Phosphinine-pyridines

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... 

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... 

In contrast to the fact that benzene or pyridine and their derivatives have an aromaticity, the phosphorus analogues (phosphinines or phos-... 

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... 

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... 

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. 

Determination of electron configurations can provide insight into reactivity. Calculations <1996JA6317> of the percentage of t character of lone pairs at the heteroatom revealed that there was a dramatic increase in s character upon going from pyridine (%s= 29.1) to phosphinine (%s 83.8), correlated with a lower basicity of the heteroatom... 

Phosphinines like pyridines behave as ligands and their coordination complexes with various metals such as titanium, vanadium, chromium, iron, manganese group, and cobalt group has been reviewed <2005AHC(89)125>. 

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. 

Phosphinines are Hiickel aromatic systems even though their properties show pronounced differences to those of the homologous pyridines. Thus, for example, phosphininium salts are not stable (in contrast to pyridinium salts), whereas the formation of phosphininyl anions and radicals as reactive intermediates is known (neither species has been described in the case of pyridine). 

Both C5H5P and C5H5As have planar structures with C—C and C—E bond lengths indicative of aromatic delocalization of the ir electrons. However, the scope of their aromatic organic chemistry is not nearly as extensive as that of pyridine. Phosphinine and its derivatives form both type complexes, for example, 10-XXXVI and 10-XXXVII. 

Dimethyl-4-phenyl- /l -phosphinine has a planar heterocyclic ring. Whereas pyridine has an almost regular hexagon, all the other... 

The first application of phosphinines in catalysis was reported by Zenneck et al. in 1996 in the case of t 6-Fe complexes [46, 98], It was shown that complex 73 could catalyse the cyclotrimerization of dimethyl acetylenedicarboxylate as well as the formation of pyridines from alkynes and nitriles. Importantly, the catalytic activity of this complex was found to be superior to that of the corresponding benzene [Fe(n6-C6FI6)(COD)] complex. Reactions of methylpropargyl ether with butyronitrile in the presence of complex 73 as catalyst in a ratio 620 2,720 1 afforded a mixture of functional benzenes and pyridines. Turn over numbers (TON) for the conversion into pyridines reached 160 and those for the formation of functional benzenes reached 326, thus corresponding to a chemoselectivity of 0.49 (Scheme 26). 

Pyridine, the diazines, 1,3,5-triazine, 1,2,4,5-tetrazine, quinoline, isoquinoline, quinolizinium ion and the benzodiazines are HtiCKEL-aromatic systems (heteroarenes). 2//-Pyrane, 4//-pyrane and the corresponding benzo compounds, 1,4-dioxin, 1,4-dithiin and 1,4-oxathiin are not aromatic. However, the cations derived therefrom, like pyrylium and thiinium cations or dioxinium and dithiinium dications are aromatic the same is true for the corresponding benzo or dibenzo systems (e.g., benzopyrylium). Phosphinin is characterized by a specific heteroaromatic situation. 

A fascinating approach to the synthesis of pyridine heterocycles has been discovered by using (arene)Fe(O) complexes to catalyze [2+2+2] cycloaddition reactions. The proposed method consists of a cross cyclization of alkynes and nitriles [45]. An example of such a reaction is depicted in Scheme 25. In the presence of (Ty -l,5-cyclooctadiene)(Ty -phosphinine)Fe(0) 85, butyronitrile 86 combines with two molecules of methyl propargyl ester 87 yielding a mixture of isomeric pyrroles 88 and 89. The reaction proceeds at ambient temperature, but relatively long reaction times are required to obtain good yields. The symmetric... 

Phosphinines, arsinines, and stibinines in sharp contrast to pyridines are ambidentate ligands, where tf(n) and >/ (E) coordination modes and their combination are common. The role of the function increases when the... 

Reaction of UX4 (X=C1, BH4) and two equivalents of [Li(Et20)] [SPS ], the lithium salt of an anionic SPS pincer ligand composed of a central hyper-valent Z4-phosphinine ring bearing two orf o-positioned diphenylphosphine sulfide side arms, formed complexes of the type [UX2(SPS )2] (Fig. 36). Crystals of the chloride derivative were obtained as the pyridine adduct, and... 

At least three names are available for the compound H3P=CH2. These are phosphine methylene, methylene phosphonium ylid and phosphonium methylide moreover, the compound can be classed as a phosphorane The compound PhjP Rj may be called triphenylphosphonium methylide, tri-phenylphosphonium methanide or triphenyl-methylenephosphorane. The pyridine analogue, C5H5P, has been known variously as phosphobenzene, phosphabenzene, phosphorin, and most recently, phosphinine. The term phosphane for (1) in Table 1.4 and its derivatives is the most recent and the most systematic, but it is difficult to see how the more widespread and firmly rooted term phosphine will ever be replaced. In addition, adoption of the term phosphane would put it in line with such little-used terms as carborane for CH4, nitrane for NH3 and oxane for HjO. 

The use of conventional valence bond formulae for unsaturated rings containing carbon and one phosphorus atom leads to the arrangements indicated in Table 6.22. Derivatives of most of these ring systems are known, but phosphinine (also named phosphorin or phosphabenzene) represents the only parent compound synthesised to date. It is the analogue of pyridine, C5H5N, and arsenin,... 

In phosphinine (also called phosphorin or phosphabenzene) there is at least a formal resemblance to the situation in pyridine and benzene, and a planar ring system is to be expected. Crystal structure... 

C5H5N (pyridine), CsHsP (phosphinine), C4H4S (thiophene)- these three ligands are most often L ligands, however, via the lone pair of the heteroatom... 

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