Phosphinine, NICS values

In two recent works the phosphorus analogues of pyridones 109-111 (Scheme 58) have been studied computationally [317, 318], 110 is planar, while for 109 and 111 the planar structures are transition states with 11.0 and 19.8 kcal moH above the nonplanar minimum, respectively. The low barriers to inversion indicate that the planar structures have considerable aromaticity, which is shown by the NICS values exhibiting 67%, 51%, and 33% of the benzene value for 109,110, and 111, respectively [317], ELF analysis also indicates a certain aromaticity in these compounds [317], The bonding situation in 109 is similar to the cyclic phosphinocarbene isomer of phosphinine 112 [319] (Scheme 59), which was shown to be planar and aromatic. Stabilization of this carbene can be achieved by incorporating it to an anne-lated ring system 113, and also by the electron donor effect of an ylide formed with an additional o4,l5-P (114) [319],... 

As a caveat to the use of NICS values as the sole determinant of aromaticity, Sastry and co-workers <2003JMT(663)145> have examined the use of this parameter for a range of heteroaromatic systems, including phosphinine 1 and its monoprotonated form 16. Their conclusion is that NICS values should not be used as the sole criterion for aromaticity for heteroaromatic molecules containing group III and IV row main group elements. 

The NICS values for pyridine have been compared with other pnictogen derivatives (P, As, Sb, Bi). Reported NICS values for phosphinine 105 vary from -6.4 to -11.4, but the values are consistently negative and large, and consistent with aromatic character . Table 3 compares NICS values for five pnicogen heterobenzenes and on magnetic criteria they are all aromatic. It should be noted, however, that on thermodynamic criteria the aromaticity decreases with stibinine 107 being very unstable and bismuthine 108 unknown <2004JMT (674)125>. NICS values are not, therefore, an indication of the thermodynamic stability associated with aromatic compounds. 

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. ... 

Substitution of the CH units in the ring by P or N results in some variation in the aromaticity. Azaphosphinines have been considered somewhat less aromatic than benzene, pyridine, or phosphinine according to NICS(0.5) values [284], With N atoms at the position adjacent to P, the aromatic delocalization is reduced and a [1,4] dipolar character is induced [284], The NICS(0) values of diphosphinines... 

Like phospholes, phosphinines can also be part of extended jr-systems. In a recent work, Mathey et al. have synthesized and characterized by X-ray crystallography a dithienophosphinine 103 (Scheme 55) [308], The electronic interaction between the fused rings has been shown on the orbitals and by the decreased NICS(l) value (from -10.8 ppm to -8.8 ppm) of the phosphinine ring. 

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