Phosphine nucleophiles reactivities

A large cationic radius generally increases the nucleophilic reactivity of the anion in an ion pair. A small cationic radius results in stronger electrostatic attraction and lesser reactivity of the anion. Catalysts with the active site separated from the polymer backbone by an aliphatic chain have higher activity than those prepared by quatemi-zation of a chloromethylpolystyrene with a tertiary amine or tertiary phosphine, leaving only one carbon atom between the active site and the aromatic ring. 

This lack of variation of selectivity with reactivity was confirmed in an independent study by Kane-Maguire and Sweigart who found that relative reactivities of amine and phosphine nucleophiles toward a range of organometallic cations were also independent of the nature of the... 

It seems likely that the magnitude of p represents the "extent of electron demand at phosphorus , in the T. S. or in other words is a measure of "electron transfer" in the T.S., a term (=z) which appears in the semi-empirical equations describing the nucleophilic reactivity of tricoordinated phosphorus. This concept is reinforced by (i) the observation (from the data of Bokonov. and Goetz ) that p-values based on the pKg values of protonated phosphines increase with increasing pKa, i.e. increase with a shift of the equilibrium (eqn. 4) to the left and... 

The orders of nucleophilic reactivity for alkylation and acylation were found to be quite different (13, 14) and in subsequent work (15) this finding was related to the extent of bond formation in the transition state as given empirically by the Brpnsted coefficient, (3. Previously, this difference was used to predict the position of bond fission in the alkaline hydrolysis of phosphinate, phosphonate, and phosphate esters (12). Jencks and Carriuolo (16) came to similar conclusions around the same time in outstanding work on the acylation of p-nitrophenyl acetate. 

Preparation and Structure.—A -Ray crystal structures of benzoylmethylene-triphenylphosphorane (1) and arsorane (2) suggest that the phosphorus ylidic bond has more double bond character, which is in agreement with the greater nucleophilic reactivity of (2). P-Chloroalkylidenephosphorane (3) and (a-chloro-alkyl)phosphine (4) isomers undergo substituent-dependent interconversion by intramolecular 1,2-chlorine shift. ... 

Nucleophilic Displacement at Acetylenic Carbon.— Among studies of nucleophilic displacement at acetylenic carbon are the reactions of phosphines and amines with halogenoacetylenes. Using phosphines, displacement occurs by competitive attack on halogen and carbon the general order of reactivity in substitution at carbon by phosphine nucleophiles is sp <- sp > sp. In... 

In the oxidative Eschenmoser sulfide contraction (Scheme 11), thioamide 59 is oxidized by benzoyl peroxide to give either a symmetrical disulfide or the O-benzoate of the thiolactam-S-oxide. In any event, the once-nucleophilic thioamide sulfur atom is now forced to adopt the role of electrophile a reactivity umpolung has, in effect, been achieved.13 The nucleophilic enamide 65 attacks the sulfur atom leading to the formation of sulfur-bridged intermediate 66. The action of a phosphine or a phosphite thiophile on the putative episulfide then gives vinylogous amidine 67. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Recently, Fossum et al. prepared several phosphine-oxide-containing monomers (Scheme 6.23).163 These monomers were used to prepare hyperbranched polymers in a typical aromatic nucleophilic substitution. However, only oligomers with M lower than 2500 g/mol were obtained. These results did not surprise us, since our previous work demonstrated that the para-hydroxyl group of the phosphonyl group is not very reactive and would require higher reaction temperatures.11... 

The reaction temperature varies between -40 and 110 °C, depending on the reactivity of both counterparts, amine and chlorophosphane. As usual, aliphatic amino groups react faster than aromatic and heteroaromatic ones due to their greater nucleophilic strength. These differences in reactivity allow chemose-lective phosphinous amide formation, as that represented in Scheme 2 where the P-N bond is formed exclusively at the aliphatic NH2 group of 2 but not at the heteroaromatic NH2, whose lone pair is extensively delocalized in the electron-withdrawing purine ring [35]. 

B. Reactions.—(/) Nucleophilic Attack at Phosphorus. A reinvestigation of the reaction between phosphorus trichloride and t-butylbenzene in the presence of aluminium chloride has shown that the product after hydrolysis is the substituted phosphinic acid (11), and not the expected phosphonic acid (12). Bis(A-alkylamino)phosphines have been reported to attack chlorodiphenyl phosphine with nitrogen, in the presence of a base, to give bis-(A-alkyl-A-diphenylphosphinoamino)phenylphosphines (13). In (13), the terminal phosphorus atoms are more reactive than the central one towards sulphur and towards alkyl halides. 

Vinylsilane to copper transmetallation has entered the literature,93 93a,93b and a system suitable for catalytic asymmetric addition of vinylsilanes to aldehydes was developed (Scheme 24).94 A copper(l) fluoride or alkoxide is necessary to initiate transmetallation, and the work employs a copper(ll) fluoride salt as a pre-catalyst, presumably reduced in situ by excess phosphine ligand. The use of a bis-phosphine was found crucial for reactivity of the vinylcopper species, which ordinarily would not be regarded as good nucleophiles for addition to aldehydes. The highly tailored 5,5 -bis(di(3,5-di-tert-butyl-4-methoxyphenyl)phosphino-4,4 -bis(benzodioxolyl) (DTBM-SEGPHOS) (see Scheme 24) was found to provide the best results, and the use of alkoxysilanes is required. Functional group tolerance has not been adequately addressed, but the method does appear encouraging as a way to activate vinylsilanes for use as nucleophiles. 

The reaction of phosphines and alkyl halides presents an alternative way to generate phosphonium electrophiles (Scheme 3.8). In particular, the combination of a phosphine and carbon tetrabromide (the Appel reaction) allows for in situ formation of a phosphonium dibromide salt (48, X = Br). Treatment of a hemiacetal donor 1 with the phosphonium halide 48 initially provides the oxophosphonium intermediate 38 (X = Br). However, the oxophosphonium intermediate 38 can react with bromide ion to form the anomeric bromide intermediate 49 (X = Br) with concomitant generation of phosphine oxide. With the aid of bromide ion catalysis (i.e. reversible, catalytic formation of the more reactive P-anomeric bromide 50) [98], the nucleophile displaces the anomeric bromide to form the desired glycoside product 3. The hydrobromic add by-product is typically buffered by the presence of tetramethyl urea (TMU). 

A range of mechanisms is possible for the acidolysis of phosphorus amides, depending on the nucleophilicity of the departing amine. A recent study of phosphinic amides (160) in acidic media demonstrated that, when R2 is aryl, the presence of an o-Me group reduced the hydrolysis rate significantly, and also that the mechanism appears to be of an associative type.128 The phosphinic halides (161 X = Cl or F R = Me) are more reactive, probably for steric reasons, than the corresponding halides (161 R = Bu1) in 5 n2(P) solvolyses with aqueous acetone and with alkali. In the case of the t-butyl compounds, the fluoride is more reactive to OH- than is the chloride.129... 

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