Comparing Neutral (Monometallic) and Anionic (Bimetallic) Aluminum Complexes in Hydroboration Catalysis: Influences of Lithium Cooperation and Ligand Set

Abstract

Bimetallic lithium aluminates and neutral aluminum counterparts are compared as catalysts in hydroboration reactions with aldehydes, ketones, imines and alkynes. Possessing Li–Al cooperativity, ate catalysts are found to be generally superior. Catalytic activity is also influenced by the ligand set, alkyl and/or amido. Devoid of an Al−H bond, iBu2Al(TMP) operates as a masked hydride reducing benzophenone through a β‐Η transfer process. This catalyst library therefore provides an entry point into the future design of Al catalysts targeting substrate specific transformations.

Organoaluminum Compounds as Catalysts for Monohydroboration of Carbodiimides

“…The organoboron compounds derived from hydroboration reactions are crucial organic intermediates in various chemical transformations and material synthesis . In recent years, the ability of organoaluminum complexes to work as efficient catalysts for the hydroboration of unsaturated bonds with C=E (E=O, C, N), and C≡E motifs (E=C, N) was discovered. Intense work was invested in the hydroboration of unsaturated bonds through the use of aluminum‐containing pre‐catalysts.…”

Silyl Anion Initiated Hydroboration of Aldehydes and Ketones

“…Delivery of a hydride from 4 to the carbonyl substrate generates an alkoxide intermediate which undergoes further coordination to HBpin, regenerating a borohydride. Coordination of the carbonyl oxygen to the potassium counterion is conceivable, and would accelerate the nucleophilic addition step …”

Catalyst‐Free and Solvent‐Free Facile Hydroboration of Imines

“…Among metal catalyzed processes, transition metal and even rare earth metal complexes have been studied (Scheme ). There is also a recent surge in utilization of s ‐ and p ‐block elements and till date, several research groups have studied the hydroboration of carbonyl and to some extent imine compounds using main‐group element (Ca, Mg, Al,– Si) based systems (Scheme ). Alkali metal based compounds for example, n BuLi and NaOH have also been utilized for the same process .…”