Reactions of an Aluminum(I) Reagent with 1,2-, 1,3-, and 1,5-Dienes: Dearomatization, Reversibility, and a Pericyclic Mechanism

Abstract

The reactions of an aluminium(I) reagent with a series of 1,2-, 1,3-and 1,5-dienes are reported. In the case of 1,3-dienes the reaction occurs by a pericyclic reaction mechanism, specifically a cheletropic cycloaddition, to form aluminocyclopentene containing products. This mechanism has been interrogated by stereochemical experiments and DFT calculations. The stereochemical experiments show that the (4+1) cycloaddition follows a suprafacial topology, while calculations support a concerted albeit asynchronous pathway in which the transition state demonstrates aromatic character. Remarkably, the substrate scope of the (4+1) cycloaddition includes dienes that are either in part, or entirely, contained within aromatic rings. In these cases, reactions occur with dearomatisation of the substrate and can be reversible. In the case of 1,2-or 1,5-dienes complementary reactivity is observed; the orthogonal nature of the C=C p-bonds (1,2-diene) and the homoconjugated system (1,5-diene) both disfavour a (4+1) cycloaddition. Rather, reaction pathways are determined by an initial (2+1) cycloaddition to form an aluminocyclopropane intermediate which can in turn undergo insertion of a further C=C p-bond leading to complex organometallic products that incorporate fused hydrocarbon rings.

Recent advances in low oxidation state aluminium chemistry

“…Interestingly, the coordination mode of the COT ligand changed to form the addition product 9aluminabicyclo[4.2.1]nona-2,4,7-triene, which is reminiscent of the reactivity of 2 and related cyclic alkenes. 46 In work published simultaneously to that of Aldridge and Goicoechea (17, Fig. 6-8), Coles and Anker sought to investigate the reactivity of 32 with CO 2 and N 2 O.…”

Catalyst control of selectivity in the C–O bond alumination of biomass derived furans

“…We have recently reported the cheletropic reaction of 1 with dienes and aromatic hydrocarbons. 26 Based on analysis of the NPA charges, the Wiberg bond indices and the bond angles and lengths from Al to the bdiketiminate ligand, this step involves a redox transformation of Al(I) to Al(III) and the dearomatisation of 2-methylfuran. The (4 + 1) intermediate rearranges via two possible isomeric transition states (TS-2) to form the isomers of the product (4b/5b) (Fig.…”

Chemoselective C−C σ‐Bond Activation of the Most Stable Ring in Biphenylene**

“…Based on cross‐over experiments, the formation of 4 is, at least, partially reversible (see the Supporting Information for further details). The formation of 4 can be explained by a pericyclic reaction, specifically a [ π 4 s + n 2 s ] cycloaddition [26–29] . Formation of 4 generates a highly strained and dearomatized hydrocarbon framework that is primed for further reactivity.…”

“…The formation of 4 can be explained by a pericyclic reaction, specifically a [ p 4 s + n 2 s ] cycloaddition. [26][27][28][29] Formation of 4 generates a highly strained and dearomatized hydrocarbon framework that is primed for further reactivity. Addition of a further equiv of 1 to 4 and heating to 100 8C generates the 2 and 3 with similar yields and selectivity to that observed directly from 1 and biphenylene.…”