Cyclooctadienes hydroboration

Borabicyclo [3.3.1] nonane [280-64-8], 9-BBN (13) is the most versatile hydroborating agent among dialkylboranes. It is commercially available or can be conveniendy prepared by the hydroboration of 1,5-cyclooctadiene with borane, followed by thermal isomerization of the mixture of isomeric bicychc boranes initially formed (57,109). 

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

Another reagent with high regioselectivity is 9-borabicyclo[3.3.1]nonane (9-BBN), which is prepared by hydroboration of 1,5-cyclooctadiene ... 

Section B of the Scheme 9.1 shows several procedures for the synthesis of ketones. Entry 6 is the synthesis of a symmetrical ketone by carbonylation. Entry 7 illustrates the synthesis of an unsymmetrical ketone by the thexylborane method and also demonstrates the use of a functionalized olefin. Entries 8 to 10 illustrate synthesis of ketones by the cyanide-TFAA method. Entry 11 shows the synthesis of a bicyclic ketone involving intramolecular hydroboration of 1,5-cyclooctadiene. Entry 12 is another ring closure, generating a potential steroid precursor. 

The reactions can be made enantioselective by using enantiomerically pure IpcBH2 for hydroboration of alkenes and then transforming the products to enantiomerically pure derivatives of 9-BBN by reaction with 1,5-cyclooctadiene.22... 

A 0.5 M solution of 9-BBN in tetrahydrofuran was purchased from Aldrich Chemical Company, Inc., and was used without additional purification. The preparation2 of the reagent by hydroboration of 1,5-cyclooctadiene with boranaAetrahydrofuran complex is reported. 

An especially selective hydroborating reagent is prepared from 1,5-cyclooctadiene and borane. The product is a bicyclic compound of structure 1 (often abbreviated as 9-BBN), in which the residual B-H bond adds to unhindered alkenes with much greater selectivity than is observed with other hydroborating reagents. It is also one of the few boranes that reacts sufficiently slowly with oxygen that it can be manipulated in air. 

Catalytic hydroboration of perfluoroalkenes 68 with catecholborane provides either terminal 69 or internal alcohols 70 regioselectively <19990L1399>. The regioselectivity is controlled by a judicious choice of catalyst. The anti-Markovnikov alcohol can be obtained with very high selectivity by using cationic rhodium catalysts such as Rh(COD)(DPPB)+BF4, while neutral Rh catalysts such as Wilkinson s catalyst provide the Markovnikov product (COD = cyclooctadiene Equation 3) <19990L1399>. 

Addition of H3B.THF to 1,5-cyclooctadiene gives a mixture of 9-borabicyclo[4.2.1]nonane and 9-borabicyclo[3.3.1]nonane. On heating, the [4.2.1] system isomerizes to the thermodynamically more stable [3.3.1] compound which is known as 9-BBN (Equation B1.12). As 9-BBN is crystalline, relatively stable to air and heat, and is available from commercial sources, this dialkylborane is a popular hydroborating agent. 

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. ... 

Hydroboration of acyclic, symmetrical, nonconjugated dienes with one equiv. of 9-BBN-H produces almost statistical mixtures of mono- and di-hydroborated species, but cyclic analogs may show substantial deviations from the statistical mixture. For example, monohydroboration of 1,5-cyclooctadiene occurs to the extent of 85% using 1 1 stoichiometry (equation 31), ° whereas disiamylborane gives predominantly dihydroboration product under such conditions. 

Another reagent with high regioselectivity is 9-borabicyclo[3.3.1]nonane (9-BBN), which is prepared by hydroboration of 1,5-cyclooctadiene, and has the advantage that it is stable in air. Borane is quite unselective and attacks all sorts of double bonds. Disiamylborane, 9-BBN, and similar molecules are far more selective and preferentially attack less-hindered bonds, so it is often possible to hydroborate one double bond in a molecule and leave others unaffected or to hydroborate one alkene in the presence of a less reactive alkene. For example, 1-pentene can be removed from a mixture of 1- and 2-pentenes, and a cis alkene can be selectively hydro-borated in a mixture of the cis and trans isomers. 

Borabicyclo[3.3.1.]nonane (9-BBN) is available by hydroboration of 1,5-cyclooctadiene with one equivalent of BH3," or commercially either as a crystalline dimer or as a THF solution. The reagent is thermally more stable than dicyclohexylborane. It is frequently used as an anchor group in organoborane reactions, allowing an efficient utilization of valuable alkenes. 

Oxabicyclo[3.3.I]noHene-l (3).13 This bridgehead alkene was synthesized in the following way. Hydroboration-oxidation of 1,5-cyclooctadiene gave n s-l,5-cycloocta-nediol, which was oxidized by Jones reagents to l-hydroxy-9-oxabicyclo[3.3.1]-nonane (1) in 49% overall yield. Attempts to prepare derivatives of (1) usually led to derivatives of the hydroxy ketone (4) however, reaction of (1) with methanesulfonyl chloride and... 

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. 

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 ... 

These reagents are prepared by hydroboration of the appropriate alkene, using control of stoichiometry to achieve the desired degree of alkylation. 9-BBN is prepared from 1,4-cyclooctadiene... 

Hydroboration of 1,3-cyclopentadiene with 9-BBN is too slow to be practicable, but monohydroboration of 1,3-cyclohexadiene is a useful reaction (equation VI). 9-BBN reacts with 1,3-cyclooctadiene to give predominantly dihydroboration products. ... 

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). ... 

A route to aldehydes from olefins via carbonylation has also been developed. The 9-BBN reagent formed on hydroboration of 1,5-cyclooctadiene is used to effect hydroboration of the olefin. Carbonylation of the resulting trialkylborane proceeds... 

The paper describes the first asym. hydroboration with a chiral catalyst. E A soln. of norbornene and catalytic amounts of chloro(l,5-cyclooctadiene)rhodium(I) dimer and DIOP (2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane) in THF flushed with argon, stirred at 20° for 15 min, cooled to —78°, 1.2 eqs. catecholborane added, stirred for 5 min, allowed to warm to —40°, maintained at this temp, for 72 h, recooled to —78°, ethanol, 3 M NaOH, and 30% H2O2 added, the mixture allowed to warm to 25° during ca. 1 h, then stirred for 12h cjco-(lR, 2R)-norborneol. Y 99% (e.e. 55%). The method is generally applicable (even to 1,1-disubst. ethylene derivs. ) and expensive chiral boron-reagents are not necessary. F.e.s. K. Burgess, M.J. Ohlmeyer, J. Org. Chem. 53, 5178-9 (1988) from allyl alcohol O-derivs. s. Tetrahedron Letters 30, 395-8 (1989). 

The hydroboration of cyclodienes is also possible at room temperature 1,5-cyclooctadiene reacts with (C2Hs)4B2H2 to give B-ethyl-9-borabicyclo-[3.3.1]nonane (XLV). 

However, the direct and convenient synthesis of (9-BBN)2 has been reported by Knights and Brown [2], and this development opened the door for its application in hydroboration [2-5]. The synthesis involves the cyclic hydroboration of 1,5-cyclooctadiene with a borane-tetrahydrofuran (THF) complex [2, 3] in a 1 1 ratio, followed by refluxing the mixture at 65 °C, thus producing a solution containing (9-BBN)2 in ca. 90% yield. 

These studies also reveal that the hydroboration of 1,5-cyclooctadiene using borane-dimethylsulfide to prepare a solution of 9-BBN can be carried out in solvents other than THF [9]. 

Cyclooctadiene undergoes hydroboration in a 2-M THF solution of BOT. The hydroboration is complete after 0.5 h at 25 °C, and thermal isomerization... 

The neat reaction is performed by the addition of 1,5-cyclooctadiene to BOT at 0 °C. Hydroboration is complete after 0.25 h at 0 °C and after 0.25 h at 25 °C. After thermal isomerization at 165-170 °C for 1 h, 1,4-thioxane is distilled completely from the reaction mixture, leaving (9-BBN)2 as a pure, stable crystalline solid. Both procedures afford (9-BBN)2 almost as a pure solid, which on recrys-tallization from THF, affords (9-BBN)2 crystals, m.p. of 153 °C. Moreover, it is now commercially available [12] in crystalline and in solution forms. 

The 1,5-cyclooctadiene gives variable results [4] on hydroboration with 9-BBN. 

These chiral alkylboronic esters are exceptionally promising intermediates for C-C bond formation reaction in the synthesis [8, 9] of a-chiral aldehydes, P Chiral alcohols, a-chiral acids, and a-chiral amines. Brown et al [10], in a real breakthrough, discovered that LiAlH readily converts these relatively inert boronic esters to a very high reactive lithium monoalkylborohydrides R BHjLi (5) of very high optical purity. The optically active monoalkylborane (R BH2) is generated, when required, by a convenient reaction with trimethylsilyl chloride [6]. Consequently, the desired B-R -9-BBN is prepared conveniently by hydroboration of 1,5-cyclooctadiene with RBHj (prepared in situ), and the desired stable 1,5-isomer is obtained by thermal isomerization. The whole sequence is illustrated in Scheme 9.1. 

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