Grignard compounds (organomagnesium halides) are among the longest-known organometallic reagents and have found a plethora of applications in organic synthesis. [2] In particular, they are used for additions to polarized or polarizable carbon-heteroatom multiple bonds. However, they are less suitable for additions to CÀC multiple bonds. Although triphosphabenzenes, for example, 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene (1) [3] exhibit a marked aromatic character according to physical-chemical measurements, [4] they are in fact highly reactive compounds. Thus, for example, Diels-Alder reactions [5] or addition reactions of lithium alkoxides with participation of all three PÀC double bonds [6] occur readily under mild conditions. Compound 1 also reacts readily with silylenes, [7] carbenes, [8] or phosphinidenes, [9] usually through a [2þ1]-cycloaddition process.
We treated the triphosphabenzene 1 with the Grignard reagents 2 a-d in THF and observed that the latter species undergo formal, regio-and stereoselective 1,4-additions to the heteroaromatic compound 1, already at À16 8C. The regioselectivity arises from the polarity of the PÀC double bonds, [10] by which the organic group R of the Grignard reagent adds to a phosphorus atom and the metal part adds to C4. The cis stereoselectivity indicates that the reaction is a synchronous or polar addition. It has been known for some time that monophosphabenzenes undergo nucleophilic addition with both organolithium and organomagnesium species. 1,2-Dihydro-or 1,4-dihydrophosphinines are obtained after hydrolysis or reaction with electrophiles. [11, 12] The structures of the novel organomagnesium compounds were deduced from their NMR spectral data and confirmed by X-ray crystallographic analysis of compound 3 d. The structures of 3 a and 3 b were derived by comparison with the NMR data of 3 d (Scheme 1).
The first unexpected result was provided by the crystal structure analysis of 3 d (Figure 1). [13] The organomagnesium compound formed did not exist as a halide with covalent bonds but rather as the salt of a diorganomagnesium compound with separated charges. To the best of our knowledge, compounds of this type have not been described previously.