Bis-phosphonio-benzophospholide

The formation of the heterocycle 1 from the xylylene-bis-phosphonium salt 2 and PCI3 proceeds via a detectable intermediate 3 in a cascade of condensation reactions that is terminated by spontaneous heterolysis of the last remaining P-Cl bond in a cyclic bis-ylide-substituted chlorophosphine formed (Scheme 1) [15]. The reaction scheme is applicable to an arsenic analogue of 1 [15] and to bis-phosphonio-benzophospholides with different triaryl-, aryl-alkyl- and aryl-vinyl-phosphonio groups [16, 18, 19], but failed for trialkylphosphonio-substituted cations here, insufficient acidity prohibited obviously quantitative deprotonation of the phosphonium salts, and only mixtures of products with unreacted starting materials were obtained [19]. The cations were isolated as chloride or bromide salts, but conversion of the anions by complexation with Lewis-acids or metathesis was easily feasible [16, 18, 19] and even salts with organometallic anions ([Co(CO)4] , [CpM(CO)3] (M=Mo, W) were accessible [20]. 

Scheme 7 Syntheses of bis-phosphonio-benzophospholides by ring metathesis...

Transformation of a phosphonio-substituted phospholide derivative under conservation of the r-electron system has been demonstrated in a number of cases which include substitution of a ring-hydrogen atom, replacement of a PRs -moiety by a hydride, or reductive de-arylation of a PhsP " - to a phosphinyl substituent PPh2. Subsequent re-quaternisation of the latter allows the formal substitution of a PhsP " - by a modified Ph2(R)P -moiety in a two-step reaction. The application of this scheme is not only perfectly suitable for the synthesis of bis-phosphonio-benzophospholides with different phosphonio-moieties but, since the last reaction stage tolerates a variety of functional groups in the electrophile, offers as well a convenient pathway for the synthesis of side-chain functionalised phosphonio-phospholide derivatives from more simple substituted precursors. 

Reaction of the bis-phosphonio-benzophospholide 1 with bromine in the presence of triethyl amine afforded the 5,6-dibromo-substituted derivative 24b [28]. Detailed studies revealed that the reaction proceeds in a similar way as the indirect electrophilic substitution of >l -phosphinines [29] where the actual substitution is preceded by oxidative addition of Br2 to the two-coordinate phosphorus (see also below). The mono-substitution product 24a was observed as intermediate, but could not be isolated (Scheme 8). 

Scheme 9 Reduction of cationic bis-phosphonio-benzophospholides and -diazaphos-pholides...

In spite of the general ambiphilicity of phosphonio-substituted phosphoHde derivatives, the aromaticity of the phosphoHde ring [10, 11] tends to reduce their electrophilicity while the intramolecular compensation of the negative charge by the phosphonio-substituents lowers at the same time their nucle-ophilicity [15, 16]. Bis-phosphonio-benzophospholides and -1,2,4-diaza-phospholides are therefore less reactive towards electrophiles and nucleophiles than other types of phosphorus containing multiple-bond systems and lack the notorious hydrolytic instabihty of many of these species [15, 16, 24]. Reactions are observed, however, with sufficiently strong electrophiles such as triflic acid or methyl triflate, or nucleophiles such as OH" or lithium alkyls, respectively. 

Oxidation and reduction of the bis-phosphonio-benzophospholide 1 were studied in some detail. Oxidation proceeds via oxidative 1,1-addition of two... 

Table 4 Catalytic activity of rhodium complexes of the bis-phosphonio-benzophospholide 23 in catalytic hydroformylation of hexane using a H2iCO (1 1) mixture (data from [44])...

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