Attack on Oxygen

Attack on Oxygen. Nitrones have been deoxygenated to the parent imine with trimethyl phosphite under vigorous conditions.45 Virtually quantitative yields of the enol-phosphates (54) and (55) were obtained from the reaction of tris(trimethylsilyl) phosphite with a-diketones and /j-benzoquinone. 4 6 

Attack on Oxygen. Cyclic phosphites or phosphonites (38) react with a-keto-acids to give cyclic acyloxyphosphoranes (39), which have previously been implicated as intermediates in the hydrolysis of phosphoenolpyruvate initial attack on keto-oxygen has been suggested, as shown. Acyclic phosphites gave the Michaelis-Arbusov product (40), presumably because they lack the five-membered ring to stabilize the phosphorane (39). 

A new method of cyclization has been developed, through the reaction of suitably or//ru-substituted diethyl benzoylphosphonates (41), or their acid chloride precursors, with trialkyl phosphites. An analogous reaction takes place with (42) to give the fluorenylphosphonate (43). The mechanism suggested involves initial attack on carbonyl oxygen, and gains support from the formation of the benzofuranylphosphon-ate (45) in the reaction of (44) with triethyl phosphite. 

The reactions of a variety of tervalent phosphorus compounds with 1,2-dioxan have been investigated. Reactions with phosphites give either phosphate and THF or the phosphorane (46), depending on the phosphite used, and so are very similar to the analogous reactions with dialkyl peroxides. Penta-alkoxyphosphoranes (47) 

The reaction of phosphites with pseudohalogens, e.g. thiocyanogen, gives the corresponding phosphoranes [e.g. (48)]. Initial attack by phosphorus on sulphur, 

Krawczyk. J. Michalski. M. Pakulski. and A. Skowrodska. Tetrahedron Letters. 1977 2019. 

Attack on Oxygen, The stoicheiometry and rate constants of the reactions of 

Trialkyl phosphites react with diphenoquinone to give a variety of products, all thought to be formed by initial attack of phosphorus at oxygen to give (51). A similar reaction with diphenoquinonedibenzenesulphonimide gives the phosphoramidate (52) as the only product. 

In the reaction of iso anato-phosphites with hexafluoroacetone, attack on oxygen gives the intermediate (53), which on cyclization and further 

Tris(dimethylamino)phosphine desulphurizes / -keto-sulphides (60) to give ketones and enol ethers as major products a mechanism involving initial 

Attack on Oxygen. Dichlorophenylphosphine reacts with propene oxide and 2-methyloxetan to give products (80) and (81) which are logically derived from initial attack at oxygen.  

The cyclohexadienone (82) reacts with two moles of sodium diethyl phosphonate to give 4-methylphenyl diethyl phosphate and the triene (83). A mechanism involving dichlorocarbene is proposed with a first step which could involve attack at either carbonyl oxygen or carbon. 

Under anhydrous conditions benzaldehyde and diphenylphosphine oxide anion give a mixture of stilbene and stilbene oxide, while a similar reaction with benzoylferrocene gives a mixture of products, including a small amount 

Ramirez has used the reaction of highly electrophilic carbonyl compounds with a variety of tervalent phosphorus compounds in a general synthesis of 1,2-oxaphosphetans [e. g. (86) ]. 

The kinetics of the oxidation of diarylphosphine oxides with perbenzoic acid have been investigated and the kinetic deuterium isotope effects 

The anhydride (72) gives quite different products, (73) and (74), with tris(dimethylamino)phosphine to those previously obtained with triethyl phosphite. The formation of (73) and (74) is suggested to involve keten intermediates and an alternative mechanism is proposed for the phosphite reaction. 

Isomeric products (75) and (76) are obtained from the reaction of perfluoroacetone with dialkyl phosphites. The relative proportions of the isomeric mixture depend on the alkyl group in the phosphite and the results are explained in terms of the polarity of the P-H bond and hence its direction of addition to the carbonyl group. Presumably, the balance in these cases is very finely adjusted, although these reactions are possibly more complicated than the results suggest. 

The reaction of aldehydes with carbon tetrachloride in the presence of excess tris(dimethylamino)phosphine has been used to prepare vinyl dihalides in yields of 50—70%. It is suggested that the reaction takes place via an intermediate salt (77), although the formation of this salt seems more likely to be analogous to the Perkow reaction than to involve attack on oxygen. 

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These give the products expected from electrophilic attack on oxygen by the electrophilic reagent atom, followed by nucleophilic opening of the cyclic oxonium ion (e.g. 42 Scheme 29) <64HC(19-1)436). 

Reactions of vinylogous amides with methanesulfonyl chloride also led to the formation of six-membered rings. Here the initial attack on oxygen produces a zwitterionic intermediate which can collapse to an enol sulfonic acid lactone (383,469). 

We would expect the C=0 linkage, by analogy with C=C (p. 178), to undergo addition reactions but whereas polar attack on the latter is normally initiated only by electrophiles, attack on the former— because of its bipolar nature—could be initiated either by electrophilic attack of X or X on oxygen or by nucleophilic attack of Y or Yt on carbon (radical-induced addition reactions of carbonyl compounds are rare). In practice, initial electrophilic attack on oxygen is of little significance except where the electrophile is an acid (or a Lewis acid), when rapid, reversible protonation may be a prelude to slow, rate-limiting attack by a nucleophile on carbon, to complete the addition, i.e. the addition is then acid-catalysed. 

However, the methyl cation (in itself a very energetic, unprobable species in the condensed state) is not expected to attack a carbon-hydrogen bond in dimethyl ether (or methanol) in preference to the oxygen atom. The more probable attack on oxygen would lead to the trimethyloxonium ion, which was observed experimentally447,463 On the action of a basic site the trimethyloxonium ion can then be deprotonated to form dimethyloxonium methylide ... 

Electrophilic attack on oxygen. 2-Azolinones are protonated on oxygen in strongly acidic media. O-Alkylation of 2-azolinones can be effected with diazomethane thiazolinone 704 forms 705. Frequently O- and N-alkylation occur together, especially in basic media where proton loss gives an ambident anion (see also Section 3.4.1.3.10). 

Direct attack on oxygen was postulated in the reaction of Grignard and lithium reagents with [E]-2-benzenesulfonyl-3-phenyloxaziridine 55. The principal organic products were alcohols and phenols (50-95% yields), derived from the organometallic reagent, and amines derived from addition of the organometallic to the intermediate A -sulfonyl imine (60-95%). 

In an earlier kinetic study of the oxaziridine-triphenylphosphine reaction, it was concluded that direct attack on oxygen by triphenylphosphino occurred because the transition state did not appear to involve much charge separation. However, the transition state for attack at nitrogen, as pictured in Ref. 95, does not involve much charge separation either. 

Similar carbene insertion reactions with ethers are also very well known. Apart from direct C-H insertion, attack on oxygen can occur to form a very reactive dialkyloxonium methylide. ... 

Olah [24] reexamined this reaction using CD2N2 with unlabelled ethers (MeOEt, MeO-n-Pr, EtOEt, EtO-n-Pr, and THF). In addition to products of C-H insertion, evidence was found for competing methylene attack on oxygen to form the ylide. The reaction of... 

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