Phosphorus-oxygen bond formation

Esters of penta- and trivalentphosphorus acids and their derivatives readily undergo cleavage of the phosphorus-oxygen bond under extremely mild conditions with formation of the phosphorus-fluorine bond Phosphates and phosphi-... 

These reagents are usually prepared by treatment of a phosphine with an alkyl halide and successive addition of base, like BuLi or NaHMDS. Reaction with the ketone takes place by attack of the carbanionoid carbon of the ylide form on the electrophilic carbon of the carbonyl group via a four-membered heterocyclic transition state, the oxaphosphetane 35. The driving force for this transformation is provided by formation of the very strong phosphorus-oxygen bond. Subsequent collapse of the oxaphosphetane furnishes the desired exomethylene group under retention of geometry. 

The carbanion centre of (6.33) is planar and so, in principle, can be approached from either side by a suitable electrophile. However, as can be seen from structure (6.33) in Figure 6.7, one side is more hindered as a result of the bulky alkyl group, R. An electrophile will therefore approach from the side of the hydrogen atom as shown. In this case, the electrophile is formaldehyde and the product of addition is the lithium alcoho-late (6.34). Rotation by 180° around the carbon-carbon bond allows the formation of the phosphorus-oxygen bond of (6.35) and subsequent elimination of the phosphine oxide to give the allylic alcohol (6.36). 

Studies by E. Vedejs (University of Michigan) indicate that the Wittig reaction takes place in two steps. In the first step (below), the aldehyde or ketone combines with the ylide in a cycloaddition reaction to form the four-membered ring of an oxaphosphet-ane. Then in a second step, the oxaphosphetane decomposes to form the alkene and triphenylphosphine oxide. The driving force for the reaction is the formation of the very strong (DH° = 540 kj moP ) phosphorus—oxygen bond in triphenylphosphine oxide. 

It is well established that in the presence of certain transition metal complexes or peroxides, that phosphines will be oxidized in the presence of oxygen via peroxy radicals to generate their thermodynamically stable phosphine oxides, the driving force being the formation of the strong phosphorus-oxygen bond. However, in the absence of such reagents, there are remarkably few studies on the oxidation of phosphines by air. There appears to be no common consensus on the first step in the mechanism of... 

The driving force for a Wittig reaction is the formation of the very strong phosphorus-oxygen bond in triphenylphosphine oxide. 

The intermediate phosphorus compound, either the betaine or the oxaphosphetane, immediately decomposes to yield the alkene and a phosphine oxide. This step is concerted all bonds break and form simultaneously in the transition state. The driving force for this reaction is the formation of a very strong phosphorus—oxygen bond. (The P—O bond dissociation energy in triphenylphosphine oxide is approximately 550 kj mole. )... 

It is now accepted that the oxaphosphetane intermediate is formed directly by a [2-1-2] cycloaddition of the phosphonium ylide with the aldehyde (or ketone) through a four-center transition state, in which the formation of the carbon-carbon bond is more advanced than that of the phosphorus-oxygen bond (Scheme 2, Path b). Although there are some exceptions [25, 26], the oxaphosphetane formation step is generally nonreversible [27, 28] and decides the stereoselectivity. 

In contrast to phosphorus esters, sulfur esters are usually cleaved at the carbon-oxygen bond with carbon-fluorine bond formation Cleavage of esteri nf methanesulfonic acid, p-toluenesidfonic acid, and especially trifluoromethane-sulfonic acid (tnflic acid) by fluoride ion is the most widely used method for the conversion of hydroxy compounds to fluoro derivatives Potassium fluoride, triethylamine trihydrofluoride, and tetrabutylammonium fluoride are common sources of the fluoride ion For the cleavage of a variety of alkyl mesylates and tosylates with potassium fluoride, polyethylene glycol 400 is a solvent of choice, the yields are limited by solvolysis of the leaving group by the solvent, but this phenomenon is controlled by bulky substituents, either in the sulfonic acid part or in the alcohol part of the ester [42] (equation 29)... 

Besides the successful development of asymmetric syntheses with carbon-carbon bond formation, methods for carbon-heteroatom bond formation were also investigated intensively. In this context we developed several practical protocols for carbon-nitrogen, carbon-oxygen, and carbon-phosphorus bond formation. 

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