TWo Phosphorus Atoms

This is the only representative of this class—there have been no reports of 1 -aza-2,3-diphosphorine, 1 -aza-3,4-diphosphorine, 1 -aza-2,4-diphosphorine, l-aza-3,5-diphosphorine, or l-aza-2,5-diphosphorine. 

Isslieb and Boettcher described a general procedure for the preparation of cyclic molecules containing the P—E—P functionality. The preparation involves reaction of metal phosphides with appropriate electrophiles, such as N,N-dichloroethylamine, to give 79 (76MI2). 

No reference was found to l,2-diaza-3,4-diphosphorine, l,2-diaza-4,5-diphosphorine, l,2-diaza-3,5-diphosphorine, l,2-diaza-3,6-diphosphorine, l,3-diaza-4,5-diphosphorine, l,3-diaza-2,5-diphosphorine, or l,4-diaza-2,6-diphosphorine. 

Fluck and co-workers described the reaction of hexachlorodiphospha-zonium chloride (80) (for which an improved synthesis was developed) with 

This class has been entered only by combination of methylenediphosphines with bifunctional reagents, such as substituted guanidines (82) (72CB2476) and silylated ureas (83) (84CZ402). Under some circumstances, the diphosphine may react with a second mole of urea to produce bicyclic products (84) (84AP1053). 

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The diphosphadithiatetrazocines 13.2 (E = S) and 13.3 (E = S) contain one more R2PN unit in the ring than 13.1. The structure of the orange-red 1,3-isomer (13.2, E = S, R = Ph) consists of an essentially planar N3S2 unit with the two phosphorus atoms located on opposite sides of the plane." Like 13.1 this isomer forms a 1 1 adduct with norbornadiene via 1,3-addition across the sulfur atoms. 

Cases of the S-coordinated rhodium and iridium are quite scarce. To complete the picture, we next consider the possibilities of S-coordination using complicated derivatives of thiophene. 2,5-[Bis(2-diphenylphosphino)ethyl]thiophene is known to contain three potential donor sites, two phosphorus atoms and the sulfur heteroatom, the latter being a rather nucleophilic center (93IC5652). A more typical situation is coordination via the phosphorus sites. It is also observed in the product of the reaction of 2,5-bis[3-(diphenylphosphino)propyl]thiophene (L) with the species obtained after treatment of [(cod)Rh(acac)] with perchloric acid (95IC365). Carbonylation of [Rh(cod)L][C104]) thus prepared yields 237. Decarbonylation of 237 gives a mixture of 238 and the S-coordinated species 239. Complete decarbonylation gives 240, where the heterocycle is -coordinated. The cycle of carbonylation decarbonylation is reversible. 

A transition structure model was proposed which accounts for the high selectiv-ities obtained for some of the substrates [80]. In the structure shown in Scheme 6.37 the two phosphorus atoms of the Tol-BINAP ligand and the two car-... 

The complex ion (Figure 2.32) contains Rh2 bound cis to two phosphorus atoms (2.216 A) and more distantly to four oxygens (2.201—2.398 A), exhibiting a distortion ascribed to the Jahn-Teller effect it is paramagnetic (fi = 1.80 fiB) and exhibits an ESR spectrum (Figure 2.33) showing rhodium hyperfine coupling as the doublet for g. ... 

Zwitterionic heterocyclic compounds with two phosphorus atoms of opposite charge and different coordination were a rarity 10 years ago. This field has emerged recently with the reports of a variety of new structures as shown in Schemes 12,13,and 14. Schevchenko and Schmutzler described a compound of... 

Studying the reactivity of lo, 2a -diphosphete 56 with o-chloranil, Bertrand and coworkers reported the synthesis of zwitterionic 57 in which the two phosphorus atoms are directly linked by a a bond (Scheme 13). Structural details of 57 were further obtained through a X-ray crystallographic analysis [70]. 

In all cases the oxidation state of phosphorus is five, and the chemical shift range observed is only about 12 ppm. Note that the two phosphorus atoms attached to the methine carbon are non-equivalent because they are chemically different (phosphonate and phosphine oxide). We can expect the coupling between aP and bP to be large, as they are separated by two bonds, while that of aP to bP or CP will be small (coupling over five bonds). 

Although the latter product is a solvated mononuclear [Rh(MeOH)2(diphos)]+ cation, in the solid state it is isolated as a binuclear complex of formula [Rh2 (diphos)2](BF4)2, in which each rhodium center is bonded to two phosphorus atoms of a chelating bis(diphenylphosphino)ethane ligand, and to a phenyl ring of the bis(diphenylphosphino)ethane ligand of the other rhodium atom. This dimer reverts to a mononuclear species on redissolving. The mechanism of hydrogenation of the prochiral alkene methyl(Z)-a-acetamidocinnamate, studied in detail by Halpern [31], is depicted in Scheme 1.7. 

The resulting product showed a single 31P chemical shift at 40 ppm with a XJ coupling to rhodium of 118 Hz, indicative of a rhodium (III) center. As this rhodium atom was shown to couple to two phosphorus atoms, it could be concluded that the phosphorus atoms are in a symmetrical position, as shown in Figure 11.3. The other Rh atom couples only to a phosphoms atom with a jphp of 196 Hz, indicative of a rhodium (I) center. 

The development of the next major class of ligands occurred during the 1990s, with Burks DuPhos (42) family of phospholane ligands [222, 223]. (An individual member of the family is named after the substituent R in Me-DuPhos, R=Me.) This structure could be considered an improvement on the DIOP-derived ligands, where the stereogenic centers are now closer to phosphorus. In addition to the aromatic spacer of DuPhos, there is also the related BPE (43) family, where the spacer between the two phosphorus atoms is less rigid. In both series the phosphorus is... 

Scheme 6.64 Calculated relative free enthalpies of various states of derivatives ofthe isobenzene 162, which contain a heteroatom or a heteroatom group of the third row of the periodic table instead of the methylene group of 162. The species 300, having two phosphorus atoms, has been included because of a known derivative thereof.

Another special case are bidentate ligands. With an appropriate distance of the two phosphorus atoms, they bind the metal like pincers (chelate structure), shifting the equilibria in Fig. 14 directly to column C. Those ligand systems are known to be appreciably slower than the corresponding bulky monophosphites, but highly regioselective (e.g., ligand 4 in Fig. 9). 

The e.s.r. spectra of phosphorus compounds have been reviewed.101 The phosphorus hyperfine splitting (ap 33.5 G) of the radical anion (77) is within the 25—36 G range of phosphorin radical anions.102 The cis- and fra/w-isomers of 1,2-bisdiphenylphos-phinoethylene gave the same radical anion (78). The unpaired electron is coupled to all the protons in the molecule as well as to the two phosphorus atoms, and shows that the electron is completely delocalized. Only when caesium was used as the gegenion in THF could a metal interaction be detected. The spectrum in this case corresponded to the association of two caesium ions with the radical anion, the third... 

It is evident that the two fullerenes are strongly electronically communicating in spite of the interposed presence of the Rh6 cluster. A partial contribution to such a strong interaction could, however, arise also from the slight unequivalence of the two C6o environments. In fact, one fullerene is linked to a Rh3 triangle coordinated to a carbon atom (of one isocyanide ligand) and a phosphorus atom (of one diphosphine), respectively, whereas the other fullerene is linked to a Rh3 triangle which coordinates two phosphorus atoms (of the two diphosphines). 

Extending the speculations presented in Section 8.2 for PPh3 and its rhodium complexes one expected that BISBI would coordinate in a bis-equatorial fashion (14) in RhH(L-L)(CO)2, thus leading to 3t only when dissociation of a CO molecule takes place (due to strain in the backbone 3t might not be completely trans). NMR and IR spectroscopy proved [57] that the structure of complexes 11 indeed contained the two phosphorus atoms in the equatorial plane and hydrogen in one of the apical positions (14, Figure 8.9). 

On electrochemical or chemical reduction, aromatic phosphaallene derivatives yield anion-radicals. These species have two equivalent phosphorus nuclei. The unpaired electron oscillates between the two phosphorus atoms (Sidorenkova et al. 1998, Alberti et al. 1999) ArP=C( )-P( ) Ar o ArP( )-C(-)=PAr. 

Clark (1988) calculated the stabilities of diverse species with odd-electron o bonds. Cataldo et al. (2001) produced evidence for the existence of the anion-radical with an intramolecular one-electron bond between two phosphorus atoms in a macrocyclic structure of the metacyclophane type. Dutan et al. (2003) observed a similar situation for the anion-radical of a di (m-silylphenyl-enedisiloxane) analog. 

In addition to the compounds listed above, a number of triazolium [36], thiazolium [37], and benzimidazolin-2-thione [32] derivatives as well as five-membered heterocycles with only one nitrogen atom [38, 39] or two phosphorus atoms [40] within the heterocycle, all of which are suitable for the generation of heterocyclic carbenes, have been described. 

It has now been clearly established 179) by a complete structure determination on Na4P2O7T0H2O that the oxygen valency angle in pyrophosphates is not 180° 172, 370), but 134° (see also Section IV,E,3). The existence of two different forms of Na2H2P207 has been reported 30, 170). It has also been proved that the two phosphorus atoms are equivalent 227). 

This synthetic approach is readily adapted to the preparation of a wide variety of three-membered rings containing two phosphorus atoms and one other p-block element from groups 13, 14, 15 or 16 [eqn (11.9)]. ... 

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