Hydroboration-oxidation of olefins

HYDROBORATION OF OLEFINS (+)-ISOPINOCAMPHEOL, 52, 59 Hydroboration-oxidation of olefins, 53, 83... 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

Preparation.—Variations continue to appear on the theme of alcohol production by hydroboration-oxidation of olefins. 5-Methoxydialkylboranes react with olefins in the presence of lithium aluminium hydride to afford a new route to trialkylboranes and thence, by carbonylation-oxidation, to trialkylcarbinols. Carbonylation with carbon monoxide is avoided in a new procedure in the presence of an excess of trifluoroacetic anhydride, trialkyl cyanoborates undergo a triple alkyl migration from boron to carbon to give, on oxidation, high yields of trialkylcarbinols (Scheme 126). Tri-... 

The hydroboration-oxidation of olefins is one example of this approach. The process involves a stereospecific ty -addition of H and OH across the olefin r-bond. This reaction sequence is regioselective with unsymmetri-cal olefins. The diastereoselective conversion of E-olefin 177 to 178, and the isomeric Z-olefin 179 to 180, illustrates this process. Note that this discussion applies to relative control of stereochemistry. Whereas use of an enantioselective hydroborating agent might afford 178 and 180 as single enantiomers (or enriched in one enantiomer), 177 would still provide 178 and the diastereomeric olefin (179) would provide the diastereomeric alcohol (180).20... 

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

Olefins, conversion to alcohols, 53, 94 from tosylhydrazones and methyllithium, 51, 69 hydroboration-oxidation of,... 

Figure 1.. The two proposed reaction pathways based on experimental results for hydroboration reactions of olefins catalyzed by the Wilkinson catalyst. (O.A. Oxidative Addition Olefin Migratory Insertion R.E. Reductive Elimination)...

Halogen-lithium exchange of iodide 71 and subsequent addition of 2-acetyl-furan (72) to the resultant organolithium intermediate yielded two diastereomeric tertiary alcohols (dr=l l), which were converted to (E)-olefin 73 with complete diastereoselectivity upon brief exposure to catalytic amounts of concentrated aqueous hydrogen chloride (Scheme 11) [18]. Diastereoselective hydroboration/oxidation of 73 gave largely the desired stereoisomer 74 due to... 

From A17(20)-Olefins The addition of an oxygen function to C-20 is best accomplished by the hydroboration-oxidation reaction on a A17(20) olefin. The ready preparation of such olefins has been discussed earlier predominantly m-olefins are obtained via the Wittig reaction on 17-keto steroids, and the /ra/w-olefins via dehydration reactions. Hydroboration-oxidation of m-A17(20)-olefins gives the 20a-alcohol in high yield, presumably by attack of diborane on the a-side of the double bond 137... 

Chiral alcohols, which are difficult to prepare in high optical purity by asymmetric hydroboration —oxidation of terminal disubstituted olefins, can be readily synthesized using the homologation approach15. 

Table 6.2. Products from the hydroboration-oxidation of representative olefins with 9-BBN in THF [2]...

Shortly after the key mechanistic papers on rhodium-catalyzed hydroboration, Marks reported a hydroboration reaction catalyzed by lanthanide complexes that proceeds by a completely different mechanism.63 Simple lanthanide salts such as Sml3 were also shown to catalyze the hydroboration of a range of olefins.64 The mechanism for this reaction was found to be complex and unknown. As in other reactions catalyzed by lanthanides, it is proposed that the entire catalytic cycle takes place without any changes in oxidation state on the central metal. 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

The iodoetherification strategy was applied to the synthesis of the smaller fragment coupling component 109 as well (Scheme 16). Silylation of alcohol 104 [30] (76% de) allowed the separation of the pure desired diastereomer, which in turn was subjected to hydroboration/oxidation, sulfide formation with thiol 105, and oxidation to give sulfone 106. The requisite y-triethylsilyloxy alkene functionality in 107 was constructed as a diastereomeric E) Z)=l.2 l mixture by another sulfone-based olefination of aldehyde 90 with 106. Treatment of 106 with... 

Addition of 4-pentenyllithium to the dione 65 gave the ds-diol 71 which was converted to the (Z)-l,2-disubstituted cyclododecene 72a. Hydroboration-oxidation and chromium trioxide oxidation provided the dialdehyde 72b whose McMurry ring closure, followed by partial catalytic hydrogenation gave the (Z)[10.10] precursor 73. Treatment of this (Z)-olefin 73 with HzS04-Ac0H in benzene was reported to effect conversion into [10.10]betweenanene (61b) of 95% purity and high yield. 

Treatment with alkaline H202 oxidizes trialkylboranes to esters of boric acid. 1 This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles. The R group does not rearrange, and this reaction is a step in the hydroboration method of converting olefins to alcohols (5-12). The mechanism has been formulated as involving a rearrangement from boron to oxygen 311... 

Various prochiral olefins are hydroborated by Rh complexes of BINAP or DIOP in up to 96% optical yield (30h, 31). Oxidation of the products provides a convenient way to produce optically active alcohols. Reaction of styrene and catecholborane in the presence of a BINAP-Rh complex at low temperature forms, after oxidative workup, 1-phenylethyl alcohol in 96% ee (Scheme 11) (31). Double stereodifferentiation occurs in the BINAP-Rh catalyzed reaction of 4-methoxy-styrene and an ephedrine-derived chiral borane (32). 

There are other stereospecific olefin addition processes which occur with cis or syn stereochemistry. Common examples include catalytic hydrogenation, hydroboration/oxidation, and dihydroxylation using osmium tetroxide. The stereospecificity of these syn additions requires that die facial properties of the olefinic bond be maintained throughout die addition process and that both new bonds are formed to the same face of the olefin. This is normally accomplished by a concerted syn addition to the n system. 

M. Zaidlewicz, Formation of C-0 Bonds by Hydroboration of Olefinic Double Bonds Followed by Oxidation, in Stereoselective Synthesis (Houben-Weyl) 4th ed. 1996, (G. Helmchen, R. W. Hoffmann, J. Mulzer, E. Schaumann, Eds.), 1996, Vol. E 21 (Workbench Edition), 8, 4519 1530, Georg Thieme Verlag, Stuttgart. 

The epoxidation-epoxide opening sequence with this reagent provides a convenient access to the products of an //-Markovnikov addition of water to olefins. Interestingly, the Cp2TiCl/H20 couple combination shows anti stereoselectivity in the reduction step [73, 74], which is complementary to the hydroboration-oxidation method (Scheme 32). 

Fig. 3.16. cis-l Iy drat ion of an olefin via the reaction sequence hydroboration/ oxidation/hydrolysis. 

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