Hydroboration catecholborane-rhodium

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... 

The differences in the steric effect between catecholborane and pinacolborane, and the valence effect between a cationic or neutral rhodium complex reverse the re-gioselechvity for fluoroalkenes (Scheme 1-4) [26]. The reaction affords one of two possible isomers with excellent regioselectivity by selecting borane and the catalyst appropriately, whereas the uncatalyzed reaction of 9-BBN or SiaiBH failed to yield the hydroboration products because of the low nucleophilicity of fluoroalkenes. The regiochemical preference is consistent with the selectivity that is observed in the hydroboration of styrene. Thus, the internal products are selectively obtained when using a cationic rhodium and small catecholborane while bulky pinacolborane yields terminal products in the presence of a neutral rhodium catalyst. 

Mannig and Noth reported the first example of rhodium-catalyzed hydroboration to C=C bonds in 1985.4 Catecholborane reacts at room temperature with 5-hexene-2-one at the carbonyl double bond when the reaction was run in the presence of 5mol.% Wilkinson s catalyst [Rh(PPh3)3Cl], addition of the B—H bond across the C=C double bond was observed affording the anti-Markovnikoff ketone as the major product (Scheme 2). Other rhodium complexes showed good catalytic properties ([Rh(COD)Cl2]2, [ Rh(PPh3)2(C O )C 1], where... 

It took another decade however before the idea of developing a rhodium-catalyzed olefin hydroboration process came to fruition. This occurred in 1985 when Mannig and Noth reported the first examples of such a process.8 They discovered that Wilkinson s catalyst 2 was effective for the addition of catecholborane 1 to a range of alkenes and alkynes, as exemplified by cyclopentene 4 (Scheme 2). 

While Josiphos 41 also possessed an element of atom-centered chirality in the side chain, Reetz reported a new class of ferrocene-derived diphosphines which had planar chirality only ligands 42 and 43, which have C2- and C -symmetry, respectively.87 Rhodium(i)-complexes of ligands (—)-42 and (—)-43 were used in situ as catalysts (0.75 mol%) for the hydroboration of styrene with catecholborane 1 for 12 h in toluene at — 50 °C. The rhodium/ i-symmetric (—)-43 catalyst system was the more enantioselective of the two - ( -l-phenylethanol was afforded with 52% and 77% ee with diphosphines (—)-42 and (—)-43, respectively. In both cases, the regioselectivity was excellent (>99 1). With the same reaction time but using DME as solvent at lower temperature (—60 °C), the rhodium complex of 43 afforded the alcohol product with an optimum 84% ee. 

Ligand 73 was prepared directly from a single enantiomer of the corresponding naphthol of QUINAP 60, an early intermediate in the original synthesis, and both enantiomers of BINOL. Application in hydroboration found that, in practice, only one of the cationic rhodium complexes of the diastereomeric pair proved effective, (aA, A)-73. While (aA, A)-73 gave 68% ee for the hydroboration of styrene (70% yield), the diastereomer (aA, R)-73 afforded the product alcohol after oxidation with an attenuated 2% ee (55% yield) and the same trend was apparent in the hydroboration of electron-poor vinylarenes. Indeed, even with (aA, A)-73, the asymmetries induced were very modest (31-51% ee). The hydroboration pre-catalyst was examined in the presence of catecholborane 1 at low temperatures and binuclear reactive intermediates were identified. However, when similar experiments were conducted with QUINAP 60, no intermediates of the same structural type were found.100... 

In an attempt to rationalize the factors that control selectivity in the Rh- and Ir-catalyzed hydroboration reactions, Fernandez and Bo [35] carried out experimental and theoretical studies on the H—B addition of catecholborane to vinylarenes with [M(C0D)(R-QUINAP)]BF4, (QUINAP = l-(2-diphenylphosphino-l-naphthyl) isoquinoHne). A considerable difference was found in the stability of the isomers when the substrate was coordinated to the iridium(I) or rhodium(I) complexes. In particular, the difference between pro-R B1 and pro-S B2 isomers was not so great when the metal center was iridium and not rhodium (Figure 7.1), which explains the low ee-values observed experimentally when asymmetric iridium-catalyzed hydroboration was performed. Structurally, the energy analysis of the n2 and Tti interactions [36] seems to be responsible for the extra stabilization of the B2 isomer in the iridium intermediates (Figure 7.1). The coordination and insertion of alkenes, then, could be considered key steps in the enantiodifferentiation pathway. 

For adoption by the synthetic-organic community, a new method must pass certain tests. If it is catalytic, then the turnover must be efficient so that the quantity of (presumably expensive) catalyst employed is small. The reaction needs to be selective in all the desirable ways, where appropriate including chemo-, regio-, and stereospecificity. Much of the work on hydroboration has been aimed at progress toward those goals. The rhodium-catalyzed reaction of catecholborane with a simple styrene frequently forms part of the standard catalytic screening procedures for a novel ligand. 

NaOH-HiOz) or to carboxylic acids (with w-chloroperbenzoic acid).-190 Double bonds can be hydroborated in the presence of triple bonds if the reagent is 9-BBN.191 On the other hand, dimesitylborane selectively hydroborates triple bonds in the presence of double bonds.192 Furthermore, it is often possible to hydroborate selectively one particular double bond of a nonconjugated diene.191 When the reagent is catecholborane, hydroboration is catalyzed by rhodium complexes, such as Wilkinson s catalyst.194 Enantioselective hydro-boration-oxidation has been achieved by the use of optically active rhodium complexes.195... 

Hydroboration of a variety of alkenes and terminal alkynes with catecholborane in the fluorous solvent perfluoromethylcyclohexane was performed using fluorous analogs of the Wilkinson catalyst.135 136 Recycling of a rhodium-based alkene hydrosilylation catalyst was also successful.137 Activated aromatics and naphthalene showed satisfactory reactivity in Friedel-Crafts acylation with acid anhydrides in the presence of Yb tris(perfluoroalkanesulfonyl)methide catalysts.138... 

In the hydroboration/oxidation reaction of 2-cyclohexen-l-ol derivatives the regio- as well as the stereoselectivities can be steered in either of two directions by use of different reagents and reaction conditions. Dialkylboranes, e.g., 9-BBN, give mainly tn rs-l,2-diols, whereas with 1,3,2-benzodioxaborole ( catecholborane") in the presence of the rhodium hydrogenation catalyst Rh(PPh3)jCl the trans-1,3-diols become predominant. The cis- 1,2-diols are usually only formed in trace amounts, but the cis-l,3-diols are always produced as byproducts in 10 — 20% yield (D.A. Evans, 1988). 

TABLE 2.5. Asymmetric Hydroboration of Olefins with Rhodium Catalysts and Catecholborane ... 

Rhodium-catalysed asymmetric cyclization/hydroboration followed either by Pd-catalysed arylation or by oxidation was applied to the synthesis of a number of chiral, non-racemic carbocycles and heterocycles. Thus, reaction of enyne (28) with catecholborane, catalysed by a 1 1 mixture of [Rh(COD)2]+ Sbly,- and (S)-BINAP (5 mol%), followed by Pd-catalysed arylation with /7-IC6H4CF3, afforded benzyli-denecyclopentane (29) in 65% yield with 88% ee.46... 

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>. 

Catalytic hydroboration is a new methodology of great synthetic potential. The reaction is usually carried out with catecholborane in the presence of rhodium, palladium, iridium and ruthenium compounds.2 In contrast to olefins, very little is known on catalytic hydroboration of conjugated dienes and enynes. Our earlier studies on the uncatalyzed monohydroboration of conjugated dienes,6 reports on the hydroboration of 1-decene with catecholborane catalyzed by lanthanide iodides,7 and monohydroboration of 1,3-enynes in the presence of palladium compounds,8 prompted us to search for other transition metal catalysts for monohydroboration of conjugated dienes and enynes 9 10... 

In contrast to olefins, little is known on catalytic hydroboration of conjugated dienes. Suzuki and Miyaura20 described a 1,4-addition of catecholborane to acyclic 1,3-dienes, catalyzed with tetrakis(triphenylphosphine)pa]ladium(0). An interesting Markovnikov type regioselectivity was observed in the enantioselective dihydroboration of (E)-1-phenyl-1,3-butadiene with catecholborane, catalyzed by chiral rhodium complexes.21 However, the scope of these reactions is not well known, and the choice of catalysts is very limited. 

Complexes of cationic rhodium compounds with asymmetric phosphane ligands catalyze the hydroboration of prostereogenic alkenes with catecholborane (see Section D.2.5.2.1.4.). The product alcohols are of 7-96% cc (Table 3). Enantioselectivity is excellent for the hydroboration/oxidation of styrenes but low for stilbenes. The small number of examples studied to date precludes generalizations, however, compared to the uncatalyzed reaction, opposite regioselec-tivity is observed for the addition to styrenes. 

SCHEME E A mechanism for the rhodium-catalyzed hydroboration of alkenes using catecholborane (HBcat)... 

Asymmetric Hydroboration. Rhodium complexes are known to catalyze hydroboration of alkenes with unreactive borane derivatives, e.g. catecholborane and oxaborolidine. This reaction proceeds enantioselectively by use of BINAP as a ligand for neutral " or cationic rhodium complexes. Reaction of styrene with catecholborane followed by oxidation affords (R)-1-phenylethanol in 96% ee in the presence of (R)-BINAP and [Rh(cod)2]Bp4 (eq 5). ... 

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