Boronate hydroboration/amination

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 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... 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Hydroboration of allylic amines.1 Hydroboration of primary and secondary allylic amines presents problems because amino groups interact with boron reagents. Hydroboration proceeds normally when the amino group is protected by trimethylsilyl groups, and deprotection can be effected by protonolysis in CH,OH. 

Hayashi et al. and Miyaura et al. have reported that far less nucleophilic aryl- and alkenyl-boronic acids can react with a variety of enones in the presence of a BINAP-rhodium catalyst to give adducts with high enantiopurity in general (Scheme 8D.5) [13], The one pot procedure, involving the hydroboration of alkynes as the first step (R = alkenyl), was achieved in the presence of amines without affecting the enantioselectivity [13]. 

The use of alkenyl boronic acid derivatives 50, which are readily prepared via hydroboration or bromoboration of alkynes, affords the corresponding p,y-unsaturated amino acids (e.g. 52-57) in a geometrically pure form [34], A variety of amines 48, including primary and secondary amines, anilines, amino alcohols and hydroxylamines can effectively participate in this process, while the alkenyl boronic acid can contain alkyl, aryl or bromo-substituents. Although the alkenyl amino acid side chain is introduced through the boronic acid component, the use of more substituted a-keto acids 49 allows the simultaneous incorporation of an additional a-substituent (e.g. 57). 

This reaction also proceeds well with a variety of other substituted stilbene derivatives. Thus, the boranes obtained by hydroboration of E and Z stilbenes 80 undergo the thermal boron migration with excellent diastereoselectivity leading to the anti and syn organoboranes 81. After amination of 81 the corresponding benzylic amines (anti and syn 82) were obtained in 66 and 40 % yield and with diastereoselectivity better than 90 10 between C(l) and C(2) (Scheme 16) [7, 8,13],... 

The normal synthetic pathway for hydroboration is reaction with an ambiphilic nucleophile of which the simplest example is hydroperoxide ion. This elicits a 1,2-migration of an alkyl group from boron to oxygen with concurrent loss of hydroxide ion. The step occurs with essentially complete retention of configuration. In similar vein, ambiphilic species with the structure NH2X may be used in amination, so that the overall reaction is an addition of ammonia to the alkene with the regio- and chemoselectivity driven by the hydroboration step. A majority of reactions of organoboranes can be rationalized in terms of these ionic mechanistic pathways, or closely related protocols (Scheme 2). 

The BH3 adduct of 2-aminopyridine seems to be very stable to internal hydroboration and polymerisation compared to borane pyridine. We attribute the extra stability to hydride-proton interactions these together with those of fluorine-proton were reported for other amine borane compounds16 (Figure 5). The MM models for the two boron complexes shown in Figure 6, present atomic distances... 

Summary Precursor-derived quaternary Si-B-C-N ceramics frequently possess an enhanced thermal stability compared to SiC, SisN4 or Si-C-N ceramics. The stability of the materials towards crystallization and/or decomposition is directly coimected to the molecular structure and the elemental composition of the polymeric precursors. This paper highlights recent investigations on the synthesis of boron-modified polysilazanes and polysilylcarbodiimides. Hydroboration of polyvinylsilazanes and dehydrocoupling reactions of boron-modified silanes with ammonia or amines as well as cyanamide are described. It is shown that simple organosilicon chemistry provides a means to efficiently optimize ceramic yields and tune elemental composition as well as thermal properties of the polymer-derived ceramics. 

Benzylic boronic esters [52]can be obtained from bromomethyl boronic esters and aryl or vinyl stannanes [53]. Alkyl and vinyl boronic esters are accessible through hydroboration of alkynes with dialkylboranes followed by oxidation with acetaldehyde [43]. Alternatively, vinylboronates can be obtained directly from alkynes in a reaction -with catecholborane without a subsequent oxidation step [53]. Further diversification in the boronic acid derivatization is achieved by binding the boronic acids bearing another derivatizable functional group (for instance an amine) to suitable polymeric supports. The modified boronic acid may then be... 

Brown showed that electronic and chelation effects can be important in the hydroboration of allylic amines (sec. 5.2.A.iv), but in this system, steric factors control the reaction, with delivery of boron to the carbon shown. 

B,B, 5"-Trichloroborazine is readily reduced with alkali metal hydroborates to yield borazine, (HBNH)3. The boron-bonded chlorine can be replaced by fluorine through the action of metal fluorides upon the trichloroborazine. With amines, the latter reacts to yield B,B, B"-triaminoborazines. The substitution of chlorine in iV-substituted B,B, B"-trichloro-borazines by organic groups is most easily achieved by the Grignard method but has also been effected by the use of organolithium compounds. 

If catecholborane 8 is used for hydroboration, then conversion of the boronic ester product to a trialkylborane with a Grignard reagent or a dialkylzinc species is needed prior to amination. Thus, asymmetric hydroboration of styrene, followed by addition of methyl magnesium chloride then hydroxylamine-O-sulfonic acid gave... 

The Inifer process developed by Kennedy can be used to functionalize vinyl monomers via a cationic route by initiating a polymerization with an alkyl halide-boron trichloride mixture R BCl. The termination by transfer to an alkyl halide leaves a halide-terminated polymer. This can be transformed to a hydroxyl terminal unit via the sequence (1) dehydrohalogenation, (2) hydroboration, and (3) oxidation and hydrolysis (Equation 5.24). These co-functional blocks may be coupled to form diblock copolymers using standard reaction techniques, e.g., diisocyanate will couple cohydroxy and co-amine blocks together. Direct reactions can also occur, and co-acid chlorides combine readily with co-hydroxy units. 

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