Acids alkenylboronic

Alkylthiocyanates and alkylselenocyanates are obtained by treatment of trialkylboranes with potassium thiocycanate (260) and sodium selenoisocyanate (261), in the presence of iron(III) compounds, respectively. Unsymmetrical trialkylboranes react preferentially at the more highly branched alkyl group. Alkenylphenyl selenides are obtained by the reaction of alkenylboronic acids with phenylselenyl bromide (262). 

The synthesis of 1-alkenylboronic acids from l-alkenylmagnesiums or -lithiums suffers from difficulty in retaining the stereochemistry of 1-aikenyl halides, but the palladium-catalyzed coupling reaction of diboron 82 with 1-aikenyl halides or tri-flates directly provides 1-alkenylboronic esters (Scheme 1-43) [157, 158]. Although the reaction conditions applied to the aryl coupling resulted in the formation of an... 

Alkenylboronic acids, alkenyl boronate esters, and alkenylboranes can be coupled with alkenyl halides by palladium catalysts to give dienes.223... 

Triflates prepared from (V-alkoxyearbonyl lactams can be coupled with aryl and alkenylboronic acids.227... 

In 1998, Miyaura reported a Rh(acac)(CO)2/dppp-catalyzed addition of aryl or alkenylboronic acids to aldehydes in aqueous organic mixtures under an inert atmosphere (Eq. 8.85).216 The use of electron-rich tri(tm-butyl)phosphine as ligand was found to be beneficial for obtaining good yields of the corresponding aldehyde addition products.217... 

In 1997, Miyaura and co-workers reported the nonasymmetric version of 1,4-addition of aryl- and alkenylboronic acids to a,/ -unsaturated ketones using rhodium-phosphine complex as the catalyst.97 Later, Hayashi and Miyaura realized the asymmetric 1,4-addition with high catalytic activity and enantioselectivity.98 In the presence of ( y)-BINAP, the reaction of 2-cyclohexenone with 2.5 equiv. of phenylboronic acid gave (A)-3-phenylcyclohexanone with 97% ee (BINAP = 2,2 -bis (diphenylphosphino)-l,l -binaphthyl Scheme 29).99... 

A broad substrate scope for the rhodium-catalyzed asymmetric 1,4-addition has been observed.98 Both arylboronic acids with either electron-donating or electron-withdrawing aryl substituents and alkenylboronic acids can be introduced into acyclic or cyclic enones with high enantioselectivities (Scheme 30). 

Not only arylboronic acids but also 1-alkenylboronic acids or esters add to aldehydes to give the corresponding allylic alcohols (Equation (39)).395 Isomerization of the allylic alcohols to saturated ketones occurs in less protic media and at higher temperatures. 

It is worth noting that conjugate addition can also be effected by the addition of aryl- and alkenylboronic acid reagents to enones in the presence of rhodium catalysts. This new catalyst system has the following advantages ... 

Addition of Organometallic Reagents to Enones in Aqueous Media Rhodium-catalyzed 1,4-addition of organometallic reagents to a,p-unsaturated compounds was first developed by Miyaura in 1997. Thus, Rh(acac)(CO)2/dppb was found to catalyze the 1,4-addition of aryl- and alkenylboronic acids to several ot,(3-unsaturated ketones in water-containing solvents at 50°C. The reaction conditions were successfully modified for the development of an asymmetric variant of this process by Hayashi and Miyaura in 1998. The important points of modification are (1) the use of Rh(acac)(C2H4)2/(5)-binap as a catalyst and... 

Figure 3.20. Scope of Rh/(5)-binap-catalyzed asymmetric 1,4-addition of alkenylboronic acid catechol esters to a,P-enones.

OBC2278>. Aqueous Suzuki reactions conducted with microwave heating were also successful for the coupling of diverse aryl- and alkenylboronic acids to unprotected 2- 6- or 8-halopurines <2006S3515>. 

A highly stereoselective synthesis of retinol via a Cm + C6 route was depicted by De Lera et al. [52]. A Suzuki reaction of a C14 alkenyliodide with a Cg alkenylboronic acid afforded retinol in 83% yield, with retention of the geometries of the coupling partners. The alkenyliodide was obtained by a zirconium-mediated methylalumination and a subsequent Al/I exchange by slow addition of ICN. Coupling with the C6 boronic acid (12 hrs to reach completion), afforded retinol in 83% yield [53], Fig. (21). 

The newest and perhaps best method of general application in which the double bond already exists is the reaction of alkenylboronic acids or their esters with (diacetoxyiodo)benzene in presence of BF3 Et20. Since the precursors are readily available from alkynes in a well defined stereochemistry, the alkenyliodonium salts may be obtained in pure E- or Z-form without difficulty and in high yield (Scheme 40) [122]. 

The (E) and (Z) forms of the vinylboron compounds 278 and 279 can be prepared by hydroboration of alkynes and haloalkynes, and their reactions with the (E)- or (Z)-vinyl iodides or bromides 280 and 281 proceed without isomerization the four possible isomers 282-285 can be prepared in high purity. However, for the efficient preparation of the (Z,Z)-dienes 287, the diisopropyl ester of (Z)-alkenylboronic acid 286 should be used. Other boron compounds give poor yields... 

This reaction can be carried out with secondary amines, sterically hindered primary amines, hydrazines or anilines in dichloromethane at room temperature. The range of potential nucleophilic partners includes alkenylboronic acids, and electroneutral as well as electron-rich (hetero-)arylboronic acids. The conversion of electron-poor boronic acids can be effected at elevated temperatures (MW) in suitable solvents. 

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