Search

A new mechanism has been proposed for the hydroboration of aldehydes by germylene and stannylene complexes, with the aid of the hydroborating agent, pinacolborane (HBpin). This new mechanism is demonstrated with computational studies with density functional theory (DFT), not only for existing systems, but also for newly designed germylene and stannylene systems. [Display omitted] The chemistry of low valent main group compounds has grown as an alternative to the chemistry of less abundant and less green transition metal complexes. It has been found that low valent compounds such as carbenes, silylenes, stannylenes and germylenes are efficient for activating small molecules and for catalysis. However, the reaction mechanism and the factors that affect the rate of reaction are not completely understood. In this computational investigation with density functional theory (DFT), we investigate and demonstrate the efficiency of a new mechanism for the hydroboration of aldehydes by germylenes and stannylenes, in the presence of the common hydroborating agent, pinacolborane, HBpin. This mechanism involves an HBpin molecule as an additional catalyst that cooperates with the germylene or stannylene catalyst to efficiently carry out the hydroboration. This mechanism is first demonstrated to work for experimentally reported systems, and then shown to be efficient for newly proposed germylene and stannylene systems. These new systems are α-Borylamido-germylene ((2,6- i Pr 2 C 6 H 3 NBCy 2) 2 Ge(II)) and α-Borylamido-stannylene((2,6- i Pr 2 C 6 H 3 NBCy 2) 2 Sn(II)). These new insights will help researchers look into low valent germylene and stannylene chemistry from a new perspective. [ABSTRACT FROM AUTHOR]

(a) The catalytic application of organolithium compounds is very limited due to their high reactivity and aggregation phenomena. However, the paradigm started to shift recently, when the groups of Okuda (J. Am. Chem. Soc. 2016, 138, 10790) and Mulvey (Chem. Eur. J., 2017, 23, 16853) reported the use of organolithium compounds for the hydroboration of aldehydes and ketones. Nonetheless, the catalytic use of organolithium compounds for reduction of more challenging and synthetically useful substrates such as carbodimides, esters etc. is deemed desirable. (b) Very recently, we have shown the use of organolithium compounds for the hydroboration of aldehydes and ketones (Chem. Commun. 2018, 54, 6843) as well as alkenes and alkynes (Chem. Commun. 2019, 55, 11711). Building upon our previous works, we demonstrate here catalytic hydroboration of ester to alcohol and carbodiimide to imine. (c) Prior to this work, there are few main-group catalysts reported for the hydroboration of carbodiimdies (See Chart 1 in the manuscript), but no well-defined group 1 complex as a single site catalyst has been used. The same is true for ester hydroboration. This is the first report of the catalytic hydroboration of esters and carbodiimides using single site catalyst based on a s -block element. (d) A thorough DFT studies were carried out to understand the mechanism of carbodiimide hydroboration. Selective and efficient hydroboration of esters and carbodiimides to alcohols and amines by two well-defined and readily accessible lithium complexes, 2,6-di-tert-butyl phenolate lithium (1a) and 1,1'-dilithioferrocene (1b) are described. A range of aliphatic, aromatic, and cyclic esters with various functional groups were selectively converted into the corresponding boronate esters. Similarly, the single hydroboration of carbodiimides with aliphatic and aromatic substituents on the nitrogen atoms was studied. A possible mechanistic pathway of the hydroboration of carbodiimides with HBpin has been proposed using NMR studies and DFT calculations. These reactions are convenient alternatives to stoichiometric hydride reduction or hydrogenation. The employing of lithium complexes is also significant, because of the need to find cheap and green alternatives to noble metal complexes. [Display omitted] We have demonstrated the use of simple and readily available lithium compounds, 2,6-di-tert-butyl phenolate lithium (1a) and 1,1'-dilithioferrocene (1b) as single site catalysts for hydroboration of diverse esters and carbodiimides in high yields and excellent functional group tolerance. DFT studies have been performed to understand the mechanism of carbodiimide hydroboration. [ABSTRACT FROM AUTHOR]