Alkynes alkenylborane

Organoboranes undergo transmetallation. 1-Hexenylboronic acid (438) reacts with methyl acrylate via the transmetallation with Pd(OAc)2, giving methyl 2,4-nonadienoate (439)[399], The ( )-alkenylboranes 440, prepared by the hydroboration of terminal alkynes, are converted into the alkylated ( )-alkenes 441 by treatment with an equivalent amount of Pd(OAc)2 and triethylamine[400]. The ( )-octenylborane 442 reacts with CO in MeOH in the... 

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. 

Its reactions with olefins, governed by steric rather than electronic factors, are very sluggish. Even simple 1-alkenes require 8 h at 25°C for complete reaction. In contrast, alkynes are hydroborated with great ease to alkenylboranes, high steric requirements of the reagent preventing dihydroboration (117). 

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. 

Regio- and stereoselective addition of 9-(phenylthio)-9-BBN to terminal alkynes is catalyzed by Pd(PPh3)4 to produce 9-[(Z)-/ -(phenylthio)alkyenyl]-9-BBN (Scheme 73) 283 Addition of styrene avoids catalyst deactivation by trapping free thiophenol generated in the reaction mixture. The produced alkenylboranes exhibit high reactivities for protonolysis with MeOH to produce 2-phenylthio-l-alkenes. 

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... 

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. 

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

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). 

Oxidation of alkenylboranes by alkaline hydrogen peroxide gives aldehydes or ketones depending on the substituent pattern of the alkenyl group thus, when alkaline hydrogen peroxide oxidation is combined with alkyne hydroboration, the resulting two-step process is a procedure for converting alkynes to carbonyl compounds (Equations B2.7 and B2.8). 

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... 

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... 

The mildness of these reagents tolerates the presence of various functional groups such as ester, ether, halogen, and nitrile. The stereospecific cis nature of hydroboration gives exclusively the tram alkenylboranes, often also in high regioisomeric purity (Eq. 53). On the other hand, highly pure (Z)-l-alkenyl-dialkylboranes are prepared without any difficulty via the monohydroboration of 1-halo-1-alkynes with disiamyl-borane or dicyclohexylborane, followed by treatment with t-butyllithium (Eq. 55)106). 

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). 

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... 

Regioselectivities appear to approach those achieved with disiamylborane e.g. see Figure 9, Section 3.10.4.3). The cis additions to alkynes give alkenylborane derivatives which are useful synthetic intermediates. 

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 cleavage of alkenylboranes may be carried out under non-acidic conditions by converting the alkenylborane into the ate complex with n-BuLi prior to hydrolysis with aqueous NaOH. " Also, alkenylboranes derived from internal alkynes may be protonated under neutral conditions using catalytic amounts of Pd(0Ac)2 in THF or acetoneor with aqueous silver ammonium nitrate [Ag(NH3)2 N03]. ... 

Hydroboration-oxidation of an alkenylborane derived from a symmetrically substituted alkyne yields a single ketone, whereas unsymmetrically disubstituted alkynes furnish mixtures of ketones. 

A )-and (Z)-alkenylboranes, or the corresponding boronic esters or potassium tri-fluoroborates, are conveniently available via hydroboration of terminal alkynes or 1-halo-1-alkynes using either disiamylborane, dicyclohexylborane, dibromoborane dimethyl sulfide, or catecholborane as hydroborating agents. ... 

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