Cooperative Activation of Isocyanates by Al–N‐Based Active Lewis Pairs and the Generation of a C₅ Chain by Simultaneous Formation of Two C–C Bonds

Abstract

The active Al/N Lewis pair, (Me C) Al-C(SiMe )=C(H)-N(CHMe-CH ) CH (2), reacted with isocyanates to afford a fascinating variety of products. One equivalent of Ph-N=C=O yielded by the release of H-C≡C-SiMe an urea-type ligand which coordinated the Al atom in a chelating manner (4). Dipp-N=C=O gave a similar product, but the bulky substituent hindered the approach of the N-aryl group to Al. A situation similar to that of frustrated Lewis pairs resulted in the coordination of the alkyne to the Al and N atoms (6) by C-H bond activation. Dual insertion was observed upon treatment of 2 with two equivalents of isocyanates (8 to 11). The preferred formation of cyclic oligomers is prevented by the specific cooperative properties of the Lewis pair. A metal-free dimeric isocyanate (13) was formed by hydrolysis. Replacement of the CMe groups in 2 by less bulky isobutyl groups (7) afforded the insertion of two isocyanate molecules into the Al-vinyl bonds without alkyne elimination. The resulting highly functionalised compound had a chain formed by two isocyanates and the organic backbone of the Lewis pair. Me C-N=C=O and 2 afforded a unique compound (14) in which an isocyanate ligand connects two molecules of 2 by the release of dimethylpiperidine. The combination of a C building block and two C groups gave an unsaturated branched C moiety by the simultaneous formation of two C-C bonds. The molecular structure showed an interaction between an Al atom and a C-C π-bond.

Cooperative activation of azides by an Al/N-based active Lewis pair – unexpected insertion of nitrogen atoms into C–Si bonds and formation of AlCN3 heterocycles

“…Usually the O or N atoms of substrates (carbonyl compounds, imines) approach the Lewis acidic Al atoms of the FLPs or ALPs as the initiating step of the respective transformation. By the interaction with the metal atom the polarity of the C=O or C=N bonds and the electrophilicity of the C atoms increase and facilitate the nucleophilic attack of the Lewis basic ALP or FLP centers at this position by ring closure [7,8] or by initiating another secondary reaction such as an insertion into a M-C bond [12,13,16]. Azides with a homonuclear N 3 group react with Al/P-based FLPs by coordination of the terminal N atom to the Lewis acidic and basic atoms (see Introduction).…”

“…A class of compounds closely related to the Al/P FLPs contain Al or Ga and N atoms. Weak bonding interactions between the Lewis acidic metal and the Lewis basic N atoms result in strained four- [11][12][13][14] or three-membered heterocyles [15][16][17][18]. Cleavage of the relatively weak donor-acceptor bonds in the presence of suitable substrates results in a reactivity similar to that of FLPs.…”

“…Due to the lack of frustration these compounds were named active Lewis pairs (ALPs) [11]. The exceptional and variable reactivity of these ALPs has been demonstrated by their reactions with isocyanates, nitriles, terminal alkynes, carbon dioxide, carbodiimide and other substrates [11][12][13][14][15][16][17][18]. Particularly noteworthy is their capability to oligomerize suitable monomeric starting materials such as cyanamides [12].…”

“…MHz; 29 Si 79.49 MHz, 15 N 40.55 MHz) or Avance III ( 1 H, 400.03 MHz; 13 C, 100.59 MHz, 29 Si, 79.47 MHz; 15 N, 40.54 MHz) and referenced internally to residual solvent resonances ( 1 H,13 C; chemical shift data δ in ppm) or to liquid NH 3 in case of 15 N 13. C NMR spectra were all protondecoupled.…”

Synthesis and Characterization of Pheox– and Phebox–Aluminum Complexes: Application as Tunable Lewis Acid Catalysts in Organic Reactions

“…Thus, hydrocarbyl ligands are promising replacements,a st his decrease in the Lewis acidity can be avoided, and modification of the carbon framework can precisely control the catalytic properties ( Figure 1A). However,f ew aluminum catalysts with well-defined hydrocarbyl ligands have been reported, [3][4][5] and simple alkyl ligandsb ehaving as spectators are often used ( Figure 1B,t ypical catalysts). [1,2] Due to the nucleophilic properties of hydrocarbyl ligandsi na ddition to their sensitivity to moisture anda ir,t he application of organoaluminum compoundsa sc atalysts is challenging.…”