A?-phosphinines

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). 

The generally accepted view about the aromaticity of A -phosphinines as summarized in the recent book is that A -phosphinines are not aromatic but they clearly are highly stable substances exhibiting very little ylidic character ." In accordance with the description used for the four-membered ring phos-phetes and diphosphetes, the resonance structures 33a—c are usually considered to describe the electronic structure." ... 

OTHER 6-MEMBERED HETARENES. PNICTOGENABENZENES PHOSPHABENZENE (A -PHOSPHININE), ARSA-, STIBA-, BISMABENZENES... 

Phosphinines can act mainly as monohapto two-electron <7-donors through the phosphorus atom(s) or as polyhapto six-electron (TT-donors with metal atoms centered above the ring system (73a,b). The cr-mode has been observed, for example, with units Cr(CO)5, Ni(PR3)2, Ni(C2H4)(PR3), Mn(cp)(CO)2, AuCl, and others. 7r-Complexation is achieved in most cases only after P-cr-complexation, as in (L )[cr-Cr(CO)5][7r-Cr(CO)3] with V = 4-Hex-C5H4P. A di(benzene)chromium analog was obtained with two molecules of 2,4,6-tri( butyl)phosphinine, while a mixed complex (cp)Mn(L") could be obtained for a phosphinine L" with a different set of substituents. ... 

The primary enamine 126 (Ar = p-MeCgHJ (see equation 54) cydizes thermally to the 4-aza-A -phosphinine 388, while potassium hydride converts it into the azapho-sphininone 389 (equation 160). ... 

Although the molecular geometries of 2 - and 2 -phosphinins are nearly identical (no significant deviations from planarity or bond alternation are observed with either system), the A -phosphinines as well as the mono-, di-, tri- and tetra-aza-A -phosphinines, the A A -diphosphinines and the A A A -triphosphinines are certainly not Hiickel-aromatic systems. Their description as cyclic phosphorus ylides with ionic resonance structures (26) is probably close to the truth. The H and C NMR evidence is particularly strong on this point. 

The 1,2/l -azaphosphinines (58a-c) react with DMAD to form A -phosphinines (66), presumably via the phosphabarrelene intermediate (65) (Scheme 7) <85CL1459>. 

Some complexes of 2-phosphinophosphinines were also synthesized. In most cases, these types of ligands, which can be regarded as the phosphinine equivalents of the ubiquitous dppm (bis(diphenylphosphino)methane), bridge two metal centres. Complexes of the [Mn(CO)4]2, [FeCp(CO)]2 and [MoCp(CO)2] fragments were synthesized and identified [56]. The combination of a phosphinine and a... 

Nucleophilic and radical reaction partners transform X -phosphinines into A. -phosphinines. For instance, with alcohols or amines in the presence of Hg(II) acetate, 1,1-dialkoxy- or l,l,-diamino->. -phosphinines 8 are formed ... 

A. -Phosphinines react as cyclic phosphonium ylides, e.g 12a 12b. In contrast to 4-methylpyridine... 

The synthesis of the parent compound is not possible by method 1. It is achieved by reaction of penta-l,4-diyne 17 with dibutyltin hydride forming stannacyclohexa-1,4-diene 18 by a cyclization/ addition sequence. Tin-phosphorus exchange by phosphorus tribromide converts 18 into dihydro-A -phosphinine 19 which, on dehydration with DBU (Ashe 1971), yields the phosphabenzene ... 

The triple bond of phosphaalkynes also acts as a dienophile with phosphole sulfides, giving a cycloadduct (167) that readily loses the bridging phosphine sulfide group and is converted to a phosphinine derivative, (168) (Scheme 34) <86ZN(B)93l>. The reaction has practical value as a phosphinine synthesis. 

Markl G (1990) A -Phosphinines, aza-A -phosphinines, and A, A -diphosphinines. In Regitz M, Scherer OJ (eds). Multiple bonds and low coordination in phosphorus chemistry. Thieme, Stuttgart, p 243... 

Substitution of a carbon atom with a pentavalent phosphorus atom in 1-alkoxy or dialkylamino hexatrienes has been reported not to hamper electrocyclization and provides a method for the synthesis of parent A, -phosphinines. ... 

Figure 6.15 Bond parameters of the phosphabenzene system in 2,6-dimethyl-4-phenyl-A -phosphinine (bond lengths in pm, bond angles in degrees).

Fig. 6.4 Occupancy angles a and of a phosphinine and cone-angle 0 of a phosphine...

Due to their particular electronic properties, phosphinines are especially suitable for the stabilization of late transition metal centers in low oxidation states. Phosphinines as ambidentate ligands possess two different potential coordination sites, the lone pair of the phosphorus atom and the aromatic Ti-system. As mentioned above, the HOMO of a phosphinine ligand is suitable for cx-coordination towards a metal center. Its energy is close to the energy of the HOMO and HOMO orbitals that can participate in / -coordination. This leads to a range of coordination modes and Fig. 6.6 represents the most common ones [36-38]. Representative examples with transition metals potentially relevant for homogeneous catalytic reactions are shown in Figs. 6.7, 6.8, 6.9, 6.10, 6.11, 6.12, 6.13, and 6.14. 

Fig. 6.11 Rh(I) complex containing a phosphinine derivative of 2,2 -bipyridine and the molecular structure of 24 in the crystal...

The first application of a phosphinine-based catalyst in homogeneous catalysis was reported in 1996 by Zenneck et al. The f/ -phosphinine-iron complex 33 was used in the catalytic [2 -h 2 -h 2] cyclotrimerization of the electron-poor alkyne dimethyl acetylenedicarboxylate to give C6(C02Me)6 and in the co-cyclotrimer-ization of methyl propargylether with butyronitrile affording functionalized pyridine derivatives (Scheme 6.8) [60]. Turnover-numbers of up to 160 for the pyridine derivatives and chemoselectivities of up to 1.4 1 (pyridinesiarenes) were observed. Nevertheless, the efficiency of 33 is significantly lower compared to classical CpCo catalysts [61, 62]. 

Reetz MT, Bohres E, Goddard R, Holthausen MC, Thiel W (1999) Synthesis, solid-state structure, and electronic nature of a phosphinine-stabilized triangulo palladium cluster. Chem Eur J 5 2101-2108... 

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