Reactions of Alkenylboranes

The reactions of trialkylboranes with 1-lithio-l-methoxypropadiene (gives ate complex), followed by treatment with acetic acid and then oxidation, provide an approach to 1-alkyl-l-methoxycyclopropanes. A similar sequence using 1,2-dimethoxyethenyl-lithium provides a route to 1,1-dialkylethenes. Further information has been given about the simultaneous 2- and 3-functionalization 

Takahashi, M. Tokuda, M. Itoh, and A. Suzuki, Chem. Lett., 1980, 461. 

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Organoboranes react with propargylic carbonates. Usually, addition of a base is essential for the Pd-catalyzed reactions of organoboranes, but the reaction with propargylic carbonates proceeds without addition of the base, because methoxide is generated in situ from carbonates. For example, the 1,2,4-triene 80 is prepared by the reaction of alkenylborane under neutral conditions[36]. 

The 3-borolenes (97) were among the several products obtained by the reaction of dichloro-(diisopropylamino)borane with the corresponding 1,3-dienes (96) in the presence of Na/K alloy (Equation (20)) <90JOM(386)i >. Reaction of alkenylboranes with alkynyllead compounds is another approach <9)ZN1207>. 

Synthesis of Alkenylboronic Acids, Aikenes, Aldehydes, and Ketones. Alkenyldibromoboranes undergo many of the characteristic reactions of alkenylboranes. The presence of dimethyl sulfide does not interfere in their transformations. Protonolysis with acetic acid in refiuxing dichloromethane gives the corresponding alkene. Oxidation leads to aldehydes or ketones (eq 4). Hydrolysis and alcoholysis yields alkenylboronic acids and esters, respectively. 

Table 30.7 Reaction conditions, products and yields for Diels-Alder reactions of alkenylboranes [40]... 

Cross-coupling Reactions of Alkenylboranes.— Alkenylboron compounds react with alkenyl or allylic bromides in the presence of Pd species to give 1,3- or 1,4-dienes in a stereocontrolled manner (e.g. Scheme 3). ... 

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. 

However, in the reaction of 1-alkenylboranes with aryl- or 1-alkenyi iodides. 2-aryl-l-alkenes 648 are obtained as the main products. When Pd metal produced from Pd(OAc)2 as a catalyst and EtjN as a weak bu.se are u.sed. abnormal products are formed. On the other hand, normal products 649 are obtained by using NaOH[5l7]. 

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]. 

The most widely used route to organoboranes is hydroboration, introduced in Section 4.5.1, which provides access to both alkyl- and alkenylboranes. Aryl-, methyl-, allylic, and benzylboranes cannot be prepared by hydroboration, and the most general route to these organoboranes is by reaction of an organometallic compound with a halo- or alkoxyboron derivative.1... 

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. 

Another method for the preparation of acetylenic carboxylates involves the reaction of 1-alkenylboranes with CO and alcohols (equation 135). The alkenyl boranes are readily prepared from alkynes by addition of boranes. PdCl2 was again the catalyst, with NaOAc as base and benzoquinone as oxidant.541... 

Like alkenes, alkynes do not react readily with trialkylboranes. Under severe reaction conditions, only hydroboration products are obtained.116 Mikhailov113 showed that triallylboranes react with various alkynes (20 C) to afford syn addition products of type (78) which rapidly cyclize (40-60 C) to give the cyclic boranes of type (79 Scheme 42). In the case of trimethylsilylacetylene and ethoxyacetylene, the reaction affords compounds of type (78 R = MesSi or OEt) which do not cyclize further.113 Recently, a transition metal silylboration reaction has been described by Oshima and cowoikers.117 A formal syn carboboration reaction leading to a variety of alkenylboranes has been reported by Suzuki and cowoikers118 (Scheme 43). Hexamethyldistannylacetylene (80) reacts readily with various trialkylboranes119 to afford syn addition products of type (81 equation 25). 

Protonolysis of alkenylboranes by carboxylic acids occurs readily. The stereochemistry of the alkenyl group is retained during the reaction and so hydroboration/protolytic cleavage of alkynes leads to cis alkenes. Deuterated... 

Cycloaddition reaction of m-chlorotributylstannane was catalyzed by BC13 or 9-Br-9-BBN at 25 °C (Equation (72)).404 A catalytic cycle involved [2+4]-reaction of 1-alkenylborane, generated in situ by Si-B transmetallation. 

Subsequent reactions of the alkenylboranes usually proceed by stereodefined pathway, thus allowing highly stereo- and regiospecific syntheses. 

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... 

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... 

Oxidation of alkenylboranes with alkaline hydrogen peroxide is an important pathway to aldehydes and ketones (equation 13). Care must be taken to inhibit hydrolysis of the alkenylborane to the corresponding alkene and hence buffered conditions are frequently us. The reaction toloates the same wide range of functionality as the oxidation of alkylboranes. 

Next, the reactions of 1-alkenyltrialkylborates will be discussed. One of the most interesting and versatile reactions of such 1-alkenylboranes readily obtainable... 

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)... 

The versatility of such cross-coupling reactions of 1-alkenylboranes has been... 

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