Alkenylboronic preparation

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). 

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

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

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]. 

One limitation of this preparative method is the concurrent formation of an alkenylboronate, which is formed presumably via a-elimination pathways involving the intermediate ate adduct of addition between the organometaUic reagent and the a-haloalkylboronate (Eq. 25). ... 

To access anti-l,2-diols, indirect methods are required for the preparation of geometrically pure, chiral E-3-alkoxy reagents. To this end, the isomerization of alkenylboronic esters described above (Eq. 41), provides a reliable route to tartrate-derived E-3-siloxy allylboronate 99 (Fig. 7). The latter shows variable enantioselectivities in additions to aldehydes, with cyclohexanecarboxaldehyde affording the highest selectivity (Eq. 70). ... 

The procedure described here Is an example of a general method for preparing conjugated alkadienes by the palladium-catalyzed reaction of 1-alkenylboranes or boronates with vinylic halides. Hydroboratlon of l-a1kynes with catecholborane Is a standard method for obtaining (E)-l-alkenylboronates (1). Several different types of alkenylboranes and boronates (2-4) are now available as reagents for the cross-coupling reaction with vinyl halides. 

Third, chromium-based reagents prepared by reduction of 1,1-dihalo compounds with CrCl2 are suitable for the transformation of aldehydes to (i4)-iodoalkenes, (it)-alkenylsilanes, " and ( )-alkenylboronic esters with one-carbon homologation (Equation (8)). The amount of CrCl2 can be reduced to a catalytic amount by using manganese (or zinc) and MesSiCl. " ... 

In 2006, Ukai et al. proposed an interesting alternative with a rhodium(I)-catalyzed carboxylation of aryl- and alkenylboronic esters proceeding under mild conditions, and leaving ancillary reactive functional groups such as carbonyl- and cyano unreacted [51] (Scheme 5.13). Considering that organoboronic esters are easily available, and that various functional groups are tolerated, this reaction appeared to be particularly useful for the preparation of functionalized arylcarboxylic acids, such as benzoic and cinnamic acid derivatives. 

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... 

A Prelude Preparation of Haloalkenes and Alkenylboronic Acid Derivatives, Important Building Blocks for Palladium-Mediated C,C Couplings Carbocupration of Alkynes... 

Fig. 16.15. Stereoselective preparations of trans-alkenyl-boronic acid esters (B) and trans-alkenylboronic acids (C) and their stereoselective conversion into c/s-bromoalkenes and trans-iodoalkenes, respectively.

Fig. 13.11. Stereoselective preparations of cw-alkenylboronic acids and the corresponding diisopropyl ester starting with cw-bromoalkenes. The first step involves a Br/Li exchange to form the alkenyllithium compound B. This organolithium compound is subsequently transmetallated to give complex C by using B(0 Pr)3.

Alkenylboronic acids and esters have been prepared by thermal or catalyzed hydroboration of 1-alkynes with catecholborane (HBcat), pinacolborane (HBpin), or dihaloboranes 41-43, followed by hydrolysis to boronic acids or alcoholysis to boronic esters. A convenient alternative to improve chemo- and regioselectivity is the hydroboration of alkynes with dialkylboranes. For selective removal of dummy groups, the oxidation of two cyclohexyl groups was conduced by treatment of l-alkenyl(dicyclohexyl)borane intermediates with Me3N-0 (Equation (7)).116 The... 

The addition of bromine or iodine to ( )-alkenylboronic acids or esters afforded intermediates, which were converted by base into (Z)-l-halo-l-alkenes.543 On the other hand, ( )-iodo-l-alkenes of high stereochemical purity were prepared by treatment of ( )-l-alkenylboronic acids or esters with sodium hydroxide followed by 1 molar equiv. of iodine.544 Iodine monochloride was used in place of iodine for such a base-induced iodination with retention or inversion of the stereochemistry (Equation (110)).545 Chloramine-T/Nal (Equation (111)),546,547 CuX2 (X = Cl, Br) (Equation (112)),548 and iV-halosuccinimides (Equation (113))549 halogenated the C-B bond with retention of stereochemistry. 

Alkenylboronic acids and esters underwent conjugated addition to ct,/ -unsaturated ketones in the presence of trifluoroborane etherate589 or cyanuric fluoride (Equation (129)).590,591 Alkadienyl trifluoromethyl ketones were stereoselectively prepared from (2-alkoxyvinyl) trifluoromethyl ketones (Equation (130)).592 Alkynyl boronates can transfer the alkynyl groups regioselectively and enantioselectively to enones (Equation (131 )).593... 

To avoid the formation of small amounts of the dihydroborated derivatives, it is desirable to use 10% excess of alkyne. Hydrolysis or alcoholysis of the resulting alkenyldichloroborane provides a simple preparation of the desired alkenylboronic acid or esters via hydroboration. 

Terminal and internal (Z)-l-alkenylboronates are prepared from (Z)-(haloalkenyl)boronic esters (4) which can be readily obtained by hydroboration of 1-halo-1-alkynes [15, 17, 18]. 

Bombykol is a well-known pheromone, first isolated from Bombyx mori L. Bombykol the three related isomers were synthesized by cross-coupling reaction. Three alkenylboronates or boronic acids (35 and 36) required for the coupling are prepared by starting from two alkynes. The stereoselective syntheses of ( )- and (Z)-l-alkenylboronic acids or esters have been discussed in Section 2.2.1. Halogenation of alkenylboronic acid 34 with either iodine or bromine provides ( )- and (Z)-haloalkenes [71]. The palladium-and base-assisted coupling of the Cj-boronate 32 or 33 with the C,-halide 35 or 36 stereoselectively provides bombykol and its three geometrical isomers (Scheme 2-29) [72]. 

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