Hydroboration with catecholborane

Hydroboration of conjugated dienes proceeds without a catalyst to give 1,2-adducts. However, the less reactive catecholborane reacts with isoprene with catalysis by Pd(PhiP)4, yielding the 1,4-adduct 73[66]. 

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.1 Hydroborations with this reagent in CH2C12 or THF (heterogeneous) are relatively slow (5-12 hours at 25°), but application of ultrasound permits complete reaction in 1-2 hours. Ultrasound also increases the rate of hydroboration with catecholborane or with 9-BBN (THF or neat). 

Scheme 3 Synthesis of Alkylboronates by Hydroboration with Catecholborane and Wilkinsons Catalyst 14-2 1...

Hydroboration proceeds without a catalyst, but hydroboration with less active catecholborane (225) is accelerated by catalysts. Usually 1,2-addition to conjugated dienes takes place, but the Pd-catalysed reaction of catecholborane (225) gives the 1,4-adduct 226. This reaction is not catalysed by Rh complexes [98], Hydroalumination of conjugated dienes catalysed by Cp2TiCl2 affords the allylic aluminium compounds 227 by 1,4-addition. The Pd-catalysed hydrostannation of isoprene with HSnBu3 affords the (Z)-2-alkenylstannane 228 with high regio- and stereoselectivities [99],... 

The rate of hydroboration with catecholborane and pinacolborane can be tremendously increased by the addition of transition metal catalysts. Hydroboration of pinacolborane 49 with alkenes 50 <1996JA909> and terminal alkynes 52 <19950M3127> proceeds with high regioselectivity in the presence of catalytic HZrCp2Cl furnishing the terminal boronates 51 and vinylboronates 53, respectively (Scheme 8). 

Hydroboration of alkenes1 (15, 91). The Rh(I)-catalyzed hydroboration provides a highly diastereoselective reaction in a synthesis of a poly ether antibiotic. Thus the derivative (1) of an acyclic allylic alcohol is converted to the primary alcohol 2 by hydroboration with catecholborane (CB) catalyzed by ClRh[P(C6H5)3]3 with 94 6 selectivity. Note that hydroboration of 1 with disiamylborane (12, 484) proceeds with the opposite selectivity at Cio (8 92). 

In 2005, Renaud and co-workers reported a novel procedure for the formal hydrogenation of alkenes via hydroboration with an excess of catecholborane, followed by treatment of the intermediate boronic acid esters with methanol in the presence of air as a radical initiator [88]. A typical example, the reduction of 43 to 44, is shown in Scheme 38. Similar results were obtained for a wide range of primary, secondary, and tertiary alkylcatecholboranes. 

Hydroboration of alkenes.2 Wilkinson s catalyst is effective for catalysis of hydroboration with catecholborane (CB), which can differ from the regioselectivity and stereochemistry of uncatalyzed hydroboration with 9-BBN. In the case of... 

Addition to C=C. Chiral boranes obtained from Rh-complex-catalyzed hydroboration with catecholborane are transformed into amines with retention of configuration on sequential treatment with MeMgCl and H NOSOjH. 

Another application of the Wilkinson-type catalyst 18 is the rhodium catalyzed hydroboration of olefins [19]. Various alkenes 20 (internal, terminal, styrenes, etc.) have been successfully hydroborated with catecholborane (21) providing the corresponding boronic esters 22 in nearly quantitative yield. Oxidative work-up (Na0H/H202) led to the corresponding alcohols 23 in 76-90% yield Eq. (11). 

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... 

Hydroboration with catecholborane (CB).2 This lanthanide is an efficient catalyst for hydroboration of a wide range of alkcncs with CB at 25°. Highest rates obtain in ihe order monosubstituted terminal > disubstituted terminal > disubstituted internal > trisubstituted. 

A recent example demonstrates that organoboranes can serve as useful precursors to carbon radicals that can then be trapped by nitroxides [14], For example, (+)-2-carene (6) was hydroborated with catecholborane and then allowed to react with two equivalents of TEMPO to produce the A-alkoxyamine 8 (Scheme 2). The first equivalent of TEMPO attacks the organoborane intermediate 7 in an overall Sh2 fashion, which probably involves the intermediacy of a boron ate -like radical species. The resulting cyclopropylcarbinyl intermediate rearranges to a homoallylic... 

Products from hydroboration with catecholborane in fluorous solvents are extractable by THF, while the catalyst [(Cf,Fi3CH2CH2)jP]3RhCl (0.04 mol%) remains in the fluorous... 

Homologation. One-carbon extension of alkenes via hydroboration (with catecholborane) is by further treatment with Me3SiCHN2 before oxidative workup. 

Hydroboration with catecholborane catalyzed by a rhodium complex of l-(2-diphenylphosphino-l-naphthyl)isoquinoline (5) is accompanied by high enantio-selectivity. Bis(l-neomenthylindenyl)zirconium dichloride (6) promotes alkylalumination of 1-alkenes through a noncyclic mechanism. 

Catecholborane hydroborations of carbonyl and related functionalities are also accelerated by RhCl(PPh3)3 (eqs 39 1) however, several related reactions proceed with similar selectivities in the absence of rhodium. ... 

Hydroboration. Pinacolborane is a stable, easily prepared and stored hydroborating agent. Unlike catecholborane which requires harsh reaction conditions for hydroboration of alkenes (100 °C) and alkynes (70 °C), hydroboration with pinacolborane proceeds under mild conditions furnishing the boronates. JCnochel and co-workers observed an excellent level of regioselectivity for hydroboration of alkynes with pinacolborane at room temperature (eq 2). Alkenes, however, react slowly with pinacolborane and often require heating for 2-3 days to furnish the terminal pinacolboronates as the major regioisomer (>98%) (eq 3). 

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). 

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