Diethyl- borane

Synthesis and Dynamic Behaviour of l-Indenyl(diethyl)borane... 

Lithium 1-alkynylboronates are attacked by many electrophiles at the position P to the boron atom. The subsequent rearrangement gives a variety of functionalized 1-alkenylboronanes (eq (28)). The stereochemistry can be either " or Z, or a mixture of the two [15]. The reaction with chloro(diethyl)borane or chloro(trimethyl)silane and -stanane stereoselectively produces c /5-bimetallic compounds [41]. 

Both isomers of perhydro-9b-boraphenalene (3a,b) are inert to water and probably to alcohols at ambient temperature. However in the presence of pivaloyl(diethyl)borane, c/5,n-a 5-perhydro-9b-boraphenalene (3b) reacts with water at room temperature to give anhydride, (C,2H22B)20 (29) (Equation (3)) <74LA54>. 

SYNTHESIS AND DYNAMIC PROPERTIES OF l-INDENYL(DIETHYL)BORANE... 

The 1-indenyl(diethyl)borane 19a was prepared from indenylHthium and di-ethylboron chloride in hexane (Scheme 2.16) [28]. In the H - H 2D EXSY... 

A sigmatropic migration of hydrogen takes place at elevated temperatures in 19a. This leads to the establishment of a dynamic equilibrium between the three isomers 19a, 19b, and 19c (Scheme 2.17). A small amount (about 6%) of 2-indenyl(diethyl)borane 19c apparently appears by a [1,5]-H shift in the Ao-indenyl intermediate 19e, which is formed from 19a via two consequent [1,3]-B shifts. 

The colorless, vacuum-distillable liquid [(2,2-dimethylpropanoyl)oxy]diethyl-borane (2,2-dimethylpropanoyl = pivaloyl) is a catalyst for the introduction of diethylboryl groups onto the oxygen atoms of hydroxy compounds using tri-ethylborane, Diethylborylations of hydroxy compounds have both preparative " and analytical (determination of the ethane value EZ ) uses. [(2,2-Dimethylpropanoyl)oxy]diethyIborane is itself a reagent for preparing aldol-condensation products" and for determining analytical pivalate values (PZ) of H-acidic compounds. ... 

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... 

Nickel can also be deposited by reduction with the aid of boranates such as sodium boranate (NaBH4) or A-diethyl borazane, i.e. (C2Hj)2NH-BH3, the basic reaction proceeding as follows ... 

Z)-2-Butenylpotassium is generated from 4.5 mL (50 mmol) of (Z)-2-butene, 2.8 g (25 mmol) or /-BuOK. and 10.8 mL (25 mmol) oT 2.3 M butyllithium in THF for 15 min at —45 JC. This solution is cooled to — 78 C and 30 mmol of a 1 M solution of methoxy(diisopinocampheyl)borane in diethyl elher is added dropwise. The mixture is stirred for 30 min at — 78 °C, then is treated with 4mL (33 mmol) of boron trifluoride-diethyl ctherate complex this removes methoxide from the intermediate ate complex. This solution is immediatelv treated with 35 mmol of an afdchyde. Isolated yields of homoallylic alcohols are 63-79%. 

A solution of potassium naphthalenide is prepared from 2.0 g (50 mmol) of potassium and 6.4 g (50 mmol) of naphthalene in 40 mL ofTHF. After 1 h at r.t. this mixture is diluted with 10 mL of diethyl ether and 10 mL of petroleum ether (bp 40-60 °C) and cooled to — 120 °C. 4.5 g (25 mmol) of ( )-l-methoxy-3-phenylthio-1-propcne arc added followed by 3.36 g (25 mmol) of chlorobis(l-dimethylamino)borane. This mixture is allowed to warm to r.t. over 3 h the solvents are removed in vacuo and the residue is carefully distilled through a 5-cm column at 10 2 Torr. The distillate, containing also naphthalene, is dissolved in 30 mL of diethyl ether and treated with 2.95 g (25 mmol) of pinacol for 3 h. The crude product is chromatographed over 30 g of basic alumina (activity 1) using petroleum ether (bp 40 -60°C) giving 9.2 g of a mixture of product and naphthalene the yield of product (89% E) is determined to be 60% by H-NMR analysis. Similarly prepared is ... 

To a — 78 C solution of diisopinocampheyl(2-propenyl)borane [theoretically 50 mmol prepared front (+ )-a-piueue as described in Section 1.3.3.3.3.1.1.1.] in 50 mL of diethyl ether are added dropwise 3.6 mL (50 mmol) of propanal. The mixture is stirred for 1 h at —78 C and then warmed to r.t. and treated with 36.6 mL (110 mmol) of 3 N sodium hydroxide and 15 mL of 30 % hydrogen peroxide solution. This mixture is refluxed for 1 It. The organic phase is separated, washed with water and NaCI and dried over MgS04. The filtrate is carefully fractionated yield 5g (74%) 86% ee (of menthyl carbonate10), bp 130-131 "C/750Torr. 

To a stirred — 78 C solution of 5.85 mL (62.5 mmol) of 3-methoxy-l-prnpene in 25 mL of THf- are added 43.1 mL (50 mmol) of 1.16 M. vcc-butyllithium in cyclohexane over a 20-25 min period. The mixture is stirred at — 78 °C for an additional 10 min, and diisopinocampheyl(methoxy)borane [50 mmol prepared from (+ )-a-pinene] in 50 mL of THF is added. This mixture is stirred for 1 h, then 8.17 mL (66.5 mmol) of boron trifluoride diethyl etherate complex are added dropwise to give a solution of diisopiuocampheyl[(Z)-3-inethoxy-2-propenyl]borane. Immediately. 2.8 mL (50 mmol) of acetaldehyde are added and the mixture is stirred for 3 h at — 78 rC and then allowed to warm to r.t. All volatile components are removed in vacuo, then the residue is dissolved in pentane. The insoluble fraction is washed with additional pentane. The combined pentane extracts are cooled to 0 JC and treated with 3.0 mL (50 mmol) of ethanolamine. The mixture is stirred for 2 h at 0rC and is then seeded with a crystal of the diisopinocampheylborane-ethanolaminc complex. The resulting crystals arc filtered and washed with cold pentane. The filtrate is carefully distilled yield 5.6 g (57%) d.r. (synjanti) >99 1 (2/ ,37 )-isomer 90% ee bp 119-120 C/745 Torr. 

R = C2H5 diethyl[(E)-1-ethyl-2-butenyI borane yield >50% R = Bu [(E)-l-butyl-2-buteny diethyiborane yield >75%... 

Reaction of 2-allylpyridine and an equilibrated 1 2 molar mixture of borane and triethylborane in tetrahydrofuran (THF) afforded l,l-diethyl-3,4-dihydro-277-pyrido[2,l-/][l,2]azaborine yield <2004LJSA0242817>. 

Since the 3-substituent may originate from borane as well as from the solvent [e.g. with triisopropyl borane in diethyl ether indenones 309 were obtained with... 

After being stirred for 2 hours at room temperature, 10 M borane-dimethyl-sulfide complex (2.0 mL) was added. The mixture was heated under reflux for 65 hours. The resulting mixture was cooled to room temperature and cautiously transferred into 2N hydrochloric acid (10 mL) in a 200 mL round-bottomed flask equipped with a magnetic stirrer bar using diethyl ether (10 mL). 

Two additional examples of the Suzuki coupling using diethyl (3-pyridyl)borane include the coupling with 4-chloro-2,3 -bipyridine l -oxide (40) to give terpyridine 41 [7] and the coupling with 1,8-dibromonaphthalene to afford a mixture of two atropisomers 42 and 43 of l,8-di(3 -pyridyl)naphthalene [33, 34], Interestingly, the N atoms in 42 and 43 provide enough steric and spectral differentiation to make the detection of atropisomers possible at room temperature. 

Li and Yue observed an ethyl transmetalation during the Suzuki coupling of diethyl (3-pyridyl)borane and 3-bromoquinoxaline 44 [35]. In the presence of a strong base (NaOH), the Suzuki reaction of diethyl (3-pyridyl)borane with 3-bromo-6,7-dichloroquinoxalin-2-ylamine (44) proceeded to give 52% of the 3-pyridylquinoxaline 45 accompanied by 21% of 3-ethylquinoxaline 46. 

Like simple aryl halides, furyl halides take part in Suzuki couplings as electrophiles [41, 42]. Young and Martin coupled 2-bromofuran with 5-indolylboronic acid to prepare 5-substituted indole 37 [43]. Terashima s group cross-coupled 3-bromofuran with diethyl-(4-isoquinolyl)borane 38 to make 4-substituted isoquinoline 39 [44]. Similarly, 2- and 3-substituted isoquinolines were also synthesized in the same fashion [45]. 

Chloropyrimidine was coupled with diethyl (3-pyridyl)borane in the presence of Pd(Ph3P)4, Bt NBr, and KOH to afford 3-(2 -pyrimidinyl)pyridine [23]. Likewise, the Suzuki coupling of 2-bromopyrimidine with diethyl (4-pyridyl)borane (47) led to 4-(2 -pyrimidinyl)pyridine (48) in 50% yield, whereas 2-chloropyrimidine produced 48 in only 20% yield under the same conditions [24], Diethyl (4-pyridyl)borane (47), on the other hand, was readily accessible from sequential treatment of 4-bromopyridine with n-BuLi and diethylmethoxyborane. 

In general the deprotonation of a polyborane B H +m (to = 4,6) leads to the anions [BnHn+m-i] or [B H +m 2]2 by removal of one or two protons from a BHB 3c2e bridge with formation of a B-B single bond. Cluster expansion with a BH3 unit, usually offered as diborane in diethyl ether or tetrahydrofuran, produces borane anions [B +1H +m+2] or [B +1H +m+1]2 and these in turn on protonation give the polyboranes Bn+iHn+m+-. These may be stable species. However, in most cases they loose H2 which results in a cluster expansion by one BH unit. Several examples of this method are described in the following sections. 

Scheme 3.2-13. Hydroboration of diethyl (propyn-l-yl)borane in the hydride bath via 23 and 24, followed by Et2BH-catalyzed condensation and self-assembly, leads to the 1-carba-arachno-pentaborane(lO) 25, having the Et2B substituent at the exopolyhedral carbon atom.

The reaction leading to 20 was used to prepare 25 (Scheme 3.2-13) [35], starting from diethyl(propyn-l-yl)borane which served as a versatile reagent for the synthesis of other carboranes (vide infra). l,l-Bis(diethyl-boryl)propene 23 can be detected and isolated, whereas it was not possible to identify l,l,l-tris(diethylboryl)propane 24 in the reaction mixture. 

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