Phosphine boranes arylation

As mentioned in Sect. 3.1.1, secondary phosphine-boranes also react efficiently with aryl iodides in palladium-catalyzed substitution reactions (Pd(PPh3)4) [73]. In all cases the boranato functional group remains unchanged. 

Nickel and palladium complexes also catalyze the formation of the carbon-phosphorus bonds in phosphorus(V) and phosphorus(III) compounds. Indeed, this chemistry has become a common way to prepare phosphine ligands by the catalytic formation of phosphine oxides and subsequent reduction, by the formation of phosphine boranes and subsequent decomplexation, or by the formation of phosphines directly. The catalytic formation of both aryl and vinyl carbon phosphorus bonds has been accomplished. 

Although more hydrolytically sensitive than the phosphine boranes, diorganochlorophosphines can be more accessible than diorganophosphines and are not pyrophoric. Thus, the reaction of a chlorophosphine with an aryl halide or aryl triflate in the presence of zinc as a reducing agent and (DPPE)NiCl2 as catalyst provides a convenient procedure for P—C coupling (Equation (49)).150 A related nickel-catalyzed process driven by electrochemical reduction has also been reported 151... 

Pd-catalyzed cross-coupling of secondary phosphine-boranes and aryl iodides has been carried out in ionic liquids using a ligand immobilized with a pyridinium substituent (Fig. 3). Catalyst recycling at least six times without significant loss of activity was possible [93]. 

Pd-catalyzed asymmetric arylation of a phosphine-borane, under kinetic resolution conditions, gave enantioenriched phosphine-borane 36. Slowing reductive elimination with a Pd-CsFs group enabled isolation and separation of the diaster-eomers of an analog of the key intermediate (Scheme 56) [94]. The stereochemical details of Pd-P bond formation and P-C reductive elimination, which both proceed with retention at phosphorus, had been elucidated earlier in related Pd(Chiraphos) complexes [95, 96]. 

Imamoto reported that Pd-catalyzed coupling of phosphine-borane with aryl halides is useful for preparation of asymmetric phosphines. Phosphines can be easily isolated from phosphine-boranes by exchange reaction with amines such as pyrrolidine and DABCO [10]. Lipshutz found that aryl nonaflates ( Nf = nonafluorobutanesulfonate) and triflates are good substrates for coupling with BHs-stabilized diaryIphosphines. Selective coupling with nonaflate without... 

Phosphorylation of At2PH-BH3 with aryl halides proceeds under mild conditions using Pd-Cu catalyst and P-chiral phosphine-boranes of high enantiopurity are prepared by this method. As an effective ligand, MePPh2 is used with Pd(OAc)2. The phosphine-borane 50 was prepared in 68 % yield by phosphorylation of optically active (Sp)-methylphenylphosphine-borane (47) via 48 with the iodide 49 in the presence of the Pd(OAc)2-MePPh2 catalyst and Cul as a cocatalyst at 0°C for 3 days. The reaction proceeded with retention of stereochemistry and the asymmetrically substituted phosphine 51 with 99% ee was obtained [15]. 

The arylation of secondary phosphines 201 with ortho-aiy iodides, catalyzed by generated in situ complex Pda (dba>3 x CHQ3, containing chiral ligand Et,Et-FerroTANE 207 and LiBr, led to the formatiOTi of corresponding tertiary phosphines with enantioselectivity of 90% cc [ 132,137]. The palladium complex 209 also showed high enantioselectivity in arylation of secondary phosphines [131,132]. Some examples of arylation reaction of secondary phosphines with low ee were described. The asymmetric arylation of phosphine boranes with anisyl iodide, catalyzed by chiral complex of oxazoline phosphine 208, led to the formation of enantiomerically enriched tertiary phosphines 206 with 45% ee [134]. The Pd complex 210 of (R )-t-Bu-JOSlPHOS ligand catalyzed arylation of PH(Me)(Ph)(BH3) by o-anisyl iodide with the formation of PAMP-BH3 with 10% ee (Table 3) [112]. 

Pican S, Gaumont A-C (2005) Palladium catalysed enantioselective phosphination reactions using secondary phosphine-boranes and aryl iodide. J Chem Soc Chem Commun 2393-2395... 

Table 4.4 gives a clear overview of the type of phosphine boranes that can be prepared with the Juge Stephan method. The method seems particularly well suited for the synthesis of phosphines containing at least one aryl group in extremely high enantioselectivities. These include a few dialkylarylphosphine boranes (entries 2-4 and 7), triarylphosphine boranes (entries 17, 18 and 35)... 

The optically pure secondary phosphine boranes are easily deprotonated, giving extremely nucleophilic phosphide borane anions, which preserve the absolute configuration at the P atom at low temperatures. This fact prompted Danjo, Imamoto and co-workers to couple the phosphide boranes derived from 9 with aryl cation equivalents. In particular, they found that fluoroarene tricarbonylchromium complexes such as 17 were excellent substrates for the SNAr process due to the high electron deficiency of the arene ring and significant electrophilicity of the ipso carbon (Scheme 5.9). 

Evans and co-workers described the preparation of 1,2-ftjXarylmethyl-phosphine borane)ethanes by oxidative coupling of anions derived from aryl-dimethylphosphine boranes with Cu(II) pivalate (Scheme 5.19). 

With phosphine boranes, a more complicated scenario was discovered by Imamoto and co-workers.Stereochemistry of the Pd-catalysed P-C bond formation ranges from almost complete retention to inversion, depending on the solvent and base used. A dramatic example of the effect of the solvent in the arylation of phosphinite borane 46 is shown in Scheme 6.21. 

More recently, Livinghouse and co-workers studied the effect of Cu(I) on Pd-catalysed arylation of secondary phosphine boranes. Apart from a marked beneficial effect on the coupling elSciency, they discovered that Cu(I) is able to suppress the base-mediated racemisation of the phosphide borane intermediate, to generate a configurationally stable metallophosphide 47 (Scheme 6.22). Consequently, tertiary phosphine boranes 48 were obtained with retention of configuration, in good yields and excellent enantioselectivities. 

Thanks to this general procedure, some classes of tetrathiahehcene-based alkyl and aryl phosphorous derivatives are now available, including phosphine—borane complexes (2015JO3921), phosphanes (2011EJO5649, 2013IC7995), phosphine oxides (2014EJO2694), and a diphosphonate (2011EJO5649). 

Imamoto and coworkers described phosphine-borane complexes as a coupling partner for the synthesis of aryl phosphines with aryl electrophiles using Pd(PPh3),j at room temperature (Scheme 20.68) [228,229], The borane moiety can be easily removed ljy excess use of diethyl amine or morpholine. Gaumont and coworkers demonstrated the palladium-catalyzed C—P cross-coupling in imidazolium-based ionic liquid and that the catalyst can be recycled up to six cycles [230]. 

Tetrasubstituted phosphinous amides of the type R2NPPh2 have been successfully arylated at phosphorus by the action of bromobenzene, in a process catalyzed by NiBr2, to give the aminophosphonium bromides [R2NPPh3] Br [109]. Other representative members of this class form phosphane-borane complexes [62], are aminated at phosphorus by chloramine to yield bis(amino)phos-phonium salts [110] and have been claimed to be protonated at phosphorus by ethereal tetrafluoroboric acid, as determined by NMR analysis [111]. 

In a modified version of the Suzuki reaction arylboronates or boranes are utilized instead of arylboronic acid. Under the action of phosphine-free palladium catalysts NaBPh4 and tra(l-naphtyl)borane were found suitable phenyl-sources for arylation of haloaromatics in fully or partially aqueous solutions at 20-80 °C with good to excellent yields (Scheme 6.12) [32-34]. Aryl halides can be replaced by water-soluble diaryliodonium salts, At2IX (X = HSO4, BF4, CF3COO) in the presence of a base both Ar groups take part in the coupling [35]. 

These nucleophilic reagents react with most common electrophiles such as organohalides, tosylates, aldehydes, ketones, epoxides, and activated alkenes. It should be noted that many workers have found much higher yields if the phosphides are protected as phosphine oxide or borane anions (see Section 3). Phosphide reagents also react with activated arenes to give mixed aryl phosphines (Protocol 2). Metal phosphides therefore provide an alternative, complementary tertiary phosphine synthesis to the electrophilic routes outlined in Section 2.1. 

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