Ether-borane complex

WORKED PROBLEM 9.15 Propose a structure for the ether—borane complex, and suggest a mechanism for its formation. 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

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]. 

The enantioselective deprotonation of the borane complex 248 of A-methylisoindoline was investigated by Simpkins and coworkers (eqnation 59) . Deprotonation with i-BuLi/(—)-sparteine (11) in diethyl ether at —78°C for 1 h, followed by quenching with chlorotrimethylsilane, yielded the silanes 251, ent-252, 252, ent-25 in a ratio of 86.3 0.4 6.3 7.0 after destroying the chiral centre at nitrogen by treatment of the whole mixture with triethylamine, an e.r. 253/ewf-253 of 86.7 13.3 is expected. 

In 2002, Grubbs and co-workers reported the first CM reactions of allyl phosphines.In an initial reaction, subjecting allyl diphenylphosphine to catalyst 5 (5 mol%) failed to produce any of the desired cross-product. However, by protecting the phosphine as its borane complex, CM reactions could be achieved in good yield with high E-selectivity (Equation (5)). Notably, catalyst 5 failed to dimerize borane-protected vinyl diphenylphosphine. This result was attributed to substrate trapping of the catalyst as an unreactive Fischer carbene, a situation analogous to that observed in the CM reactions of alkyl vinyl ethers. 

Enantioselective condensation of aldehydes and enol silyl ethers is promoted by addition of chiral Lewis acids. Through coordination of aldehyde oxygen to the Lewis acids containing an Al, Eu, or Rh atom (286), the prochiral substrates are endowed with high electrophilicity and chiral environments. Although the optical yields in the early works remained poor to moderate, the use of a chiral (acyloxy)borane complex as catalyst allowed the erythro-selective condensation with high enan-tioselectivity (Scheme 119) (287). This aldol-type reaction may proceed via an extended acyclic transition state rather than a six-membered pericyclic structure (288). Not only ketone enolates but ester enolates... 

Asymmetric aldol reactions.4 The borane complex 3 can also serve as the Lewis acid catalyst for the aldol reaction of enol silyl ethers with aldehydes (Mukaiyama reactions).5 Asymmetric induction is modest (80-85% ee) in reactions of enol ethers of methyl ketones, but can be as high as 96% ee in reactions of enol ethers of ethyl ketones. Moreover, the reaction is syn-selective, regardless of the geometry of the enol. However, the asymmetric induction is solvent-dependent, being higher in nitroethane than in dichloromethane. 

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). 

SCHEME 4. Total reductive hydrolysis of a galactan of the agar group containing 3,6-anhydro-L-galactose and its 2-methyl ether (borane-4-methylmorpholine complex in 2 M CF3C02H, 8 h at 100 °C, according to refs.213 214... 

Kabalka and co-workers have shown that an attenuated form of BI3, BI3 NEt2Ph, will cleave a variety of compounds containing C-O single bonds at elevated temperatures [20]. Solutions of this reagent are prepared by reacting the commercially available amine-borane complex with I2 in benzene at 80 °C for several hours. This reagent cleaves ethers [21], esters [20], and geminal diacetates [21]. Esters [20] are cleaved to an activated acyl intermediate RCOX which can be used to prepare acids, other esters, and amides (Eq. 10). 

The borane complex, [NaOEt2][Zn(BH )j], formed by reaction of xs NaBH on ZnClj in ether is stable to 80°C. Anionic complexes are synthesized ... 

The first studies on the reaction of olefins and acetylenes with diborane met with little success. In the gas phase, olefins react sluggishly over long periods at elevated T to give mixtures . Acetylene gives polymeric organoboranes . However, in ether solvents, the addition is fast and quantitative at 0°C The catalytic effect of ethers is attributed to the formation of weak, reactive borane complexes ... 

Addition to vinyl aminopropyl ethers. Boranes add to the double bond in the anti-Markovnikov sense under the influence of the Rh catalyst. However, intramolecular hydroamination to give tetrahydro-2-methyloxazine occurs in the presence of Pd or Pt complexes. 

In 1974,Mukaiyama and co-workers reported the first examples of Lewis acid-catalyzed Michael reactions between silyl enolates and a,p-unsaturated carbonyl compounds [33]. Evans and co-workers developed a catalytic asymmetric Michael reaction of silyl enol ethers of thiol esters to alkylidene malonates. For example, the reaction of alkylidene malonate 23 with 2.2 equiv of silyl enol ether 22 was carried out in the presence of 10 mol % of catalyst 25 and 2 equiv of hexa-fluoro-2-propanol (HFIP) in PhMe/CH2Cl2 (3 1) at -78 °C to give the expected adduct 24 in 93% ee (Scheme 5) [34]. Borane complex-catalyzed asymmetric Michael addition has also been reported [35]. 

Borane complexes can also be formed with other ethers, such as diethyl ether (as just discussed) or diglyme (diethylene glycol dimethyl ether).These complexes form readily because the ether, acting as a Lewis base (electron donor), can satisfy the electron-deficient boron atom, which acts as a Lewis acid (electron acceptor). Borane reacts rapidly with water, and therefore procedures using the BHs-THF complex must be conducted under anhydrous conditions. 

However, BH3 forms stable Lewis acid-base complexes with ethers. Borane is most often used as a commercially available solution of BH3 in THF. 

The insoluble polymeric pyridine-borane system (20) has been reported to reduce aldehydes and ketones in high yields at room temperature in the presence of boron trifluoride-etherate, whereas acid derivatives and a,/3-enones are not reduced. The polymer is used in benzene, and reducing power is better if the polymer beads are swollen in benzene before use. The corresponding polymeric 4-pyridyl-borane (21) is sluggish in reductions but reactivity can be increased using a borane complex of the quaternary polymer (22), which is assumed to... 

Borane—dimethyl sulfide complex (BMS) (2) is free of these inconveniences. The complex is a pure 1 1 adduct, ca 10 Af in BH, stable indefinitely at room temperature and soluble in ethers, dichioromethane, benzene, and other solvents (56,57). Its disadvantage is the unpleasant smell of dimethyl sulfide, which is volatile and water insoluble. Borane—1,4-thioxane complex (3), which is also a pure 1 1 adduct, ca 8 Af in BH, shows solubiUty characteristics similar to BMS (58). 1,4-Thioxane [15980-15-1] is slightly soluble in water and can be separated from the hydroboration products by extraction into water. 

The products are Hquids, soluble in various solvents and stable over prolonged periods. Monochloroborane is an equiUbtium mixture containing small amounts of borane and dichloroborane complexes with dimethyl sulfide (81). Monobromoborane—dimethyl sulfide complex shows high purity (82,83). Solutions of monochloroborane in tetrahydrofuran and diethyl ether can also be prepared. Strong complexation renders hydroboration with monochloroborane in tetrahydrofuran sluggish and inconvenient. Monochloroborane solutions in less complexing diethyl ether, an equiUbtium with small amounts of borane and dichloroborane, show excellent reactivity (88,89). Monochloroborane—diethyl etherate [36594-41-9] (10) may be represented as H2BCI O... 

Free borane (2) exists as gaseous dimer—the diborane BaHg. In addition Lewis acid/Lewis base-complexes, as for example formed in an ethereal solvent, e.g. 4, are commercially available ... 

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