Norbornanols 211

Reduction of either the exo or endo isomer of 2-phenyl-2-norbornanol with trifluoroacetic acid and triethylsilane, triphenylsilane, or phenylsilane in dichloro-methane gives endo-2-phenylnorbomane quantitatively (Eq. 24).164 The stereospecific formation of only the endo-hydrocarbon can be understood on the basis that only exo approach by organosilicon hydride toward the 2-phenylnorbornyl cation intermediate is kinetically competitive for product formation.164... 

Using two types of specially synthesized rhodium-complexes (12a/12b), pyruvate is chemically hydrogenated to produce racemic lactate. Within the mixture, both a d- and L-specific lactate dehydrogenase (d-/l-LDH) are co-immobilized, which oxidize the lactate back to pyruvate while reducing NAD+ to NADH (Scheme 43.4). The reduced cofactor is then used by the producing enzyme (ADH from horse liver, HL-ADH), to reduce a ketone to an alcohol. Two examples have been examined. The first example is the reduction of cyclohexanone to cyclohexanol, which proceeded to 100% conversion after 8 days, resulting in total TONs (TTNs) of 1500 for the Rh-complexes 12 and 50 for NAD. The second example concerns the reduction of ( )-2-norbornanone to 72% endo-norbor-nanol (38% ee) and 28% exo-norbornanol (>99% ee), which was also completed in 8 days, and resulted in the same TTNs as for the first case. 

When norbornene is treated with trichlorosilane in this manner, quantitative yield of evo-2-trichlorosilylnorbornane is obtained, and (lS,2S,4R)-exo-2-norbornanol can be obtained in 96% ee upon hydrogen peroxide oxidation.28 This reaction can be extended to other olefinic substrates such as 2,5-... 

The dehydration of norbornanols was carried out at 280° and 310° over acidic alumina and over alumina modified by piperidine (73). The rate of dehydration of norbornanols is about three to six times slower than that of the corresponding bornanols. 2-ea o-Norbornanol dehydrates six times faster than 2-ewdo-norbornanol. These results agree with those obtained by Winstein and Trifan (74) from the solvolysis studies of the corresponding j-toluenesulfonates and chlorides. 

Nortricyclene is the only product of dehydration of 2-ewdo-norbornanol in the presence of the modified alumina. With longer contact time especially at higher temperature, the nortricyclene isomerizes to norbornene. 2-ea o-Norbornanol forms 70% nortricyclene and 30% norbornene. 

The greater rate of dehydration of 2-exo- over 2-endo-norbornanol can be interpreted by an anchimeric assistance which involves the delocalization of C(l)-C(6) bonding electron pair this helps in the removal of a hydroxyl ion and facilitates dehydration. This delocalization is probably responsible for the formation of norbornene as one of the primary products of dehydration. 

In order to explain the formation of nortricyclene from 2-ea o-norbornanol, it is necessary to assume a back side attack at the hydrogen attached to carbon 6. The general mechanism here is similar to the trans elimination reaction as discussed under menthol, 1,4-cyclohexanediol, and bornanols. 

By VPC based on RCOR. Isolated yields are in parentheses. 100% Excess monolithiutn acetylide was used. The product was >99% 2-ethynyl-endo-2-norbornanol by VPC and NMR examination. 

M. Mark Midland, Jim I. McLoughlin, and Ralph T. Werley, Jr., u PREPARATION AND USE OF LITHIUM ACETYLIDE 1-METHYL-2-ETHYNYL-endo-3,3-DIMETHYL-2-NORBORNANOL... 

Preparation and Use of Lithium Acetylide 1-Methyl-2-Ethynyl-endo-3,3-Dimethyl-2-Norbornanol... 

NMR measurements in the presence of lanthanide shift reagents are very popular when assigning bicyclo[2.2.1]heptanes, e.g., for 2-norbornanols and their methylated derivatives 591 597. The same is true for related tricyclic compounds such as derivatives of dicyclopen-tadienes598-600 1 and benzo-7-phosphanorbornenes601 2. 

Interestingly, when solvents possessing a lower polarity than benzene—such as heptane—are employed, a substantial acceleration of the oxidation can be observed. Thus, endo-2-norbornanol (70) is oxidized 11 times faster in heptane than in benzene.5 In fact, even weak ligands such as alkenes can produce a substantial slowing of the oxidation. For example, endo-2-norbornenol (71) reacts 50 times slower than endo-2-norbornanol (70) with Fetizon s reagent.5... 

In the reactions discussed and exemplified above, reactants, transient species and products are related by linear sequences of elementary reactions. The transient species can be regarded as a kinetic product and, if isolable, subject to the usual tests for stability to the reaction conditions. Multiple products, however, may also occur by a mechanism involving branching. Indeed, the case shown earlier in Fig. 9.5b, where the transient is a cul de sac species, is the one in which the branching to the thermodynamic product P and kinetic product T occurs directly from the reactant. In the absence of reversibility, the scheme becomes as that shown in Scheme 9.8a, where the stable products P and Q are formed as, for example, in the stereoselective reduction of a ketone to give diastereoisomeric alcohols. The reduction of 2-norbornanone to a mixture of exo- and cndo-2-norbornanols by sodium borohydride is a classic case. The product ratio is constant over the course of the reaction and reflects directly the ratio of rate constants for the competing reactions. The pseudo-first-order rate constant for disappearance of R is the sum of the component rate constants. 

Less direct precedents are also available for the rearrangement of both exo-1,2-trimethylenenorbornane (32) and emfo-2,6-trimethylenenorbornane (33) to adamantane. Solvolysis of exo-1,6-trimethylene-exo-2-norbornyl tosylate in aqueous acetone at room temperature gives nearly quantitative yields of endo-2,6-trimethylene-ejco-2-norbornanol [Eq. (12)], which, when treated with sulfuric acid, produces 1-adamantanol43). 

The same method also results in quantitative yields from the oxidation of 1,2-diphenylethanol and in much improved yields from oxidation of 7-norbornanol. ... 

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. 

Recently, much better results have been obtained using the palladium catalyst D [with ( + )-(/ )-MOP], Hydrosilylation of norbomene with trichlorosilane in the presence of this catalyst gives a quantitative yield of e.vo-2-trichlorosilylbornane which upon oxidation produces (l.S .2.S.4f )- .w-2-norbornanol (25) with 96% ee30. 

Gas chromatographic analysis shows this material to have a purity of about 96%. Besides a small amount of water (up to 0.5%) there are two minor impurities. Neither 2-cx< -norbornyl formate nor 2-ew-norbornanol is present, however. Oxidation of 2-ep o-norbornanol with chromic acid, under a variety of conditions, gives 2-norbornanone contaminated with some starting material. 

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