Cyclic hydroboration

The double bonds in a conjugated diene are hydroborated separately, that is, there is no 1,4 addition. However, it is not easy to hydroborate just one of a conjugated system, since conjugated double bonds are less reactive than isolated ones. Thexylborane °(48) is particularly useful for achieving the cyclic hydroboration of dienes, conjugated or nonconjugated, as in the formation of 53." ... 

Stereoselective cyclic hydroboration. The hydroboration of l,4 and 1,5-dienes with thexylborane generally results in boracycles.2 If the intial hydroboration produces an asymmetric center, the second intramolecular hydroboration can be effected with remote asymmetric induction. Thus hydroboration of the diene 1 with thexylborane (1.25 equivalent) followed by oxidation results in a 6 1 mixture of the diastereomers 2 and. 3. An even higher induction is obtained in the case of the 1,4-... 

Simplified preparation. 9-BBN (dimeric) can be prepared in excellent yield (88%) and high purity by cyclic hydroboration of 1,5-octadiene with BH3 S(CH3)2 in monoglyme (1,2-dimethoxyethane) as the solvent, in which dimeric 9-BBN has very low solubility. One crystallization from this solvent gives large needles melting at... 

Cyclic hydroboration (10, 397). Two laboratories1,2 have reported stereoselective syntheses of the Prelog-Djerassi lactonic acid (4), both of which involve cyclic hydroboration of a slightly different 1,5-diene. One of these syntheses is outlined in... 

Good yields of the title compounds have been obtained when the cyclic hydroborating agents are used. The versatility of this reaction is illustrated in Eq. 103, where the reaction of 3,5-dimethylborinane with 1-bromo-l-hexyne affords l-pentylidene-3,5-dimethylcyclohexane 161). 

Cyclic hydroboration of ( + )-limonene with thexylborane yields the pure borabicyclic compound (138), which is oxidized to the diol (32 X = OH) or protonol-ysed with acetic acid to yield pure (—)-carvomenthol (32 X = H) after the usual oxidation.227 The treatment of dialkylcyanothexylborates with trifluoroacetic anhydride followed by oxidation as a route to ketones has been used with ( + )-limonene to give the two bridged-ring ketones (139).228... 

Treatment of Cp2ZrCl2 with LiAlH4 and then HsGaNMes or reaction of the preformed hydride Cp2ZrH2 with H3 AINMes affords the heterobimetallic complex (15). Boron-containing heterobimetallic complexes (16) can be prepared by reaction of metallocene dichlorides with the cyclic hydroborate anion [H2BC5H10] then reduction of MX l bond with LiH. 

Table 1 Selected nB chemical shift (ppm) of cyclic hydroborate complexes...

Monochloborane-dimethyl sulfide coexists with small amounts of the borane and dichloroborane complexes, but the bromoborane-dimethyl sulfide complex appears to be almost pure. - These complexes react readily with alkenes at 25 °C and can be used for hydroborations in a variety of solvents. Dialkylha-loboranes are obtained in high yield as their dimethyl sulfide complexes (equation 24), but dimethyl sulfide is readily removed under reduced pressure if required. - The reagents are also useful for cyclic hydroborations of dienes such as cyclooctadiene (equation 25). An alternative approach to dialkylbro-moboranes involves the reaction of dialkyl(methylthio)boranes with bromine. ... 

The use of thexylborane (1,1,2-trimethylpropylborane) as the hydroborating agent permits (a) the synthesis of mixed trialkyboranes, and (b) cyclic hydroboration of dienes. When followed by carbonylation, these hydroboration-carbonylation sequences generate a variety of unsymmetrically substituted ketones and cyclic ketones, respectively. Since the thexyl moiety exhibits a low migratory aptitude in carbonylation reactions, it serves as an anchor group. 

Brown, H. C., Negishi, E. Cyclic hydroboration of dienes. Simple convenient route to heterocyclic organoboranes. Pure Appl. Chem. 1972, 29, 527-545. 

A tandem carbocyclization/hydroboration sequence has been disclosed by Molander [83], The carbocyclization of a,co-dienes proceeds smoothly in the presence of the divalent samarocene Cp 2Sm(THF) and the metal-alkyl intermediate is trapped by a l,3-diaza-2-boracyclopentaneto afford the cyclic hydroboration product (29). 

Borabicyclo[3.3.1]nonane (9-BBN) has been prepared by the thermal redistribution of 9-n-propyl-9-BBN, and the hydroboration of 1,5-cyclooctadlene with borane-tetrahydrofuran complex followed by thermal isomerization of the mixture of dialkylboranes at BS C. Solutions of 9-BBN have been prepared from the hydroboration of 1,5-cyclooctad ene with borane-methyl sulfide in solvents other than THF.6 The present procedure involves the cyclic hydroboration of 1,5-cyclooctadiene with borane-methyl sulfide in 1,2-dimethoxyethane.7 Distillative removal of the dimethyl sulfide in this special solvent system provides a medium that gives high purity, large needles of crystalline 9-BBN dimer in excellent yield. The material can be handled in air for brief periods without measurable decomposition. 

Cyclic organoboron halides can also be prepared by redistribution. This is important when the triorganoborane is not readily available by metal-metal exchange (see 5.3.2.3.1) or in cyclic hydroboration (see 5.3.2.5), e.g., in the preparation of... 

Octadiene and higher a,co-dienes with 1 1 HjB-THF leads to polymeric material. Low yields of cyclic products are also obtained in the hydroboration of mixed dienes, e.g., 4-vinylcyclohexene or limonene (see Table 7), but 1,5-cyclooctadiene undergoes cyclic hydroboration in high yield. A mixture of 9-borabicyclo[3.3.1] nonane (9-BBN) and 9-borabicyclo[4.2.1]nonane is formed. Other intermediates are also observed by B NMR. Upon heating, the latter compound is transformed " to the more thermodynamically stable 9-BBN ... 

The difunctionality of monoalkyl- and monoarylboranes suits them well for cyclic hydroboration of dienes. The B-alkyl- and B-arylboracycloalkanes can be formed in excellent yields ... 

Limonylborane (LimBH, 1) is best prepared by cyclic hydroboration of limonene with a mono-chloroborane-diethyl ether complex followed by hydridation with lithium aluminum hydride76. 

The most common methods of ring construction employ cyclic hydroboration, radical additions of H2Y to unsaturated organoboranes and transmetallation procedures. Specific examples of these processes are presented in the following sections. 

The cyclic hydroboration of a,oj-dienes with monoalkylboranes leads to boracycles, often producing these ring systems in high isomeric purities. An early example of this process is represented by the cyclic hydroboration of divinyl ether (1) with t-butylborane-trimethylamine complex (Equation (1)) <61JA2541>. 

The cyclic hydroboration of divinyl compounds can provide a convenient entry to borinanes (e.g. 

Asymmetric cyclic hydroboration. As illustrated in the following equation, the facile accessibility of this reagent by in situ reduction of the dichloroborane derivative with MejSiH and the high optical yield of (+)-trans-decalone prepared from the hydroboration route suggest a general approach to such ketones ... 

Hydroboration of a.,oi-dienes. Cyclic hydroboration of ,cu-dienes can be effected with this reagent. The initial product is often polymeric, but six- to eight-membered cyclic B-chloroboracycloalkanes can be obtained after depolymerization by distillation in good yields. The B-chloroboracycIoaikanes (2) and (3) from 1,4-pentadiene (1) were transformed into the cyclic ketones (6) and (7) by reaction with a,a -dichloromethyl methyl ether (5, 200 203) followed by oxidation. ... 

Organoboranes from 9-BBN-H. First identified by Kdster, 9-BBN-H dimer is prepared from the cyclic hydroboration of 1,5-cyclooctadiene (eq 1). As a dialkylborane, 9-BBN-H exhibits extraordinary steric- and electronic-based regioselectivities which distinguish these derivatives from the less useful polyhydridic reagents (Table 1). ... 

Diols from dienes via stereospecific cyclic hydroboration C = C CHC(OH)... 

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