Triethylborane

It was claimed that the Z-form of the allylic acetate 430 was retained in homoallylic ketone 431 obtained by reaction with the potassium enolate of 3-vinylcyclopentanone (429), after treatment with triethylborane[282]. Usually this is not possible. The reaction of a (Z)-allylic chloride with an alkenylaluminum reagent to give 1,4-dienes proceeds with retention of the stereochemistry to a considerable extent when it is carried out at -70 C[283]. 

This derivative is stable to acetic acid and CF3C02H. The related diethylborinic acid derivative has been prepared from triethylborane (THF, reflux). ... 

Ishikura and co-workers (95H2437) have extended the scope of this reaction, combining our results with their borate chemistry. Thus, the reaction of 2-lithio-l-methoxyindole (72), prepared from 1-methoxyindole (71) and -BuLi (92H1285, 92H1295, 94H31), with triethylborane generates an indolylborate 73 in situ (Scheme 11). Subsequent treatment of 73 with aqueous NaOH and 30%... 

Carbon-centered radicals have been shown to undergo addition reactions with azirine-3-carboxylates. Methyl 2-(2,6-dichlorophenyl)azirine-2-carboxylate thus reacts with alkyl and aryl iodides in the presence of triethylborane to give aziridines in good yields. The radical approaches from the opposite face to the aryl substituent, giving the cis products as single diastereoisomers (Scheme 4.43) [63],... 

The anti diastereoselectivity is improved to a 6 1 ratio by the addition of triethylborane to the reaction mixture83. NMR-spectroscopic investigations indicate that a boronate complex is the decisive intermediate84, since it can also be prepared by the addition of alkyllithium to the dialkyl(2-butenyl)borane (path ). 

The reactions of aldehydes with 2-butenylboron ate complexes generated, for example, by the addition of 2-butenyllithium to triethylborane, have also been investigated. These reagents exist mainly as the E-isomer and display moderate selectivity (68-85%) for the antf-diastereomer40. 

The (TMS)3Si radical addition to terminal alkenes or alkynes, followed by radical cyclization to oxime ethers, were also studied (Reaction 50). The radical reactions proceeded effectively by the use of triethylborane as a radical initiator to provide the functionalized pyrrolidines via a carbon-carbon bond-forming process. Yields of 79 and 63% are obtained for oxime ethers connected with an olefin or propargyl group, respectively. 

The amidinato(pyridine) complexes also react with the triethylborane adduct of the NHC l,3-diisopropylimidazol-2-ylidene in toluene at reflux temperature to give the corresponding carbene complexes (Scheme 128). ... 

During the study of the reaction products between six different co-lactams and triethylborane, monomeric (for 36, 39), dimeric (for 38), tetrameric (for 40) and oligomeric (for 35) species have been detected in solution by cryoscopic methods. For 37 [X = -CH2)5-] only lateral reaction products could be observed (Fig. 14) [66]. 

While 1,2- and 1,3-alkanediols form only monomeric 1,3,2-dioxaborolanes and 1,3,2-dioxaborinanes with triethylborane, 1,4- and 1,5-alkanediols give mainly dimeric macrocyclic boronates 41 and 42, which are obtained in yields of 70-75% after purification by vacuum distillation. As lateral products oligo-and polymeric molecules are formed. The main products have a 14- and 16-membered heterocyclic ring, respectively (Fig. 15) [84]. 

Trialkylboranes, especially triethylborane, are used in conjunction with 02 to generate radicals.297 The alkyl radicals are generated by breakdown of a borane-oxygen adduct. An advantage this method has over many other radical initiation systems is that it proceeds at low temperature, e.g., -78°C. 

An alternative system for initiating radical cyclization uses triethylborane and oxygen. Under these conditions, fm-(trimethylsilyl)silane is an effective hydrogen donor.330... 

Triethylborane can also be used for radical initiation and the low temperature can lead to improved yields and stereoselectivity. 

Radical addition to alkenes has been used in cyclizations in aqueous media. Oshima and co-worker studied triethylborane-induced atom-transfer radical cyclization of iodoacetals and iodoacetates in water.121 Radical cyclization of the iodoacetal proceeded smoothly both in aqueous methanol and in water. Atom-transfer radical cyclization of allyl iodoacetate is much more efficient in water than in benzene or hexane. For instance, treatment of allyl iodoacetate with triethylborane in benzene or hexane at room temperature did not yield the desired lactone. In contrast, the compound cyclized much more smoothly in water and yielded the corresponding y-lactone in high yield (Eq. 3.31). 

Clerici and Porta reported that phenyl, acetyl and methyl radicals add to the Ca atom of the iminium ion, PhN+Me=CHMe, formed in situ by the titanium-catalyzed condensation of /V-methylanilinc with acetaldehyde to give PhNMeCHMePh, PhNMeCHMeAc, and PhNMeCHMe2 in 80% overall yield.83 Recently, Miyabe and co-workers studied the addition of various alkyl radicals to imine derivatives. Alkyl radicals generated from alkyl iodide and triethylborane were added to imine derivatives such as oxime ethers, hydrazones, and nitrones in an aqueous medium.84 The reaction also proceeds on solid support.85 A-sulfonylimines are also effective under such reaction conditions.86 Indium is also effective as the mediator (Eq. 11.49).87 A tandem radical addition-cyclization reaction of oxime ether and hydrazone was also developed (Eq. 11.50).88 Li and co-workers reported the synthesis of a-amino acid derivatives and amines via the addition of simple alkyl halides to imines and enamides mediated by zinc in water (Eq. 11.51).89 The zinc-mediated radical reaction of the hydrazone bearing a chiral camphorsultam provided the corresponding alkylated products with good diastereoselectivities that can be converted into enantiomerically pure a-amino acids (Eq. 11.52).90... 

Homoallylation of Aldehydes with Dienes Promoted by Triethylborane. . 184... 

Remarkably, 1,3-dienes have been shown to serve as homoallyl anion equivalents and react with aldehydes in the presence of a catalytic amount of Ni(acac)2 and a stoichiometric amount of triethylborane to provide bis-homoallyl alcohols, 4-pentenols 40, exclusively (Eq. 11) [27,28]. 

The reaction is quite general for dienes with a variety of combinations of substituents R R3. Some representative examples for the reaction with benzaldehyde are summarized in Table 3. The reaction is performed uniformly using benzaldehyde (1 equiv.), a diene (4 equiv.), Ni(acac)2 (0.1 equiv.), and triethylborane (2.4 equiv.) in THF at room temperature under nitrogen atmosphere. 

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