Phosphaalkenes, structure

The aim of the reaction of 15a with Cl—P=C(SiMe3)Ph was to synthesize the 2,3,1-diphosphasilabuta-1,3-diene derivative 42 (Scheme 11). However, it turned out that the Cl—P bond of the phosphaalkene exclusively adds to the Si=P bond, forming the addition product 43, which probably has the ( >isomeric structure.45 The corresponding (Z)-isomer 43 was not formed. On heating of 43, even for 8 hours at 110°C, no elimination of /Pr3SiCl was observed. Instead, ( )/(Z)-isomerization simply occurred (43 and 43 in the molar ratio of 1 0.8). 

Let us now direct our attention to the P=C bond in phosphaalkene ion-radicals. The literature contains data on two such anion-radicals in which a furan and a thiophene ring are bound to the carbon atom, and the 2,4,6-tri(tert-butyl)phenyl group is bound to the phosphorus atom. According to the ESR spectra of anion-radicals, an unpaired electron is delocalized on a n orbital built from the five-membered ring (furanyl or thienyl) and the P=C bond. The participation of the phosphaalkene moiety in this MO was estimated at about 60% and some moderate (but sufficient) transmission of the spin density occurs through the P=C bridge (Jouaiti et al. 1997). Scheme 1.6 depicts the structures under discussion. 

Phenylmethylsilanediol, synthesis, 42 155 Phenylsilanetriol, monosodium salts, 42 169 4 -Phenyl-2,2 6, 2 -terpyridine bis nickel complex, 30 74 molecular structure, 30 74 PhjtfluorenyllSiOH, 42 197 (Ph(Me2N)C-=Nli], 37 59-65 orientation of imino ligand, 37 61-63 (PhMe SiljCSiH OH, 42 244-245, 248 (PhMe SiljCsiMeHlOH), 42 191 Phosphaalkenes acyclic, 33 338-353 butadienes, 33 346-349 cumulenes, 33 352... 

Phosphaalkenes display a number of unusual properties. An investigation of phosphaalkenes with several aryl substituents, revealed that bis(diphosphene) 19 forms a triplet dianion211. A cumulene structure was demonstrated for the Ar-P = C=C=P-Ar radical anion. The unpaired electron is predominately localized on the P atoms212. 

Phosphaalkene (RP==CR2) complexes, whose structures are similar to that of complex (44), have recently been reported.195,203... 

The synthesis and structural study of the stable P-heterocylic carbene 49 and related structures (e.g., structures 48 and 52 see Figure 3) have attracted some recent research activity <2005AGE1700, 2002JA2506, 2006AGE2598, 2006AGE7447>. The synthesis of the stable P-heterocylic carbene 49 was accomplished in two steps (1) a formal [3+2] cycloaddition of the readily available phosphaalkene 123 with acetonitrile in the presence of silver triflate afforded salt 124, and (2) the isolated and recrystallized salt 124 was deprotonated by lithium hexamethyldisilazide in tetrahydrofuran (THF) to afford carbene 49 as relatively stable light-yellow crystals (Scheme 10) <2005AGE1700>. 

With respect to the structure of phosphaalkenes, considerable progress has been achieved by the X-ray crystal structure determination of Ui (2) and of its Cr(C0>5 complex 2 (3) Both the structural data and the close similarity of spectral data confirm that la as such and as a ligand in 2 has essentially the same structure of a planar, non-delocalized phosphaethene (Table 1). 

Binuclear cumulenes, synthesis, 230 Bis(methylene)propenylphosphorane, synthesis with phosphaalkenes, 19 [ ( -BuSe)Fe2(CO)6h(/i4-Se)], synthesis and structure, 275, 276 Butene 1,4-diones, synthesis, 224... 

The proofs to justify defining real pjt-pjt-double bonds within these compounds are found in the parameters of the chemical structure as well as in properties that show the evident affinity to the classical CC double bond, thus confirming the designation phosphaalkenes not only for formal reasons. Ocassionally the term alkyliden(methylene)phos-phanes is used for these types of compounds. 

Independently of the configuration of the phosphaalkene unit within the l,6-diphospha-l,5-hexadiene structure the four-centered transition state (seat configuration) for the rearrangement favors an E-configured CC double bond for the 1,2-diphosphacycloalkane. 

Complexation of phosphorus-containing multiple-bond systems to transition metals has been investigated extensively in recent years. In this regard, bonding between the phosphaalkene and transition metal carbonyls can be achieved via the free electron pair of the phosphorus (tjl coordination, type A) (149-151) as well as via the n system (r 2 coordination, type B) (152-154). The latter reaction type can be explained by two different electronic resonance structures. Type C is a combination of the two possibilities and can be occasionally observed (155, 156) (Fig. 23). 

The carbene-pnictinidene (93-97) adducts can be represented by two extreme canonical forms (98 and 99). The canonical form (99) represents a conventional phosphaalkene with a C=P double bond, whereas form (98) corresponds to a phosphinidene adduct of a carbene and features a C-P dative bond order of one. Structural data for compounds (93-97) showed that the E-C(Carbene) bond is 4% longer than typical single bonds and that the E-C(R") moiety is twisted out of the carbene plane by 26-46°. These data, along with the high-held chemical shifts of (93,94) and (96), indicate the predominance of structure (98). 

The scope of three-membered rings with two heteroatoms of heavier group 15 elements includes essentially heterocyclopropanes and heterocyclopropenes with two phosphorus atoms. This chapter also covers bicyclic and polycyclic molecules containing the structural motif of the rings under discussion. Such species frequently result from transformations of phosphaalkenes or oligomers. Monocyclic diphosphirane and diphosphirene derivatives are synthesized from acyclic precursors with a P-C-P linkage or alternatively via [2-1-1] cycloadditions. A few examples... 

---