Phosphine borane complexes

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

Standard cyclisation methodology was used to access the cyclic monophosphinic acid derivative 78 by reaction of ammonium phosphonate and ethyldiisopropylamine, followed by the addition of chlorotrimethylsilane, with 2,2 -bis (bromomethyl)-l,l -biphenyl. Silane reduction of 78 gave the secondary phosphine. The secondary phosphine borane complex 79 could be used in alkylation or Michael addition reactions. For example the Michael adduct 80 was produced in high yield by treatment of 78 with a NaH suspension in THF followed by the addition of diethylvinylphosphonate . 

Tris(trimethylsilyl)silane reacts with phosphine sulfides and phosphine selen-ides under free radical conditions to give the corresponding phosphines or, after treatment with BH3-THF, the corresponding phosphine-borane complex in good to excellent yields (Reaction 4.45) [82]. Stereochemical studies on P-chiral phosphine sulphides showed that these reductions proceed with retention of configuration. An example is given in Reaction (4.46). 

The rhenium complex 76 related to 74b was also prepared recently by Labinger and Bercaw using another synthetic strategy. 2 In this case, the pendant borane moieties were introduced by hydroboration of unsaturated phosphines in the coordination sphere of the metal. The cationic rhenium complex 75 featuring two diphenyl(vinyl)phosphines was readily converted into the corresponding bis(phosphine-borane) complex 76 (Scheme 45). The coordination mode of 76 was substantiated spectroscopically (5 nB = 87.7 ppm) and crystallographically. 

The residue obtained after evaporation of the solvents was purified by flash chromatography (hexanes ether CH2CI2, 17 1 2) affording the phosphine-borane complex as a white solid (0.93 g, 70%). Characterize the product by 1H NMR, 13C NMR, 31P NMR, IR spectroscopy, mass spectrometry, elementary analysis, and determine the optical rotation. [a]D25 = -38.2° (c = 3.55, CHCI3). 

The enantiotopic methyl groups of the phosphine-borane complexes 429 and 430 can similarly be desymmetrised by rc-BuLi-(-)-sparteine. On coupling and deprotection, valuable chiral diphosphines 431 are formed.185... 

Many phosphine-borane complexes Y3P BZ3 have been characterized. They include compouuds where Y = alkoxy, aUcyl, amino, halide, and hydride groups, and Z = alkyl, halide, and hydride. The stabilities of these complexes vary widely depending on the Lewis acidity and basicity of the boron and phosphorus moieties, respectively. The relative stabilities of Lewis acid-base complexes with BH3 are R3P > R3N > R3AS > R3Sb, but with BF3 the order is R3N > R3P > R3AS > RsSb. The stabihties of the borou halide complexes of phosphines follow the same order as the amine complexes BI3 > BBrs > BCI3 > BF3. 

The xanthate required to implement the transformation was synthesized by first treating alcohol 134 with NaH and CS2, followed by the addition of Mel to give a 98% yield of the desired product (Scheme 34). Reaction of the xanthate with tributyl phosphine-borane complex and AIBN in refluxing dioxane gave the reduced product 135 in a 69% yield. 

Use of Chiral Phosphine-Borane Complexes in the Enantioselective Borane Reduction of Ketones. 84... 

Barton and his students [24] and, more recently, by other groups [25], The second transformation in Scheme 12 illustrates the conversion of a tartrate-derived thio-carbonate into a malate using hypophosphorous acid [26]. Phosphine-borane complexes have also been proposed as convenient hydrogen atom donors for the deoxygenation [27],... 

The chemistry of boron-phosphorus compounds has been reviewed. Numerous boron-phosphorus derivatives have been reported, but relatively few boron-arsenic or boron-antimony compounds have been described. Boron-phosphorus compounds are similar in many ways to boron - nitrogen derivatives, but the tendency to share bonding electrons in covalent tetrahedral compounds is much more evident with phosphorus than with nitrogen. In fact, most boron-phosphorus chemistry involves tetrahedral boron. They are typically either phosphine-borane complexes, such as RgP-BRj, or phosphinoboranes (R2PBR 2) , cyclic or polymeric derivatives of the hypothetical H3P BH3. The chemistry of these compounds and that of boron phosphate and thiophosphate is described below. Boron phosphides are discussed in Section 2.6. 

Dephenylation of phosphine-borane complexes. A P-phenylphospholane-borane complex is converted to the P-lithio reagent by treatment with lithium. The latter species are used for the synthesis of chiral ligands. 

There are no reports on the enantioselective reaction of the carbanion a to non-activated phosphines due to the low acidity of their a-protons. However, formation of phosphine-borane complexes enables deprotonation at the a position [83]. Evans and coworkers have foimd that enantiotopic methyl groups of phos-phine-boranes can be efficiently discriminated by s-BuLi-(-)-sparteine [Eq. (30)] [84]. This result is in accord with many other reports that the chiral complex composed of s-BuLi and (-)-sparteine is generally the most efficient enantioselective deprotonating combination [1,2,3,85]. However, in the reaction of phosphine sulfides this is not the case. The n-BuLi-(-)-sparteine combi-... 

Kobayashi and coworkers have demonstrated that optically active phos-pholane-2-carboxylic acid [n=l,Eq. (33)] and phosphorinane-2-carboxylic acid (n=2) can be prepared by asymmetric carboxylation of the phosphine-borane complexes. High trans selectivity and excellent enantiomeric excesses of the trans isomers were achieved on treatment of 1-phenylphospholane-borane and 1-phenylphosphorinane-borane complexes with s-BuLi-(-)-sparteine and subsequently with CO2. Notably, enantioselectivity was improved up to 92% ee when 1-phenylphospholane-borane was treated with s-BuLi-(-)-sparteine at 25°C, compared with 83% ee at -78°C [87]. 

Chiral phosphines are widely used as auxiliaries for various metal-catalyzed asymmetric reactions and can be prepared from stable phosphine-borane complexes. Secondary P-chiral phos-phine-boranes can be prepared by reductive lithiation of the corresponding tertiary phosphine-borane using LN (eq Likewise, P-chiral tertiary phosphine ligands can be produced by the reductive lithiation of phosphinite-boranes followed by alkylation, both proceeding with retention of configuration (eq 18). ... 

It is difficult to know exactly how to draw the bonding in metal complexes and there are often several different acceptable representations. There is no problem when the metal forms a a bond to atoms such as Cl or C as the simple line we normally use for covalent bonds means exactly what it says. The problems arise with ligands that form a bonds by donating both their electrons, and with Jt complexes. Everyone writes phosphine-borane complexes with two charges but we normally draw the same sort of bond between a phosphine and, say, Pd as a simple line with no charges. 

Such phosphine borane complexes have been tested in the catalytic asymmetric allyhc alkylation of l,3-diphenylpropene-2-yl acetate 120 with dimethyl malonate as nucleophile (Scheme 32). It was fbimd that the enantioselectivities strongly depend on the bite angle which is deeply affected by the nature of the spirocychc spacer and the direction of the coordinating phosphine groups. Representative X-ray stmctures of bisphosphine boranes 122 and 123 are shown in Scheme 32. Bisphosphine 119 afforded up to 74% ee while only 34% and 45% ee were obtained from the reactions using 122 and 123, respectively. 

A quite new class of catalysts based on early main group metals (Ca, Sr, and K) was recently reported to promote general conversion of conjugated double bond (137). The catalytic reaction is initiated by the formation of a highly reactive metal hydride that adds either to an alkene or to a silane. The regiochemistry for the hydrosilylation of 1,1-diphenylethylene (DPE) catalyzed by calcium complex can be completely controlled by the polarity of the solvent. Amine borane and phosphine borane complexes were successfully used as effective catalysts for hydrosilylation of organic compounds with internal unsaturated bond (138) that cannot be selectively hydrosilylated in the presence of Pt catalysts. 

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]. 

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