Tetrahydrofuran Lewis base

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). 

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

Many organic syntheses requHe the use of stericaHy hindered and less nucleophilic bases than //-butyUithium. Lithium diisopropylamide (LDA) and lithium hexamethyldisilazide (LHS) are often used (140—142). Both compounds are soluble in a wide variety of aprotic solvents. Presence of a Lewis base, most commonly tetrahydrofuran, is requHed for LDA solubdity in hydrocarbons. A 30% solution of LHS can be prepared in hexane. Although these compounds may be prepared by reaction of the amine with //-butyUithium in the approprite medium just prior to use, they are also available commercially in hydrocarbon or mixed hydrocarbon—THF solvents as 1.0—2.0 M solutions. 

Germenes readily form complexes with such Lewis bases as diethyl ether,11,51 tetrahydrofuran,52 and triethylamine.51 The germanimine18 and... 

Since Lewis base additives and basic solvents such as tetrahydrofuran are known to deaggregate polymeric organolithium compounds, (21,23,26) it was postulated that ketone formation would be minimized in the presence of sufficient tetrahydrofuran to effect dissociation of the aggregates. In complete accord with these predictions, it was found that the carbonation of poly(styryl)lithium (eq. 9), poly(isoprenyl)-lithium, and poly(styrene-b-isoprenyl)lithium in a 75/25 mixture (by volume) of benzene and tetrahydrofuran occurs quantitatively to produce the carboxylic acid chain ends (8 ). 

The typical B-NMR signals of iminoboranes are essentially unaltered when iminoboranes are dissolved in liquids with Lewis base activity (e.g., tertiary amines, tetrahydrofuran) (19). I conclude that equilibria like Eq. (28) are shifted far to the left, even in the presence of an excess of the Lewis base D. 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

The behavior toward Lewis bases was studied. The compound [R2A10CR NPh]2 did not form a stable complex with Lewis bases such as pyridine, tetrahydrofuran and triethylamine, as evidenced by IR spectroscopy, but did form with a strong bases such as trimethyl-amine oxide, Me3NO and hexamethylphosphoramide (Me2N)3PO. This fact means that the Lewis acidity of this organoaluminum is rather weak, because A1R3 forms the stable complex with a relatively weak electron donor compound such as diethyl ether and tetrahydrofuran, and R2A1NR2 with an electron donor such as triethylamine. 

The behavior of the aldehyde complex toward Lewis bases was examined. The acetaldehyde moiety in the acetaldehyde complex Me2A10CPhNPh MeCHO was not displaced by a large excess of Lewis base such as pyridine or tetrahydrofuran, but was replaced by a strong electron donor substance such as trimethylamine oxide to give a crystalline trimethylamine oxide complex, Me2A10CPhNPh ONMe3, which is identical to that obtained from trimethylamine oxide and [Me2AlOCPhNPh]2. 

Few detailed studies have been made on the effect of Lewis bases on the propagation rate, but the effect of tetrahydrofuran is known for... 

The Tebbe reagent functions as a nucleophilic carbenoid in its reactions with carbonyl groups. The carbenoid is activated in the presence of a Lewis base which presumably complexes with the aluminum atom. Tetrahydrofuran is the Lewis base in the reactions described above. If the reaction is performed in the absence of added tetrahydrofuran, the carbonyl oxygen atom can function as a weak Lewis base, although the methylenation process is considerably slower. 

In the case of germanium and tin, reactions such as (162) lead to the production of GeMe - or SnMejj-metal complexes, stabilized by electron donation from the Lewis base, tetrahydrofuran (315). 

Lappert and coworkers also found that such rare-earth metal aluminate complexes can be readily transformed into methyl derivatives by addition of equimolar amounts of donor Lewis base molecules such as pyridine or tetrahydrofuran (Lappert s concept of donor-induced aluminate cleavage, Scheme 1) [10]. Depending on the Lewis acidity of the Ln(III) center homo-... 

It seems likely that tetrahydrofuran, acting as a Lewis base toward silicon centers, promotes charge separation and incipient formation of carbonylate anions, just as described in Section III,D. The oxygen atoms in coordinated carbonyl groups are thereby made more nucleophilic, leading to formation of Si- 0- C-M links, as discussed above. Consistent with this, Cl3SiCo(CO)4 is unchanged after a solution of it in tetrahydrofuran has been refluxed it is also known to be unreactive towards Lewis bases such as NMea (Table XIII, entry 26) and Co(CO)4 (336). 

In the case of the Tebbe reagent, the titanium methylidene is presumably released by some Lewis base. That base is often the heteroatom of the substrate. A Lewis base such as tetrahydrofuran or pyridine may be added to the reaction and usually enhances reactivity relative to reaction with the substrate alone- ... 

Borane (BH3) is a reactive gas that exists mostly as the dimer, diborane (B2H6). Borane is a strong Lewis acid that reacts readily with Lewis bases. For ease in handling in the laboratory, it is commonly used as a complex with tetrahydrofuran (THF). 

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