Deprotonation of aminophosphaalkenes (RMe(2)Si)(2)C=PN(H)(R') (R=Me, iPr; R'=tBu, 1-adamantyl (1-Ada), 2,4,6-tBu(3)C(6)H(2) (Mes*)) followed by reactions of the corresponding Li salts Li[(RMe(2)Si)(2)C=P(M)(R')] with one equivalent of the corresponding P-chlorophosphaalkenes (RMe(2)Si)(2)C=PCl provides bisphosphaalkenes (2,4-diphospha-3-azapentadienes) [(RMe(2)Si)(2)C=P](2)NR'. The thermally unstable tert-butyliminobisphosphaalkene [(Me(3)Si)(2)C=P](2)NtBu (4 a) undergoes isomerisation reactions by Me(3)Si-group migration that lead to mixtures of four-membered heterocyles, but in the presence of an excess amount of (Me(3)Si)(2)C=PCl, 4 a furnishes an azatriphosphabicyclohexene C(3)(SiMe(3))(5)P(3)NtBu (5) that gave red single crystals. Compound 5 contains a diphosphirane ring condensed with an azatriphospholene system that exhibits an endocylic P=C double bond and an exocyclic ylidic P((+))-C((-))(SiMe(3))(2) unit. Using the bulkier iPrMe(2)Si substituents at three-coordinated carbon leads to slightly enhanced thermal stability of 2,4-diphospha-3-azapentadienes [(iPrMe(2)Si)(2)C=P](2)NR' (R'=tBu: 4 b; R'=1-Ada: 8). According to a low-temperature crystal-structure determination, 8 adopts a non-planar structure with two distinctly differently oriented P=C sites, but (31)P NMR spectra in solution exhibit singlet signals. (31)P NMR spectra also reveal that bulky Mes* groups (Mes*=2,4,6-tBu(3)C(6)H(2)) at the central imino function lead to mixtures of symmetric and unsymmetric rotamers, thus implying hindered rotation around the P-N bonds in persistent compounds [(RMe(2)Si)(2)C=P](2)NMes* (11 a, 11 b). DFT calculations for the parent molecule [(H(3)Si)(2)C=P](2)NCH(3) suggest that the non-planar distortion of compound 8 will have steric grounds.

Metalation of the aminophosphaalkene (iPrMe 2 Si) 2 C=PN(H)-SiMe 3 (2) with lithium diisopropylamide (LDA) in THF solution, followed by the reaction of the lithium salt 3 with the Pchlorophosphaalkenes (RMe 2 Si) 2 C=PCl (1a, R = Me; 1b, R = iPr; 1c, R = Ph), furnishes the first N-silylimino-bridged bis (phosphaalkenes)

Hydrolytic cleavage of the P-chlorophosphaalkenes (RMe2Si)2C=PCl (R = Me: 1a; R = iPr: 1b) in the presence of triethylamine leads to di(phosphavinyl) ethers (2,4-diphospha-3-oxapentadienes) [(RMe2Si)2C=P]2O (2a, 2b) as main products, accompanied by alkylphosphinic acids (RMe2Si)2(H)CP(H)(O)OH (3a, 3b). The hydrolysis of (PhMe2Si)2C=PCl (1c) proceeds less selectively. Reactions with metal oxides under aprotic conditions provide 2a [impure, from 1a with (nBu3Sn)2O] and 2b [from iodophosphaalkene (iPrMe2Si)2C=PI with Ag2O] as oils. 1H, 13C, 29Si and 31P NMR spectra, however, allow unambiguous characterisation of 2a and 2b. Formation mechanisms, structure, and C=P-O π stabilisation of the oxabisphosphaalkene [(H3Si)2C=P]2O (2ʹ) were studied with DFT methods. The double [2+4] cycloaddition reaction of 2a with two equivalents of cyclopentadiene leads to the phosphinous anhydride 7 as a mixture of diastereomers whereas the addition of two equivalents of tetrachloro-o-benzoquinone proceeds in a diastereoselective fashion. An X-ray crystal structure determination of the resulting oxo-bridged bis(2-phospha-2,5-dioxa-3,4-benzophospholene) derivative 8 revealed the presence of a racemic mixture of (R,R)- and (S,S)-configurated molecules. The solid state structure of a by-product, bisylphosphonic tetrachlorocatechol monoester (Me3Si)2CH-P(=O)(OH)-o-OC6Cl4OH 9, was also determined crystallographically.

Reactions of P-chlorophosphaalkenes (RMe 2 Si) 2 C=PCl (1a: R = Me; 1b: R = Ph) with the disilane Me 3 SiSiCl 3 (5) furnish diphosphenes (Cl 3 Si)(RMe 2 Si) 2 C-P=P-C(SiCl 3 )(SiMe 2 R) 2 (4a: R = Me; 4b: R = Ph) by Me 3 SiCl elimination. The structure of the new compound 4b was confirmed by X-ray diffraction; it displays crystallographic inversion symmetry. Monitoring the reactions with 31 P-and 29 Si-NMR spectroscopy detected P-(trichlorosilyl)phosphaalkenes (RMe 2 Si) 2 C=PSiCl 3 (2a, R = Me; 2b, R = Ph) as the primary intermediates from reductive P-silylation of 4a, 4b, and P- [(trichlorosilyl)phosphanyl]phosphaalkenes (RMe 2 Si) 2 C=P-P(SiCl 3 )C(SiCl 3 )(SiMe 2 R) 2 (3a: R = Me; 3b: R = Ph) as unsymmetric dimerisation products that

Highly unsaturated organophosphorus compounds are rich in energy, and tend to rearrange to more stable rings, cages, and polymers. Nieckes 2,3,4-triphosphapentadienide anion [{(Me 3 Si) 2 C = P} 2 P] À (1), [1] the only p-donor heteroatombridged bisphosphaalkene anion that allows electronic communication between the neighboring P=C bonds, is known to undergo thermal elimination/re-addition reactions to furnish anionic P 3 C 2 heterocyles, [1] but its synthetic potential remains unexplored. The uncharged isoelectronic sulfide [(Me 3 Si) 2 C= P] 2 S (2), however, might be a promising synthetic precursor. We began our studies with 2 and the related selenide 3 to evaluate if and how p communication through the heteroatoms sulfur and selenium might influence the reactivity of bifunctional phosphaalkenes.2,4-Diphospha-3-thia-and 2,4-diphospha-3-selenapentadienes 2 and 3 are formed in the reaction of (Me 3 Si) 2 C = PCl (4) [2] with bis(trialkylsilyl)chalcogenides [Eq. (1)]. Redbrown 3 crystallizes as a monomer. The W-shaped CPSePC moiety of 3 ( Figure 1) shows a close analogy to that of Nieckes anion 1.[1] The central angle PSeP of 3 is 90.78, whereas in the isoelectronic arsenide ion [{(Me 3 Si) 2 C=P} 2 As] À (5), the angle PAsP shrinks to 85.88.[3] The P = C bonds in 3 (1.664 and 1.668 ) are slightly shorter than in 1 (1.687 ), and the PÀSe bonds appear to be normal single bonds.Energies of isodesmic reactions of the model compound [(H 3 Si) 2 C = P] 2 S with H 2 S calculated at the B3LYP/6-31 + G* level (leading to 2 (H 3 Si) 2 C = PSH; + 0.1 kcal mol À1 ) and with PH 3 (leading to (H 3 Si) 2 C = PH and (H 2 P) 2 S; + 17.6 kcal mol À1 ) [4] suggest that in the C=P-S unit there is p stabilization (ca. 9 kcal mol À1 for each P=C moiety), but there is no further energy gain when the second P=C unit is attached to sulfur.Two equivalents of elemental sulfur or selenium add to compounds 2 and 3, respectively, in a surprising fashion: [5] norbornane-type P 2 Se 5 -related [6] [(Me 3 Si) 2 CP] 2 E 3 cages (E = S, Se) are formed. Bisphosphiranes (Scheme 1, type III), or thermodynamically somewhat less stable bis-s 3 ,l 5 -phosphoranes (V), are not observed.31 P NMR spectra of the reaction mixture of 2 with sulfur (see Experimental Section) exhibit a weak AX pattern at early stages of the reaction (d( 31 P) = + 353.3, À25.0 ppm; J = 276.3 Hz); this AX pattern is assigned to a P-(phosphaalkenylthio)thiaphosphirane (6, type I, see Scheme 1).[7] The main product is the C 2 -symmetric compound [(Me 3 Si) 2 C] 2 P 2 S 3 (7; P III ,P III ) [Eq. (2)], accompanied by partially oxidized [(Me 3 Si) 2 C] 2 P 2 S 4 (8; P III ,P V ), which has an AB pattern (J31 P 31 P = AE 17.0 Hz) in the 31 P NMR spectrum, and C 2 -symmetric [(Me 3 Si) 2 C] 2 P 2 S 5 (9, P V ,P V ). Compound 9 was isolated as a few single crystals. In a similar way, compound 3 reacts smoothly with two equivalents of selenium to give [(Me 3 Si) 2 C] 2 P 2 Se 3 (10, P III , P III ) in high yield [Eq. (3)] which is however not further oxidized by selenium. Crude 10 is