Issue 17, 2023

Catalyst-free diboration and silaboration of alkenes and alkynes using bis(9-heterofluorenyl)s

Abstract

Diboration and silaboration reactions are prominent tools to introduce valuable functional groups into organic substrates. To date, most diboranes(4) and silylboranes used for this purpose are electronically and/or kinetically stabilized and require activation by a catalyst. We show here that the tetraaryl (μ-hydrido)diborane(4) anion [3] and the silyl (hydrido)borate ([4])/Me3SiBr system react spontaneously with the archetypal olefin ethylene in the absence of a catalyst. The actual active species in both cases are the valence isoelectronic intermediates [FluB–B(H)Flu] ([1]) and FluB–Si(H)Flu (2), which consist of two 9-heterofluorenyl halves that get attached to the 1 and 2 positions of ethylene. At room temperature, [1] is present in a dynamic equilibrium with its isolable isomer [3], while 2 has to be released in situ at low temperatures by H abstraction from [4]. Quantum-chemical calculations show qualitatively identical reaction mechanisms for [1] and 2. Since the reactions start with π coordination of the ethylene molecule to a vacant B(pz) orbital, the high Lewis acidity and low steric hindrance of the 9-borafluorenyl fragments are the keys to success. As the reaction proceeds, back-donation from the B–E bond into the ethylene π* orbital becomes increasingly important (E = B, Si). The scope of the reactions has been extended to tBu(H)C[double bond, length as m-dash]CH2 and tBuC[triple bond, length as m-dash]CH on the one hand and FluB–Si(Cl)Flu as well as FluB–Si(Cl)Ph2 on the other.

Graphical abstract: Catalyst-free diboration and silaboration of alkenes and alkynes using bis(9-heterofluorenyl)s

Supplementary files

Article information

Article type
Edge Article
Submitted
16 Marts 2023
Accepted
31 Marts 2023
First published
31 Marts 2023
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2023,14, 4589-4596

Catalyst-free diboration and silaboration of alkenes and alkynes using bis(9-heterofluorenyl)s

J. Gilmer, T. Trageser, L. Čaić, A. Virovets, M. Bolte, H. Lerner, F. Fantuzzi and M. Wagner, Chem. Sci., 2023, 14, 4589 DOI: 10.1039/D3SC01395B

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