Phosphaalkenes as Building Blocks in Ene Reactions:  Synthesis and Reactivity of 3-Amino-1,2-dihydro-1,2-diphosphetes

Abstract

Phospha-ene reactions of the type II between methylidenephosphanes as enophiles and Caminophosphaalkenes possessing allylic hydrogen atoms proceed by P-P bond formation to furnish the corresponding functionalized diphosphanes. Thus, the reaction of the methylidenephosphane 1 with the C-amino-substituted ethylidenephosphane 3a runs smoothly at room temperature to afford the unsymmetrical 1,2-diphosphane 4 as a 60:40 mixture of two diastereomers in 64% yield. Rotation about the P-N bond in 4 is hindered at room temperature, but free enthalpies of activation of ∆G q (PN) ) 16.3 kcal‚mol -1 (major diastereomer) and 15.2 kcal‚mol -1 (minor diastereomer) were determined by variable temperature 1 H NMR spectroscopy. In contrast, reactions of the ethylidenephosphanes 3a-c with the chlorophosphane 2 as enophile follow an unusual course involving a regiospecific ene reaction of the type II and a subsequent intramolecular ring closure reaction to furnish the 3-amino-1,2-dihydro-1,2-diphosphetes 7a-c in good yields (53-79%). The reactivity of this new class of heterocyclic compounds has been studied exemplarily for product 7a. The trans form of 7a obtained by synthesis can be converted to the corresponding cis form photochemically. Complexation of both phosphorus centers in 7a is possible by reaction with 2 equiv of diiron nonacarbonyl and gives rise to complex 8, the constitution of which has been confirmed by X-ray crystallography. Unexpectedly, hydrolysis of 7a does not proceed by attack at the endocyclic enamine unit but rather by way of P-P bond cleavage to afford the functionalized phosphaalkene 9. On the other hand, oxidation of 7a by bis(trimethylsilyl) peroxide takes place with retention of the P 2 C 2 skeleton: a selective increase in coordination at P-1 results in the formation of the 1-oxo-1,2-dihydrodiphosphete 10, the structure of which was confirmed by X-ray crystallography. The reactions of 7a with the electron-poor acetylenes 11a-d also follow an unusual course. Nucleophilic attack of P-1 at the CtC triple bond and proton shift lead stereospecifically to the 1-methylene-1,2-dihydro-1,2-diphosphetes 12a-d as thermolabile addition products. The constitution and configuration of 12c were unambiguously confirmed by X-ray crystallography.

Phospha‐Organic Chemistry: Panorama and Perspectives

“…(25)]. [59] The analogy between C¼C and P¼C bonds is completed by the participation of P¼C bonds in ene reactions, [60] [1,5], [61] and [3,3] sigmatropic shifts, [62] as well as 4 p [63] and 6 p electrocyclizations. [64] Finally, numerous conjugated or cumulated phosphapolyenes and phosphaketenes show chemistry which quite nicely parallels that of their all-carbon analogues.…”

Phosphaorganische Chemie: Panorama und Perspektiven

“…Die Beteiligung von PC‐Bindungen in En‐Reaktionen,60 [1,5]‐61 und [3,3]‐sigmatropen Umlagerungen62 sowie in 4π‐63 und 6π‐Elektrocyclisierungen64 vervollständigt unseren Blick auf die Analogie von C‐C‐ und P‐C‐Doppelbindungen. Abschließend sei erwähnt, dass auch die Chemie zahlreicher konjugierter oder kumulierter Phosphapolyene beeindruckende Parallelen zur Chemie der analogen reinen Kohlenstoffverbindungen aufweist 2…”

The Ene Reaction in the Chemistry of Low-Coordinate Phosphorus

“…The two components react smoothly to give the multifunctionalized 1,2-diphosphine 19 as the ene adduct (Scheme 11). 33 Since this type II ene reaction proceeds with the formation of two new asymmetric centers at the two phosphorus atoms, two diastereomers in a ratio of 60:40 are observed. 34,35 The primary addition of the enophile 22a proceeds exclusively or with high selectivity at the substituted, electron-rich C-C double bond of 23, but always with P-C bond formation.…”