Trialkylboranes halides

Addition to Carbonyl Compounds. Unlike Grignard and alkykitliium compounds, trialkylboranes are inert to carbonyl compounds. The air-catalyzed addition to formaldehyde is exceptional (373). Alkylborates are more reactive and can transfer alkyl groups to acyl halides. The reaction provides a highly chemoselective method for the synthesis of ketones (374). 

Treatment with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid. This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles. The R group does not rearrange, and this reaction is a step in the hydro-boration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving a rearrangement from boron to oxygenr ... 

Coupling of aryl or alkenyl halides with trialkylboranes.1 This reaction can be carried out in refluxing THF with a base (NaOH or NaOCH3,1 equiv.) catalyzed by PdCl2(dppf). A B-alkyl-9-BBN is more useful than a trialkylborane, since only one alkyl group is utilized. 

Extension of this reaction to electrophiles other than aldehydes was unsuccessful [22, 23], However, propargylic boronates were found to react with allylic halides and various carbonyl compounds [23], The boronates were prepared by lithiation of a methyl-substituted alkyne with t-butyllithium followed by treatment with a trialkylborane. The propargylic boronate preferentially reacts with the electrophile at the y-position to yield propargylic products (Eq. 9.20). The methodology has also been applied to alanates with comparable results. 

Alkylation of thienyllithium with 5-alkyl halide may lead to a competitive elimination reaction. This has now been overcome the reaction with trialkylboranes, followed by treatment with iodine, gives the corresponding alkyl derivatives in good yields (Scheme 172) (79BCJ1865, 81BCJ1587). 

Pailer and Fenzl lx) replaced the trialkylboranes in this pyrolysis reaction by organoboron dihalides, RBX2, and obtained the 1,3,2-dia-zaboracycloalkane system under evolution of hydrogen halide. 

In 2007 the scope of the trialkylborane/water system was extended to the dehalogenation of alkyl iodides and the chemoselective deoxygenation of secondary alcohols in the presence of alkyl and aryl halides [86]. The rate constants for the hydrogen-atom transfer from this reagent to secondary radicals (Scheme 37) are substantially lower than those of the Ti(III) aqua-complex [78, 87]. 

Trialkylboranes, including 9-alkyl-9-BBN, underwent cross-coupling with 1-alkenyl or aryl halides or triflates.750,751 The reaction was limitedly used for primary alkylboranes thus, hydroboration of terminal alkenes with 9-BBN was the most convenient to furnish the desired boron reagents in the presence of a base and PdCl2(dppf)750,751 or PdCl2(dppf)/2Ph3As (Equation (200)).752... 

Although the reaction of alkylboronic acids is significantly slower than that of trialkylboranes, methylboroxine (MeBO)3 or methylboronic acid exceptionally alkylates bromo- and chloroarenes [125, 126]. Methylation of electron-poor bromoarenes was efficient, whereas prolonged reaction time was required for more electron-rich substrates. The reaction times can be shortened by use of the polar solvent DMF at 115 °C. B-Methyl-9-BBN and its oxy-derivative (48) [127] undergo methylation of aryl and vinyl halides under more mild conditions (Scheme 22). 

The fluorovinyllithium reagents are thermally unstable (vu/e supra) and are reacted with electro-philes at low temperatures. These reagents undergo usual reactions with a variety of electrophiles including ketones, esters, alkanoyl chlorides, epoxides (e.g.. formation of 6 ), oxetanes, carbon dioxide, sulfur dioxide, tin and silicon halides, and trifluoromethancsulfonates. The reaction of 2,2-difluoro-1 -(tosyloxy)vinyllithium (5) with trialkylboranes followed by a second electrophile is an attractive route to functionalized 1.1-difluoroalkenes. e.g. 7. ... 

Selective a-alkylalion of ketones Potassium enolates of ketones and an unhindered trialkylborane such as triethylborane form a potassium enoxytriethyl-borate, which undergoes selective a -monoalkylation with alkyl halides in high yield. Lithium enolates do not form the corresponding borates. In the absence of triethylborane dialkylated products are formed and some of the original ketone is recovered. 

An alternative to dialkylcuprates in these addition reactions are copper(I) methyltrialkylborana-tes which are readily prepared from trialkylborane, methyl lithium and copper(I) halides. When these reagents were reacted with 1-benzoyl-2-vinylcyclopropane (30), 1,7-addition products were isolated in high yields. The y,i5-unsaturated ketones 31 formed were shown to be selectively trans-isomers. 

Organomagnesium-halide, organolithium and organoaluminum reagents are preferred to other organometaiiics. Commercially available trialkylaluminum compounds are convenient starting materials for the lower trialkylboranes. Solvents are not required, and trialkylborane can be isolated by distillation directly from the mixture ... 

Besides the methyl compounds a number of other alkyldiboranes have been prepared from diborane and trialkylboranes (111, 128). The alkylated diboranes can also be obtained by treating at elevated temperatures NaBH4, LiBH4, or LiAlH4 with R3B, and either u hydrogen halide or a boron trihalide (83b). 

Cross-coupling reactions are now possible between aryl (or vinyl) halides and trialkylboranes by the use of catalytic... 

There are many examples of the use of 2- and 3-lithiofurans in reactions with various electrophilic species, such as aldehydes and ketones,and halides. It has been shown that treatment of 2-lithiofuran with copper(II) chloride leads to 2,2 -bifuran, and with trialkylboranes to borates, subsequent treatment of which with halogen provides an excellent method for the overall introduction of alkyl groups at the furan a-position. ... 

A second means of effecting alkylations of organoboranes involves reactions with highly reactive alkyl halides, especially a-halocarbonyl compounds. For example, ethyl bromoacetate has been found to alkylate a number of trialkylboranes in excellent yield. This synthetic transformation is more efficiently carried out using a... 

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