Hydroboration of norbornene

Table 2 Hydroboration of norbornene, indene, and styrene using catecholborane...

Table 4 Hydroboration of norbornene using phosphoramidite ligands 35-38...

Asymmetric hydroboration of norbornene (27) is a synthetically useful transformation forming optically active norbornanol (28) which is an important chiral synthon. The catalytic enantioselective hydroboration with catecholborane was examined using rhodium complexes coordinated with several chiral phosphine ligands (Scheme 7 and Table 4) [14,15,17,23,24,27]. For this reaction, DIOP (10) and its derivatives 21 and 22, which are modified on the diphenyl-phosphino group, are more enantioselective Hgands than BINAP (7) or chira-phos (9). The highest enantioselectivity was observed in the reaction at -25 °C... 

Table 4.Catalytic asymmetric hydroboration of norbornene (27) with catecbolborane forming e co-norbornanol (28) (Scheme 7)...

A 500-ml flask is equipped with a thermometer, a magnetic stirrer, and a dropping funnel, and all openings are protected by drying tubes. The system is flushed with nitrogen and a solution of 2.84 g (0.075 mole) of sodium borohydride in 150 ml of diglyme is introduced followed by 28.3 g (0.30 mole) of norbornene. The flask is immersed in an ice-water bath and the hydroboration is achieved by the dropwise addition of 27.4 ml (0.10 mole) of boron trifluoride diglymate. The solution is stirred... 

Early work in the field of asymmetric hydroboration employed norbornene as a simple unsaturated substrate. A range of chiral-chelating phosphine ligands were probed (DIOP (5), 2,2 -bis(diphenyl-phosphino)-l,l -binaphthyl (BINAP) (6), 2,3-bis(diphenylphosphino)butane (CHIRAPHOS) (7), 2,4-bis(diphenylphosphino)pentane (BDPP) (8), and l,2-(bis(o-methoxyphenyl)(phenyl)phos-phino)ethane) (DIPAMP) (9)) in combination with [Rh(COD)Cl]2 and catecholborane at room temperature (Scheme 8).45 General observations were that enantioselectivities increased as the temperature was lowered below ambient, but that variations of solvent (THF, benzene, or toluene) had little impact. 

Application of these ligands to the hydroboration-oxidation of styrene proceeded with moderate yields and much lower enantioselectivities than for norbornene. The best result was with (R, R)-38, which afforded (A)-l-phenyletha-nol in 61% yield and 42% ee. However, Bianchini s (R, 7 )-BDPBzP 39 was even less efficient for this substrate, and gave both poor yields (29%) and poor enantioselectivities (26% ee) for the hydroboration of styrene at 0°C.84... 

The hydrosilylation-oxidation of simple unfunctionalized alkenes has not been widely used for the diastereoselective preparation of alcohols, probably because it would not in general be expected to give very different results to hydroboration oxidation. One example which has been reported is the exo-selective hydrosilylation of norbornene (1) with trichlorosilane and hexachloroplatinic acid6, followed by oxidation to c.vo-norbornanol (2)1. 

Thus hydroboration-oxidation of norbornene yields 99.5% exoiV> (10). Similarly, the base-catalyzed deuterium exchange of norcamphor gives an exo endo ratio of 71535 (32). It is now suggested that ionization in such U-shaped structures likewise... 

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). 

Diblock copolymers having both polydiene and polyalcoholic blocks have been synthesized by the hydroboration of butadiene-isoprene diblock copolymer, followed by H2O2 oxidation [2]. Hydroxyl groups containing polymers have been prepared by the hydroboration of 2,5-norbomadiene with 9-BBN in the presence of a WClg-Me4Sn catalyst, followed by alkaline HjOj to afford poly(exo-5-hydroxy norbornene) (Eq. 21.1) [3]. 

Diels-Alder reaction of cyclopentadiene and butadiene affords a mixture of exo-5-vinyl-2-norbornene (la) and e do-5-vinyl-2-norbornene (lb) [1], Preparative GC separation [1] of these isomers encounters difficulties in obtaining the individual isomer in pure form and in large quantities. An alternative approach of separation via thermal isomerization [2], in which lb gets transferred to 4,7,3a,7a-tetrahydro-lff-indene, whereas la remains unchanged, is also not successful. This is because it is difficult to prevent la being contaminated by unreacted lb. As no other method is available for their separation, Inoue has reported [3] that hydroboration of 1 with 9-BBN, followed by oxidation with alkaline hydrogen peroxide results in the formation of alcohols 2a and 2b. The iodoether cyclization only of the endo isomer takes place. The sequence of approach is delineated in Scheme 29.1. 

In a 200-ml three-necked flask fitted with a dropping funnel (drying tube) is placed a solution of 13.4 g (0.12 mole) of 1-octene in 35 ml of THF. The flask is flushed with nitrogen and 3.7 ml of a 0.5 M solution of diborane (0.012 mole of hydride) in THF is added to carry out the hydroboration. (See Chapter 4, Section I regarding preparation of diborane in THF.) After 1 hour, 1.8 ml (0.1 mole) of water is added, followed by 4.4 g (0.06 mole) of methyl vinyl ketone, and the mixture is stirred for 1 hour at room temperature. The solvent is removed, and the residue is dissolved in ether, dried, and distilled. 2-Dodecanone has bp 119710 mm, 24571 atm. (The product contains 15 % of 5-methyl-2-undecane.) The reaction sequence can be applied successfully to a variety of olefins including cyclopentene, cyclohexene, and norbornene. 

Formal hydration of the double bond appeared by the hydroboration-oxidation sequence. Desymmetrization reactions with catalytic asymmetric hydroboration are not restricted to norbornene or nonfunctionalized substrates and can be successfully applied to meso bicyclic hydrazines. In the case of 157, hydroxy derivative 158 is formed with only moderate enantioselectivity both using Rh or Ir precatalysts. Interestingly, a reversal of enantioselectivity is observed for the catalytic desymmetrization reaction by exchanging these two transition metals. Rh-catalyzed hydroboration involves a metal-H insertion, and a boryl migration is involved when using an Ir precatalyst (Equation 17) <2002JA12098, 2002JOC3522>. 

Attempts to expand the range of substrates to include cyclopropenes20 and norbornene-derived meso-bicyclic hydrazines25 have recently been reported. A further noteworthy contribution has been the development of a recyclable hydroboration process by the group of Fernandez26-28 These and other approaches toward... 

A norbornene molecule labeled with deuterium is subjected to hydroboration-oxidation. Give the structures of the intermediates and products. 

A similar approach uses the co-polymerization of ethylene with 5-ethylidene-2-norbornene, followed by hydroboration/oxidation of the unreacted vinyl group. The hydroxylic functions in the co-polymer are then converted into -OAlEt2 groups and used as catalysts for -caprolactone polymerization, thus leading to poly(ethylene-fo-ENB)-graft-polycaprolactone co-polymers.600... 

Consequently, the interconversion of classical cations at a rate comparable with capture by a nucleophile (Brown) is not valid here since it is in chlorides and not in acetates that the less homogeneous deuterium distribution should have been expected. The authors maintain that the HX-type reagents are added to nor-bomene and some other similar bicyclic olefins both through intermediate formation of nonclassical ions and by simultaneous cyclic processes. As concerns Brown s data on the addition of HQ and CHjCOOH to 7,7-dimethylnorbomene, Cristol believes them to be inapplicable to unsubstituted norbornenes since in the 7,7-di-methylnorbornene the simultaneous processes are strongly hindered. Cristol s objections, however, are hardly convincing because epoxidation, hydroboration etc. proceed in the case of 7,7-dimethylnorbomene as weU though they only result in endo products. 

Hoveyda et al. reported the enantioselective synthesis of (+)-africanol through asymmetric olefin metathesis followed by one-carbon contraction via decarbonylation (Scheme 8.16) [65]. When symmetrical norbornene 84 was treated with a chiral molybdenum catalyst, desymmetrization took place to afford the chiral bicycle 85 in an enantioselective fashion. The resulting vinyl terminus was subsequently transformed into a methyl group via hydroboration, oxidation to aldehydes, and decarbonylation. Further manipulations of the functional groups gave (+)-africanol. 

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