Borane complexes chiral boron

The chemistry of secondary phosphine oxides, R2P(H)0 and their phosphi-nous acid tautomers, R2POH, has continued to attract attention. The study of the phosphinous acid tautomers has been aided by the development of stereoselective procedures for direct conversion of secondary phosphine oxides to the phosphinous acid-boranes (83). Treatment of the secondary phosphine oxide with either a base-borane complex or boron trifluoride and sodium borohyd-ride provides the phosphinous acid-borane with predominant inversion of configuration at phosphorus. The phosphinous acid tautomers are usually trapped as ligands in metal complexes and further examples of this behaviour have been noted. Discrimination of enantiomeric forms of chiral phosphinous acids, Ph(R)OH, coordinated to a chiral rhodium complex, has been studied by NMR. °° Palladium complexes of di(t-butyl)phosphinous acid have found application as homogeneous catalysts.A lithium salt of the tellurophos-phinite Ph2PTeH has been prepared and structurally characterised. ... 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

The iridium(I) salt containing 185 has also been employed as catalyst in hydrogenation of imines. In the hydrogenation of 2-substitufed quinolines to provide chiral tetrahydro derivatives iridium complexes of SYNPHOS and DIFLUORPHOS prove to he effective catalysts. Oximes undergo enantioselective reduction (and N—O bond cleavage) on treatment with borane and spirocychc boronate 186. ... 

MeOTf has been reported to effect the dealkoxylation of a perfluoroalkyltrialkoxyboronate to generate the corresponding boronic ester, l Conversely, an alkenyl boroxycarbene complex was reacted with MeOTf to remove the borane-based chiral auxiliary yielding a Fischer carbene complex. l l ferf-Amide substituted oxetanes rearranged in anhydrous nitrobenzene at 150 °C with a catalytic amount of MeOTf to produce ester-substituted azetidines (eq 16).i Other acids such as boron trifluoride ether-ate, trifluoromethanesulfonic acid, and benzylthiolanium hexaflu-oroantimonate led to low yields of the desired azetidines. 

The reducing ability of NHC-borane complexes was later expanded to the use of chiral NHC-borane complexes in the asymmetric reduction of ketones carried out by Lindsay and McArthur [89]. They used borane complexes of the NHCs shown in Figure 15.19 to reduce acetophenone to chiral 1-phenylethanol. Though initially enantioselectivities were lower than would be desired, they then tested the effect of substitution around the boron center. A bulkier NHC coupled with a smaller Lewis acid additive (Bp3.0Et2) led to a 90% yield and 56% enantiomeric excess (ee) for acetophenone, with variations on these yields and enantioselectivities depending on the ketone. 

A study has appeared which describes the preparation and properties of hydrolytically stable cyanohydro(pyrrolyl-l)borates and chiral boron-containing amine-cyano(pyrrolyl-l)borane complexes. Finally, two reports have appeared concerning the preparation, thermal stability, and stereochemistry of silyl-substituted aminoboranes. ... 

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

Stereoselective reduction of some triazolodiazines (derivatives of ring systems 33 and 37) bearing chiral terpene residues has been elaborated by Groselj el al. <2006TA79>. With catalytic hydrogenation, partial saturation of the six-membered ring was experienced, while reaction with borane-methyl sulfide resulted in formation of triazole-boron complexes. 

The characteristic feature of the aforementioned oxazaborolidine catalyst system consists of a-sulfonamide carboxylic acid ligand for boron reagent, where the five-membered ring system seems to be the major structural feature for the active catalyst. Accordingly, tartaric acid-derived chiral (acyloxy)borane (CAB) complexes can also catalyze the asymmetric Diels-Alder reaction of a,P-unsaturated aldehydes with a high level of asymmetric induction [10] (Eq. 8A.4). Similarly, a chiral tartrate-derived dioxaborolidine has been introduced as a catalyst for enantioselective Diels-Alder reaction of 2-bromoacrolein [11] (Eq. 8A.5). 

Chiral C2-symmetric boron bis(oxazolines) act as enantioselective catalysts in the reduction of ketones promoted by catecholborane.321 DFT calculations indicate that the stereochemical outcome is determined by such catalysts being able to bind both the ketone and borane reducing agent, activating the latter as a hydride donor, while also enhancing the electrophilicity of the carbonyl. X-ray structures of catalyst-catechol complexes are also reported. 

Chiral modification is not limited to boronate and aluminate complexes. Boranes or alanes are partially decomposed with protic substances such as chiral amines, alcohols or amino alcohols to form useful reagents for enantioselective reduction of carbonyl compounds. For example, reduction of acetophenone with borane modified with the amines (65) to (67) gives (5)-l-phenylethyl alcohol with 3.5-20%... 

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