Diphosphine sulfides

Almost no attention has been paid to diphosphine sulfides employed as chiral ligands for palladium-catalysed nucleophilic substitution reactions. In this context, enantiomerically pure diphosphine sulfides derived from 2,2 -biphosphole, which combined axial chirality and phosphorus chiralities, were synthesised, in 2008, by Gouygou et al. through a four-step synthetic sequence. Among various palladium catalytic systems derived from this type of ligands and evaluated for the test reaction, that depicted in Scheme 1.62... 

Scheme 1.62 Test reaction with 2,2 -biphosphole-derived diphosphine sulfide.

This procedure was applied to the preparation of mono- and diphosphine sulfides with the opposite configuration to those prepared by the standard method (Scheme 5.45). ... 

The trimethylsilyl derivative 109 was chosen because its optical purity can be easily increased by recrystallisation until a virtually enantiopure compound is obtained in 56% yield. Some screening showed that polyamines such as PMDETA in combination with. s-BuLi constitute the optimum conditions in order to minimise undesired functionalisation of the methylene group. Under these conditions, 109 was regioselectively lithiated and the carbanion further elaborated by known methods to afford, after a standard desilylation step, a-hydroxyphosphine sulfide 110 (55% yield, 94% ee) and p-hydroxyphosphine sulfide 111 (50% yield over two steps, 96% ee). The method was also adapted to yield diphosphine sulfides 112 (69% yield, 90% ee) and 113 (46% yield, 99% ee). The latter compound is indeed the disulfide of (5, 5)-t-Bu-MiniPHOS. During its synthesis it was observed that no meso compound was formed. 

Rob6 E, Perlikowska W, Lemoine C, Diab L, Vincendeau S, Mikolajczyk M, Daran J-C, Gouygou M (2008) Diphosphine sulfides derived from 2,2 -biphosphole novel chiral S,S ligands for palladium-catalyzed asymmetric allylic substitution. Dalton Trans 2894-2898... 

In contrast, synthesis of 3,4-diphosphorylthiophenes requires more elaboration because of low reactivity of 3,4-positions of thiophene and unavailability of 3,4-dihalo or dimetallated thiophenes. Minami et al. synthesized 3,4-diphosphoryl thiophenes 16 as shown in Scheme 24 [46], Bis(phosphoryl)butadiene 17 was synthesized from 2-butyne-l,4-diol. Double addition of sodium sulfide to 17 gave tetrahydrothiophene 18. Oxidation of 18 to the corresponding sulfoxide 19 followed by dehydration gave dihydrothiophene 20. Final oxidation of 20 afforded 3,4-diphosphorylthiophene 16. 3,4-Diphosphorylthiophene derivative 21 was also synthesized by Pd catalyzed phosphorylation of 2,5-disubstituted-3,4-dihalothiophene and converted to diphosphine ligand for Rh catalysts for asymmetric hydrogenation (Scheme 25) [47],... 

Other related compounds are the diphosphine disulfides R2P(S)P(S)R2 14, which contain a direct phosphorus-phosphorus bond, and polysulfur fe(dior-ganophosphine sulfides) R2P(S)-S -P(S)R2 in which the two phosphorus centres are connected by a chain of one of more sulfur atoms. R2P(S)P(S)R2 compounds are accessible from heating secondary phosphine sulfides with thiophosphinic chlorides (Equation 10) or by reaction of fephosphines with... 

The metal and ammonium salts of dithiophosphinic acids tend to exhibit far greater stability with respect to this thermal decomposition reaction, and consequently these acids are often prepared directly in their salt form for convenience and ease of handling. Alkali-metal dithiophosphinates are accessible from the reaction of diphosphine disulfides with alkali-metal sulfides (Equation 22) or from the reaction of alkali-metal diorganophosphides with two equivalents of elemental sulfur (Equation 23). Alternatively, they can be prepared directly from the parent dithiophosphinic acid on treatment with an alkali-metal hydroxide or alkali-metal organo reagent. Reaction of secondary phosphines with elemental sulfur in dilute ammonia solution gives the dithiophosphinic acid ammonium salts (Equation 24). 

For monosulfonation of PPh3 the reaction mixture can be heated for a limited time [1-3] while multiple sulfonation is achieved hy letting the solution stand at room temperature for a few days [4-10], In this simplest way of the preparation several problems may arise. Under the harsh conditions of sulfonation there is always some oxidation of the phosphine into phosphine oxide and phosphine sulfides are formed, too. Furthermore, selective preparation of TPPMS (1) or TPPDS (2) requires optimum reaction temperature and time and is best achieved by constantly monitoring the reaction by NMR [10] or HPLC [7]. Even then, the product can be contaminated with unreacted starting material. However, 1 can be freed of both the non-sulfonated and the multiply sulfonated contaminants by simple methods, and in the preparation of TPPTS (3) contamination with PPh3, 1 or 2 is usually not the case. Direct sulfonation with fuming sulfuric add was also used for the preparation of the chelating diphosphines 34-38, 51, 52. 

Preparation. - The diphosphines (223) can be oxidised to the related dioxides on treatment with oxygen or hydrogen peroxide. Treatment of the diphosphine (223, R = Ph) with an excess of sulfur in hot toluene affords the corresponding bis(sulfide). X-ray studies of representative dioxides and the above disulfide reveal considerable steric strain in the molecule, resulting in out of plane displacement of phosphorus atoms. " The reaction of (223, R = Ph) with an... 

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