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

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

Synthesis of trans-mono(silyl)palladium(II) bromide complexes

The stoichiometric reaction of cis-[Pd(ITMe)2(SiR3)2], where (SiR3 = SiMe3 and SiMe2Ph and ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) with allyl bromide affords the corresponding allylsilanes along with complexes of the type trans-[Pd(ITMe)2(SiR3)(Br)]. The structure of trans-[Pd(ITMe)2(SiMe2Ph)Br] 2b has been determined in the solid state and displays a slightly distorted square-planar geometry with the two N-heterocyclic carbene ligands in a trans-configuration

Comparison of the reactivity of the low buried-volume carbene complexes (ITMe)2Pd(PhC≡CPh) and (ITMe)2Pd(PhN=NPh)

The novel Pd(0)-azobenzene complex (ITMe)2Pd(PhN=NPh) (5) (ITMe = 1,3,4,5-tetramethylimidazol 2-ylidene) has been isolated and characterized in the solid state and by cyclic voltammetry. Its reactivity towards E-E’ bonds (E, E’= Si, B, Ge) has been compared with that of the known palladium carbene complex (ITMe)2Pd(PhC≡CPh) (2). Whereas 2 reacts with all E-E’ bonds studied, 5 only reacted with B-B and B-Si moieties, echoing our previous catalytic studies on azobenzenes

Transition metal catalyzed element–element′ additions to alkynes

The efficient and stereoselective synthesis of, or precursors to, multi-substituted alkenes has attracted substantial interest due to their existence in various industrially and biologically important compounds. One of the most atom economical routes to such alkenes is the transition metal catalyzed hetero element–element′ π-insertion into alkynes. This article provides a thorough up-to-date review on this area of chemistry, including discussions on the mechanism, range of Esingle bondE′ bonds accessible and the stoichiometric/catalytic transition metal mediators employed

(N-Heterocyclic Carbene)₂‑Pd(0)-Catalyzed Silaboration of Internal and Terminal Alkynes: Scope and Mechanistic Studies

Pd­(ITMe)₂(PhCCPh) acts as a highly reactive precatalyst 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, <i>cis</i>-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

Comparison of the Reactivity of the Low Buried-Volume Carbene Complexes (ITMe)₂Pd(PhCCPh) and (ITMe)₂Pd(PhNNPh)

The novel Pd(0)-azobenzene complex (ITMe)₂Pd­(PhNNPh) (<b>5</b>) (ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) has been isolated and characterized in the solid state and by cyclic voltammetry. Its reactivity toward E–E′ bonds (E, E′ = Si, B, Ge) has been compared with that of the known carbene complex (ITMe)₂Pd­(PhCCPh) (<b>2</b>). Whereas <b>2</b> reacts with all E–E′ bonds studied, <b>5</b> only reacted with B–B and B–Si moieties, echoing our previous catalytic studies on azobenzenes