Boranes transmetallation

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. 

The transmetallation of the lithiated anion, generated from the tosylate of trifluoro-ethanol, is performed at low temperature with a borane or with cyclopentadienylzir-conium. A further transmetallation affords a second organometallic species that is more stable or more reactive (zinc or copper) and that has a broader synthetic potential... 

In equation 7.1. a 4-chloropyridine was coupled with diethyl(3-pyridyl)borane.3 The reaction was run in aqueous THF in the presence of potassium carbonate. The role of the base is to facilitate the transmetalation step through the formation of a borate ion, as organoboranes are usually not nucleophilic enough to transfer their organic moiety onto the palladium. An alternate function of the base is to increase the electrophilicity of the palladium through exchange of the halide to carbonate. 

A similar transformation was reported using a polyfunctional phthalazine derivative (7.2.) 4 It is interesting to note that the transmetalation of the borane was not selective with respect to the pyridyl group, formation of the product resulting from the transfer of the ethyl group was also observed. 

Albert S.C. Chun of the Hong Kong Polytechnic University reports (J. Org. Chem. 68 1589, 2003) two important transformations. The three-component (Mannich) condensation of 10 with 11 and 12 proceeds with high diastereoselectivity, to give the amino alcohol 13. Hydroboration of the alkyne 14 followed by transmetalation of the intermediate vinyl borane gives a zinc species, which under catalysis by the easily-prepared 3-naphthol 13 adds to aromatic and branched aldehydes with high . The product allylic alcohols are useful intermediates for organic synthesis. 

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). 

Preparation of alkyl boranes RB(C6F5)2 which do not contain (3-hydrogens can be accomplished by transmetallation between Cp2ZrR2 and ClB(C6F5)2.48b The methyl, benzyl and trimethylsilyl methyl bis-(pentafluorophenyl) boranes have been prepared in... 

Preparation of 4-cyanophenylzinc bromide by transmetallation and its reaction with phosphorus trichloride preparation of tris (4-cyanophenyl)phosphine-borane complex8... 

Arylboronic acids are readily available via transmetalation between ArSnMe3 and borane in THF. Consequently, a variety of arylpolyboronic acids is achievable from the stannanes previously synthesized by the SRN1 mechanism from ArCl or ArOH in around 80% yields [50, 51]. These arene di- and tri-boronic acids can be used as starting materials for the synthesis of polycyclic aromatic systems via double or triple Suzuki crosscoupling reactions [50, 51], as shown in Scheme 10.34 [51],... 

Aryltrimethyl stannanes are easily transformed into boronic acids by transmetalation with borane [106]. Thus, 1,4- and 1,3-Z>fy(trimethyl-stannanyl)benzenes as well as 2,5- and 2,6-Z /s(trimethylstannyl)pyridines synthesized by the SRN1 mechanism, react with borane in THF to give intermediates which on hydrolysis lead to benzene- and pyridinediboronic acids (Sch. 35) [107]. 

The synthesis of 9-alkenyl-9-BBN via hydroboration of terminal alkynes with 9-BBN suffered from the formation of 1,1-diborylalkanes via dihydroboration along with desired monohydroboration products. Alternatively, selective monohydroboration of terminal alkynes with dicyclohexylborane was followed by transmetallation with 9-MeO-9-BBN (Equation (193)).710 Treatment of the hydroboration intermediates with DIBAL-H in the presence of a borane-trapping reagent such as 1-hexene gave 1-alkenylaluminum compounds with complete retention of the (E)-stereochemistry (Equation (194)).711... 

An example for the use of the boron-zinc exchange reaction for copper-mediated SN2 -substitutions of allylic electrophiles is the hydroboration of nitroolefin 130 with diethylborane, followed by successive transmetallation of the borane 131 with diethylzinc and CuCN-2LiCl, and final trapping with allyl bromide to give the product 133 with 83% yield over four steps (Scheme 34).34,34a This transformation again demonstrates the tolerance of the method towards functional groups and acidic hydrogen atoms. 

As mentioned in Section 9.12.2.1.1, the boron-zinc exchange can be performed stereoselectively if diisopropyl-zinc instead of diethylzinc is used. For example, hydroboration of the chiral, racemic endocyclic olefin 134 with diethylzinc, followed by twofold transmetallation and electrophilic capture of the resulting copper intermediate with allyl bromide was used for the highly diastereoselective formation of the stereotriad in product 136 (Scheme 35).35,35a 103 QorreSp0nding enantioselective transformations were carried out with chiral boranes and catalytic amounts of copper salts (see Section 9.12.2.2.2).36... 

Synthesis of pure borane 24 was acomplished by transmetallation of the tin derivative 26 (Eq. 11). In borane 24 both possible [1,3]-B shifts are observed however, the migration of the dipropylboryl group to position 9 proceeds much faster than the similar rearrangement to position 3. 

The first two and last two steps for the catalytic cycle of Suzuki cross-coupling are much the same as those for the Stille reaction (Scheme 12.18) the transmeta-lation step, however, is unique. Transmetalation involves transfer of R to Pd from a borane, borate ester, or boronic acid. Both recent experimental investigations and analysis using DFT calculations indicate that transmetalation is not simply a concerted process as suggested by transition state structure 38. 

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