Phosphorin, 2- synthesis

Another A3-phosphorin synthesis from phospholes involves a 1,5-shift of the phenyl group of 1-phenylphosphole into the 2-position. 3,4-Dimethyl-l-phenylphosphole and 1,2,3-triphenylphosphole react with diphenylacetylene at 170 and 230 °C, respectively, in this manner (equation 66) (81TL4713, 81JA4595). 

Phosphorinanium iodide, 1-ethyl-l-phenyl-synthesis, 1, 500 Phosphorins, 1, 506-513 structure, 2, 7... 

The synthesis of the planar, tricyclic- A5-phosphorin (179) has been described.161 It reacts with methyl iodide to give the salt (180) as a mixture of stereoisomers, and is in equilibrium with the ylide (181), which reacts with benzaldehyde to give the oxide (182). 

The direct synthesis of 1.1-substituted X -phosphorins requires many tedious steps (see p. 76). Cleavage of the 1.1-substituents is possible in some cases (see p. 90), but on the whole this method has no preparative value. 

On the basis of CNDO/2 calculations on the model compounds 2.4.6-trimethyl-pyridine and 2.4.6-tri-methyl-X -phosphorin, Schweig and coworkers found that the MO sequences of pyridine and X -phosphorin do not correspond to each other. For simplicity, methyl- rather than tert-butyl groups were used in the calculations (whereas the PE and UV spectra of the tert-butyl compounds have been recorded the synthesis of the unsubstituted X -phosphorin was unknown at the... 

The position of phosphorus with respect to nitrogen in the periodic table led to the expectation that it should be much easier to remove one electron from X -phos-phorins than from pyridines (see also p. 37). Indeed, soon after the synthesis of 2.4.6-triphenyl-X -phosphorin 20 by Markl, we discovered that addition of 2.4.6-triphenoxyl 57 in benzene induces oxidation to the very stable radical cation 55... 

Alkylation with triethyl oxonium tetrafluoroborate yields two products (in the proportion 3 1) 1.2-dihydro-l-ethoxy-2-ethyl-2-4.6-triphenyl-phosphorin oxide 92 m.p. 161-162 °C, and l.l-diethoxy-2.4.6-triphenyl-X -phosphorin 71, m.p. 106 "C. 71 is known from Stade s synthesis (p. 84), 92 was identified by H-NMR. No diastereoisomers were isolated... 

As early as 1963 Markl prepared the first representatives of this class of compounds via multi-step synthesis l,l-diphenyl-X -phosphorin 118 ( 1,1-diphenyl-phosphabenzene ) and 1,l-diphenyl-2,3-benzo-X -phosphorin i20 ( 1,1-diphenyl-phospha-naphthalene ). Neither compound could be obtained in crystalline form. Instead, treatment of the crystalline phosphonium salts iiP and 121 with aqueous alkali affords very reactive, air-sensitive yellow or orange powders 118 and 120). Acid treatment leads back to the phosphonium salts. 

The unsubstituted compounds 118 and 120 must be prepared via a large number of steps involving simple building blocks. This method has little preparative value. Moreover, the unsubstituted X -phosphorins appear to be so reactive that they are rather difficult to handle. Thus, the emphasis in synthesis has been placed on the preparation of 2.4.6-tri-substituted derivatives, which will be dealt with exclusively in this section. 

Finally, we point to the possibility of P = 0 bond formation from 1-alkoxy-X -phosphorin derivatives 124 or 125 by cleavage of alkyl cations. Also the reverse process, /. e. alkylation of the P = O moiety to form P—O—R groups is possible. The synthesis of X -phosphorins having functional groups at the C-atoms of the phosphorin ring was first made possible by the preparation of new stable X -phosphorin carbenium ions 140. Here again, the fundamental difference between phosphorin and pyridine systems comes to light Whereas carbanionic structures 139 b are stabilized in the pyridine series, in the X -phosphorin series carbenium ions as 140 b are stabilized. 

The l,l-diphenyl-X -phosphorins 720a-c are colored, easily autoxidizable, noncrystalline compounds which were prepared by multistep synthesis, the last step being conversion of the corresponding 1.1-diphenyl-2.3-benzophosphonium perchlorates 727 a—c by aqueous alkali. 

The reactions are also smooth with 2- or 4-alkyl-substituted X -phosphorins 1.1-dialkoxy-or l.l-bis-dialkylamino4-methyl-2.6-diphenyl-X -phosphorins, which can be readily obtained from 2.6-diphenyl4-methyl-X -phosphorin, are very useful for further synthesis (see p. 87). 

Such nucleophilic substitutions can also be carried out on analogous 1,1-di-arylamino-X -phosphorins and on oxy-bis -X -phosphorinsii4 or acetates 153, the latter can be viewed as mixed anhydrides in which phosphinic acid or acetic acid are easily displaced by nucleophiles. These reactions have a broad application in synthesis. 

A 3-Phosphorin derivatives of benzo annelated rings were intensively studied by Bickel-haupt and coworkers. One of his preferred methods consists in the introduction of the final P=C double bond from appropriate ring systems by basic elimination of HC1 with DBU or other bases. Many benzo-A 3- phosphorins have been reported. An example is the synthesis of 2-phosphanaphthalene (equation 26) (75T1097). Some other examples are shown in (32)-(35). 

Three different methods exist for the synthesis of As-phosphorin derivatives from A3-phosphorins. 

Another synthesis recently reported by Markl (81LA870) involves rearrangement of l-R-4-R1-4-methoxy-l,4-dihydrophosphorins (cis and trans isomers) under acid conditions. The method gives rise to many 2,6-unsubstituted A5-phosphorins (equation (31)). 

Following the development of synthetic methods for phosphorins in the 1960s, the last decade has seen the synthesis of the remaining Group V analogues of pyridine with Ashe and Markl being particularly active contributors to the field. 

Phosphorinanones have been utilized as substrates for the preparation of alkenes,11 amines,12 indoles,5,13 and in the synthesis of a series of secondary and tertiary alcohols via reduction,10 and by reaction with Grignard6,11 and Refor-matsky11,14 reagents. Phosphorinanones have also been used as precursors to a series of 1,4-disubstituted phosphorins.15 The use of 4-amino-l,2,5,6-tetrahydro-l-phenylphosphorin-3-carbonitrile for the direct formation of phosphorino-[4,3-d] pyrimidines has been reported.16... 

---