main Author: Ansell, Melvyn B.
• Published: University of Sussex 2017
• Subjects: 547; QD Chemistry
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.714786

The focus of this thesis is the synthesis of novel palladium(0) complexes bearing the ligand 1,3,4,5-tetramethylimidazol-2-ylidene (ITMe), a small percentage buried volume N-heterocyclic carbene. These complexes have been assessed as mediators for the 1,2-additions of hetero element-element bonds to unsaturated organic moieties. In particular, Si-Si, Si-B and B-B bond additions to alkynes and azobenzenes were chosen as reactions of interest due to their challenging nature. Chapter 1 introduces the concept of transition metal mediated element-element additions to alkynes and includes a thorough review on the current literature state. Chapter 2 describes the first solution based synthesis of [Pd(ITMe)2] and its in situ reactivity with Me3SiSiMe3 under mild conditions to form the novel complex cis-[Pd(ITMe)2(SiMe3)2], the first NHC-bearing complex resulting from the oxidative addition of hexamethyldisilane to a palladium centre. The use of this complex as a pre-catalyst for the bis(silyl)ation of electronically and sterically challenging internal acetylenes using non-activated disilanes is reported. A series of novel 1,2-disilylstilbenes were synthesized in high yield and with 100% Z-stereoselectivity. Chapter 3 details the use of [Pd(ITMe)2(PhC≡CPh)], the first bis(N-heterocyclic carbene)Pd(0)-alkyne complex, as a highly reactive pre-catalyst in the silaboration of terminal and internal alkynes to yield a number of known and novel 1-silyl-2-boryl alkenes. Unprecedented mild reaction temperatures for terminal alkynes, short reaction times and low catalytic loadings are reported. During mechanistic studies, cis-[Pd(ITMe)2(SiMe2Ph)(Bpin)] was directly synthesized by oxidative addition of PhMe2SiBpin to [Pd(ITMe)2(PhC≡CPh)]. This represents a very rare example of a (silyl)(boryl)palladium complex. A plausible catalyst decomposition route was also examined. In Chapter 4, [Pd(ITMe)2(PhC≡CPh)] acts as a highly reactive pre-catalyst in the unprecedented homogeneous catalyzed diboration of terminal and internal alkynes, yielding a number of novel and known syn-1,2-diborylalkenes in a 100% stereoselective manner. DFT calculations conducted by our collaborators suggest that a similar reaction pathway to that proposed for platinum phosphine analogues is followed, and that destabilization of key intermediates by ITMe is vital to the overall success for the palladium-catalyzed B-B addition to alkynes. Chapter 5 reports the use of [Pd(ITMe)2(PhC≡CPh)] as a highly active pre-catalyst in the diboration and silaboration of azobenzenes to synthesize a series of novel functionalized hydrazines. The reactions proceed using commercially available diboranes and silaboranes under mild reaction conditions. Preliminary investigations into further reactivity of [Pd(ITMe)2(PhC≡CPh)], [Pd(ITMe)2] and cis-[Pd(ITMe)2(SiR3)2] (SiR3 = SiMe2Ph or SiMe3) are reported in Chapter 6. This includes the oxidative cleavage of Me3GeGeMe3 by [Pd(ITMe)2(PhC≡CPh)] to form the novel cis-[Pd(ITMe)2(GeMe3)2] and an initial study into the catalytic alkyne digermylations. The hydrogenation of diphenylacetylene to form Z-stilbene using an amine-borane and catalytic quantities of [Pd(ITMe)2(PhC≡CPh)] was also investigated. Finally, the stoichiometric reactions of allyl bromides with cis-[Pd(ITMe)2(SiR3)2] to form the novel complexes trans-[Pd(ITMe)2(SiR3)(Br)] are detailed.