The planar 3,4-diphosphinidenecyclobutene (DPCB) can be remarkably twisted into a C2 -type helical structure by dual coordination of a AuCl moiety. A prompt chirality control of the twisted DPCB skeleton ligated by the digold units affords the enantiopure structure by exchanging the chloride ligands for chiral [1,1'-binaphthalene]-2,2'-dicarboxylate. The chirality of the diaurated 2,2'-bis(diphenylphosphanyl)-1,1'-biphenyl (BIPHEP) system can be controlled prior to that of DPCB. Mixing of a DPCB-bis(chlorogold) complex with the chiral silver salt dynamically leads to a single diastereomer, which was characterized by the (31) P NMR spectrum and the CD couplet patterns in the visible (DPCB) area. The absolute configuration of the singly induced helical structure was assigned by the theoretical CD spectra determined by TD-DFT calculations. Intramolecular alkoxycyclization of hexa-4,5-dien-1-ol catalyzed by the asymmetric DPCB-digold structure were also attempted.
The π‐accepting property of alkene‐like phosphaethenes (–P=C<) is attractive for the development of Lewis acidic transition‐metal catalysts. The stereoselective monoalkynylation of a sterically encumbered gem‐dibromophosphaethene by a Sonogashira process has been accomplished, and the corresponding 2‐alkynyl‐2‐bromo‐1‐phosphaethenes were obtained. Subsequent arylation of the 2‐alkynyl‐2‐bromo‐1‐phosphaethenes by the palladium version of the Kumada–Tamao–Corriu (KTC) reaction gave the corresponding 2‐alkynyl‐2‐aryl‐1‐phosphaethenes through an inversion of the configuration. The 2‐alkynyl‐2‐aryl‐1‐phosphaethenes were converted into the corresponding chloro‐gold(I) complexes, and screening of their catalytic activity revealed that both appropriate π‐conjugative substituents and the stereochemistry were decisive for the efficiency of the gold‐catalyzed reactions under activation‐free conditions. Chloro‐gold complexes bearing 2,2‐diaryl‐1‐phosphaethene ligands showed moderate‐to‐good catalytic activity.
Dedicated to Prof. Masaaki Yoshifuji on the occasion of his 75th birthdayThe trans bromide of 2,2-dibromo-1-(2,4,6-tri-t-butylphenyl)-1phosphaethene (Mes*P=CBr 2 ; Mes* = 2,4,6-tBu 3 C 6 H 2 ) can be successfully substituted with an aryl group by using a palladium version of the Kumada-Tamao-Corriu cross-coupling process. Predominant formation of the 2-aryl-2-bromo-1-phosphaethene [(Z)-Mes*P=C(Br)Ar] required suitable conditions including optimization of the ancillary phosphine ligand, thereby retarding the dual elimination of bromides leading to phosphaalkyne (Mes*C P). The 2-aryl-2-bromo-1-phosphaethenes hold promise as versatile synthons for functional p-conjugated molecules, and stereospecific transformations of the bromine atom by halogen-metal exchange and palladium-catalyzed arylations were demonstrated.[a] Prof.
Butadiene is the simplest neutral acyclic closed‐shell π‐conjugated system and is typically sufficiently stable enough to avoid electrocyclization to cyclobutene. In contrast, most congeners of butadiene containing heavier elements are easily converted into the corresponding 4‐membered cyclobutene system. Herein, we demonstrate that the gauche 1,4‐diphosphabutadiene (P=C−C=P) skeleton in a sterically encumbered 2,3‐bis(phosphanylidene)‐1,4‐disilinane can be remarkably perturbed due to “incomplete electrocyclization” where P=C−C=P partially form the corresponding 1,2‐dihydrodiphosphete (3,4‐diphosphacyclobutene) by [2+2] electrocyclization. 31P NMR data obtained in solution indicated that the coexistence of a closed ring substantially reduces the open‐ring characteristics of the P=C−C=P moiety. However, the 31P CP‐MAS spectrum of 2,3‐bis(phosphanylidene)‐1,4‐disilinane showed that the P=C−C=P structure is predominant in the solid‐state. Single‐crystal X‐ray analysis revealed that decreasing the temperature promoted the generation of small amounts of incomplete 1,2‐dihydrodiphosphete system in the crystalline state. Furthermore, the 1,2‐dihydrodiphosphete units disappeared upon warming the single crystal, and this unique solid‐state electrocyclization reaction was reversible.
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