Borane complexes preparation

Indeed, by immobilization of optically active a- or )0-amino alcohols on cross-linked polystyrenes as in 6a-d, utilization of chiral borane complexes becomes feasible. These functionalized polymers were incorporated into simple columns and enantioselective reductions of aldehydes and ketones were performed. Thus, reduction of acetophenone with a borane complex prepared from 6d yielded optically active (-)-l-phenyl-2-propanol in high optical yield (>99% ee) [31]. In addition, the flow system also served for continuous regeneration of the immobilized complex. Injection of borane and valerophenone into the column, which was loaded with polymer 6a, was followed by collection of fractions every 30 min. The individual batches of collected 1-phenylpentanol were analyzed and the enantiomeric excess was determined to be 87,93, and 91% for three batches [32]. 

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... 

A comprehensive review of the preparation, reactions, and n.m.r. spectra of phosphorus-fluorine compounds has appeared. This year s literature has been notable for the first detailed applications of ab initio SCF-MO calculations to the problems of bonding in halogenophosphines and their derivatives. - Comparison of the results of such theoretical calculations with experimental data obtained from photoelectron spectra shows a good correlation in the case of phosphorus trichloride and phosphoryl chloride, and of phosphorus trifluoride and its borane complex. ... 

Very recently, we reported liquid imidazole-borane complexes (scheme 5)57 that are air stable. Judging from their polarity and viscosity (Table 3), they are expected to be a new class of solvent or electrolyte. Preparation of polymer homo-logues of imidazole-alkylborane complexes will also be reported elsewhere in the near future. 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

High stereoselectivities (94-100 %) are attained in the reduction of aromatic ketones by use of a new chiral borane complex with (S)-2-amino-3-methyl-l,l-diphenylbutan-l-ol,(S-68) readily prepared in two steps from (S)-valine, in an experimentally convenient procedure961. (S)-Valine methyl ester hydrochloride was converted with excess of phenylmagnesium bromide into (S-68). The same treatment of (R)-valine gave (R-68). In a typical asymmetric reduction the reagent, prepared from (S-68) and borane, and the ketone (69) in tetrahydrofuran were kept at 30 °C for some hours. The corresponding alcohols were obtained in high optical purity. (S-68) could be recovered to more than 80% without racemization 96). 

The stability and equilibrium properties of neutral borane complexes are not well known. Instead, the recent chemistry of the borane complexes is predominantly characterized by preparative and structural investigations. This is even more obvious for the charged borane complexes. 

The majority of the L-BHnX3- (X = halogen, R, OR, SR, CN, C02H, etc.) borane complexes were described in the last 20-25 years. There exist various possibilities for their preparation, summarized in Table 3. 

The rhenium complex 76 related to 74b was also prepared recently by Labinger and Bercaw using another synthetic strategy. 2 In this case, the pendant borane moieties were introduced by hydroboration of unsaturated phosphines in the coordination sphere of the metal. The cationic rhenium complex 75 featuring two diphenyl(vinyl)phosphines was readily converted into the corresponding bis(phosphine-borane) complex 76 (Scheme 45). The coordination mode of 76 was substantiated spectroscopically (5 nB = 87.7 ppm) and crystallographically. 

Bis(phosphoranimine) ligands, chromium complexes, 5, 359 Bis(pinacolato)diboranes activated alkene additions, 10, 731—732 for alkyl group functionalization, 10, 110 alkyne additions, 10, 728 allene additions, 10, 730 carbenoid additions, 10, 733 diazoalkane additions, 10, 733 imine additions, 10, 733 methylenecyclopropane additions, 10, 733 Bisporphyrins, in organometallic synthesis, 1, 71 Bis(pyrazol-l-yl)borane acetyl complexes, with iron, 6, 88 Bis(pyrazolyl)borates, in platinum(II) complexes, 8, 503 Bispyrazolyl-methane rhodium complex, preparation, 7, 185 Bis(pyrazolyl)methanes, in platinum(II) complexes, 8, 503 Bis(3-pyrazolyl)nickel complexes, preparation, 8, 80-81 Bis(2-pyridyl)amines... 

Diphosphaferrocene, with chromium carbonyls, 5, 220 l,l -Diphospha[2]ferrocenophane, synthesis, 6, 210 Diphosphazanes, in dinuclear Ru complexes, 6, 674 l,l -Diphosphetanylferrocenes, preparation, 6, 200-201 Diphosphine borane complex, polypyrrole support for,... 

Two different approaches can be followed to prepare and use the catalyst. The first is to prepare it in situ by mixing (R)- or (S)-diphenylprolinol (DPP) (4) and a borane complex (Scheme 16.2). This route is advantageous because there is no need to use boronic acids (or boroxines) and to remove water to form the catalyst. Another possible way is to use preformed catalysts, some of which are commercially available from suppliers such as Callery. 

The asymmetric reduction of ketones by borane catalyzed by oxazaborolidines has been widely studied since the beginning of the 1980s. Despite the use of borane complexes, which are hazardous chemicals, this reaction is an excellent tool to introduce the chirality in a synthesis and has demonstrated its usefulness in industrial preparation of chiral pharmaceutical intermediates. As a result of its performance, versatility, predictability, and scale up features, this method is particularly suitable for the rapid preparation of quantities of complex chiral molecules for clinical trials. 

As shown in Equation (19), l-(2-dimethylaminoethyl)-8,9-dihydropyrano[3,2-e]indoles (44), rotationally restricted phenolic analogues of the neurotransmitter serotonin, were prepared in good yield from 5-hydroxyindoles in a cyclization reaction using an intramolecular variant of the Mitsunobu reaction <91TL3345, 92T1039). The borane complex could be dissociated with CsF and Na2C03 in refluxing EtOH. 

Al complexes prepared in situ from Al[OCH(CH3)2]3 and two equivalents of (K)-BINAPHTHOL (9) and (i )-H8-BINAPHTHOL (10) promoted the enanti-oselective reduction of propiophenone with borane-dimethyl sulfide and gave the S alcohol in 83% and 90% ee, respectively (Scheme 7) [47]. The reaction was much slower and afforded a racemic product in the absence of Al[OCH(CH3)2]3 under otherwise identical conditions. The addition of a catalytic amount of Al(OC2H5)3 increased both the rate and enantioselectivity in the hydroboration of ketones with a chiral amino alcohol [48]. 

The oxidation of non-basic and monophenolic tetrahydrobenzylisoquinolines with VOF3-TFA is a superior method for aporphine preparation. N-Trifluoroacetylnorcodamine (32), under these conditions, gives a 70% yield of N-trifluoroacetylnorthaliporphine, while the codamine-borane complex (33) provides thaliporphine (28) in 80% yield. The reaction has also been extended to the preparation of homoaporphines.2... 

A wide variety of phosphorus-containing heterocycles were prepared using RCM. For example, Gouverneur and co-workers reported the synthesis of borane complexes of cyclic... 

In 1979, Johnson reported the enantioselective reduction of ketones with stoichiometric amounts of optically active (1-hydroxy sulfoximine-borane complexes.131 Prochiral alkyl phenyl ketones (RCOPh) undergo enantioselective reduction with enantiomerically pure p-hydroxy sulfoximine borane complexes (301 and 302). These complexes are prepared by reaction of the corresponding P-hydroxy sulfoximine with borane at -78 °C. The structures 301 and 302 have been suggested for these complexes. In the case of the borane complex 301, the enantioselectivity increased as the steric bulk of the R substituent of the ketone (RCOPh) was decreased from IV to Me. The analogous reductions of methyl alkyl ketones (MeCOR) with these borane complexes were less enantioselective (3-27% ee).131... 

C. The reported procedure provides a practical preparation of (S)-tetrahydro-i-methyl-3,3-diphenyl-lH,3H-pyrrolo[i,2-c][l,3,2]oxazaboroie and conversion to its more stable borane complex.13 The oxazaborolidine-borane complex has also been prepared by treatment of a toluene solution of the free oxazaborolidine with gaseous fiborane followed by recrystallization from a dichloromethane-hexane bilayer.14 This nd other chiral oxazaborolidines have been used to catalyze the enantioselective eduction of prochiral ketones.15 The yield and enantioselectivity of reductions using catalytic amounts of the oxazaborolidine-borane complex are equal to or greater than those obtained using the free oxazaborolidine.13... 

Thus, using L-amino add oxidase from P. myxcfaciens and various amine-borane complexes or D-amino acid oxidase from porcine kidney and sodium cyanoboro-hydride, the preparation of several natural and non-natural enantiopure D- and L-amino adds was achieved, respectively [51]. In a more recent report, several P- and y-substituted a-amino adds were deracemized using D-amino add oxidase from Trigonopsis variahilis and sodium cyanoborohydride or sodium borohydride [52] (Scheme 13.20). 

Another important subgroup of monohydrido complexes can be prepared by the oxidative addition of nido-heteroborane anions to [RhCl(PPh3)3] (Scheme 17). Further, the c/oTO-dicarborane complex is the starting material for the preparation of other rhodium(III) borane complexes (Scheme 18). They are also weakly active hydrogenation catalysts. 

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