Alkenylboranes synthesis

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]. 

Several methods for stereoselective alkene synthesis are based on boron intermediates. One approach involves alkenylboranes, which can be prepared from terminal alkynes. Procedures have been developed for the synthesis of both Z- and E-alkenes. 

A number of procedures for stereoselective synthesis of alkenes involving alkenylboranes have been developed. For each of the reactions given below, show the structure of the intermediates and outline the mechanism in sufficient detail to account for the observed stereoselectivity. 

Hydroboration of an alkyne followed by treatment of the alkenylborane with basic peroxide provides a method of synthesis of aldehydes and ketones. 

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]. 

The Zweifel trans-alkene synthesis from dialkyl-1-alkenylboranes involves the hy-droboration of 1-halo-l-alkynes with dialkylborane (Eq. 69) 126). It is of interest that the base produces a transfer with inversion 126 127). 

As discussed in the last sections, addition of sodium methoxide-alkenylborane complexes to copper bromide-dimethyl sulfide at 0 °C immediately gives conjugated dienes. However, if the temperature is lowered to —15 °C, the dark blue-black complex formed is stable and can be trapped by allylic halides to afford a stereochemically defined synthesis of (4 )-1,4-dienes (Eq. 93) 148). 

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 ... 

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. 

The currently available methods for the synthesis of the title compounds are confined to the preparation of homo-1,1-dihalo-1-alkenes 180 while only a few reports are available for mixed 1,1-dihalo-1-alkenes of defined stereochemistry 18u. As the hy-droboration reaction proceeds in a stereospecific manner, the hydroboration-oxi-dation-bromination-debromoboration sequence of 1-chloro-l-alkynes produces selectively (Z)-l-bromo-l -chloro-l-alkenes (Eq. 116),82>. The oxidation with anhydrous trimethylamine oxide of the alkenylborane prior to the addition of bromine is necessary to avoid the competing transfer of one of 1,2-dimethylpropyl group from boron to the adjacent carbon atom. Similar reaction sequence provides 1,1-dibromo-l-alkenes (Eq. 117)182). 

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... 

VI Synthesis via 1-Alkenylboranes in the Presence of PaUadium Catalysts and Bases... 

Protonolysis of Alkylboranes with Carboxylic Acids Synthesis cf Alkanes 3.10.73 Protonolysis cf Alkenylboranes Synthesis ofAlkenes and Dienes... 

Cross-coupling reactions between 1-alkynyl halides and 1-alkenylboranes, which are readily available via hydroboration of alkynes, can also be catalyzed by Pd-phosphine complexes. Stereoselective synthesis of conjugated ( )-enynes 143 was achieved by the coupling of the alkynyl bromide with an alkenylborane [Eq. (48)] [69]. 

Hydroborations of alkynes directed toward the synthesis of alkenylboranes furnishes versatile intermediates for a wide array of chemical transformations. For example, protonolysis of alkenylboranes provides a noncatalytic method for the semireduction of alkynes. 

The regio- and stereoselective monohydroboration of 1 -alkynes or functionally substituted alkynes with dialkylboranes followed by protonolysis of the resultant alkenylboranes with a carboxylic acid is a powerful tool for the synthesis of 1-alkenes, and cA-disubsti-tuted and functionally substituted alkenes. ... 

Like hydroalumination and hydrozirconation, hydroboration of alkynes also provides a convenient and Stereospecific route to alkenyl metal reagents. However, initial attempts to achieve palladium-catalyzed cross-coupling of alkenylboranes with alkenyl halides were unsuccessful, due to the poor carbanionic character of these reagents. Later, Suzuki discovered that the desired transformation could be effected in the presence of an alkoxide or hydroxide base weaker bases, such as sodium acetate or triethylamine, were not generally effective. The reaction is suitable for the preparation of ( , )-, ( ,Z)- and (Z,Z)-dienes. Since reactions of alkenylboronates are higher yielding than those of alkenylboranes, the recent availability of (Z)-l-alkenylboronates " substantially improves the Suzuki method for the preparation of (Z)-alkenes. An extension of the methodology to the synthesis of trisubstituted alkenes has also been reported. " ... 

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