Metalation, chlorophosphine

Exchange of chlorine by fluorine can be easily effected by reacting the chlorophosphine-metal complex with antimony trifluoride (250), arsenic trifluoride (281), zinc fluoride (281), potassium fluorosulfinate (281, 284, 295), or potassium fluoride in tetramethylene sulfone (250). For example,... 

By far the most ubiquitous intermediates in synthesis of this class of phosphines are the alkali metal phosphides which can be prepared by either the KOH/DMSO method, by reaction of tertiary phosphines or chlorophosphines with alkali metals, or in the reaction of BuLi with appropriate secondary or tertiary phosphines. A number of the ligands in Figures 6 and 7 were prepared this way (60-69,72-74). 

The general synthesis of the Daniphos ligands starting from enantiomerically pure [(R)-l-(phenylethyl)dimethylamine]chromiumtricarbonyl 1, is depicted in Scheme 1.4.1 [15]. A directed ortho-metallation (DOM) and subsequent quench with a chlorophosphine leads to an enantiomerically pure planar-chiral complex, which after chlorination using ACE chloride (1-chloroethyl chloroformate) is transformed into the desired diphosphine by a nucleophilic substitution without any loss of optical purity (Scheme 1.4.1) [6, 10]. 

Knowing all these facts, especially the difficult access to fluorophosphines and the poor donating abilities of phosphorus trifluoride (5, 6), we decided to use another approach, which readily led to a number of coordination compounds with fluorophosphine ligands—namely, the fluorination of chlorophosphines already coordinated to the transition metal, where the 3s electrons of phosphorus are blocked by the complex formation. There was no reaction between elemental nickel and phosphorus trifluoride, even under extreme conditions, whereas the exchange of carbon monoxide in nickel carbonyl upon interaction with phosphorus trifluoride proceeded very slowly and even after 100 hours interaction did not lead to a well defined product (5,6). 

At the time of our investigation the only known coordination compounds of chlorophosphines (aside from phosphorus trichloride complexes) were the nickel-(0) compounds, tetrakis(methyldichlorophosphine)nickel-(0) (20) and tetrakis-phenyldichlorophosphine) nickel- (0) (17). Tetrakis (methyldichlorophosphine) -nickel-(0) is noteworthy in that it represents a still rare example of the direct reaction of a ligand with an elemental transition metal to give a complex, while tetrakis (phenyldichlorophosphine) nickel- (0), like tetrakis (trichlorophosphine) -nickel-(0), was obtained readily via the carbonyl. AD chlorophosphine-nickel-(O) complexes, including the phosphorus trichloride complex, Ni(PCl3)4, are compounds relatively stable in the atmosphere, but show poor stability in almost any organic solvent, even under strictly anaerobic conditions. 

In the course of a study of the formation of fluorophosphoranes from chloro-phosphines (22) we observed one exception in the compound chloromethyldi-chlorophosphine, which reacted smoothly with antimony trifluoride to give the flammable fluorophosphine, C1CH2PF2, under conditions where many other chloro-phosphines were invariably converted into fluorophosphoranes. As this fluorophosphine is readily available, its interaction with a metal carbonyl derivative was studied, and cycloheptatriene molybdenum tricarbonyl, obtained from the reaction of molybdenum hexacarbonyl with cycloheptatriene (I, 2), was chosen as a starting compound. 

Direct Synthesis. Some chlorophosphines and arsines react directly with metallic nickel in the same way as carbon monoxide to form the tet-rasubstituted derivatives. This reaction was described initially for PCUMe and PCUPh by Quinn (156)] soon afterwards Maier (125) obtained analogous reactions with AsBr2Me and PBr2Me. 

X -systems, and other low coordination phosphorus species, in particular phosphenium ions, R2P , and phosphinidenes, RP . The reactions of phosphenium ions with isocyanides, 1,3-dienes and o-quinones, and amidines, have been investigated. The coordination chemistry of phosphenium ions also continues to stimulate interest.The thermal decomposition of phosphirene and phosphirane P-complexes provides a new approach for the synthesis of terminal phosphinidene complexes, e.g., (180), which can be trapped with a variety of reagents. Evidence of the formation of surface phosphinidene intermediates has been adduced in the heterogeneous dechlorination of alkyldi-chlorophosphines by magnesium metal at 600K. ... 

Mixed phosphine-fluorophosphine (160, 247, 164, 173, 174), phos-phite-fluorophosphine (174), tertiary amine-fluorophosphine (174), carbonyl-fluorophosphine (160, 173, 56, 58, 59), tertiary arsine-fluoro-phosphine (174,175), and stibine-fluorophosphine complexes are readily formed directly from the appropriate fluorophosphine-metal complex. Only in the case of carbon monoxide, chlorophosphines, and phosphites is it possible to completely displace all the fluorophosphine ligands from the metal. Some typical examples are quoted below ... 

Lithiation Reactions. One of the earliest reactions of this type made use of metal-halogen exchange reactions carried out on poly[bis(p-bromophenoxy)phosphazene]. Polyphosphazenes that bear p-bromophenoxy side oups are normally unreactive. However, they can be lithiated, as shown in Scheme III, and the lithio derivatives react with a wide variety of electrophiles that range from chlorophosphines (19) to organometallic halides (42-45), This provides an access route to polymer-bound transition metal catalysts and other metallated or silylated polymers. 

The final step in Scheme 69 is similar to that suggested for nickel or palladium-catalyzed cross-coupling of terminal alkynes with chlorophosphines to give phos-phinoalkynes. After P-Cl oxidative addition, Sonogashira-style formation of a metal-alkynyl group and P-C reductive elimination were proposed [123, 124]. 

A few papers have reported the use of various aluminium-based reducing agents in phosphine synthesis. Lithium aluminium hydride reduction of phosphine oxide precursors provides a route to new 2-phosphinomethyl-1/7-pyrroles and diisobutylaluminium hydride (DIBAL) has been found to be an excellent reagent for the reduction of phosphinites, phosphinates and chlorophosphines. 2-Chloroethylphosphine (CICH2CH2PH2) has been prepared for the first time by a chemoselective reduction of diethyl 2-chloroethylphosphonate with dichloroalane (HAICI2), prepared in situ from LiAlH4 and AlCls. A patent has described the use of metallic aluminium... 

The chlorophosphine boranes 97 are efficient starting reagents for the synthesis of various classes of P-chiral phosphorus compounds. Reactions of chlorophosphine boranes 97 with nucleophiles, such as carbanions, phenoxides, phenylthiolates, or amides, leads to the formation of corresponding organophos-phorus compounds 100-103 in yields of 53-99% and with up to 99% ee. This method was also used for the preparation of various classes of symmetric and asymmetric P-chiral ligands useful for asymmetric reactions, catalyzed by complexes of transition metals (Scheme 31) [52, 60, 61]. 

Methylphosphinite boranes react smoothly with organolithium reagents to afford the corresponding tertiary phosphine boranes, as will be discussed in the next section. However, phosphinite boranes are not electrophilic enough to react with other weaker nucleophiles such as alcohols, amines or thiols. More reactive precursors, capable of producing a wide variety of phosphorus compounds, were needed. In phosphorus chemistry halophosphines, and chlorophosphines 27 in particular, are essential synthons (Scheme 4.12) as nucleophilic (after transformation into metal phosphides 28) and electrophilic building blocks. 

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