Recent developments in catalytic cross-couplings with unsaturated carboxylates

Catalytic cross-couplings through C-O bond-cleavage of unsaturated carboxylates with organometallics have emerged as a powerful method for sustainable syntheses. Over the last decade, remarkable achievements have been made with the development of transition metal-catalyzed cross-couplings with the readily available phenol and enol derivatives as suitable coupling electrophiles beyond unsaturated halides. Therefore, this perspective describes the recent advances in the field of transition metal-catalyzed C-O bond activation of unsaturated carboxylates with organometallics, including B, Mg, Zn, Al, and Si reagents, until May 2021.

Palladium-Catalyzed Synthesis of 1,2-Diketones from Aryl Halides and Organoaluminum Reagents Using tert-Butyl Isocyanide as the CO Source

In this work, an interesting and practical procedure for the synthesis of 1,2-diketones from aryl halides and organoaluminum reagents has been developed. Employing tert-butyl isocyanide as the CO source and palladium as the catalyst, the desired 1,2-diketones were isolated in good to excellent yields with good functional group tolerance. Concerning the reaction partners, besides aryl halides, both alkyl- and arylaluminum reagents were all suitable substrates here.

Palladium-Catalyzed Carbonylative Coupling of Aryl Iodides with Alkenylaluminum Reagents

A highly reactive catalytic system for the carbonylative coupling of aryl iodides with alkenylaluminum reagents has been developed. Various β-substituted γ,δ-unsaturated ketones were produced under mild conditions in good to excellent yields even under ppm level of palladium catalyst. Notably, this also represents the first example on carbonylative transformation of alkenylaluminum compounds. Additionally, by the addition of zinc salt, the selectivity of the product can be modified.

Donor-influenced Structure-Activity Correlations in Stoichiometric and Catalytic Reactions of Lithium Monoamido-Monohydrido-Dialkylaluminates

A series of heteroleptic monoamido‐monohydrido‐dialkylaluminate complexes of general formula [iBu₂AlTMPHLi⋅donor] were synthesized and characterised in solution and in the solid state. Applying these complexes in catalytic hydroboration reactions with representative aldehydes and ketones reveals that all are competent, however a definite donor substituent effect is discernible. The bifunctional nature of the complexes is also probed by assessing their performance in metallation of a triazole and phenylacetylene and addition across pyrazine. These results lead to an example of phenylacetylene hydroboration, which likely proceeds via deprotonation, rather than insertion as observed with the aldehydes and ketones. Collectively, the results emphasise that reactivity is strongly influenced by both the mixed‐metal constitution and mixed‐ligand constitution of the new aluminates.

Trans-Metal-Trapping: Concealed Crossover Complexes En Route to Transmetallation?

Defined as the transfer of ligands from one metal to another, transmetallation is a common reaction in organometallic chemistry. Its chemical celebrity stems from its role in important catalytic cycles of cross-coupling reactions such as those of Negishi, Sonogashira, Stille, or Suzuki. This article focuses on trans-metal-trapping (TMT), which could be construed as partially complete transmetallations. On mixing two distinct organometallic compounds, of for example lithium with aluminium or gallium, the two metals meet in a crossover co-complex, but the reaction ceases at that point and full transmetallation is not reached. Though in its infancy, trans-metal-trapping shows promise in transforming failed lithiations into successful lithiations and in stabilising sensitive carbanions through cooperative bimetallic effects making them more amenable to onward reactivity.

LiTMP Trans-Metal-Trapping of Fluorinated Aromatic Molecules: A Comparative Study of Aluminum and Gallium Carbanion Traps

Fluoroaromatic scaffolds pose a challenge to lithiation due to low stability of lithiated intermediates. Here we apply trans‐metal‐trapping (TMT) to a series of key fluorinated aromatics. In TMT, LiTMP performs the metalation, while an organometallic trap intercepts the emergent carbanion. This study contrasts the trapping abilities of iBu₂AlTMP and Ga(CH₂SiMe₃)₃, structurally mapping their TMT reactions and probing relative stabilities of metalated fluoroaromatic intermediates by NMR studies. Results show the installed Al−C(aryl) bonds are more prone to decomposition by benzyne formation and Li‐F liberation, than the Ga−C(aryl) species. The latter are thus better for onward reactivity as demonstrated in cross‐coupling reactions with benzoyl chloride that produce ketones.

TMP (2,2,6,6-tetramethylpiperidide)-aluminate bases: lithium-mediated alumination or lithiation–alkylaluminium-trapping reagents?

Surprisingly lithium TMP-aluminate reagents are not capable of directly aluminating anisole as previously thought but operateviasequential lithiation–alkylaluminium trapping.