5-Endo-trig cyclizations are generally considered to be kinetically unfavourable, as described by Baldwin's rules. Consequently, observation of this mode of reaction under kinetic control is rare. This is usually ascribed to challenges in achieving appropriate approach trajectories for orbital overlap in the transition state. Here, we describe a highly enantio- and diastereoselective route to complex indanes bearing all-carbon quaternary stereogenic centres via a 5-endo-trig cyclization catalysed by a chiral ammonium salt. Through computation, the preference for the formally disfavoured 5-endo-trig Michael reaction over the formally favoured 5-exo-trig Dieckmann reaction is shown to result from thermodynamic contributions to the innate selectivity of the nucleophilic group, which outweigh the importance of the approach trajectory as embodied by Baldwin's rules. Our experimental and theoretical findings demonstrate that geometric and stereoelectronic constraints may not be decisive in the observed outcome of irreversible ring-closing reactions.

Herein, we report the first 1,4‐diphosphinine‐1,4‐diide compound [(ADCPh)P]2 (5‐Ph) (ADCPh=PhC{(NDipp)C}2; Dipp=2,6‐iPr2C6H3) derived from an anionic dicarbene (ADCPh) as a red crystalline solid. Compound 5‐Ph containing a 16π‐electron planar fused‐tricyclic ring system was obtained by the 4e reduction of [(ADCPh)PCl2]2 (4‐Ph) with Mg (or KC8) in a quantitative yield. Experimental and computational results imply that the central 8π‐electrons C4P2 ring of 5‐Ph, which is fused between two 6π‐electrons C3N2 aromatic rings, is antiaromatic. Thus, each of the phosphorus atoms of 5‐Ph has two electron‐lone‐pairs, one in a p‐type orbital is in conjugation with the C=C bonds of the C4P2 ring, while the second resides in a σ‐symmetric orbital. This can be shown with the gold complex [(ADCPh)P(AuCl)2]2 (6‐Ph) obtained by reacting 5‐Ph with (Me2S)AuCl. A mixture of 5‐Ph and 4‐Ph undergoes comproportionation in the presence of MgCl2 to form the intermediate oxidation state compound [(ADCAr)P]2(MgCl4) (7‐Ph), which is an aromatic species.

It has been shown that heating of 3-R-[1,2,4]triazino[2,3-c]quinazolin-2-ones with 5-fold excess of hydrazine hydrate in propanol-2 gave the corresponding 3-(2-aminophenyl)-6-R-1,2,4-triazin-5-ones with high yields. The mechanism of the reaction has been proposed based on the results of theoretical investigation at B3LYP and MP2 levels of theory. According to calculations, an attack of the hydrazine molecule on the C6═N7 double bond leads to ring-opening with N5-C6 bond cleavage. The process continues by addition of the second nucleophile molecule and elimination of the formazan fragment through a series of transformations that yield the target product.

A theoretical study of the hydrolysis mechanism of A-234 [EtO–P(O)(F)–NC(Me)–N(Et)2]; the suspected novichok agent in the Skripal attack.