Chiral catalyzed hydroboration

The field of transition metal-catalyzed hydroboration has developed enormously over the last 20 years and is now one of the most powerful techniques for the transformation of C=C and C=C bonds.1-3 While hydroboration is possible in the absence of a metal catalyst, some of the more common borane reagents attached to heteroatom groups (e.g., catecholborane or HBcat, (1)) react only very slowly at room temperature (Scheme 1) addition of a metal catalyst M] accelerates the reaction. In addition, the ability to manipulate [M] through the judicious choice of ligands (both achiral and chiral) allows the regio-, chemo-, and enantioselectivity to be directed. 

An extensive array of chiral phosphine ligands has been tested for the asymmetric rhodium-catalyzed hydroboration of aryl-substituted alkenes. It is well known that cationic Rh complexes bearing chelating phosphine ligands (e.g., dppf) result in Markovnikoff addition of HBcat to vinylarenes to afford branched boryl compounds. These can then be oxidized through to the corresponding chiral alcohol (11) (Equation (5)) ... 

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. 

As a greater understanding has emerged of the control factors at play in enantioselective rhodium-catalyzed hydroborations, an increased number of novel chiral diphosphine ligands have been reported in the chemical literature (Figure 3), the majority of these within the last five years. 

Although the vast majority of centrally chiral diphosphine ligands to be employed in enantioselective rhodium-catalyzed hydroborations possess -symmetry, there are a few examples of ( -symmetric diphosphine ligands. Buono prepared bis(aminophosphine) ligands 35-38,81 while Bianchini reported (R, i )-BDPBzP 39 (Figure 5).82... 

An asymmetric version of the Pd-catalyzed hydroboration of the enynes was reported in 1993(118]. The monodentate phosphine (S)-MeO-MOP was used as a chiral ligand for the palladium catalyst. Enantioselectivity of the asymmetric hydroboration was estimated from the enantiopurity of homopropargyl alcohols, which were obtained from the axially chiral allenylboranes and benzaldehyde via an SE pathway (Scheme 3.78). 

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). 

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. 

Hydroboration. Although hydroboration seldom requires a catalyst, hydrobora-tion with electron-deficient boron compounds, such as boric esters, may be greatly accelerated by using transition-metal catalysts. In addition, the chemo-, regio- and stereoslectivity of hydroboration could all be affected. Furthemore, catalyzed hydroboration may offer the possibility to carry out chiral hydroboration by the use of catalysts with chiral ligands. Since the hydroboration of alkynes is more facile than that of alkenes the main advantage of the catalytic process for alkynes may be to achieve better selectivities. Hydroboration catalyzed by transition-metal complexes has become the most intensively studied area of the field.599... 

Asymmetric reaction is one of the most exciting features of catalyzed hydroboration since optically active phosphine ligands are the chiral auxiliaries most extensively studied for metal-catalyzed reactions (Scheme 13).134 The chiral ligands used for asymmetric hydroboration of alkenes include BINAP,136 1 03-106,167-170 QUINAP,171-173 107-109,172,174-176 and BDPP.177,178... 

Probably the most important developments in this field over the past 10 years, however, have been in the area of enantioselective see Enantioselectivity) hydroborations using cationic rhodium complexes of the type [Rh(diene)L ]+ (L = chiral ligand). An excellent review on this topic has recently been published. New chiral see Chiral) catalyst systems are typically tested in hydroborations of vinyl arenes. Although catalyzed hydroboration of vinyl arenes can be used as a mild and efficient route to preparing 1-arylethanol... 

Chiral chromans are redueed stereoselectively by oxazaborolidine-catalyzed hydroboration to give the potent antiarrhythmia agent MK-0499, which acts as a potassium channel blocker [46]. 

Brunei, J.-M., Buono, G. Enantioselective rhodium catalyzed hydroboration of olefins using chiral bis(aminophosphine) ligands. Tetrahedron Lett. 1999, 40, 3561-3564. 

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. 

The development of enantioselective rhodium-catalyzed hydroboration of olefins (chiral bidentate P,P and P,N ligands) 05ASC609. 

Substrate-controlled diastereoselective hydroboration of protected chiral allylic alcohols [25-27] or amines [28, 29] with 9-BBN gives almost always anti selective products. On the other hand, catalyzed hydroboration in most of the cases using catecholborane as hydroborating agent tends to be syn selective [28-30] (Eq. 5.9). 

The excellent syn selectivity of chiral allylamine derivatives, using catalyzed hydroboration with 9-BBN and borane-THF complex, can be explained by the following reactive conformations (Fig. 5.3). 

Burgess and Ohlmeyer [30] have reported that electronic effects are important in catalyzed hydroboration, e.g., allylic acetates are hydroborated with less syn selectivity than allylic trifluoroacetate is [25], and proposed the general model [A] (Fig 5.4) for catalyzed hydroboration of chiral allylic alcohols. The model predicts that the OCOCF, substituent (good a acceptors) will preferentially orientate anti to the approaching rhodium complex. The largest of the other two substituents on the chiral center will occupy the outside position, and the smallest will reside in the inside (crowded) site and thus, syn selectivity will result... 

The proposed model [B] implies that syn selection in catalyzed hydroboration should decrease as the o-accepting character of the anti substituent decreases. This explains the higher syn selectivity in catalyzed hydroboration of allylic trifluoroacetates as compared with reactions of allylic acetates and carbamates [25]. The less diastereoselectivity of cationic complexes in hydroboration of chiral allylic systems than in the neutral catalyst systems [29, 30] is rationalized, as the latter have more electron density to shed via back-bonding. The anti selectivity in catalyzed hydroboration of the allylic acetate and trifluoroacetate is rationalized in terms of competition between the phenyl and acetate groups for the role of a acceptor. The further evidence is obtained in the case of pentafluo-... 

The enantioselective synthesis of axially chiral P—N ligands was also accomplished by rhodium-catalyzed [2 + 2+-2] cycloaddition. The reactions of 1,6-diynes 75 with diphenylphosphinoyl-substituted isoquinolinyl acetylenes 76 furnished diphenylphosphinoyl-substituted axially chiral 1-arylisoquinolines 77 with high yields and ee values (Scheme 9.28) [23], The new diphenylphosphinoyl-substituted axially chiral 1-arylisoquinoline 77 (Z = NTs, R = Me) was derivatized to the corresponding axially chiral P—N ligand 78 and isoquinoline A-oxide 79 without racemization, which could be used in the rhodium-catalyzed hydroboration and Lewis base-catalyzed allylation, respectively [23],... 

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