Alkenylboron

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 organoborate intermediates can also be generated from alkenylboronic esters and alkyllithium or Grignard reagents, or from ttialkylboranes and alkenyllithium compounds. Conjugated symmetrical and unsymmetrical diynes (289—291), stereochemically pure 1,3-dienes (292,293), and 1,3-enynes (294) including functionali2ed systems can be prepared (289,295). 

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

An alkyl- or alkenylboron compound, as a suitable organoboron component (a borane, boronic acid or ester) can be prepared through hydroboration of an... 

Z)-2-Alkenylboronates are available in good yield by palladium-catalyzed hydroboration of 1,3-dienes with catechol borane100. 

I J Hydroboration, Diboration, Silylboration and Stannylboration Tab. 1-5 Dehydrogenative Coupling giving 1 -Alkenylboronates... 

Hydroboration of alkenes or alkynes followed by cross-coupling with organic electrophiles provides a straightforward method for the carbon-carbon bond formation (Scheme 1-19). The hydroboration of thioalkynes with catecholborane in the presence of a nickel or palladium catalyst yields P-(aLkylthio)-l-alkenylboronates (72a)... 

The utility of the stepwise, double-coupling procedure is demonstrated in the parallel synthesis of Tamoxifen derivatives on solid support [127] (Scheme 1-29). 1-Alkenylboronates thus obtained by a diboration-cross coupling sequence are further coupled with p-silyUodobenzene supported on polymer resin. Using this strategy, each position about the ethylene core is modified by the appropriate choice of alkyne, aryl halide, and cleavage conditions for the synthesis of a library of Tamoxifen derivatives. 

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... 

Alkenylboronic esters undergo regio- and stereoselective 1,3-dipolar cycloadditions with nitrones. These reactions lead to boronic ester-substituted isoxazolidines, which can be converted by oxidation with H202 to the corresponding 4-hydroxy derivatives (Eq. 8.48).69 The high selectivity could be the result of a favorable interaction between the boronic ester and the amino group. 

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). 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

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 ... 

The alkenylboronic esters were synthesized according to a literature procedure [56], Hydrozirconation of alkenylboronic esters with zirconocene hydrochloride, Cp2Zr(H)Cl, prepared by Buchwald s procedure [57], took place smoothly in CH2C12, providing in each case the corresponding borazirconocene 1,1-alkane 18 [34], Addition of propargyl bromide and a catalytic amount of copper(I) cyanide was accompanied by the disappearance of the yellow color associated with these compounds and by carbon—carbon bond... 

As representatives of this class of compounds, one hindered and one non-hindered gem-boriozirconocene alkene were prepared [54]. Hydrozirconation of a 1-alkynyldioxaborolane with Cp2Zr(H)Cl in either 1,4-dioxane or THF proceeded readily by a syn addition to give the essentially pure ( )-l,l-bis-metallic boriozirconocene 45 [12]. This bis-metallic complex 45 was also reacted with various electrophiles, thereby generating the alkenylboron derivatives 53—57 as shown in Scheme 7.17. 

As an example of the selective reactivity of borazirconocene alkenes, their hydrolysis was examined [1]. The carbon—zirconium bond is more reactive than the carbon—boron bond towards various electrophiles, and so hydrolysis can be expected to occur with preferential cleavage of the former bond. Since hydrolysis of alkenylzirconocenes is known to proceed with retention of configuration [4,127—129], a direct utility of 45 is the preparation of (Z)-1-alkenylboronates 57 (Scheme 7.17) [12]. Though the gem-dimetalloalkenes can be isolated, in the present case it is not necessary. The desired (Z)-l-alkenylboronates can be obtained in a one-pot procedure by hydrozirconation followed by hydrolysis with excess H20. The reaction sequence is operationally simple and is compatible with various functional groups such as halides, acetals, silanes, and silyloxy protecting groups [12]. 

A formal trans-hydroboration of terminal alkynes with catecholborane and pinacolborane to yield cis-l-alkenylboronates has also been carried out in the presence of Rh(I) and Ir(I). The dominant factors for reversing the conventional cis-hydroboration to the trans-hydroboration were the use of NEts and the use of bulk phosphines such as P Pr3 and PCy3, and an excess of alkyne in front of the boron reagent [46]. 

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.

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