[4 + 1]- and [4 + 2]-cycloadditions of a thiazole-2-thione-based 1,4-diphosphinine - broadening the scope.

A broad study on [4 + 1]- and [4 + 2]-cycloaddition reactions of a thiazole-2-thione-based 1,4-diphosphinine (1) is reported, with a special focus on reversible reactions. Reactions of 1 with group 13 carbenoids DippNacNacM (M = Al and Ga) afford [4 + 1] adducts that can be classified as Al and Ga phosphides or as 7-metalla-1,4-norbornadienes. Reactions of 1 with alkynes and alkenes result in [4 + 2]-cycloaddition, affording 1,4-diphosphabarrelenes. The effect of different dienophiles on the formation of 1,4-diphosphabarrelenes and their thermal [4 + 2]-cycloreversion reactions is studied from an experimental as well as theoretical point of view, opening the door for protection/deprotection strategies in this chemistry.

Tricyclic 1,4-Diphosphinines: Local vs. Global Aromaticity.

1D and 2D NICS π . zz SOM ${{{\rm { NICS}}}_{{\rm { {\rm \pi}{}}}{\rm { .zz}}}^{{\rm { SOM}}}}$ (Sigma only model) calculations were performed on recently established tricyclic 1,4-diphosphinines as well as related benzene and pyrazine derivatives. The study was extended to evaluate the effect of the fused rings on the overall aromatic properties with a special focus on functional groups such as carbenes. The effect of non-aromatic heterocycles on the local ring current of the central ring is small, while aromatic heterocycles (e. g. NHC, imidazolium) lead to a global aromaticity. A higher sulfur content of the adjacent five-membered rings reduces the central ring current. The comparison to related tricyclic benzene and pyrazine derivatives showed that the 1,4-diphosphinine systems resemble more closely the situation in the benzene derivatives than the pyrazines. The effect of charged systems was studied using bis(TTF)-fused 1,4-diphosphinines and, according to NICS π . zz SOM ${{{\rm { NICS}}}_{{\rm { {\rm \pi}{}}}{\rm { .zz}}}^{{\rm { SOM}}}}$ values, the neutral form doesn't possess significant aromaticity but the tetracation resembles the global aromatic situation observed for other heterocycles in this study.

Elimination reactions in 1,4-diphosphinine chemistry: mixed-valence intermediates and 1,1- vs. 1,4-elimination pathways.

Reactions of tricyclic 1,4-diphosphinines 1,4 with LiOH, followed by protonation are reported. 1,3-Thiazole-2-thione-derived 1 enabled only observation of the first anionic addition product 2/3. On the other hand, imidazole-2-selone-annelated derivative 4 enabled the identification of the first (5) and second product (6) at low temperature. As water was eliminated upon warming in both cases, DFT calculations were performed to gain more insight into the reaction pathway(s).

Access to and reactions of P-functional 1,4-dihydro-1,4-diphosphinines fused to two TTF units.

P-Functional phosphanylated tetrathiafulvalenes 3a-f were synthesised via stepwise lithiation and phosphanylation of TTF derivatives, and then reacted with PCl3 to form the related P-chloro compounds 4a-f. Reactions of 4c-f with LDA resulted in the formation of the corresponding 1,4-dihydro-1,4-diphosphinines 5c-f. As a case in point, P-oxidation reactions of 5d,f with elemental chalcogens were performed, and the former were also converted into 1,4-dichloro-1,4-dihydro-1,4-diphosphinine 9f. The latter was reduced to form the related 1,4-diphosphinine 10f which could not be isolated but formed the corresponding 1,4-diphosphabarrelene 11f in a [4 + 2]-cycloaddition with 1-hexene. All compounds were characterised by multinuclear NMR spectroscopy and mass spectrometry and also by single crystal X-ray diffraction studies in some cases. Intensive cyclic voltammetry studies were performed for all isolated compounds with the special focus on using TTF units as sensors to study the substituent effects on oxidation potentials and, hence, the degree of electronic communication between redox active moieties in the bis-TTF species. E.g., 5d possesses four quasi-reversible one-electron oxidation steps thus forming a tetracation species at highest potential (+0.54 V vs. Fc+/0). Additionally, high level DFT calculations were undertaken to get a deeper understanding of various aspects of this novel combination of phosphorus and TTF chemistry.

Dehydrogenation reaction of triethylamine by an electrophilic terminal phosphinidene complex.

Reaction of a transiently formed terminal phosphinidene complex with triethylamine resulted in the formation of an sp3 C-H insertion product, as revealed by 31P NMR spectroscopy, which was isolated as semi-solid compound. However, if the reaction was continued for 24 h, a primary phosphane complex was obtained eventually. The compounds were characterised by NMR spectroscopy and mass spectrometry. Formation of the final products is explained by a mechanistic proposal based on DFT calculations.

New frontiers: 1,4-diphosphinines and P-bridged bis(NHCs).

This review describes synthetic concepts and breakthroughs in 1,4-diphosphinine and related P-bridged bis(NHCs) chemistry, covering the last four decades, starting from monocyclic 1,4-dihydro-1,4-diphosphinines in the early 80s to the most recent and promising achievements of tricyclic 1,4-dihydro-1,4-diphosphinines and tricyclic 1,4-diphosphinines. Theoretical aspects are presented for 1,4-dihydro- and 1,4-diphosphinines considering HOMO LUMO situations as well as the degree of aromaticity. Moreover, fundamental characteristics of analytical data of these compounds are highlighted with special focus on substituent effects, structural aspects and trends of electrophilicity. The two P-centers and the heterocyclic rings of 1,4-dihydro- and 1,4-diphosphinines constitute a broad platform for substitution, reduction/oxidation, alkylation, complexation and cycloaddition reactions, i.e., a comprehensive compilation of reactivity aspects is presented. Furthermore, very recent developments in the synthesis and reactivity of tricyclic PV/V- and PIII/III-bridged bis(imidazole-2-ylidenes) will be discussed together with new perspectives derived from an antiaromatic middle ring. In total, our intention is to show new frontiers, i.e., new synthetic paths, thus creating novel opportunities for potential applications in molecular and materials chemistry.

A case study on the conversion of Li/Cl phosphinidenoid into phosphinidene complexes.

N,N-Diphenylamino- and N,N-dicyclohexylamino-substituted dichlorophosphane W(CO)5 complexes 1a,b were used to generate thermally very labile Li/Cl phospinidenoid W(CO)5 complexes 2a,b. The formation of transient complex 2a was confirmed via low-temperature 31P{1H} NMR spectroscopy, but further strong evidence for the formation of transient complexes 2a,b was obtained from reactions with methanol and methylamine as formal E-H insertions (E = O, N) furnished complexes 3a,b and 4a. By using toluene in the absence of donor ligands, the primarily nucleophilic complexes 2a,b were converted into electrophilic terminal phosphinidene complexes 5a,b which was deduced from specific trapping reactions using tolane, 1-pentene and 1-hexene and thus obtained 1H-phosphirene 6 and phosphirane complexes 7 and8. The state-of-the-art DFT calculations reveal insights into the possible reaction pathways and exclude a direct reaction of 2a,b with alkene trapping reagents.

[4 + 2]-Cycloadditions of a thiazol-based tricyclic 1,4-diphosphinine and a new easy 1,4-diphosphinine protection deprotection strategy.

Diels-Alder-reactions of a thiazol-2-thione-based, tricyclic 1,4-di-phosphinine were investigated, showing that the central aromatic π-system can react with various dienophiles. The reaction with 4-phenyl-1,2,4-triazoline-3,5-dione was special as the product revealed a remarkable sensitivity towards light, thus enabling the photochemical deprotection of the tricyclic 1,4-diphosphinine.