Monohydroboration

Scheme 27).43b A regiospecific monohydroboration of enyne 107 with disiamylborane furnishes (A)-vinylborane 108, a substance that combines stereospecifically with vinyl iodide 109 under the indicated conditions to give conjugated triene 110 (52% overall yield). Trisporol B benzyl ether (111) is obtained after acid-induced hydrolysis of the dioxolane ketal functions.

Triple bonds can be monohydroborated to give vinylic boranes, which can be reduced with carboxylic acids to cis alkenes or oxidized and hydrolyzed to aldehydes or ketones. Terminal alkynes give aldehydes by this method, in contrast to the mercuric or acid-catalyzed addition of water discussed at 15-4. However, terminal alkynes give vinylic boranes (and hence aldehydes) only when treated with a hindered borane such as 47, 48, or catecholborane (p. 798)," or with BHBr2—SMe2. The reaction between terminal alkynes and BH3 produces 1,1-... 

Conjugated dienes exhibit a marked decrease in reactivity toward 9-BBN relative to alkenes and nonconjugated dienes. Structurally different double bonds, however, may allow clean monohydroboration.368 Homoallylic boranes are usually formed, but certain dienes yield allylic boranes. 

The same reagents may be used to conduct monohydroboration of internal acetylenes as well. Unsymmetrically disubstituted compounds usually produce a... 

Thus, 1.7-octadiene (79), which was subjected to monohydroboration followed by asymmetric dihydroxylation of the remaining double bond to give triol 80 with approximately 80% ee. Further transformations then afforded the desired butenolide 81. Double asymmetric dihydroxylation of diene 83 and subsequent protection gave hydroxy lactone 84 [98], which was then converted into acetylenic bis(hydroxy)bistetrahydrofuran 82 as the required intermediate for the (+)-asimicin synthesis. Mitsunobu inversion at C-24 gave rise to the diastereomeric (+)-bullatacin precursor. 

BBN hydroborates internal alkynes cleanly (Equation B1.13) and thus for this reaction it is superior to borane which tends to give intractable polymers when added to alkynes. The reaction is less useful for terminal alkynes as monohydroboration can only be achieved if an excess of alkyne is used. 

The synthesis of 9-alkenyl-9-BBN via hydroboration of terminal alkynes with 9-BBN suffered from the formation of 1,1-diborylalkanes via dihydroboration along with desired monohydroboration products. Alternatively, selective monohydroboration of terminal alkynes with dicyclohexylborane was followed by transmetallation with 9-MeO-9-BBN (Equation (193)).710 Treatment of the hydroboration intermediates with DIBAL-H in the presence of a borane-trapping reagent such as 1-hexene gave 1-alkenylaluminum compounds with complete retention of the (E)-stereochemistry (Equation (194)).711... 

A series of books 7) and reviews 8) written on the utility of these reactions highlights the synthetic applicability of this field. However, there was no attempt to systematize the otherwise vast literature available for selective hydroboration, e.g., hydroboration of double bond in the presence of triple bond and vice versa, hydroboration of one of the double bonds of dienes, monohydroboration of alkynes, and selective hydroboration of carbon-carbon multiple bonds in the presence of many other functional groups. These topics are the aim of this review. Included here also is a brief account of the developing field of haloboration. 

Double bonds with different geometry behave in a unique way. For example, the monohydroboration of caryophyllene (9) with dicyclohexylborane followed by oxidation brought about the participation of the (E)-trisubstitutcd double bond in preference to the exocyclic double bond during hydroboration to give the corresponding alcohol (10) (Eq. 10)50t. However, under similar conditions, isocaryophyllene (11), which has a (Z)-endocyclic double bond, affords a mixture of unsaturated alcohols... 

In general, the hydroboration of terminal olefins is complete within 1 h at 25 °C. The reagent is, however, limited to the preparation of thexylalkylchloroborane derivatives containing primary or unhindered secondary alkyl groups. Thus a chemist has in hand thexylborane, which cleanly leads to monohydroboration stage in case of hindered olefins, while on the other hand thexylchloroborane-methylsulfide hydroborates only unhindered olefins (Eq. 26). As the hydridation sequence provides thexyl-... 

The a,p-acetylenic ketones can be synthesized in good yields by the selective mono-hydroboration-oxidation process of conjugated diynes. The monohydroboration of conjugated diynes with disiamylborane places boron preferentially at the internal triple position of the diyne system. The resultant organoboranes on treatment with sodium hydroxide and 30% H202 afforded the a,P-acetylenic ketones (Eq. 33) 79). 

Hydroboration of conjugated diene, e.g., monohydroboration of 1,3-cyclohexa-diene with disiamylborane95 yields the corresponding allylic boranes. A similar reaction with 9-BBN affords B-2-cyclohexenyl-9-BBN in 95% yield (Eq. 41). 

Monohydroboration of allenes with 9-BBN places the boron atom exclusively at the terminal position. Thus, 3-methyl-l,2-butadiene gives B-3-methyl-2-butenyl-9-BBN as the only product (Eq. 43)96). However, such a procedure for the preparation of allylic boranes by the hydroboration of conjugated dienes and allenes is not a general... 

However, many mono-, dialkylboranes and heterosubstituted boranes (Chart 1) exhibit highly selective behavior in hydroboration of alkynes7). Thus, they appeared attractive for the controlled monohydroboration of alkynes for the synthesis of the desired vinylic boranes from terminal and internal alkynes (Eqs. 53 and 54). 

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 monohydroboration of alkynes can also be achieved with BH2C1 OEt2 at 0 °C using an internal alkyne or a terminal alkyne (Eqs. 56 and 57, Table 8)37), while dihydroboration is a significant reaction when THF is used as a solvent. This is due to the reason as the nucleophilic character of THF is higher as compared to diethyl ether. 

Monohydroboration of terminal alkynes with relatively unhindered dialkylborane is accompanied by competing dihydroboration, while for thexylmonoalkylboranes both thexyl and R group migrates competitively. 

The synthesis of iridomyrmecin 247) took advantage of the high transannular reactivity which frequently operates in eight-membered rings (vide supm). To this end, 1,5-dimethyl-1,5-cyclooctadiene 243) was monohydroborated, treated with... 

Hydroboration ofallenes Monohydroboration of cyclic allenes with catecholborane (1) followed by oxidation results in a single ketone arising from regiospecific attack of boron at the central carbon atom of the allene (equation I). The reaction with diborane or disiamylborane is not selective. 

A second BH3 molecule attacks the monohydroborated product most likely in syn position due to the easy formation of BHB bonds with the BH2 group to yield 9b, which loses "B2H4 to give 7b. The postulated diborane(4) is unstable and decomposes in the absence of suitable Lewis bases. 

Catalytic hydroboration is a new methodology of great synthetic potential. The reaction is usually carried out with catecholborane in the presence of rhodium, palladium, iridium and ruthenium compounds.2 In contrast to olefins, very little is known on catalytic hydroboration of conjugated dienes and enynes. Our earlier studies on the uncatalyzed monohydroboration of conjugated dienes,6 reports on the hydroboration of 1-decene with catecholborane catalyzed by lanthanide iodides,7 and monohydroboration of 1,3-enynes in the presence of palladium compounds,8 prompted us to search for other transition metal catalysts for monohydroboration of conjugated dienes and enynes 9 10... 

Based on the additions described above, alkenyldichloroboranes were used as dienophiles, equivalents of 1-alkenes. The organoboranes were conveniently prepared by monohydroboration of 1-alkynes with dichloroborane, generated in situ19... 

Table 2 Monohydroboration Products of Conjugated Dienes and Enyrtes with Catecholborane Catalyzed by NiCltfdppe) in THF at 20 °C...

In contrast to palladium and rhodium catalyzed monohydroboration of 1,3-dienes,20 no 1,4-addition and only 1,2-addition products were obtained with anri-Markovnikov regioselectivity. Acyclic dienes were more reactive than cyclic dienes, and styrene was unreactive. Among iron(II) and cobalt(II) chloride complexes with dppe, dppp and dppb only CoClz(dppp) showed catalytic activity albeit lower than NiCUIdppe). 

Aldehydes can be synthesized by the monohydroboration of alkynes followed by oxidation (Eqs. 13 14 and 15... 

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