Phosphetanes—

Parent phosphetane, (CH2)3PH (Table 6.18) is not yet known, but many derivatives have been prepared. Some phosphetanes are of interest because of their restrictive effect on pseudorotation (Chapter 14.3). 

Phosphetane chemistry originated in 1962 with the synthesis of (6.823). Another synthesis is (6.824) 

Phosphetanic acid can be prepared in low yield by reacting bromopropyl phosphonic dichloride with magnesium in ether (6.825). 

Trichlorsilane will reduce 1-phenyl phosphetanium oxide to 1-phenyl phosphetane, which with benzyl bromide gives the phosphetanium bromide. Hydrolysis of this phosphonium compound gives the original 1-phenyl phosphetanium oxide (6.827). However, hydrolysis of benzyl methyl phosphetanium bromide results in ring expansion to a phospholane oxide (6.828). 

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Phosphepin, 4,5-dihydro-l-phenyl synthesis, 1, 527 Phosphepin, 1-oxo-1-phenyl-synthesis, 1, 527 Phosphetanes, 1, 523 NMR, 1, 524-525 stereochemistry, 1, 524... 

In triaziridine 23, the o-relaxation is appreciable due to the lone pair effect (Sect. 2.1.4) as is the case with triphosphirane 14 which has almost the same SE as tetra-phosphetane 15. Triaziridine 23 has lower SE (31.8 kcal moF ) than tetraazetidine 24 (SE = 33.4 kcal moF ) (Scheme 12) [12]. Lone pairs on nitrogen atoms have high s-character. The hybrid orbitals for the N-N bonds have lower s-character (sp ) in 23 than that (sp ) in triazane 25 [12]. The geminal a-a interaction is less antibonding in 23 (IBP, = -0.004) than in 25 (IBP. = -0.014). The geminal... 

The phosphetans (51) react with one equivalent of chlorine to give but-3-enylhalogenophosphines, which can be cyclized by heating or by treatment with halogen or aluminium chloride to the A -phospholen... 

In the solvolysis of a series of derivatives of phosphetan-1-oxide (80) it has been observed that, when X = NMeg or Cl, rates were lower than for comparable acyclic analogues whereas when X = OR the converse was true. Making the assumption that all these displacements proceed by way... 

A potential source for generating monomeric 23 is found in the l,2Xs-oxa-phosphetanes 21 and 22 18-20>, Their mass spectra contain peaks at M+ — 140, corresponding to [2 + 2]-cycloreversion to olefin and 23 however, the latter fragment (m/e = 140) was not found in the mass spectra. Although it cannot be explicitly stated whether this fragmentation is induced by electron-impact or thermally, a thermal reaction in the mass spectrometer certainly appears plausible. Such a reaction can indeed be accomplished on a preparative scale under milder conditions, as previously reported for 21 (R = C6HS) (Sect. 2.1). 

The most active palladium catalyst system developed for the asymmetric hydrosilylation of cyclopentadiene (Scheme 23) involves the use of the (/ )-MOP-phen ligand (38), which shows significant enhancement of enantioselectivity compared to (R)-MeO-MOP (80% ee from (38), 39% ee from (36a)).114 Other phosphine ligands that afford active palladium catalysts for the same transformation include the /3-7V-sulfonylaminoalkylphosphine (39) and phosphetane ligand (40) (Equation (13)).115-117 A comparison of the enantioselectivities of these ligands for the palladium-catalyzed hydrosilylation of cyclopentadiene is given in Table 8. 

The phosphetane ring is a useful synthon in the preparation of optically active ligands. Chiral l,2-bis(phosphetano)benzenes 38 are easily prepared from dilithiophenylphosphine 36 by reaction with a cyclic sulfate 37 <00T95>. 

The paper devoted to the phosphetan synthesis26 describes optimum conditions for the cyclization process leading to (22), and explains the effects of other conditions in terms of competing reactions, such as alkene polymerization or addition of hydrogen chloride. Assignment of structure (23) to the product from hept-l-ene (24)... 

The thio-Wittig reaction, like the Wittig itself, may involve (thia)phosphetane or betaine-type structures as intermediates. A combined experimental and theoretical study over a wide range of conditions and of substrates (aliphatic vs aromatic, aldehyde- vs ketone-derived) suggests a mechanistic continuity, with solvent polarity and substrate electronic effects being the main influences on the transition from one mechanism to another. ... 

Phosphetanes have previously been discussed with respect to the stereospecific coupling found in these compounds (III.B.2).a967,82 1968,67,121... 

The present review is concerned with four-membered heterocycles that contain one sulfur atom either alone or associated with another heteroatom. The classes of compounds that will be discussed are the thietanes (1) and thietes (2), the 1,2-oxathietanes (3), the / -sultines and -sultones (4 and 5), the 1,2- and 1,3-thiazetidines (6 and 7) and thiazetes (8), the 1,2- and 1,3-thia-phosphetanes (9 and 10), and the 1,3-silathietanes and 1,3-selenathietanes (11 and 12). 

The phosphetanes are rather scarce and present an open field of research. The parent phosphetane and its phosphorus derivatives (P-CN = 3 or 4) are still unknown. The only synthesis, first developed in 1962 (62JOC606), involves addition of PC13 in the presence of AICI3 to a number of highly substituted alkenes in CH2CI2 at 0 to -10 °C (equation 72). 

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