Boranes cross-coupling

Scheme 5-81. Transmetalation for tin or boranes cross-coupling of zinc...

Moderate yields of acids and ketones can be obtained by paHadium-cataly2ed carbonylation of boronic acids and by carbonylation cross-coupling reactions (272,320,321). In an alternative procedure for the carbonylation reaction, potassium trialkylborohydride ia the presence of a catalytic amount of the free borane is utilized (322). FiaaHy, various tertiary alcohols including hindered and polycycHc stmctures become readily available by oxidation of the organoborane iatermediate produced after migration of three alkyl groups (312,313,323). 

Recently, Caddick and Cloke have developed an extension of this procedure that allows the use of alkyl bromides as coupling partners. The basic changes consist of a stoichiometric amount of the bulkier KO Bu instead of KOMe to activate the borane, and the addition of AgOTf to the reaction mixture [119]. These results, although poor in terms of yield, clearly confirm that sp -sp Suzuki-Miyaura cross-couplings are possible and should be further developed (Scheme 6.35). 

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]. 

Early findings by Suzuki and co-workers [109] showed that the palladium-catalyzed iminocarbonylative cross-coupling reaction between 9-alkyl-9-BBN derivatives, t-butylisocyanide, and arylhalides gives access to alkyl aryl ketones 132 after hydrolysis of the corresponding ketimine intermediates 131. Presumably, the concentration of free isocyanide is kept to a minimum by its coordination with the borane. Formation of an iminoacylpalladium(II) halide 130 by insertion of isocyanide to the newly formed arylpalladium complex followed by a transmetallation step afford the ketimine intermediates 131 (Scheme 8.52). 

Boranes and, to a lesser extent, boronic acids can undergo slow hydrolysis (protode-boration) in the presence of protic solvents. This unwanted reaction can become predominant if a cross-coupling reaction only proceeds slowly (e.g. with electron-rich, sterically demanding, or unreactive halides Scheme 8.20 see also Scheme 8.14) or if the boron derivative is particularly sensitive, for example 2-formylphenylboronic acid. In such instances the reaction should be performed under anhydrous conditions in an aprotic solvent with a boronic acid ester [151] or a stannane. 

Miyaura, N. Yamada, K. Suginome, H. Suzuki, A. Novel and convenient method for the stereo- and regiospecific synthesis of conjugated alkadienes and alkenynes via the Pd-catalyzed cross-coupling reaction of 1-alkenyl-boranes with bromoalkenes and bromoalk-ynesj. Am. Chem. Soc. 1985, 107, 972-980. 

Scheme 31 Cross-coupling of alkyl (sp3-C) boranes via hydroboration-coupling sequence. The left parts come from terminal alkenes and the right parts from halides or triflates (Equation (200)).

Palladium-catalyzed cross-coupling of (2-ethoxyvinyl)borane readily prepared by the hydroboration of ethoxyethyne, with aryl and benzylic halides provides a convenient method for conversion of such halides into aldehydes with two more carbon atoms (Eq. 18)57). 

Trialkylboranes including 9-alkyl-9-BBN and alkyl(disiamyl)borane readily undergo the cross-coupling with 1-alkenyl or aryl halides or triflates [8] (Scheme 20). The reaction is limitedly used for primary alkylboranes thus, hydroboration of terminal alkenes with 9-BBN or HB(Sia)2 is the most convenient to furnish the desired boron reagents. Trialkylboranes thus obtained are... 

Vinylic ethers (44) can be synthesized in high yields by the cross-coupling of aryl or benzyl halides with tris(2-ethoxyvinyl)borane (4S) in the presence of 1 molej% of a palladium compound such as tetrakis(triphenylphosphine)palladium and a base (Eq. 108) Since vinylic ethers (44) thus obtained can readily be hydrolyzed to aldehydes, this reaction provides a convenient procedure for converting aryl or benzylic halides into the corresponding aldehydes with two more carbon atoms. 

The search for potential BNCT drugs has re-awakened interest in the organic chemistry of the 12-vertex polyhedral boranes.1 The most widely studied chemistry is of o-carborane, where two carbon atoms are available for a number of organic reactions.2 Most substituted carboranes are prepared either by interaction of alkynes with decaborane,3 cross-coupling of Li, Na, Mg-carboranes with electrophiles 4 or by the organic reactions on the side chains (Scheme 1). ... 

Hydroboration of acetylenic selenides with 9-BBN led to the regio- and stereoselective formation of a-selanylalkenyl boranes which were then converted into Z-a, -disubstituted vinyl selenides by cross-coupling reaction with aryl bromides [80] (Scheme 58). With unsubstituted acetylenic selenides, an inversion of regioselectivity during the hydrozirconation was observed [81,82]. 

The introduction of the aminoethyl side chain is accomplished by Suzuki cross-coupling of dienyl triflate 10 with the alkyl borane generated by hydroboration of A -vinyl- er butyl carbamate (11) with 9-BBN. ... 

Conjugated enynes are of importance for themselves, as well as for the synthesis of conjugated dienes. The cross-coupling reaction of 1-alkenyl(disiamyl)boranes (3c) with 1-bromo-l-alkynes (Scheme 2-34) provides conjugated enynes in high yields [45]. 

The cross-coupling reaction of tris(2-ethoxyethenyl)borane (37) [96] or 2-(l-ethoxy-1-alken-2-yl)-l,3,2-benzodioxaboroles (38) with iodoarenes produces styryl ethers in high yields in the presence of Pd(PPh3)4 and powdered NaOH suspended in THF (Scheme 2-37) [97, 98]. Since 37 and 38 have a tendency to undergo base-induced decomposition on prolonged heating, it is desirable to use an iodoarene derivative as a substrate or an excess... 

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