Hydroboration catalytic asymmetric

There have been several reviews of asymmetric synthesis via chiral organoboranes (6,8,378,382,467—472). Asymmetric induction in the hydroboration reaction may result from the chiraHty present in the olefin (asymmetric substrate), in the reagent (asymmetric hydroboration), or in the catalyst (catalytic asymmetric hydroboration). 

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... 

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>. 

However, by far the most common approach toward catalytic asymmetric hydroboration is to use a chiral catalyst and an achiral borane source. Both homobidentate F Pand heterobidentate P,N ligand classes have been employed in this transformation with varying degrees of success. 

The development of chiral catalysts for use in enantioselective rhodium-catalyzed hydroborations was pioneered by Burgess9, Suzuki,77 and Hayashi.78 The chiral diphosphine ligands employed in their preliminary investigations 23-26 (Figures 2(a) and 2(b)), had previously been successfully applied in other catalytic asymmetric transformations. 

The third major class of diphosphine ligands employed in catalytic asymmetric hydroboration are those possessing planar chirality (Figure 6). These ligands are chiral by virtue of the non-symmetrical disubstitution of one of the cyclopentadienyl rings. 

In 1993, Hayashi and co-workers reported a catalytic asymmetric synthesis of alle-nylboranes 256 by palladium-catalyzed hydroboration of conjugated enynes 253 (Scheme 4.66) [105]. Reaction of but-l-en-3-ynes 253 with catecholborane 254 in the presence of a catalyst, prepared from Pd2(dba)3 CHC13 (1 mol%) and a chiral mono-dentate phosphine ligand (S)-MeO-MOP 255 (1 mol%), gave an allenylborane 256. The ee of 256 was determined by the reaction with benzaldehyde affording the corresponding optically active homopropargyl alcohols 257 with up to 61% ee (syn anti= 1 1—3 1). 

Fig. 2.9 Template structures for the intermediate in catalytic asymmetric hydroboration with alkene, ligand, catechoborane and hydride coordinated, (a) R-BINAP and (b) R-QUINAP. E-propenylbenzene is illustrated to show the increased steric hindrance to the alkene in the BINAP case.

Thanks go especially to the students who have contributed over the past several years to the Oxford publications on catalytic asymmetric hydroboration, most recently Antonia Black, Christophe Pichon, and Kenji Maeda. 

As an alternative to the stoichiometric enantioselective hydroboration, catalytic hydroboration using chiral catalysts has been also developed for enantioselective hydroboration The catalytic hydroboration-amination methodology has been successfully applied as a one-pot reaction for the asymmetric synthesis of primary... 

Many different organoboranes have been developed and are in use at present to control selectivities, which makes hydroboration a particularly valuable and flexible process. A further important advantage of this method is that a wide range of functional groups tolerate hydroboration. Catalytic hydroboration, a recent development, is a new way of asymmetric hydroborations. Detailed treatment of hydroboration can be found in monographs321 323 324 331 and review papers.320 325 332-335... 

In this chapter, recent advances in asymmetric hydrosilylations promoted by chiral transition-metal catalysts will be reviewed, which attained spectacular increase in enantioselectivity in the 1990s [1], After our previous review in the original Catalytic Asymmetric Synthesis, which covered literature through the end of 1992 [2], various chiral Pn, Nn, and P-N type ligands have been developed extensively with great successes. In addition to common rhodium and palladium catalysts, other new chiral transition-metal catalysts, including Ti and Ru complexes, have emerged. This chapter also discusses catalytic hydrometallation reactions other than hydrosily-lation such as hydroboration and hydroalumination. 

Asymmetric hydroboration of CtHsCH=CH2.2 The reaction of cate-cholborane with styrene provides, after oxidation, 2-phenylethanol. Hydroboration catalyzed by a cationic rhodium catalyst, [Rh(COD)2]+BF4 and dppb, provides 1-phenylethanol (99 1) in 86% yield. A catalytic asymmetric hydroboration is possible with BINAP. Use of (-)-BINAP as ligand and a temperature of -78° provides 1-phenylethanol in 96% ee (equation I). 

The catalytic asymmetric hydroboration reaction can also be applied to OTei o-bicyclic hydrazines. The resulting alcohols are of great synthetic interest and can lead to cyclopentanic diamino alcohols with good enantiomeric purity (equation 14). Interestingly, a reversal of enantioselectivity is observed between reactions employing rhodium and iridium catalysts. ... 

Without question, the most important developments in this field over the past 10 years have been in the area of enantioselective hydroborations. New chiral catalyst systems are typically tested in hydroborations of vinyl arenes, as reactions using HBcat and a cationic rhodium catalyst are well known to give selective formation of the unusual branched isomer. In related studies, enantiopure 2,2-disubstituted cyclopropyl boronates were easily prepared via the catalytic asymmetric hydroboration of 3,3-disubstituted cyclopropenes using a number of chiral neutral rhodium complexes (equation 13). Directing groups, such as esters and alkoxymethyl substituents, were necessary for achieving... 

Table 3. Selected Chiral Alcohols via Catalytic Asymmetric Hydroboration/Oxidation of Alkenes with 1,3,2-Benzodioxaborole...

Catalytic asymmetric hydroboration has been most extensively studied with styrene (4) as the substrate which produces 1-phenylethanol (6) after treatment of the hydroboration product, l-phenyl-l,3,2-benzodioxaborole (5), with alkaline hydrogen peroxide (Scheme 2). The regioselectivity favoring the branched isomer 5 over the linear isomer 5 is usually high when the reaction is carried out with rhodium complexes coordinated with chelating ligands such as bisphos-... 

Table 2. Catalytic asymmetric hydroboration of styrene derivatives (16) with catecholborane (Scheme 3) ...

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

Table 5. Catalytic asymmetric hydroboration of 1,1-disubstituted olefins (29) with catecholb-orane (Scheme 8)...

The catalytic asymmetric hydroboration of 1,1-disubstituted alkenes R R C=CH2 with pinacolborane has been attained (<92% ee) by using complexes of iridium with chiral phospine-oxazolidines as catalysts (2.5 mol%). The same class of compounds has been shown to facilitate hydroboration (<98% ee) catalysed by chiral complexes of CuCl with a bidentate A)-heterocyclic carbene (NHC) containing SOg" as an additional ligating group. ... 

---