Reactivity of a Monomeric Aluminium Hydrazide towards Isocyanates and Isothiocyanates: Active Lewis Pair Behaviour versus Classical Insertion Reactions

Abstract

Abstract:Hydroalumination of H 10 C 5 N-N=C(Ad) with HAliBu 2 yielded the hydrazide H 10 C 5 N-N(AliBu 2 )Ad (4; Ad = adamantyl, NC 5 H 10 = piperidinyl). Compound 4 is monomeric and contains a highly strained AlN 2 heterocycle formed by a donor-acceptor interaction between the Al atom and the -N atom of the hydrazine. Two equivalents of the hydride afforded an adduct with an HAliBu 2 molecule coordinated to 4 by an Al-N and a 3c-2e Al-H-Al bond (5). tBuN=C=O did not insert into the relatively weak Al←N donor-acceptor bond of 4; instead, insertion

A Dimeric Gallium Hydrazide as an Active Lewis Pair – Complexation and Activation of Me2GaH and Various Heterocumulenes

“…The formation of compound 6 clearly demonstrates the reactivity of 5 as a masked FLP and its ability to act as an ambiphilic ligand. Similar compounds have been obtained with an Al i Bu 2 analogue of 5 and several Al/P based FLPs . NMR experiments showed that 5 and 6 could be quantitatively interconverted by combining stoichiometric quantities of 5 and Me 2 GaH or 6 and H 10 C 5 N–N=C(C 9 H 14 ).…”

“…The sterically less demanding i Bu 2 AlH gave both types of products upon treatment of the metal hydride with the hydrazone in the appropriate stoichiometric ratio of 1:1 or 2:1. Interestingly the reaction of the aluminium hydrazide with excess metal hydride to give the hydride adduct failed, while treatment of the hydride adduct with stoichiometric amounts of hydrazone at elevated temperatures afforded the hydrazide in reasonable yield …”

“…Monomeric dialkylaluminium or dialkylgallium hydrazides are promising candidates in this field. They have strained AlN 2 heterocycles with a long donor‐acceptor and a relatively short polar covalent Al–N bond and are mostly generated by the reaction of hydrazones with dialkylaluminium or ‐gallium hydrides . They were shown to be highly efficient active Lewis pairs for the coordination and insertion of small molecules, which are both favored by the cooperative functionality of the metal and N atoms.…”

“…The bulky substituents are advantageous for the growing of single crystals and the purification of the hydrazides and their activation products by recrystallization. Recent publications detailed the influence of the steric bulk of the substituents R on Al on the structures and reactivity of the hydrazides R 2 Al–N(2‐Ad)–NC 5 H 10 ( R = t Bu, CH 2 t Bu, i Bu, Me; 2‐Ad = 2‐adamantyl) . Monomeric compounds featuring strained AlN 2 heterocycles were obtained for R = t Bu, CH 2 t Bu, and i Bu, whereas a dimer with a four‐membered Al 2 N 2 heterocycle and exocyclic N–N bonds was formed in the case of R = Me .…”

“…Although the Al–N bond lengths in the AlN 2 heterocycles differed considerably (ca. 180 and 200 pm) surprisingly both Al–N bonds were reactive and allowed the insertion of suitable substrates such as CO 2 , CS 2 , R NCO, or R NCS ( 1 to 3 ) (Scheme ) , . The reaction of t Bu 2 Al–N(2‐Ad)–NC 5 H 10 with Ph–C≡C–H afforded by cooperative C–H bond activation followed by unexpected alkane elimination a trisalkynylaluminium compound, (PhC≡C) 3 Al · (H 10 C 5 N‐N(H)(2‐Ad) ( 4 ) (Scheme ) …”