Expected and Unexpected Reactivities of Homoleptic LiNacNac and Heteroleptic NacNacMg(TMP) β-Diketiminates toward Various Small Unsaturated Organic Molecules

Abstract

Homoleptic LiNacNac forms simple donor–acceptor complexes with N , N ′-dicyclohexylcarbodiimide (CyN=C=NCy), triphenylphosphine oxide (Ph 3 P=O), and benzophenone (Ph 2 CO). These crystallographically characterized compounds could be regarded as model intermediates en route to reducing the N=C, P=O, and C=O bonds of unsaturated substrates. Heteroleptic NacNacMg(TMP) intriguingly functions as a TMP nucleophile both with t -BuNCO and t -BuNCS, producing a urea or thiourea derivative respectively attached to Mg, though the NacNac ligand in the former reaction also engages noninnocently with a second t -BuNCO molecule via insertion at the reactive NacNac backbone γ-carbon site.

Crystallographic Characterisation of Organolithium and Organomagnesium Intermediates in Reactions of Aldehydes and Ketones

“…In addition to reporting γ‐inserted NacNac species, we have recently presented some examples of model intermediate donor–acceptor complexes, involving lithium NacNac, which provided crystallographic confirmation for previously proposed intermediates from a range of reactions [46] . Here, NacNac refers to the aminoimine, 2,6‐diisopropylphenyl‐β‐methyldiketimine (NacNacH (Me, Dipp), labelled for brevity throughout this text as NacNacH) when its N−H bond has been deprotonated.…”

“…In previous publications by the Mulvey, Ma and Hill groups, a range of heteroleptic NacNacMg(R) species have shown interesting dual insertion patterns when reacted with unsaturated small molecules, where R = TMP [46] , n Bu [50] or SiMe 2 Ph. [52] In light of this previous work, we have reacted the heteroleptic magnesium species NacNacMg(TMP), first synthesised by the Hevia group, with a range of small aldehydes and ketones.…”

“…There was thought to be potential to either reproduce the known lithium pinacolone enolate hexamer [{ t BuC(=CH 2 )OLi} 6 ] (Figure 2), capture of an aldol condensation type intermediate analogous to 1 or forming a donor -acceptor type complex. [46] Pleasingly, we had success in growing yellow crystals of [Li{(MeCN-2,6-i Pr 2 C 6 H 3 ) 2 CH}{O=C-(Me) t Bu}] (2; 63 % yield), a presumed donor -acceptor intermediate in the formation of the lithium pinacolone enolate (Figure 4). This was produced from a 1 : 3, LiNacNac/pinacolone reaction stoichiometry, though other stoichiometries were attempted.…”

“…The recent widespread use of β‐diketiminates (commonly abbreviated as NacNacs or BDIs) within the realm of main group metal chemistry, as bulky and adaptable ligands, enables us to explore a wide range of reactivities and provides such a degree of stabilisation that they have become key to characterising previously unknown main group metal bonding as well as novel catalytic applications [30–39] . Initially considered to be innocent ligands, an increasing number of examples of non‐innocent behaviour by NacNac ligands have been reported, leading to unexpected, and often undesired, side reactivity of NacNac supported complexes [40–47] . Whilst most non‐innocent behaviour generally does not find any useful purpose, some studies have begun to deliberately characterise insertions at the γ‐carbon position on the NacNac backbone [48–53] .…”

“…Such studies show that a range of small molecules can be used either to block off this position to prevent non‐innocent behaviour, in particular through the work of the Jones group, or simply to generate novel new structures [54] . Our group has previously presented a selection of examples of such γ‐carbon‐inserted NacNac species in recent publications [45,46] …”