Phosphabenzene aromaticity

The aromatic heterocycle phosphabenzene C5H5P (analogous to pyridine) was reported in 1971, some years after its triphenyl derivative 2,4,6-Ph3C5H2P. See also HP=CH2 29) nd [P(CN)2] ° (p. 484). The burgeoning field of heterocyclic phosphorus compounds featuring... 

Phosphorus. Substituted phosphorus analogues of pyridine (phosphinines, A3-phosphabenzenes, also called phosphonins or phosphorins) were first prepared by Markl starting from pyrylium cations their chemical properties suggest that their aromaticity is lower than that of pyridine (e.g. phosphinine 186, Scheme 72).230-232 Molecular calculations for other six-membered jr-systems with planar tricoordinate phosphorus, such as phospininines 186 and 187, have evidenced their aromaticity (Scheme 72).156... 

One of the most obvious examples is strong deshielding of the a-protons in the series pyridine (8 8.29 ppm), phosphabenzene (8.61), arsabenzene (9.68), stibabenzene (10.94), and bismabenzene (13.25), although other data unambiguously point to a falling off of the aromaticity in this sequence. Here the contribution by crAringcurr is mostly obscured by local effects connecting with nonuniform distribution of the electron density and by the anisotropy of the heavier heteroatoms. 

There are a number of heteroatom six-membered aromatic rings which are analogous to benzene in that they can donate six electrons to a metal. These include phosphabenzene. borabenzene anion, bomzire. and arsabenzene shown in Fig. I5.42.uv... 

A final remark to close the section dealing with five-membered rings is that Mathey recently reported a one-pot procedure converting a phospholide ion (which is highly aromatic, as discussed above) to the six-membered ring phosphabenzene (which is also highly aromatic, as will be discussed below) as shown in Scheme 49 [278],... 

The aromaticity of the six-membered ring phosphinine is similar to that of benzene (different aromaticity measures all exhibit about 90% of the benzene values), as reviewed previously [4, 278]. These characteristics reflect in the frequently used term phosphabenzene (for a recent review on its chemistry, coordination, and catalytic behavior see [279]). The numerical values for some aromaticity indicators... 

Table 3 Aromaticity measures of benzene and phosphinine (phosphabenzene)...

We carried out ab initio EOM-CCSD calculations to evaluate one-, two-, and three-bond 13C-13C, 15N-13C, 31P-13C coupling constants in benzene (161), pyridine (164, X = H), pyridinium (188), phosphinine (also called phosphabenzene and A,3-phosphorin) (189), and phosphininium (190). The introduction of N or P heteroatoms into the aromatic ring not only changes the magnitudes of the corresponding... 

Keywords aromaticity, arsabenzene, boratabenzene, germabenzene, heterocycles, metallabenzene, phosphabenzene, pyridine, pyridinium, pyrylium, silabenzene, thiopyrylium... 

It has been a well-entrenched dogma that elements beyond argon are reluctant to form multiple covalent bonds, citing as examples the difference between C02 and Si02. One should note, however, that the formation of multiple bonds for such atoms occurs with increasing relative easiness in the order Si < P < S indeed, although silabenzene is unstable under normal conditions, phosphabenzene and thiopyrylium are normal, stable aromatic systems. 

Of course, with atoms from lower periods of the periodic system, such as P or S, it is possible to have aromatic systems that are not simply homologs of pyridine or pyrylium, (i.e., phosphinine and thiopyrylium, respectively) but also systems with P(V) and S(IV), i.e. phosphabenzene (/. -phosphinine) and thiabenzene, respectively. However, in some cases the aromaticity is strongly perturbed, as for instance in selenabenzene derivatives stabilized by electron-withdrawing groups, when these compounds are shown to be zwitterionic. 

A striking difference to pyridine is the lack of basicity and nucleophilicity for phosphabenzene and arsabenzene, which are protonated at the a- and y-carbon atoms (this reaction is responsible for deuteration). Neither alkyl halides nor trialkyloxo-nium salts can alkylate phosphabenzene, therefore there will be no discussion of quaternary salts for these pnictogena-hetarenes. Whereas pyridine can be oxidized to the zwitterionic N-oxide, phosphabenzene affords non-aromatic -oxidation products 35 and 36 with tetracoordinated P(V) phosphorus atoms, similar to phosphin-oxides and phosphonic acids, respectively. 

The bottom line on monocyclic six-membered aromatic compounds is that, so far, only benzene, the azines with one through four nitrogen atoms, phosphabenzene and arsabenzene, pyrylium, azapyrylium and chalcogenopyrylium cations (with or... 

Benzene (CH)6, of course, is the most prototypal aromatic system. When one or more of the CH groups are replaced by other atom(s), a heterocyclic aromatic system is obtained. Well-known examples include pyridine N(CH)5 and pyrimidine N2(CH)4, while lesser known cases are phosphabenzene P(CH)5 and arsabenzene As(CH)s. There are also systems where the heteroatom is a heavy transition metal examples include L Os(CH)s and L Ir(CH)5. 

Strong nucleophiles such as organolithium or organomagnesium derivatives do not react with substituted or unsubstituted phosphabenzene or arsabenzene (Y = P or As) by nucleophilic substitution as in the case of pyridines, but by addition to the heteroatom forming intermediate anions. These anions can then be converted into non-aromatic compounds by reaction with water yielding 1-alkyl-1,2-dihydro-derivatives, or they can be alkylated by an alkyl halide with the same or a different alkyl group, when two products may result a l,2-dialkyl-l,2-dihydro-derivative, or a 2 -derivative (Figure 17). The former products are kinetically controlled, whereas the latter compounds are thermodynamically controlled. 

Interestingly, the bond distances and angles of l,l-dimethyl-2,4,6-triphenyl-/l -phosphorin determined by X-ray diffraction are similar to those of /( -derivatives (Figure 18), yet the physical and chemical properties show that the 2 -phosphorins with electron-donor substituents (H, Me, SiHs) at the P heteroatom are not aromatic, and that the tetracoordinated phosphorus atom interrupts the ring current in such compounds. Flowever, when the P-substituents of /l -phosphorins have higher electronegativities (F, OFl, Cl, Br) the NlCS(l) values indicate aromaticity, but somewhat less than phosphabenzene (01HCA1578). 

The bottom line on monocyclic aromatic compounds with re-electron sextet is that so far, among six-membered systems only benzene, the azines with 1 through 4 nitrogen atoms, phosphabenzene and arsaben-zene, pyrylium, azapyrylium, chalcogenopyrylium cations (with or without exocyclic groups such as hydroxy, amino and corresponding tautomeric or prototropic forms), and the metallabenzenes with platinum family metals have been proved to afford stable molecules under normal conditions. The list is richer for five-membered systems. 

Bachrach, S. M. Aromaticity of annulated benzene, pyridine and phosphabenzene, 7. Organomet. Chem. 2002, 643-644, 39-46. 

Phosphabenzene, the phosphorus analogue of pyridine, is a species that, as with the methylenephosphines, formally contains a doubly coordinated phosphorus singly bound to one carbon and doubly bound to another. The first isolation of a phosphabenzene derivative was reported in 1966 using the reaction as shown below (equation 10). The parent phosphabenzene molecule was first isolated by the reaction (equation 11) of phosphorus tribromide with a staimane derivative, albeit in low yield.Further spectroscopic investigation of this parent molecule indicated it to be planar with indications of multiple bond character between phosphoms and carbon and aromatic delocalization. ... 

The syntheses, as well as physical and theoretical studies regarding the aromatic character of heterobenzenes, such as phosphabenzene " , arsabenzene and bismabenzene , have been extended also for stibabenzene . The benzenelike structure has been attributed to stibabenzene on the basis of experimental study and theoretical interpretation of the microwave spectra of SbCjH5, SbCjH5, ) -dideuterio SbC5H3D2 and 8-dideuterio SbC5H3D2 in the region 26.5-40.0GHz. 

The aromatic heterocycle phosphabenzene C5H5P (analogous to pyridine) was reported in 1971, some years after its triphenyl derivative... 

Fig- 15.42 Complexes of phosphabenzene. borabenzene. borazine. and arsabenzene. These six-membered rings are analogous 10 benzene, i.c.. they are six-eleciron donors and are aromatic. 

Since 1965 arsenic chemistry and especially the chemistry of heterocyclic arsenic compounds has become revitalized because of the synthesis of the first six-membered, cyclic conjugated, Hiickel-aromatic phosphabenzenes (2) and arsabenzenes (3) <71JA3293>. Then, 50 years after the salvarsan period, a second boom in arsenic chemistry, especially in heterocyclic, unsaturated arsenic compounds began. 

Thus the strong similarity of the UV spectra of borazarenes to those of the corresponding carbocyclic compounds is considered evidence for considerable aromatic character of the former63 64 (see also Section III,D, 10). The ultraviolet spectra of phosphabenzenes,65 arsabenzenes,66 and stibabenzenes66 also reflects the considerable delocalization in these rings (Sections III,D, 4 and 5). 

The planarity of the phosphabenzene nucleus has been demonstrated by X-ray analyses of 2,6-dimethyl-4-phenylphosphabenzenc284 and of a 2-butyl-4-aryl-5,6-dihydronaphtho[l, 2-6]phosphonin.286 The bond lengths, which clearly imply aromatic character, and bond angles are summarized in Fig. 3. 

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