A Comparison of Two Zinc Hydride Catalysts for Terminal Alkyne C–H Borylation/Hydroboration and the Formation of 1,1,1-Triborylalkanes by Tandem Catalysis Using Zn–H and B–H Compounds

Abstract

The synthesis of 1,1,1-triborylalkanes from terminal alkynes and pinacolborane (HBPin) is reported. This transformation proceeds via initial Zn-catalyzed alkyne C-H borylation, which can be achieved using a NacNacZnH complex. Combinations of a NacNacZn-alkynyl formed via C-H zincation of a terminal alkyne and HBPin exist in equilibrium with the alkynyl-BPin and NacNacZnH. The consumption of NacNacZnH by irreversible reaction with a terminal alkyne evolving H2 is essential for driving alkyne C-H borylation to completion. The alkynyl-BPin compounds undergo hydroboration catalyzed by Zn-H complexes at raised temperatures with a {7DIPP}ZnH(NTf2) complex (7DIPP = 1,3-bis(2,6-diisopropylphenyl)-4,5,6,7tetrahydro-1H-1,3-di-azepin-3-ium-2-ide) a more active catalyst for hydroboration than a NacNacZnH complex. Calculations indicate the {7DIPP}Zn-H congener has more pronounced biphilic character than NacNacZnH (greater electrophilicity at Zn while maintaining a sufficiently basic hydride). Of the two hydroboration steps, the hydroboration of alkynylBPin is catalyzed by Zn-H complexes, whilst the hydroboration of 1,1-diborylalkenes is catalyzed more effectively by B-H containing species, including boranes formed in-situ from HBPin. These observations led to a one-pot protocol being developed for converting terminal alkynes into 1,1,1-triborylalkanes that utilizes {7DIPP}ZnPh(NTf2) as a precatalyst for the formation of 1,1-diborylated alkenes with subsequent addition of BH3-THF as catalyst for the final step.

Cross‐Dehydrogenative Alkynylation: A Powerful Tool for the Synthesis of Internal Alkynes

“…Four years later, Ingleson extended the substrate scope to H−Bpin with the use of a bulky NHC‐zinc hydride complex as a catalyst . By increasing the loading of H−Bpin and elevating the reaction temperature, 1,1‐diborylated alkenes and 1,1,1‐triborylated alkanes were also selectively generated . Very recently, Pucheault achieved the cross‐dehydrogenative alkynylation of i‐ Pr 2 NBH 2 forming i‐ Pr 2 NB(H)−≡− R ( R =Aryl and alkyl) through magnesium catalysis .…”

Zwitterion‐Initiated Hydroboration of Alkynes and Styrene

“…As the para ‐methoxy substituted zwitterion 4 b exhibited increased thermal stability compared to the unsubstituted parent compound, we next investigated catalyst turnover using this species. However, stoichiometric studies were hampered by pinacol rearrangement [16] of the dioxaborolane (HBpin) diol, presumably mediated by the high Lewis acidity of 4 b (Scheme 2, D and see SI, S−R6). BH 3 was not observed under these conditions, but an unobserved catalytic role cannot be ruled out [8k] .…”

Computational aspects of the combined finite‐discrete element method in static and dynamic analysis of shell structures

“…The 2-TRIELS numerical algorithm [24] is based on the assumption that the curvature k between two adjacent triangular finite elements A and B in direction orthogonal to the common edge 1-2 in initial configuration is given by:…”

“…Besides large deflections, in boundary element methods post-buckling behaviour can also be taken into consideration [21,22]. In order to combine the advantages of finite and discrete element methods there has been a development of a shell analysis by using a combined finitediscrete element method (FDEM) [23][24][25]. This approach uses an explicit time integration scheme (there is no need for stiffness of mass matrices), making it robust and suitable for parallel programing.…”

“…The main purpose of this paper is to demonstrate computational aspects of the two numerical models (2-TRIELS [24] and 4-TRIELS [25]) for a static and dynamic analysis of shells based on the combined finite-discrete element method. The dynamic and static response of shells under various boundary conditions and mesh refinements was analysed and compared with reference solutions in order to illustrate the accuracy and the field of application of the proposed numerical model.…”