Alkenylborane coupling

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

Organoboranes are reactive compounds for cross-coupling[277]. The synthesis of humulene (83) by the intramolecular cross-coupling of allylic bromide with alkenylborane is an example[278]. The reaction of vinyiborane with vinyl-oxirane (425) affords the homoallylic alcohol 426 by 1,2-addition as main products and the allylic alcohol 427 by 1,4-addition as a minor product[279]. Two phenyl groups in sodium tetraphenylborate (428) are used for the coupling with allylic acetate[280] or allyl chloride[33,28l]. 

Various electrophiles other than iodine have been used to induce alkenyl coupling (9). Alkyl haUdes and protic acids react with alkynylborates to yield mixtures of stereoisomeric alkenylboranes. Nevertheless, oxidation of these products is synthetically useful, providing single ketones (296—298). Alcohols are obtained from the corresponding alkenylborates. 

The postulated steps that constitute the Suzuki coupling process are shown in Scheme 25. After oxidative addition of the organic halide to the palladium(o) catalyst, it is presumed that a metathetical displacement of the halide substituent in the palladium(ii) complex A by ethoxide ion (or hydroxide ion) takes place to give an alkoxo-palladium(ff) complex B. The latter complex then reacts with the alkenylborane, generating the diorganopalladium complex C. Finally, reductive elimination of C furnishes the cross-coupling product (D) and regenerates the palladium(o) catalyst. 

Alkenylboranes (R2C=CHBZ2 Z — various groups) couple in high yields with vinylic, alkynyl, aryl, benzylic, and allylic halides in the presence of tetra-kis(triphenylphosphine)palladium, Pd(PPh3)4, and a base to give R C CHR. 9-Alkyl-9-BBN compounds (p. 1013) also couple with vinylic and aryl halides " as well as with a-halo ketones, nitriles, and esters.Aryl halides couple with ArB(IR2 ) species with a palladium catalyst. ... 

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

Trisubstituted alkenes. The (Z)-2-bromo-l-alkenylboranes (1), obtained by bromoboration of 1-alkynes with BBr, (13, 43), undergo coupling with organozinc chlorides in the presence of Cl2Pd[P(C6H5)3]2 to provide, after protonolysis, disub-stituted alkenes (3). However, the intermediate alkenylborane (2) can undergo a... 

Many of the modern vinylallene syntheses involve metal-mediated coupling reactions, as shown by several typical examples summarized in Scheme 5.11. The treatment of the propargyl carbonate 83 with the alkenylborane 84 in the presence of a palladium catalyst furnished the substituted vinylallene 85 in excellent yield (Scheme 5.11) [28],... 

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. 

Coupling of 1-alkenylboranes with 1-bromoalkenes or -alkynes.4,s This reaction fails when catalyzed by Pd(0) alone, but proceeds in high yield in the presence of an added sodium alkoxide or sodium hydroxide in refluxing benzene (or THF). This coupling can be used to prepare (E,E)-, (E,Z)-, (Z,E)-, or (Z,Z)-1,3-dienes stereo- and regiospecifically. 

The coupling of alkenylboranes with alkenyl halides is particularly useful in the stereoselective synthesis of 1,3-diene systems of four possible double-bond isomers... 

The (Z, , )-triene systems in leukotriene and DiHETE were constructed by the coupling of the (ZfT)-dienylborane with the (Z)-alkenyl iodide [129,130]. In the total synthesis of the naturally occurring large molecule palytoxin, which has numerous labile functional groups, Suzuki coupling gives the best results for the creation of the (E,Z)-, 3-diene part (290) by the coupling of the alkenylborane 288 with the (Z)-alkenyl iodide 289. In this case, thallium hydroxide as the base accelerates the reaction 1000 times more than KOH [131]. 

Potential difference in reactivity between two G-B bonds allowed the transformation of l,2-bis(boryl)-l-alkenes to 1-alkenylboranes via a cross-coupling with the aryl, 1-alkenyl, benzyl, and cinnamyl halides (Equation (23)).211-213 This tandem procedure synthetically equivalent to a yy/z-carboboration of alkynes was used for synthesizing Tamoxifen derivatives via stepwise double coupling with two of the G-B bonds.212,213 Hydrogenation of the resulting bisborylalk-enes with a chiral rhodium catalyst is synthetically equivalent to an asymmetric diboration of alkenes (Equation (24)).214... 

Recently, Rossi et al. have applied the cross-coupling of 1-alkenylboranes with 1-alkynyl halides for the synthesis of natural products. Thus, (7 , 9Z)-7,9-dodecadien-1-yl acetate (32), the sex pheromone of Lobesia botrana, has been synthesized by the following sequence involving ... 

As described in previous sections, the palladium catalyzed cross-coupling reaction of stereodefined 1-alkenylboranes with stereodefined 1-alkenyl halides in the presence of relatively strong bases such as sodium alkoxides and hydroxide ( head-to-head... 

The stereodefined alkenyl halides are of prime importance due to the recent developments of di- or trisubstituted alkene synthesis by cross-coupling reactions between organometallics and alkenyl halides catalyzed by transition metal compounds 171). These alkenyl halides can be conveniently obtained from alkenylboranes or alkeneboronic acids. B-Alk enylcatecholboranes undergo rapid hydrolysis when stirred with excess water at 25 °C (Eq. 109)102). The alkeneboronic acids are usually crystalline solids of low solubility in water and can be easily isolated and handled in air without significant deterioration. 

Alkenylboron compounds couple with the representative organic halides or triflates (Scheme 28). Hexaalkylbenzene was synthesized by sixfold alkenylation (55) of hexabromobenzene followed by catalytic hydrogenation of the double bonds 11521. The reaction of 1-alkenylborane with 1-bromo-l-alkyne stereose-lectively provided ( )-enyne (56) which was then converted into ( ,Z)-hexa-deca-10,12-dienal, a sex pheromone of the melonworm 11531. Due to the difficulty of purification of a geometrical mixture, the stereoselective synthesis is critical for such dienes or trienes. The PGEi derivatives (57) were synthesized... 

The above methylcopper-induced reaction is also applicable for cross-coupling of alkenylboranes with organic halides. The treatment of dialkenyichloroboranes (36) with 3 equivalents of n thylcopper at —30 to —40 °C followed by addition of allylic halides gives 1,4-dienes (Eq. 80)In the case of simple alkyl halides, the cross-coupling products are obtained with the aid of PhSLi or P(OPh)3 (Eq. 80)... 

Most recently, Rossi et al. have applied the cross-coupling of 1-alkenylboranes... 

Most recently, it has been disclosed that 1-alkenylboranes react with 3,4-epoxy-1-butene in the presence of a pall ium or nickel complex to form the two corresponding coupling products, 2-ethenyl-3-alken-l-ol (52) and 2,5-alkadien-l-ol (ii) (Eq. 115). ... 

Cross-coupling of aryl halides and 1-alkenylboranes. Aryl iodides and bromides couple efficiently with 1-alkenylboranes (prepared from acetylenes and catecholborane, 4, 70) in the presence of this Pd(0) complex and a base (NaOC2Hs) (equation I). The reaction proceeds with retention of configuration with respect to the alkenylborane to form only (E)-alkenes. Yields are typically 81-100%. One example is reported where the alkenylborane was prepared with disiamylborane (yield 41%)."... 

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