Phosphinines—

Replacement names are much less popular, and are not commonly used for the smaller monocycles, with the exception of analogs of pyridine having a heavier Group V element in place of nitrogen. Phosphabenzene occurs more frequently in primary publications than phosphinine , for example, probably because of the ambi uous state of Hantzsch-Widman... 

Phosphinine Derivatives and their Use as Versatile Intermediates in P-Heterocyclic... 

I.3.4.2.6. Compounds with Unusual Double Bonds 1,3-Dipolar cycloaddition of l-chloro-2-phenyl-2-trimetkylsilyl-l-phosphaethene with nitrile oxides, followed by elimination of Me SiCl, results in 3,5-diphenyl-l,4,2-oxaphosphazole 190 (356). Chromium, molybdenum, and tungsten pentacarbonyls of 3,5-diphenyl-).3-phosphinins react with nitrile oxides to give the corresponding 1,3-dipolar cycloadducts, at the P = C bond, see 191 (Ar = Ph, Mes) (357). 

Although fused phosphinines have been first synthesized only in the recent years, almost every study in this area also involved some reactions with the ring phosphorus atoms. These transformations are compiled in Table 5. In this table, the starting and product P-functions, the reagents, yields (if published), and references are listed. 

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... 

Derivatives of monoaza- and diaza-phosphinines 194—198 have been synthesized by Frison and Le Floch (Scheme 75).235 Calculations performed on such... 

Other congeners of phosphinins—arsenin, antimonin, and bismin—have been shown to be definitely less aromatic than benzene by diverse theoretical treatments that have been reviewed.236 For instance, the Bird aromaticity index for arsenin was found to be 67, compared to 100 for benzene.123 Table 4 summarizes a few parameters used to estimate the aromaticity of heterobenzenes resonance energies... 

The first phosphinine derivative was synthesized by Markl in 1966, and the parent unsubstituted phosphinine itself was first reported by Ashe. The... 

Figure 2. B3LYP-6-311+G(d,p) structure of phosphinine (3), with calculated bond lengths and experimental H NMR chemical shifts.

From the investigation of all these data it is clear that the aromaticity of phosphinine is nearly equal to that of benzene. Their chemical reactivity, however, is different. Most important is the effect of the in-plane phosphorus lone pair, which (i) is able to form a complex and (ii) can be attacked by electrophiles to form A -phosphinines. Thus, electrophilic substitution reaction on the ring carbon is impossible. In Diels—Alder reactions, phosphinines behave as dienes, providing similar products to benzene but under less forcing condition (the reaction with bis(trifluoromethyl) acetylene takes place at 100 °C with 3, while for benzene 200 °C is required). 

While phosphinine is easily complexed 7 -bound at the phosphorus, resulting in a complexes, r-complex-... 

This is just slightly smaller than for benzene (28.2 kcaFmol) or for phosphinine (27.2 kcal/mol). Bach-rach and Magdalinos noted the planarity Dsh symmetry) of C3H3P3. By comparing the MP2/6-31G bond leng (1.739 A) with that of the PC bond length in phosphinine, it has also been assumed that triphosphinine is aromatic. ... 

Schleyer et al. very recently also investigated the stabilization and NIGS of triphosphinine. For the symmetric triphosphinine and for 1,3-diphosphinine, the (HF/6-3H-G(d)) NICS(O) is —5.9 and —7.2, respectively. The HF/6-31H-G(d,p) NICS(l) values are —9.6 and —10.3, matching favorably with the —10.8 and —11.3 values for phosphinine and benzene, respectively. This behavior indicates the rather large influence of the PC o contributions, which die out above the ring plane. Schleyer et al. also observed that with reference to the homodesmotic eq 10, for benzene (X = CH) the variation is 2—4 kcal/mol if a CH unit is replaced by phosphorus. ... 

Investigating the high Diels—Alder reactivity of phosphinines (eq 11). the transition-state energy was much smaller for X = P than for X = CH (17.8 vs 44.6 kcal/mol—MP4/6-31G //MP2/6-31G ). Bachrach... 

Carbocations have similar electronic structures to carbenes. The P-protonated derivative of phosphinine should also be similar to 23. Indeed, while investigating the proton affinity of 3. the most preferred protonation site was phosphorus and not carbon, whereby the cyclic jt system would be interrupted. ... 

---