Hydroboration/amination

The QUINAP ligands are highly active for reaction of -substituted vinylarenes, thus, p-methoxystyrene gives the corresponding branched alcohol in 95% yield (57% ee) within 15 min at room temperature. Similarly, a combination of hydroboration/amination using (13) has allowed primary amines to be formed in a one-pot reaction of vinylarenes upon reaction of the B H addition product with MeMgCl/H2N0S03H (Scheme 10).5°... 

Fig. 2.13 Examples of hydroboration/amination using Rh-QUINAP complexes and electrophilic aminating agents. (I) Catecholborane, 1 mol% (S)-QUINAP-Rh catalyst, THE, RT, 1 h 2 equiv. MeMgCI in THE, 30 min solid H2NOSO3H,...

Hydroboration-amination methodology affords chiral amines from chiral mono- or diorganoboranes and HOSA in high yields and enantiomeric purity (Scheme 30) . ... 

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... 

Sulfonylhydroxylamines and hydroxylamine O-sulfonic acid have found wide apph-cation in synthesis of amines from achiral or chiral organoboranes and boronate esters and the hydroboration-amination methodology is successfully used for direct amination of alkenes. 0-Sulfonyloximes were also found to be good reagents for synthesis of amines from organomagnesium, -copper and -zinc reagents. 

The hydroboration amination sequence in diglyme is a general procedure for the conversion of olefins to primary amines without rearrangement and with predictable stereochemistry.5 An alternative procedure, using tetrahydrofuran as solvent and either hydroxylamine-O-sulfonic acid or chloramine, is applicable with terminal olefins and relatively unhindered internal and alicyclic olefins.6 O-Mesitylenesulfonylhydroxylamine also gave desired amines in comparable yield.7 Alternative procedures for the hydroboration of olefins use commercially available solutions of di-borane in tetrahydrofuran8 or dimethylsulfide.9... 

Olefins may be converted to primary amines by the Ritter reaction10 or by reaction with mercuric nitrate in acetonitrile solution.11 In both cases regiospecificity for the formal addition of ammonia across the double bond is opposite to that observed in the hydroboration-amination sequence. 

Interestingly, hydroboration-amination as an isotope incorporation methodology has led to the synthesis of isomerically pure 13N- and 15N-labeled primary amines from labeled ammonia [23-25] (Scheme 8). These labeled amines are valuable intermediates in pharmacology. 

Similarly, N-(p-toluenesulfonyl)-protected primary amines have been prepared using two different protocols, via hydroboration - amination with chloramine-T... 

The asymmetric synthesis of enantiomerically pure primary amines has received considerable attention in recent years due to applications of the chiral amines, either as chiral auxiliaries for the synthesis of optically active molecules [33] or as a deri-vatizing agent for the resolution of racemic carboxylic acids [34], Hydroboration -amination is also a convenient synthetic route to epimerically clean amine derivatives in a simple one-stage reaction. Interestingly, rrans-2-phenylcyclopentylamine (cypenamine), which is an antidepressant [35], can be obtained as a pure isomer in good yields by the hydroboration of 1-methylcyclopentene [7,10,36] (Scheme 13). 

This hydroboration- amination methodology shown in the second part of Scheme 14 can be extended to the synthesis of a series of enantiomerically pure terpenyl-amines [38,39], embracing (—)-cis-caran-tranj-2-amine, (-)-cis-camn-trans-4-amine, (—)-longifolamine, and (+)-ci s-myrtanylamine. These are all formed in satisfactory yields by hydroboration of the corresponding alkene, if necessary purified... 

The influence of steric factors on the organoborane also explains the unexpected insertion into 9-BBN observed in the hydroboration - amination of the dicyclopen-tadiene entity contained in the EPDM polymer [42] (Scheme 17). 

Finally, the hydroboration-amination method via azides might be the key step in the synthesis of alkyl aryl ketones involving a free radical mechanism as opposed to the usual ionic pathway [64,65] (Scheme 23). 

This possibility of intimate association of rhodium with the aromatic ring suggests further experiments. A logical extension of asymmetric syntheses involving prochir-al reactants is a kinetic resolution with related chiral reactants under similar conditions. In the one case of hydroboration-amination where this has been applied, it has proved to be very effective. The reactant was prepared directly by a Heck reaction on 1,2-dihydronaphthalene, and under the standard conditions of catalytic hydrobora-tion gave >45% of both enantiomerically pure recovered alkene with (after oxidative work-up) the alcohol of opposite hand, mainly as the trans-isomer. This procedure forms a simple and potentially useful route to pharmacologically active substances, demonstrated by the racemic synthesis shown [105] (Scheme 34). 

Following the demonstration of catalytic hydroboration-amination employing hydroxylamine-O-sulfonic acid as the electrophile, extension to the synthesis of secondary amines was considered. 

The successful method was demonstrated for the examples shown in Table 2.3 both the amine and the borane can be varied without detriment. The ees observed are consistently 1 - 5 % lower than in the case of simple hydroboration-oxidation or the hydroboration-amination reactions discussed earlier [107],... 

The covalent C-B bonds of organoboronic acids and esters are very inert to ionic and radical reactions, thus allowing functionalization of remote sites other than the B-G bond (Equations (94)-(97)). Bulky diols such as pinacol have been used as the protecting group of B(OH)2 because of their high stability to nucleophiles and water and silica gel in amination of 316,474 hydroboration-amination of 317,475 Wittig reaction of 3 18,476 and oxidation-alkylation of 319.477... 

The most common methods of C—B—X ring construction employ hydroboration/amination, aminolysis of 4-bromoalkylborinate esters, electrophilic additions of boron halides to o-amino-styrenes and -biphenyls, insertion of hydrazines into 1,2,5-thiadiborolenes, and aldol-type condensations. Specific examples of these processes are presented in the following sections 6.28.3.1 and... 

The hydroboration/amination protocol has been used successfully for the preparation of 1-aza-... 

A) and used in organic synthesis. a-Lithio sulfides give useful carbanion reactions and sulfur ylids can be formed (secs. 8.6, 8.8.B). The difficulties in preparing primary and secondary amines by traditional methods (from halides or tosylates via reaction with imides or azides, sec. 2.7) make the hydroboration-amination sequence a mild and useful alternative for the synthesis of amines. 

Fernandez, E. Maeda, K. Hooper, M. W. Brown, J. M. Catalytic Asymmetric Hydroboration/Amination and Alkylamination with Rhodium Complexes of l,l -(2-Diarylphosphino-l-naphthyl)isoquinoline. Chem. 2000, 6, 1840-1846. 

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