Phosphamethin-cyanines

A brief history of (3p-2p)7i bonds between phosphorus and carbon followed by an introduction to the methods of phosphaalkene synthesis that are pertinent to this review will be provided. The earliest stable compound exhibiting (3p-2p)7x bonding between phosphorus and carbon was the phosphamethine cyanine cation (1) [33]. An isolable substituted phosphabenzene (2) appeared just two years later [34]. The parent phosphabenzene (3) was later reported in 1971 [35]. These were remarkable achievements and, collectively, they played an important role in the downfall of the long held double bond rule . The electronic delocalization of the phosphorus-carbon multiple bond in 1-3, which gives rise to their stability, unfortunately prevented a thorough study of the chemistry and reactivity of the P=C bond. 

Before 1964 no stable compound with a localized or delocalized carbon-phosphorus double bond was known. Indeed, it was generally assumed that the atomic radius of phosphorus, being larger than that of carbon or nitrogen, would not provide sufficient 2pn—3pir overlap for such a ir system to be stable Our first communication, written jointly with Peter Hoffmann which described the synthesis of a stable phosphamethin-cyanine 1 with a delocalized P—C double bond was therefore received with skepticism However, after Allmann confirmed the structure by X-ray analysis the existence of a new type of phosphorus bond in a cationic delocalized tt system was unambigously established... 

All presently known phosphamethin-cyanines were prepared according to our original procedure (1964) in which two quaternary salts of a heterocyclic base (e. g. 4) are condensed with tris-hydroxymethyl-phosphine 5 in the presence of a proton-abstracting base The preparation ofbis-[N-ethyl-benzothiazole(2)]-phosphamethin-cyanine-tetrafluoroborate 6 illustrates the synthetic sequence. A mixture of 2 moles of N-ethyl-2-chlorobenzothiazolium-tetrafluoroborate 4 and 1 mole of tris-hydroxymethyl-phosphine 5 in dimethylformamide is slowly reacted with ethyl-di-isopropylamine or pyridine at 0 °C. Addition of water immediately affords the crystalline cyanine dye 6 in ca. 45% yield ... 

Quaternary salts of other heterocyclic bases can also be employed. For example, symmetrically substituted phosphamethin-cyanines having quinoline and benzimidazole moieties, 7 and 8, respectively, have been prepared... 

During formation of the addition compounds 12 and 16, no free formaldehyde accumulates. We assume that the liberated formaldehyde immediately reacts with tris-hydroxymethyl-phosphine, forming the quaternary tetrakis-hydroxymethyl-phosphonium salt 13. The addition compounds 12 and 16 are relatively unstable, but can be purified for analysis. Intermediates 12 and 16 can also be employed in the synthesis of symmetrical or unsymmetrical phosphamethin-cyanines. For example, Klapproth synthesized 18 in 60% yield by condensing 16 with 77. 

It is likely that intramolecular dealkylation of 12 and 16 leads to 14 and 7 7, respectively, since these are sometimes formed in considerable amounts as side-products in the synthesis of phosphamethin-cyanines. 

A simUar phosphamethin-cyanine synthesis starting from tris-trimethylsilyl-phosphine has been described by Markl. Using this procedure, arsamethin-cyanines can also be prepared from tris-trimethylsilyl-arsin... 

Table 1 lists all phosphamethin-cyanines which have been obtained in analytically pure form, together with some of their physical properties. 

The spectral features in the UV and visible re ons of phosphamethin-cyanines resemble those of the correspondingly substituted methin- and azamethin-cyanines. The position and extinction coefficient of the maxima as well as the general shape are quite similar (Fig. 1). 

In the phosphamethin-cyanine series with benzimidazolium substituents, the effect of the size of the N-alkyl groups on the absorption spectra (Fig. 2) was also investigated Table 1,8a-8e, shows that with increasing alkyl size, the long-... 

No temperature dependence has yet been observed. However, the solubility of the phosphamethin-cyanines is generally so limited that little variation is possible with respect to solvent, concentration or temperature. Comparative experiments with conesponding methin-and azamethin-cyanines are currently under way in our laboratories ... 

In the case of unsymmetrical phosphamethin-cyanines 18, the signal of the CH3 protons of the more basic imidazole ring appears at lower field than that of the CH3 protons of the quinoline ring. 

Table 2. chemical shifts of phosphamethin-cyanine tetrafluoroborates in DMF and chemical shifts of analogous methin-cyaninc tetrafluoro-boiates... 

Allmann and Kawada and Allmann carried out X-ray analysis on bis-fN-ethyl-benzthiazole]-phosphamethin-cyanine perchlorate 6b and bis-[N-ethyl-quinoline]-phosphamethin-cyanine perchlorate 7b. In 6b the two heterocycles lie roughly in the same plane. The C—P—C bond angle is 104,6°. The P-C bond lengths, 1,754 and 1,757 be regarded as identical, proving that the C-P-C bond network is... 

Phosphamethin-cyanines, like methin-cyanines, can be protonated by strong acids, forming colorless dications. These can be converted back to the original phosphamethin-cyanines by careful addition of weak bases such as tert.-butanol. This acid-base reaction is least successful in the case of the weakly basic bis-benzthiazole-phosphamethin-cyanine 6. For bis-quinoline-phosphamethin-cyanine 7 b, we obtained vnth perchloric acid in glacial acetic acid the absorption spectrum of the N-ethyl-quinolinium salt. 

The more basic benzimidazole-phosphamethin-cyanines can be easily protonated. In acetonitrile with 50% ethereal HBF4 the cyanine absorption bands at 440—421 nm and 347—335 nm(Table 1) of bis-[l,3-dimethyl-benzimidazole-2]-phosphacyanine disappear completely, A new sharp absorption at 300 nm (e = 30000) can be observed. Addition of tert-butanol restores 9C of the starting material, as can be seen by the spectrum. If the protonation is carried out in ethylene chloride with an excess of ethereal HBF4, the product can be isolated as a colorless, stable, but not analytically pure salt. Ethyldiisopropylamine or more ether will regenerate the cyanine. 

Absorption maxima of phosphamethin-cyanines 8 in comparison to their Ag-addition compounds... 

By treating benzimidazole-phosphamethin-cyanines 8a with methanolic silver tetrafluoro-borate solutions, Greif isolated colorless, crystalline silver complexes. Their absorption bands lie at shorter wavelengths. The extinction coefficients are httle changed. Here, too, a clear dependence of the position of the absorption maxima on the nature of the N-alkyl substituents can be observed (Table 3). 

Addition of excess methanol destroys the silver complexes and regenerates the phosphamethin-cyanine salts. It is more advantageous to bind the Ag ions by addition of tris-j cyanoethyl-phosphine. 

Mercuric chloride in methanol also reacts with compounds 8 (in dichloro-methane), forming colorless mercury complexes, which can in turn be reconverted to the cyanines 8. Such addition compounds are stable only as solids, decomposing rather quickly in solution. Mercuric acetate in methanol reacts rapidly with the formation of elemental mercury, where by the phosphamethin-cyanines are destroyed uniform products from this reaction have not as yet been isolated. 

The reaction of diazonium salts with methin- and phosphamethin cyanines gives interesting results which are now investigated in detail. 

In summary, the chemistry of the phosphamethin-cyanines as far as it has been investigated to date resembles that of the methincyanines. However, the phosphamethin-cyanines are considerably more reactive. The smooth cleavage of the P-C bond has no counterpart in metliincyanine chemistry. 

The chemistry of phosphorus compounds with a delocalized P-C double bond proves to be very versatile. Whereas the physical properties of phosphamethin-cyanines are similar to the corresponding methin- or azamethin-cyanines, their chemical properties are distinguished by the higher reactivity of the phosphorus atom and the phosphorus-carbon double bond. 

Delfino, A. B., and Buchs, A. Mass Spectra and Computers, 39 109-137 (1973) DePuy, C. H. Stereochemistry and Reactivity in Cyclopropane Ring-Cleavage by Electrophiles. 40, 73-101 (1973) Dimroth, K. Delocalized Phosphorus-Carbon Double Bonds. Phosphamethin-cyanines, X3-Phosphorins and 5-Phos-phorins. 38,1—150 (1973)... 

In order to lower the energy of the PC double bond for stabilization, ring systems are rather suitable indeed, for the first time, Dimroth and Hoffmann (4), with the phosphamethine cyanin cation, and later on Markl (5) and Ashe (6), with phosphabenzene, succeeded in synthesizing such molecules as stable species. 

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