Pilar Sanchez-Andrada

Tandem 1,5-Hydride Shift/1,5-S,N-Cyclization with Ethylene Extrusion of 1,3-Oxathiolane-Substituted Ketenimines and Carbodiimides. An Experimental and Computational Study†

Under thermal activation in solution, N-[2-(1,3-oxathiolan-2-yl)]phenyl ketenimines and carbodiimides were converted into 2,1-benzisothiazol-3-ones bearing a pendant N-styryl or imidoyl fragment, respectively. These processes should occur with the concomitant formation of ethylene as result of the fragmentation of the 1,3-oxathiolane ring. The conversions of ketenimines took place under softer thermal conditions, toluene 110 degrees C, than those of carbodiimides, o-xylene 160 degrees C. A computational DFT study unveiled the mechanistic course of these transformations, rare tandem processes consisting of an initial 1,5-hydride shift of the acetalic hydrogen atom to the central carbon atom of the heterocumulene function leading to the respective o-azaxylylene. This transient intermediate then converts, in a single step, into ethylene and the experimentally isolated benzisothiazolone. This latter stage of the mechanism is rather peculiar, combining a 1,5-cyclization by S-N bond formation, aromaticity recovery at the benzene nucleus, and the fragmentation of the oxathiolane framework originating a new carbonyl group. It can be related with a vinylogous retro-ene reaction and shows pseudopericyclic characteristics. The computations also revealed that the alternative 6pi electrocyclization of the transient o-azaxylylenes cannot compete, on kinetic and thermodynamic grounds, with the experimentally observed reaction channel. The two alternative reaction paths of a number of ketenimines and carbodiimides were computationally scrutinized, the results being in accord with the experimental outcomes. In addition, sulfur extrusion from the benzisothiazolones by the action of triphenylphosphine under two different reaction conditions led to three different types of heterocyclic products, 4(3H)-quinolones, quinolino[2,1-b]quinazolin-5,12-diones, and dibenzo[b,f][1,5]diazocin-6,12-diones, whose formation is explained by the initial formation of an intermediate imidoylketene. This reactive species could be trapped by a nucleophilic solvent, ethanol.

Unexpected formation of 2,1-benzisothiazol-3-ones from oxathiolano ketenimines: a rare tandem process.

A rare one-pot reaction, a tandem [1,5]-H shift/1,5 electrocyclization/[3 + 2] cycloreversion process, leading from N-[2-(1,3-oxathiolan-2-yl)]phenyl ketenimines to 1-(beta-styryl)-2,1-benzisothiazol-3-ones and ethylene, is disclosed and mechanistically unraveled by means of a computational DFT study. The two latter stages of the tandem process are calculated to occur in a single mechanistic step via a transition structure of pseudopericyclic characteristics.

Highly stereocontrolled [2+2] cycloaddition versus unprecedented imino-ene reactions of imino-ketenimines

Abstract N-[(2-Benzylideneamino)phenyl]-C-methylketenimines undergo intramolecular cyclization via two different reaction pathways, a [2+2] cycloaddition and a rare imino-ene reaction, to yield azeto[1,2-a]benzimidazoles and 2-(α-styryl)benzimidazoles, respectively. The results are interpreted in terms of a two-step mechanism involving two stereoisomeric conjugated betaines as intermediates.

Polar Hetero-Diels−Alder Reactions of 4-Alkenylthiazoles with 1,2,4-Triazoline-3,5-diones: An Experimental and Computational Study

The hetero-Diels-Alder reactions of 4-alkenylthiazoles with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) lead to new heteropolycyclic systems in excellent yields and high levels of stereocontrol through an exclusively suprafacial approach. 4-Alkenylthiazoles with a stereogenic center placed at the alkenylic substituent react with PTAD giving the corresponding chiral cycloadducts in moderate diastereomeric excesses. The stereochemical course is dominated by the steric interactions at the two diastereomeric transition states. A computational study of these processes with structurally simpler reagents has been carried out. A concerted pathway via a highly asynchronous transition state is preferred for 2-unsubstituted 4-vinyl and 4-styrylthiazoles. However, two alternative and equally likely pathways, concerted and stepwise, have been found to be followed by 2-methyl- or 2-phenyl-substituted 4-styrylthiazoles. The concerted pathway features a highly asynchronous transition state. For the stepwise pathway, the rate-determining step is the first one, as the energy barrier for the second step is virtually nonexistent.

Synthesis and Molecular Structure of β‐Phosphinoyl Carboxamides: An Unexpected Case of Chiral Discrimination of Hydrogen‐Bonded Dimers in the Solid State

A series of N,P,P-trisubstituted aminophosphanes react with diphenylcyclopropenone to afford an easily separable mixture of two diastereomeric alpha,beta-diphenyl-beta-phosphinoyl carboxamides in good yields. X-ray crystal structures reveal that these compounds associate into dimers, formed from two enantiomeric units linked by two bifurcated hydrogen bonds, whereby the oxygen atom of the phosphoryl group acts as a dual acceptor for the vicinal NH and CH of a carbonyl group of a neighbouring molecule. Studies on the interconversion between diastereomeric phosphinoyl carboxamides in basic solution show that the thermodynamically most stable isomer depends on the nature of the substituent at the nitrogen atom. Simple computational calculations explain this phenomenon.

Medium- and large-membered rings from bis(iminophosphoranes): An efficient preparation of cyclic carbodiimides

Abstract The new cyclic carbodiimides 3 are synthetized from bis(iminophosphoranes) 2 and Boc2O in the presence of DMAP; similar results can be achieved from the N-Boc-protected bis(iminophosphoranes) and carbon dioxide.

An efficient preparation of macrocycles containing two tetra-coordinate phosphorus atoms

Abstract The Staudinger reaction between bis(azides) and bis(phosphines) under mild conditions provides macrocyclic bis(iminophosphoranes) (at least 12-membered rings) in excellent to moderate yield. The method also allows the incorporation of a heteroatom into the chain connecting the two aromatic or heteroaromatic rings.

A generalized and efficient preparation of a novel class of macrocyclic bis(guanidines)

Abstract Reaction of readily available macrocyclic bis(carbodiimides) with ammonia, primary or secondary alkylamines and α,ω-diamino compounds provided a novel class of macrocyclic bis(guanidines), which were isolated as crystalline solids in high yields (74–98 %).