Tertiary A -oxide

Treatment of a tertiary A -oxide with an activating agent such as acetic anhydride, resulting in rearrangement where an A, A -disubstituted acetamide and an aldehyde are generated. 

Another very important reaction initially involving nucleophilic attack on an aldehyde carbonyl is the Wittig reaction. An yUd adds to the carbonyl forming a betaine intermediate which then decomposes to produce an olefin and a tertiary phosphine oxide. 

Amine oxides, known as A[-oxides of tertiary amines, are classified as aromatic or aliphatic, depending on whether the nitrogen is part of an aromatic ring system or not. This stmctural difference accounts for the difference in chemical and physical properties between the two types. 

Trialkyl- and triarylarsine sulfides have been prepared by several different methods. The reaction of sulfur with a tertiary arsine, with or without a solvent, gives the sulfides in almost quantitative yields. Another method involves the reaction of hydrogen sulfide with a tertiary arsine oxide, hydroxyhahde, or dihaloarsorane. X-ray diffraction studies of triphenylarsine sulfide [3937-40-4], C gH AsS, show the arsenic to be tetrahedral the arsenic—sulfur bond is a tme double bond (137). Triphenylarsine sulfide and trimethylarsine sulfide [38859-90-4], C H AsS, form a number of coordination compounds with salts of transition elements (138,139). Both trialkyl- and triarylarsine selenides have been reported. The trialkyl compounds have been prepared by refluxing trialkylarsines with selenium powder (140). The preparation of triphenylarsine selenide [65374-39-2], C gH AsSe, from dichlorotriphenylarsorane and hydrogen selenide has been reported (141), but other workers could not dupHcate this work (140). 

Amine oxides, prepared to protect tertiary amines during methylation and to prevent their protonation in diazotized aminopyridines, can be cleaved by reduction (e.g., SO2/H2O, 1 h, 22°, 63% yield H2/Pd-C, AcOH, AC2O, 7 h, 91% yield Zn/HCl, 30% yield). Photolytic reduction of an aromatic amine oxide has been reported [i.e., 4-nitropyridine A-oxide, 300 nm, (MeO)3PO/CH2Cl2, 15 min, 85-95% yieldl. ... 

To return to a more historical development the mercuric acetate oxidation of substituted piperidines (77) should be discussed next. This study established that the normal order of hydrogen removal from the aW-carbon is tertiary —C—H > secondary —C—H > primary —C—H, an observation mentioned earlier in this section. The effect of substitution variations in the piperidine series can be summarized as follow s l-mcthyl-2,6-dialkyl and 1-methyl-2,2,6-trialkyl piperidines, as model systems, are oxidized to the corresponding enamines the 1,2-dialkyl and l-methyl-2,5-dialkyl piperidines are oxidized preferentially at the tertiary a-carbon the 1-methyl-2,3-dialkyl piperidines gave not only the enamines formed by oxidation at the tertiary a-carbon but also hydroxylated enamines as found for 1-methyl-decahydroquinoline (48) (62) l-methyl-2,2,6,6-tctraalkyl piperidines and piperidine are resistant to oxidation by aqueous mercuric acetate and... 

Bohlmann and Arndt (S3) have separated the possible stereoisomers of hexahydrojulolidine (78-80) and subjected them to mercuric acetate oxidation. The rates, which were followed by the precipitation of mercurous acetate, showed that isomer 78 reacted about five times faster than isomer 79, while isomer 80 reacted very slowly. The difference in rates between 78 and 79, both of which have tertiary a-hydrogens trans to the nitrogen electron pair, was explained by pointing out that greater relief of non-classical strain occurs in the oxidation of 78 as compared to 79. Isomer 80 has no tertiary a-hydrogens trans to the nitrogen electron pair except when it is in an unfavorable boat conformation. 

Conformational effects appear to be important in determining which tertiary a-hydrogen is removed. For example, 4-phenyl-, 4-p-nitrophenyl-, and 4-/)-methoxyphenylquinolizidine (83) all are oxidized to the corresponding /j < -iminium salts (84) and not to the conjugated zJ -iminium salts (85) (86). The authors Judged that steric hindrance was responsible or that the conformation of the 4-sabstituted quinolizidines did not contain ideal... 

Six categories of A/-hetero atom derivatives are considered N-M (M = boron, copper) N-N (e.g., A/-nitro, A-nitroso) A-oxides (used to protect tertiary amines) N-P (e.g., phosphinamides, phosphonamides) N-SiR3 (R = CH3), and N-S (e.g., sulfonamides, sulfenamides). 

Pyridlne-A -oxide may be formed in a manner analogous to the reaction of dibenzoyl peroxide with tertiary phosphines, i.e., Eq, (18)... 

This mechanism is the same as that of 19-23 the products differ only because tertiary amine oxides cannot be further oxidized. The mechanism with other peroxyacids is probably the same. Racemic (3-hydroxy tertiary amines have been resolved by oxidizing them with t-BuOOH and a chiral catalyst one enantiomer reacts faster than the other.This kinetic resolution gives products with enantiomeric excesses of > 90%. 

With a-dfsubstituted yUdes the reaction results in the formation of a carbonyl compound and a tertiary phosphine oxide (Scheme 6). 

A. Nucleophilic Reactions of the P=0 Group.—Tris(trifluoromethyl)-phosphine oxide (33) reacts with hexamethyldisiloxane to give a phos-phorane, whose n.m.r. spectrum at — 140 °C shows non-equivalent trifluoromethyl groups. Although this unusual reaction clearly involves nucleophilic attack of the phosphoryl oxygen on silicon at some stage of the reaction, a full study of the mechanism has not been published. Tertiary phosphine oxides can be converted cleanly into dichlorophos-phoranes (34) by treatment with two moles of phosphorus pentachloride. Alkylation of the sodium salt of tetraphenylmethylenediphosphine dioxide (35) with alkyl halides, in dimethyl sulphoxide, has been reported to... 

A modification of this procedure allowed the isolation of 1,3,2-oxazaphospholidine 52a as a single diastereomer [41] and its application to asymmetric synthesis of enantiomerically and diastereomerically pure phosphinic acid derivatives 53 and 54 and tertiary phosphine oxides 55 (Scheme 20) [45], A few years later, a similar approach for the synthesis of enantiomerically pure tertiary phosphine oxides 55... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

For example, direct treatment of red phosphorus with potassium hydroxide in a mixture of dioxane and water with a phase-transfer catalyst (benzyltriethylammonium chloride) allows direct reaction with primary haloalkanes to form the trialkylphosphine oxide in moderate (60-65%) yield.1415 Allylic and benzylic halides are similarly reported to generate the corresponding tertiary phosphine oxides. When the reaction is performed with a,(o-dihalides, cyclic products are generated only with four- and five-carbon chains the third site... 

A Michael-type addition reaction of phosphine generated from red phosphorus in concentrated aqueous KOH solution has been noted to provide moderate isolable yields of pure organophosphorus products.27 For example, tris-(2-cyanoethyl)phosphine is produced in 45% isolable yield from acrylonitrile, and tris-(2-[y-pyridyl]ethyl) phosphine oxide is isolated in 40% yield from 4-vinylpyridine under these conditions. Excellent yields of the tertiary phosphine oxide, tris-(2-cyanoethyl)phosphine oxide, have been reported using white phosphorus in absolute ethanol with KOH at ice/salt-bath temperatures.28 A variety of solvent systems were examined for this reaction involving a Michael-type addition to acrylonitrile. Similarly, tris-(Z-styryl)phosphine is produced from phenylacetylene under these conditions in 55% isolated yield. It is noteworthy that this last cited reaction involves stereospecific syn- addition of the phosphine to the alkyne. 

Preparation of a tertiary phosphine oxide from white... 

Gusarova, N.K., Shaikhudinova, S.I., Reutskaya, A.M., Tartarinova, A.A., and Trofimov, B.A., One-step synthesis of unsymmetrical tertiary phosphine oxides from red phosphorus and organyl halides, Russ. Chem. Bull., 49, 1320, 2000. 

The use of m-CPBA allows the formation of nitrones in the oxidation of tertiary amines. The resulting amines A-oxides are subject to either Cope or Meisenheimer rearrangements, providing formation of nitrones. Thus, the generated corresponding nitrones in the oxidation of bicyclic aziridines give nitrones as a result of a Meisenheimer rearrangement (Scheme 2.14) (93). 

Oxidations of a range of p-cyanoethyl tertiary amines (44) with m-CPBA in CH2CI2 give the corresponding A-oxides (45), which can be isolated or undergo Cope elimination affording hydroxylamines (46) in high yields (Scheme 2.16) (Table 2.1) (96). Hydroxylamines (46) can be easily oxidated into nitrones (see Section 2.2.1.3). 

---