Borane sulfides

Various polymer-supported hydrides have been applied successfully to reductions of both carbonyl and olefin groups. Rajasree and Devaky13 describe a cross-linked polystyrene-supported ethylenediamine borane reagent for the selective reduction of aldehydes in the presence of ketones (entry 9). This borane reagent is easily prepared and can be recycled after completion of the reaction. This is a practical alternative to standard borane reagents such as diborane, borane-amine, or borane-sulfide complexes. 

Odorless borane-sulfide complexes 36a-e65-68 and borane-amine complexes 37a-g69-75 have been recently prepared because of the growing importance of diborane for the synthesis of pharmaceuticals or other compounds and certain inconveniences of well-established reagents, for example, the low concentration and instability of BH3-THF and high volatility, flammability, and unpleasant odor of dimethyl sulfide from BH3-SMe2. Trialkylsilanes, for example, Me3SiH, were found to be an efficient reducing reagent for the in situ preparation of... 

The desired pyridylamine was obtained in 69 % overall yield by monomethylation of 2-(aminomethyl)pyridine following a literature procedure (Scheme 4.14). First amine 4.48 was converted into formamide 4.49, through reaction with the in situ prepared mixed anhydride of acetic acid and formic acid. Reduction of 4.49 with borane dimethyl sulfide complex produced diamine 4.50. This compound could be used successfully in the Mannich reaction with 4.39, affording crude 4.51 in 92 % yield (Scheme 4.15). Analogous to 4.44, 4.51 also coordinates to copper(II) in water, as indicated by a shift of the UV-absorption maximum from 296 nm to 308 nm. 

Pyridyl)hydrazine (Aldrich), 4-acetylpyridine (Acros), N,N,N -trimethylethylenediamine (Aldrich), methylrhenium trioxide (Aldrich), InQj (Aldrich), Cu(N0j)2-3H20 (Merck), Ni(N03)2-6Il20 (Merck), Yb(OTf)3(Fluka), Sc(OTf)3 (Fluka), 2-(aminomethyl)pyridine (Acros), benzylideneacetone (Aldrich), and chalcone (Aldrich) were of the highest purity available. Borane dimethyl sulfide (2M solution in THE) was obtained from Aldrich. Methyl vinyl ketone was distilled prior to use. Cyclopentadiene was prepared from its dimer immediately before use. (R)-l-acetyl-5-isopropoxy-3-pyrrolin-2-one (4.15) has been kindly provided by Prof H. Hiemstra (University of Amsterdam). 

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). 

Retardation of the reaction rate by the addition of dimethyl sulfide is in accord with this mechanism. Borane—amine complexes and the dibromoborane—dimethyl sulfide complex react similarly (43). Dimeric diaLkylboranes initially dissociate (at rate to the monomers subsequentiy reacting with an olefin at rate (44). For highly reactive olefins > k - (recombination) and the reaction is first-order in the dimer. For slowly reacting olefins k - > and the reaction shows 0.5 order in the dimer. 

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. 

Monohalogenoboranes are conveniendy prepared from borane—dimethyl sulfide and boron trihahdes (BX where X = Cl, Br, I) by redistribution reaction, eg, for monochloroborane—dimethyl sulfide [63348-81-2] (9) (81—83). Other methods are also known (84—87). 

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... 

Dihalogenoboranes are conveniently prepared by the redistribution of borane—dimethyl sulfide with boron trihaUde—dimethyl sulfide complexes (82,83), eg, for dibromoborane—dimethyl sulfide [55671-55-1] (14). 

Dibromoborane—dimethyl sulfide is a more convenient reagent. It reacts directly with alkenes and alkynes to give the corresponding alkyl- and alkenyldibromoboranes (120—123). Dibromoborane differentiates between alkenes and alkynes hydroborating internal alkynes preferentially to terminal double and triple bonds (123). Unlike other substituted boranes it is more reactive toward 1,1-disubstituted than monosubstituted alkenes (124). 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Certain base adducts of borane, such as triethylamine borane [1722-26-5] (C2H )2N BH, dimethyl sulfide borane [13292-87-OJ, (CH2)2S BH, and tetrahydrofuran borane [14044-65-6] C HgO BH, are more easily and safely handled than B2H and are commercially available. These compounds find wide use as reducing agents and in hydroboration reactions (57). A wide variety of borane reducing agents and hydroborating agents is available from Aldrich Chemical Co., Milwaukee, Wisconsin. Base displacement reactions can be used to convert one adduct to another. The relative stabiUties of BH adducts as a function of Group 15 and 16 donor atoms are P > N and S > O. This order has sparked controversy because the trend opposes the normal order estabUshed by BF. In the case of anionic nucleophiles, base displacement leads to ionic hydroborate adducts (eqs. 20,21). 

Borane complexes are the most widely used commercial boron compounds, after sodium borobydride. Examples used in organic synthesis are amine borane complexes and borane complexes of tetrahydrofuran and dimethyl sulfide. 

Reduction of 3,5,5-tris-aryl-2(5// )-furanones 115 (R, R, R = aryl) with dimethyl sulfide-borane led to the formation of the 2,5-dihydrofurans 116 in high yields. However, in the case of 3,4-diaryl-2(5//)-furanones 115 (R, R = aryl R = H or r = H R, R = aryl), the reduction led to a complicated mixture of products of which only the diarylfurans 117 could be characterized (Scheme 36) (88S68). It was concluded that the smooth conversion of the tris-aryl-2(5//)-furanones to the corresponding furan derivatives with the dimethylsulfide-borane complex in high yields could be due to the presence of bulky aryl substituents which prevent addition reaction across the double bond (88S68). 

The oxazaborolidines are easily prepared by heating ephedrine with borane dimethyl sulfide or the appropriate boronate ester. The aluminum reagent C is obtained by mixing ephedrine and trimethylaluminum. Borolidinc A is superior to its methyl derivative B and to the aluminum analog C. The diastereomeric borolidine obtained from borane and (S,S)-pseu-doephedrine failed to show any cnantioselectivity25. A variety of aromatic aldehydes can be enantioselectively alkylated in the presence of A, however, with heptanal the enantioselectivity is poor25. 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

Borane-methyl sulfide complex (neat) was purchased from Aldrich Chemical Company, Inc. and was used as received. 

Scheme 17 Reduction of nitriles using borane dimethyl sulfide and microwave irradiation...

Aldehydes and ketones have been converted to sulfides by treatment with thiols and pyridine-borane, RCOR -I- R"SH —+ RR CHSR", in a reductive alkylation reaction, analogous to 16-6. 

P-Chirogenic phosphine/sulfide hybrid phosphine-boranes 80 were synthesized from the reaction between (l )-tosylates 79 [94] and sodium thiolate in DMF at ambient temperature as depicted in Scheme 12, or alternatively by a one pot synthesis consisting of the nucleophilic attack of the chirally induced hthium salt of 74 on phenyl disulfide. Both methodologies provided the desired sul-fide/phosphine boranes in excellent yields [10]. 

In aprotic solvents that can act as electron pair donors such as ethers, tertiary amines, and sulfides, borane forms Lewis acid-base adducts. 

Borane dissolved in THF or dimethyl sulfide undergoes addition reactions rapidly with most alkenes. This reaction, which is known as hydroboration, has been extensively studied and a variety of useful synthetic processes have been developed, largely through the work of H. C. Brown and his associates. 

A novel, unprecedented sulfoxide-directed borane reduction of the a,P-unsaturated sulfoxide to the saturated sulfide was discovered. 

The kinetics of the reaction was measured by NMR studies and the reaction was found to be first order relative to borane-dimethyl sulfide and to the substrate (Figure 5.3). This result was consistent with a bimolecular reaction in the rate-limiting step. 

Primary amino methylene substituents were introduced by a sequence of cya-nodehalogenation and subsequent reduction of the resulting nitrile with borane dimethyl sulfide. To incorporate tertiary aminomethylene substituents into the 2-pyri-done framework, a microwave-assisted Mannich reaction using preformed iminium salts proved to be effective. 

Diazadiphosphacyclooctane (51) interacts with only two molecules of borane with participation of the phosphorus atoms [Eq. (50)]. The number of borane molecules added is likely to be determined by the conformation of the eight-membered ring, which exists in a distorted boat-boat form similar to that of the analogous sulfide. 

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