Phosphaalkenes double bond

We note that the P=C—P unit is a 1,3-diphosphavinyl system giving the phosphorus lone pair the opportunity to be in conjugation with the phosphaalkene double bond. For effective conjugation to occur, the substituents on the tertiary phosphorus atom have to be coplanar with the phosphaalkene unit. However, hyperconjugation operates when the dihedral angle is different from 90° and is angle dependent. We observe that the effect is both steric and electronic in natnre, and operates on both phosphous atoms in parallel. 

A brief history of (3p-2p)7i bonds between phosphorus and carbon followed by an introduction to the methods of phosphaalkene synthesis that are pertinent to this review will be provided. The earliest stable compound exhibiting (3p-2p)7x bonding between phosphorus and carbon was the phosphamethine cyanine cation (1) [33]. An isolable substituted phosphabenzene (2) appeared just two years later [34]. The parent phosphabenzene (3) was later reported in 1971 [35]. These were remarkable achievements and, collectively, they played an important role in the downfall of the long held double bond rule . The electronic delocalization of the phosphorus-carbon multiple bond in 1-3, which gives rise to their stability, unfortunately prevented a thorough study of the chemistry and reactivity of the P=C bond. 

Geometrical parameters provide much more information than simple identification. The phosphorus-carbon distance of 1.686(6) A in this tungsten complex is consistent with there being a double bond, as it is close to those observed in open chain phosphaalkenes. But how much better it would have been if the parent compound could have been studied uncomplexed, so that the strain in this unusual ring system could have been investigated. The geometrical parameters would then have provided very useful information about the likely reactivity of the compound, and information about the changes on... 

Phosphaalkenes (continued) reactions at PC double bond, see PC double bond... 

In contrast to the P=C bond of phosphaalkenes (Section 8.2.2), double bonds between phosphorus and a heteroatom have not been used much as dipolarophiles. Most of the studies reported so far were devoted to the reactivity of the (X )P=N bond of iminophosphanes. Amino(iminophosphanes) react with diazoalkanes to form 4,5-dihydro-3//-l,2,3,4-triazaphospholes or, by N2 loss from the latter, to imino(alkylidene)-X -phosphoranes (5,238). With P-halogeno-(arylimino)phos-phanes 174 and the appropriate diazo compounds, 3//-l,2,3,4-triazaphospholes 175 (167) and 176 (239) (Scheme 8.40) were obtained as the major products after cycloaddition and eliminative aromatization. 

NMR spectroscopy of phosphaalkenes and other compounds containing C—P double bonds has been reviewed309. Considerable differences in the 31P chemical shifts have been found for diastereomeric pairs of phosphaalkenes, for example310,311 ... 

The isolation of compounds containing simple C=P double bonds parallels the triple-bond work. The first stable acyclic phosphaalkene was synthesized over fifteen years ago.17 Again, base-induced dehydrohalogenation and stabilization by bulky groups is important ... 

Three years ago, it was shown that phosphaalkenes (F), which are not substituted at the P=C double bond by hetero atoms and owe their thermal stability mainly to steric hindrance around this bond can be obtained by base-induced elimination of HC1 from 2 (1). 

Phosphaalkenes are tervalent phosphorus derivatives with a double bond between carbon and phosphorus. The observed P=C bond lengths range from 161 to 171 pm (average 167 pm), appreciably shorter than the single P-C bond length of 185 pm. 

In the case of the phosphaalkenes—in analogy to the phosphaalkynes—kinetic stabilization of the localized double bond by bulky substituents such as, tert-butyl-, trimethylsilyl-, adamantyl-, or 2,4,6-tri-ferr-butylphenyl groups has again proved useful (Scheme 3). In addition, however, electronic effects also have a significant influence on the stability thus, it has been shown that a sufficient overlap of the p orbitals in such double bond systems can be realized by reducing the polarity of the P—C bond—for example, by the presence of electron-withdrawing substituents at the phosphorus atom. 

The class of phosphaalkenes with isolated P=C double bonds was first synthes ized by Becker.33 His synthetic strategy starting from trimethylsilylphosphines and acyl chlorides is still the most versatile (Protocol 3). The principle is based on the easily achievable, 1,3-silatropic migration of a silyl group bonded to phosphorus to a doubly bonded element such as nitrogen, oxygen, or sulfur. The process is favoured energetically by the construction of the P=C double bond with concomitant formation of a very stable silicon-element bond. 

It is possible in many cases to create PC double bonds via condensation in separations of halosilane, siloxane (22), or even water (23a-c). Likewise, formaldehyde or benzaldehyde reacts with 2,4,6-fri-t-butyl-phenylphosphane, forming the phosphaalkene, supported by dehydration agents such as P4Ol0 or CaO/CaCl2 [Eq. (3)]. This method is excep-... 

As a stable compound the basic HP=CH2 is still unknown. Nevertheless there exist phosphaalkenes that are entirely hydrogen substituted either at the phosphorus or at the carbon atom. Compounds stable up to room temperature should have at least one bulky substituent on either side of the double bond, although compounds shielded by bulky substituents still demonstrate an amazingly strong reactivity. 

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. 

The reactions of phosphaalkenes given here are restricted to those that are characteristic for the PC bond. The double bond holds two reactive centers one is the P atom with a coordination number of two and the other is the n bond. 

A reaction at the P atom preserving the double bond and culminating in an oxidative increase of the coordination can be realized with special phosphaalkenes using ozone, sulfur, selenium [Eq. (9a-c)] (42-46), or some carbenes [Eq. (9d)] (47-49). 

The second characteristic center of reactivity within phosphaal-kenes is the (p-p)-n bond. The reactivity is more similar to the C=C than to the corresponding C=N or P=N double bond. This is in accordance with the highest occupied molecular orbital (HOMO) energy levels of the phosphaalkenes, which frequently are n orbitals, as with olefins (51). The olefinic character is also demonstrated by the lower polarity of the P—C bond in comparison with the C—N or P—N bond. 

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. 

The kind of bond with respect to type B may be interpreted analogous to the ethylene complexes as a dative x-alternating effect (Fig. 24). As opposed to type A complexes, the properties of type B complexes are determined in a characteristic way by the back donation into the unoccupied n molecular orbital (LUMO) of the double bond (Fig. 24). This strengthens the metal-ligand bond, as proved by a shorter interatomic distance, and weakens the double bond (occupying anti bonding molecular orbitals), as is evident by a widened PC distance. The phosphaalkene P atom has, so to speak, more of a phosphane... 

With respect to the reaction mechanism a nucleophilic attack of the bis(trimethylsilyl)phosphanide anion at the sp -hybridized carbon of diethyl carbonate, followed by an elimination of one molecule of ethoxy trimethylsilane and formation of a still undetected phosphaalkene is supposed. Splitting off a second molecule ethoxytrimethylsilane the P-C double bond of the intermediate is converted into a triple bond thereafter (Eq. 10). 

Starting from the phosphinidene complex (22) and ketones, the double-bond metathesis reaction provides access to a phosphaalkene and the zirconocene oxide (Cp2ZrO) . A similar situation is also observed in the reaction with epoxides to give the corresponding phosphiranes (Scheme 6). ... 

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). 

Summary We report on DFT calculations (geometries, activation energies, reaction enthalpies) and bond strength deso-iption of an isomerization of bis(trimethylsilyl)-methyl-substituted phosphinidene complexes to corresponding phosphaalkene complexes under formation of a C-P double bond via neutral l,2-(C->P)-silyl migration. 

Several structurally different types of phosphaalkenes containing a phosphorous-carbon double bond have been found to undergo [4 + 2]... 

Little attention has been paid over the last decadeto the synthesis of fused phosphinines. However, in 2008, the preparation of a dithienophosphinine was reported. Compound 71 was obtained in three steps from dithienophosphole 70 by reaction successively with acetyl chloride, triethylamine and water [43], This transformation involves the transient formation of the oxide 69 (Scheme 17). DFT calculations reveal that the fused derivative is less aromatic than the parent compound C5H5P and suggest that a substantial electronic delocalization takes place within the three rings. Both the HOMO and the LUMO are localized on the P = C-Ph double bond and thus resemble those of a phosphaalkene derivative. 

Phosphinines are a special group of phosphaalkenes in which the P=C double bond becomes integrated into the aromatic system of benzene and related arenes. The majority of examples are based on the mono-phospha-benzenes, but di-and triphospha-benzenes in various substitution patterns have also been employed as ligands. The coordination chemistry of this class of ligands has been reviewed. ... 

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