Quasiphosphonium intermediates

To obtain more information on the nature of the quasiphosphonium intermediates involved in these systems we have studied the reactions gf sterically hindered neopentyl esters by means of 1P nmr spectroscopy. Trineopentyl phosphite and a-bromoacetophenone gave rise to a peak at +41 ppm due to the ketophosphonium intermediate 3 (R = Me.CCH, R = Ph X = Br ) within half an hour of mixingJthe reactants in acetone-dfi at 27 °C ( p nmr shifts are relative to 85% H-PO. down-field positive). Peaks due to the ketophospnonate 4 +19 ppm and the vinyl phosphate 7 (-7 ppm) were also observed (compound 4 and 7 have satisfactory elemental analysis and spectroscopic data ). The concentration of the intermediate reached a maximum after about two hours when it was precipitated from acetone solution by the addition of anhydrous ether to give white crystals of trineopentyloxy (phenacyl)phosphonium bromide, identified by elemental analysis and nmr spectroscopy ( XP 6+41, in CDCl ). When redissolved in acetone-dg, deuterochloroform, acetic acid, or acetic acid-acetone mixtures, the intermediate decomposed to yield keto-phosphonate 4 but none of the vinyl phosphate 6 (Perkow product). Nor was the course of reaction affected by the addition of chloride ion or of a-chloro-acetophenone in acetonitrile. 

The increase in stability of quasiphosphonium intermediates Ph P(0R)n-3MeX, with increasing n, is attributed mainly to inductive effects of the substituent groups. Phosphorus-carbon bond lengths for Ph - P and Me - P in the intermediates do not support the view that mesomeric release from phenyl to phosphorus is involved (1). On this basis, the stability should increase with decreasing n, if p -d interaction between oxygen and phosphorus is a significant factor (8), whereas exactly the opposite is observed. 

Hudson, H. R., Kow, A., Roberts, J. C. Quasiphosphonium intermediates. Part 3. Preparation, structure, and reactivity of alkoxyphosphonium halides in the reactions of neopentyl diphenylphosphinite, dineopentyl phenylphosphonite, and trineopentyl phosphite with halomethanes and the effect of phenoxy-substituents on the mechanism of alkyl-oxygen fission in Michaelis-Arbuzov reactions. J. Chem. Soc., Perkin Trans. 2 1983, 1363-1368. 

Petnehazy, I., Szakal, G., Tdke, L., Hudson, H.R., Powroznyk, L., and Cooksey, C.J., Quasiphosphonium intermediates. Part 4. Isolation and identification of intermediates in the Arbuzov and Perkow reactions of neopentyl esters of phosphorus(III) acids with a-halogenoacetophenones. Tetrahedron, 39, 4229, 1983. 

Hudson, H.R., Matthews, R.W., McPartlin, M., Pryce, M.A., and Shode, O.O.. Quasiphosphonium intermediates. Part 7. The preparation of trinorborn-l-yl phosphite and its reactions with halogeno compounds. Stable intermediates of the Arbuzov and Perkow reactions and then sU uctural characterization by X-ray diffraction, NMR spectroscopy, and fast-atom-bombardment mass spectrometry, J. Chem. Soc., Perkin Trans. 2, 1433, 1993. 

Quasiphosphonium intermediate species are found to be involved in the transition metal-catalyzed reaction of haloaromatics and vinylic halides with trivalent phosphorus-containing esters. These reactions, while giving products that appear to be from a Michaelis-Arbuzov type reaction, actually involve addition-elimination reactions. 

Exceptional behaviour is seen in the reaction of diethyl N,N-bis (trimethylsilyl) phosphoramidite with acetyl chloride. This reaction was reported to give diethyl N-(trimethylsilyl) acetylphosphonimidate (equation 35). Apparently, one of the silicon atoms is attacked preferentially over the phosphorus in the quasiphosphonium intermediate formed in the first step of the Arbuzov reaction, and the product is formed by the loss of chlorotrimethylsilane. 

In this context, it was suggested [53] that the reaction involved a tertiary carbocation intermediate that was in equilibrium with quasiphosphonium salt. The elimination of HC1 from the latter, leads to the formation of 2,5-dihydro-l,2-oxaphosphole derivatives. Macomber [54], however, has shown that bromination of optically pure... 

It is generally understood that the reaction proceeds in two stages, the first one generating an intermediate quasiphosphonium ion followed by attack of the associated counteranion on an ester linkage at the atom attached to phosphorus through oxygen (Equation 3.2). 

Under commonly used reaction conditions, the intermediate species is indicated to be a quasiphosphonium ion. Isolation of the intermediates from reactions in which the initial substrate is sufficiently reactive not to require heating, thereby precluding the second step from occurring rapidly, has provided evidence (nuclear magnetic resonance [NMR] and x-ray) of a quasiphosphonium ion species rather than a phosphorane species.17-26... 

Quasiphosphonium ions involving oxygen directly bound to the positive phosphorus site are generated as intermediates in numerous other reactions. For example, the Michaelis-Arbuzov reaction (see Section 3.5) produces a quasiphosphonium ion as an intermediate that undergoes attack by the associated amon at the carbon end of the C T P linkage. The reaction follows a different course when aryl ester linkages are present on the starting trivalent phosphorus acid derivative. In such instances, a quasiphosphomum ion is... 

Using P NMR, the intermediacy of the quasiphosphonium ion bearing the phosphorus-oxygen linkage has been demonstrated experimentally. In the absence of added acid, the same product formation occurs, but the intermediate quasiphosphonium species has not been detected experimentally. However, evidence of an intermediate oxyphosphorane (see Section 5.3) has been demonstrated under these conditions, as it has under acidic conditions, and such an intermediate has independently been shown to provide the usual products of the reaction. Evidence is available in other systems for the equilibrium of oxyphosphoranes with oxygen-containing quasiphosphonium salts. 

Griffiths, D.V., Griffiths, P.A., Whitehead, B.J., and Tebby, J.C., Novel quasiphosphonium ylides from the reaction of trialkyl phosphites with dialkyl benzoylphosphonates. Evidence for carbene intermediates in the intramolecular cyclization of 2-substituted dialkyl benzoylphosphonates. 7. Chem. Soc., Perkin Trans. 1, 479, 1992. 

Sal keeva et al. studied the reactions of ethyl and /-butyl phosphorodiamidites with chloroacetone, bromoacetone, and a-chloroethyl acetate. The reaction pathway is determined by the structure of the intermediate quasiphosphonium compound responsible for the formation of the Arbuzov products as well as for the occurrence of the anomalous reaction yielding vinyl phosphate (Perkow product).26 The yield of the Arbuzov product increases from chloroacetone to bromoacetone, because the anions of low basicity are more rapidly ionized, which facilitates the formation of the ionic form of the intermediate. Also /-butyl phosphorodiamidite forms via Arbuzov reaction as the major product, because the second stage of the S l reaction is accelerated.26... 

Evidence for the formation of intermediate quasiphosphonium salts comes from the isolation of the ion 436 (A = B = EtO R = H, R = R = Me E = SPh) as the hexa-chloroantimonate and further evidence stems from reactions with cyclic esters of the (alka-l,2-dienyl)phosphonic acids. The reactions of the l,3,2-dioxaphosph(V)olanes 444, of known geometry, with CI2, Br2 and RSCl or RSeCl " are highly stereoselective and would be expected to proceed through the quasiphosphonium salts 445 such salts have been isolated from other reactions. The 1,2-oxaphosph(V)ol-3-enes 446 have been isolated (66-75%) when EY is RSCl or RSeCl, but in the former case, were accompanied by 449 (65-73%) Phosphonium salts 436 (A = B = alkyl) have been obtained from reactions of propadienyldialkylphosphineoxides. ... 

FAB-MS and ESI-MS have proved to be useful in the direct detection of intermediates formed in a number of solution reactions. As such, these techniques nicely complement P NMR studies on the same systems. For example, the iodine-induced cyclization reaction of the unsaturated phosphoamidate 51a, phosphonates 51b and c and phosphate 51d (illustrated in Scheme 7) was monitored by removing aliquots of the reaction mixture and analysing them via FAB-MS"". The FAB mass spectra revealed the presence of the reactant 51 (observed as an [M + H] ion), the initial diiodo addition product 52 (observed as an [M + H] ion) and the quasiphosphonium ion 53 (observed as the intact quasiphos-phonium ion) and the iodolactone 54 (observed as an [M + H] ion). 

FAB-MS has also been used to identify quasiphosphonium halide intermediates (55) of the Arbuzov and Perkow reactions The quasiphosphonium ions fragment differently from conventional phosphonium ions under FAB conditions, preferring to undergo H transfer rearrangement with concomitant p cleavage (equation 84). 

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