Phosphine amide

Acryhc esters dimerize to give the 2-methylene glutaric acid esters catalyzed by tertiary organic phosphines (37) or organic phosphorous triamides, phosphonous diamides, or phosphinous amides (38). Yields of 75—80% dimer, together with 15—20% trimer, are obtained. Reaction conditions can be varied to obtain high yields of trimer, tetramer, and other polymers. 

The Chemistry of Phosphinous Amides (Aminophosphanes) Old Reagents with New Applications... 

Abstract This chapter is devoted to phosphinous amides, a particular class of tervalent aminophosphanes. First, attention is focused on their stability and synthetic procedures. Reports dealing with their prototropic equilibrium and main group chemistry are also considered. Last but not least the really important applications of these species as metal ligands in the field of catalysis are reviewed, including asymmetric variants. 

Keywords Phosphinous amides Aminophosphanes Phosphanamines Catalysis... 

Tervalent organophosphorus compounds containing one single P-N bond with the valency of each atom saturated by protons or carbons (but no other heteroatoms) have been known since their discovery by MichaeUs more than one century ago [ 1 ] and named indistinctly as aminophosphanes, phosphanamines, phosphazanes, or phosphinous amides. This last chemical nomenclature is the one used by the Chemical Abstracts Service (CAS) for indexing these compounds and is also the one that best delimits the scope of this review those species derived from the parent H2P-NH2 (phosphinous amide in CAS nomenclature) by partial or total substitution of protons by hydrocarbon radicals (Table 1). 

Between all the classes of substituted phosphinous amides summarized in Table 1, those with the P atom totally substituted are more stable than others with H-P bonds, as in many other classes of organophosphorus compounds. Between those, the trisubstituted or fully substituted (typed in boldface) are by far the most stable and the main actors of the chemistry described in the following pages. 

Also covered by this review are those compounds bearing two or three phosphane units at the same N atom, that is those derived from (H2P)2NH (N-phosphino phosphinous amide in CAS nomenclature) and from (H2P)3N, NJsl-bis(phosphino) phosphinous amide, but not those bearing more than one amino... 

This chapter will also deal with compounds containing two or three phosphinous amide units, which, for simpUcity, will be named here as bis(amino-phosphanes) or tris(aminophosphanes) but not with phosphinous amides containing other additional organophosphorus functionaUties as, for instance, the so-called aminophosphine phosphinites (AMMP), which have been the subject of increasing attention in the Uterature dealing with catalytic asymmetric transformations and have been treated in other reviews [2,3]. 

This chapter will not cover compounds containing heteroatoms hnked to the P and/or N atom of the P-N unit, with the notorious exception of those with N-Si bonds (A/-silyl phosphinous amides) due to its particular relevance in terms of chemical reactivity. 

The stability of phosphinous amides depends, to a large extent, on the substituents at phosphorus and nitrogen. Normally, tetrasubstituted and N,P,P-trisubstituted phosphinous amides are stable and well-known compounds. The parent compound H2PNH2 is a volatile compound that is formed on hydrolysis of a solid state solution mixture of magnesium phosphide and magnesium... 

M,JV-Bis(phosphino) phosphinous amides N(PR2)3, commonly called triphos-phinoamines, are still virtually unknown, with the exceptional reported existence of N(Pp2)3 and N[P(CH3)2]3 [15,19,20]. The usual synthetic approaches to N(PPh2)3 give instead the isomeric Ph2P-P(Ph2)=N-PPh2 [25]. 

The P-N bond in phosphinous amides is essentially a single bond, so the lone pairs on N and P are available for electrophiUc reagents and for donor bonding towards metal atoms. Proton addition to the N atom of HjPNHj has been calculated to loosen the P-N bond, whereas protonation at P renders this bond stronger than in the parent molecule [26]. NH-Phosphinous amides are practically not associated by intermolecular hydrogen bonds [27]. 

Hindered rotation around the P-N bond has been observed at low temperature in tetrasubstituted phosphorus amides [28]. For PhjPNJSiMCjJj, two different Me3Si groups are observed below -65 °C, the calculated activation energy for P-N rotation being 10.2 Real moT [29]. Chiral phosphinous amides with stereogenic phosphorus atoms have been prepared [30,31 ]. 

Among the routes for preparing phosphinous amides, the most frequently used method is the aminolysis of halophosphanes, most usually chlorophosphanes [32-34], because a number of such halophosphanes are easily accessible from commercial sources. These reactions usually provide the target species, i.e., trisubstituted compounds 1 in Scheme 1, in high yield. The HCl liberated from the reaction forms a salt with an organic base (either excess of the starting amine or externally added as, for example TEA or DBU, sometimes in the presence of DM AP) which is insoluble in the reaction solvent, typically diethyl ether... 

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]. 

The generalized application of the aminolysis of halophosphanes has been the method of choice for the preparation of a wide variety of chiral phosphinous amides by starting from enantioenriched primary amines [36]. The aminolysis reaction occurs efficiently even when the halophosphane is placed in the coordination sphere of a metal, as in the palladium and platinum complexes of the type ds-M(Ph2PCl2)2Cl (M=Pd, Pt) [37,38]. 

Not only N-H bonds from amines can participate in the aminolysis reaction, but also less nucleophilic urea, thiourea and biuret NH units can react with halophosphanes in an effective manner, forming the corresponding phosphinous amides with additional functionalities at the nitrogen atom [39-44]. 

N-Unsubstituted imines have been similarly converted into AT-alkylidene phosphinous amides, as 3, in reactions run in the presence of triethylamine [45] (Scheme 3). 

An alternative method for preparing phosphinous amides makes a profit on the high affinity between silicon and halogen atoms. This is the driving force of the reactions between halophosphanes and Ar-(trimethylsilyl)anilines, AT-(tri-methylsilyl)amides or AT-(trimethylsilyl)ureas and thioureas, as represented in the Scheme 6. In these processes the desired P-N bond and an halosilane are simultaneously formed [53,58-60]. 

Species 5 (Scheme 8), commonly known as dialkylaminodifluorophosphines, are readily synthesized via the selective cleavage of the phosphorus-carbon bond of difluoro(trichloromethylphosphane) by the action of secondary amines [65,66]. Compounds 5 show selective F/H exchange with LiAlH4/HN(Tr)2 to give the respective PH2 (P-unsubstituted) phosphinous amides [13]. 

The amino interchange reaction is another method commonly used for preparing phosphinous amides [67] (Scheme 9). The low boiling points of di-methylamine and diethylamine allow their displacement from Ar,AT-dimethyl and AT,iV-diethyl phosphinous amides, respectively, by other less volatile amines, leading to new members of the same class. High reaction temperatures are nevertheless required. 

Phosphinous amides bearing protons at the nitrogen atom, that here we will call NH phosphinous amides, such as 6, may be involved in prototropic equilibria with their PH phosphazene forms 7 [18,68-70] (Scheme 10). This kind of prototropic equilibria, paralleling that between phosphinous acids R2P-OH and phosphane oxides R2P(0)H [4], have been evidenced in some particular... 

This prototropic equilibrium has been also studied in substrates bearing metals linked to the nitrogen atom in the form of CP2MCI or CpMCl2 groups [76-78], and the influence of different ligands at the metal center on the resulting equilibrium between the kinetically favored NH phosphinous amide and the thermodynamically stable tautomeric PH phosphazene form has been discussed (Scheme 11). 

Phosphinous amides of general structure R R PNHR are easily converted to their respective anions by metals or bases. For instance, they can be easily deprotonated by alkyllithium reagents to give the [R R PNR ] anions (8-A or 8-B, illustrated in Scheme 12). 

These phosphinous amide anions are presumably responsible for the formation of the by-products AT-phosphino phosphinous amides 11 and mono-phosphazenes derived from diphosphanes 12 in the sequential treatment of primary amines with n-BuLi and chlorophosphanes for preparing NH phosphinous amides [75,88] (Scheme 14). Compounds 11 and 12 are presumably derived from anions 9 and 10, respectively, generated by deprotonation of the newly formed phosphinous amide with the lithiated amine R NHLi. In solution, 9 can establish a metallotropic equilibrium with 10. 

A number of NH phosphinous amides have been P-alkylated by previous conversion to their corresponding anions [59,74]. A particular case of double alkylation takes place with the anion derived from the AT-phosphino phosphinous amide NH(PPh2)2 yielding the diphosphonium salt 17 [102] (Scheme 17). When neutral, its methylation is reported to give the P-H phosphazene-phos-phonium salt 18 [103]. 

NH-Phosphinous amides are also alkylated at phosphorus by electrophilic olefins, such as acrylonitrile and acrylamide, with concomitant formation of a... 

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