Hydroboration of cyclohexene

With increasing steric hindrance around the double bond, the second and third hydroboration steps become increasingly sluggish and so, for example, hydroboration of cyclohexene with H3B.THF may be stopped at the dialkylborane stage, and hydroboration of 1,2-dimethyIcyclopentene with H3B.THF does not proceed beyond the monoalkylborane. 

Ethylene and BjHn similarly form both CjHjBjHm and (C2H5)2B5Hg. Hydroboration of cyclohexene with BioHi2[S(CH3)2]2 produces 9-cyclohexyl-5(7)-dimethylsul-fide-nido-decaborane(ll), B,oH,(CfiH,)[S(CH3)2]. Also, propylene reacts with (H30)2Bi2H,2-5 H2O to afford [C3H7-Bi2H,J -. 

In addition a few groups can direct Rh(l)- or lr(l)-catalyzcd hydroboration. Thus an amide group can direct hydroboration of cyclohexene-derived alkcncs (equation 111). 

Table 4.6 Rate data and rate constants for the hydroboration of cyclohexene (0.400 M) with (9-BBN)2 (0.200 M) in carbon tetrachloride at 25 C [3]...

RjBOTf reagents are prepared [13] by the controlled addition of trifluoro-methane sulfonic acid (1 equiv) to R BH (1 equiv). RjBOMs reagents are obtained by the controlled addition of methanesulfonic acid (1 equiv) to RjBH (1 equiv) [11]. ChXjBBr is prepared by the direct hydroboration of cyclohexene with monobromoborane methylsulfide [11]. B-Br-9-BBN is synthesized by treating 9-BBN with HBr gas. 

A soln. of 1,4-benzoquinone in ether added dropwise under Ng to refluxing soln. of crude tricyclohexylborane (prepared by hydroboration of cyclohexene) in the same solvent, and refluxing continued 0.5 hr. 2-cyclohexyl-1,4-hydro-quinone. Y 99%. F. e. and limitations s. M. F. Hawthorne and M. Reintjes, Am. Soc. 86, 951 (1964). 

The preparation of Pans-1,2-cyclohexanediol by oxidation of cyclohexene with peroxyformic acid and subsequent hydrolysis of the diol monoformate has been described, and other methods for the preparation of both cis- and trans-l,2-cyclohexanediols were cited. Subsequently the trans diol has been prepared by oxidation of cyclohexene with various peroxy acids, with hydrogen peroxide and selenium dioxide, and with iodine and silver acetate by the Prevost reaction. Alternative methods for preparing the trans isomer are hydroboration of various enol derivatives of cyclohexanone and reduction of Pans-2-cyclohexen-l-ol epoxide with lithium aluminum hydride. cis-1,2-Cyclohexanediol has been prepared by cis hydroxylation of cyclohexene with various reagents or catalysts derived from osmium tetroxide, by solvolysis of Pans-2-halocyclohexanol esters in a manner similar to the Woodward-Prevost reaction, by reduction of cis-2-cyclohexen-l-ol epoxide with lithium aluminum hydride, and by oxymercuration of 2-cyclohexen-l-ol with mercury(II) trifluoro-acetate in the presence of ehloral and subsequent reduction. ... 

Although considerable selectivity for the less-hindered face is shown in the hydroboration of a-pinene (see equation 1) borane-THF shows only moderate selectivity for the different faces of most simple substituted cyclohexenes and methylenecyclohexanes (e.g. Figure... 

When an alkene reacts with BH3 in THF solution, rapid addition to the double bond occurs. Since BH3 has three hydrogens, addition occurs three times, and a trialkylborane, R3B, is formed. For example, 1 molar equivalent of BH3 adds to 3 molar equivalents of cyclohexene to yield tri-cyclohexylborane. When tricyclohexylborane is then treated with aqueous hydrogen peroxide (H202i in basic solution, an oxidation takes place. The three C-B bonds are broken, -OH groups bond to the three carbons, and 3 equivalents of cyclohexanol are produced. The net effect of the two-step hydroboration/oxidation sequence is hydration of the alkene double bond ... 

Scheme 1). Preparation of 1,3-diols by hydroboration of allylic alcohols with thexylborane has been shown to proceed with high stereoselectivity when the substrates are 1-(1-hydroxyalkyl)-cyclohexenes. Stereocontrol in the hydrosilation of allylic and homoaHylic alcohols has been studied jointly by the groups of Tamao... 

Hydroboration and carbonylation of cyclohexene gives an intermediate acylborane, which is reduced by the borohydride reagent. Oxidative work-up gives the aldehyde... 

Hydroboration of representative olefins—such as 1-octene, 1-decene, styrene, o -methyl-styrene, 2-methyl-1-pentene, c/5 -4-methyl-2-pentene, 2-methy 1-2-butene, of-pinene, cyclohexene, 3-carene, 1-pheny 1-2-methyl-1-propene, 2,3-dimethy 1-2-butene and 1,2-dimethylcyclopentene—with dioxane-BH2Cl was carried out in dioxane and dichloromethane solvents. The procedure followed for all the olefins in both the solvents are same. The procedure followed for 1-decene in dichloromethane is representative. 

The kinetics for hydroboration of aikenes are conducted in various solvents such as carbon tetrachloride, hexane, cyclohexane, benzene, and THE 9-BBN exists predominantly as the dimer (9-BBN)2 [2]. After the addition of olefins, at 25 °C, the aliquots from the reaction mixture are removed after appropriate intervals of time, quenched with an excess methanol, and analyzed by GLC for residual olefin. All operations are performed under nitrogen until identical rates are observed for more reactive olefins such as 1-hexene, 2-methyl-1-pen-tene, 3,3-dimethyl-1-butene, and cyclopentene, and variation of olefin concentration does not alter the rate. These results establish that the reaction is first order (Eq. 4.1). Typical data for cyclopentene and cyclohexene are presented in Table 4.1 [1]. 

The relative rates of hydroboration [7] of cyclic alkenes depend on the presence of strain [11] on the carbon-carbon double bond. The double bond of cyclopen-tene is considerably more strained than the double bond of cyclohexene, and this strain is responsible for higher reactivity of cyclopentene. 

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