Hydroboration/boron-zinc exchange

Cross-coupling between functionalized zinc-copper reagents and l-iodoalkynes or l-bromoalkynes is very fast [83]. This smooth cross-coupling occurs at low temperatures (—55 "C) and offers hi stereoselectivity in reactions with chiral secondary organozinc-copper reagents such as 108 (obtained by a hydroboration/ boron-zinc exchange sequence), producing the alhyne 109 in 42% overall yield (Scheme 2.40) [73]. 

A quinidine alkaloid derivative containing an olefin, such as 20, has been readily allylated via a hydroboration, boron-zinc exchange, and copper(I)-catalyzed allylation, leading to the alkaloid derivative 21 (see Section 9.6.3 Scheme 9-21) [51]. 

Progress from a synthetic point of view has been achieved in the selective oxidation of alkyl zinc reagents with oxygen in perfluoro-hexane solutions to give good yields of either hydroperoxides or alcohols. [28] As many zinc organometallics are easily obtained from olefins via a hydroboration/boron-zinc exchange sequence or by a nickel catalyzed hydro- or carbozincation reaction, this method may proof its synthetic potential in the future development. 

A combination of a substitution reaction with a chlorophosphine followed by a hydroboration, boron-zinc exchange allows the preparation of the mixed 1,2-diphosphine 207 in good yield (Scheme 7.62) [145]. 

Functionalized olefins may be converted to the corresponding diorganozinc compounds via hydroboration and subsequent boron-zinc exchange, as shown in Scheme 4.28... 

For internal olefins, the hydroboration step is slower, taking days rather than hours, and in this case the boron-zinc exchange requires the use of Zn(Pr )2 (Scheme 5). The reaction is stereospecific, provided that the diisopropylzinc is metal halide free.29,30... 

The RzZn reagents are readily prepared from the corresponding (functionalized) alkene by hydroboration and subsequent boron-zinc exchange, according to the... 

Scheme 7.8. Nickel-catalyzed 1,4-addition, using alkene hydroboration and boron-zinc exchange.

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... 

Organocuprates. The preparation of cuprates from organozincs is viable due to the tolerance of many functional groups (e.g., esters) in the reagents. There are also more synthetic pathways to the organozincs for example, from an alkene it is possible to achieve hydroboration and boron-zinc exchange. 

Pd(0)-catalyzed reactions also occur with retention of configuration. Thus, the hydroboration of 3-methylindene 10, followed by a boron-zinc exchange reaction with /-PraZn, gives the secondary dialkylzinc reagent 11 which undergoes a Pd(0)-catalyzed reaction with alkenyl iodides, such as 12, providing only the trans-indene 13 (Scheme 5) (10). 

A quinine alkaloid derivative containing a vinyl group, such as 252, has been hydroborated, undergo a boron-zinc exchange and copper(I)-catalyzed allylation, leading to the alkaloid derivative 253 103b]. A new route to hydrophobic amino-... 

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