Amines hydroboration-amination

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

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). 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

The reduction of an aromatic system under controlled conditions is an important source of cycloalkanes. The procedure given here employs a solution of lithium in a mixture of low-boiling amines to accomplish that end and affords a mixture of octalins as product. The mixture may be separated by selective hydroboration (Chapter 4, Section III). 

As previously described, a mixture of and J -octalins can be prepared by the reduction of naphthalene or Tetralin. Another route to this mixture is the dehydration of a mixture of 2-decalol isomers. This latter route has certain advantages in that one can avoid the handling of lithium metal and low-boiling amines. Moreover, 2-decalol is available commercially or can be prepared by the hydrogenation of 2-naphthol (5). In either case a comparable mixture of octalins is obtained, which can be purified by selective hydroboration to give the pure J -octalin (Chapter 4, Section III). 

Organoboranes react with a mixture of aqueous NH3 and NaOCl to produce primary amines. It is likely that the actual reagent is chloramine NH2CI. Chloramine itself,hydroxylamine-O-sulfonic acid in diglyme, and trimethyl-silyl azide " also give the reaction. Since the boranes can be prepared by the hydroboration of alkenes (15-16), this is an indirect method for the addition of NH3 to a double bond with anti-Markovnikov orientation. Secondary amines can be prepared by the treatment of alkyl- or aryldichloroboranes or dialkylchlorobor-anes with alkyl or aryl azides. 

Ammonia can be added to double bonds (even ordinary double bonds) in an indirect manner by the use of hydroboration (15-16) followed by treatment with NH2CI or NH2OSO2OH (12-29). This produces a primary amine with anti-Markovnikov orientation. An indirect way of adding a primary or secondary amine to a double bond consists of aminomercuration followed by reduction (see 15-3 for the analogous oxymercuration-demercuration procedure), for example. 

Amine-borane complexes are not very reactive toward hydroboration, but the pyridine complex of borane can be activated by reaction with iodine.160 The active reagent is thought to be the pyridine complex of iodoborane. 

An alternative procedure for oxidation to ketones involves treatment of the alkylborane with a quaternary ammonium perruthenate salt and an amine oxide186 (see Entry 6 in Scheme 4.9). Use of dibromoborane-dimethyl sulfide for hydroboration of terminal alkenes, followed by hydrolysis and Cr(VI) oxidation gives carboxylic acids.187... 

Scheme 4.9 gives some examples of the use of boranes in syntheses of alcohols, aldehydes, ketones, amines, and halides. Entry 1 demonstrates both the regioselec-tivity and stereospecificity of hydroboration, resulting in the formation of trans-2-methylcyclohexanol. Entry 2 illustrates the facial selectivity, with the borane adding anti to the endo methyl group. 

Combining catalytic enantioselective hydroboration (see p. 342) with animation has provided certain amines with good enantioselectivity. In this procedure the catechol group is replaced by methyl prior to the animation step. 

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°... 

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. 

A series of polyether macrocycles [59]—[66] (Fig. 33) that contain a coordinated reducible, redox-active 16-electron molybdenum nitrosyl (Mo(NO)(3+ group have been prepared (Al-Obaidi et al, 1986 Beer et al., 1987). Compounds [59]—[63] were synthesized from the reactions between [Mo(NO)LX2] (L = tris(3,5-dimethylpyrazolyl)hydroborate X = Cl or I ) and the appropriate amine substituted benzo-crown ether. Compounds... 

Hydroboration of alkenes generates organoboranes, which react with sodium azide in the presence of an aqueous acid to give primary amines (equation 58). Thus 1-nonene... 

Phase-transfer generated diborane has been used for the hydroboration of alkenes and their conversion into alcohols [1,2] and the procedure has also been employed for the cleavage of formamido compounds to yield the amines [3]. Cyclododecan-1,3- and 1,4-diones have been obtained in a 3 1 ratio and overall yield of 59% via... 

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) . ... 

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