Borinic from boranes

Trimethylamine oxide Borinic acid esters from boranes... 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

Iminoboranes derived from borine or alkylboranes and related aldimino-boranes are listed in Table 3. 

From Hard Allylic Organometallics. The most common preparation of allylic boranes and boronates is the addition of a reactive allylic metal species to a borinic or boric ester, respectively (Eqs. 10 and 11). Preparations from allyllithium, " " allylmagnesium, and allylpotassium " ° reagents are all well known. These methods are popular because the required allylic anions are quite easy to prepare, and because they generally lead to high yields of products. 

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

The reaction undergone by alcohols with trialkyl- and triaryl-boranes in the presence of pivalic acid, to give borinic esters, and the thermal cyclization of bis(dialkylborinates) to boronates, are discussed briefly in Section II. Many borinates have been prepared in quantitative yield from mono-, di-, oligo-, and poly-saccharides,338,108 and mixed... 

Just as trialkylboranes add to simple alkenes (15-16), they rapidly add to the double bonds of acrolein, methyl vinyl ketone, and certain of their derivatives in THF at 25°C to give enol borinates (also see, p. 631), which can be hydrolyzed to aldehydes or ketones." If water is present in the reaction medium from the beginning, the reaction can be run in one laboratory step. Since the boranes can be prepared from alkenes (15-16), this reaction provides a means of lengthening a carbon chain by three or four carbons, respectively. Compounds containing a terminal alkyl group, such as crotonaldehyde (CH3CH=CHCHO) and 3-penten-2-one, fail to react under these conditions, as does acrylonitrile, but these compounds can be induced to react by the slow and controlled addition of O2 or by... 

The trialkylborane is derived from monoalkyl-(l,l,2-trimethylpropyl)borane by hydroboration. Care must be taken in the oxidation step leading to the a-chiral ketones and buffering is necessary to avoid racemization. However, even with these precautions, 2-6% racemization is observed in some cases9,20. The intermediate alkenyl-(l,l,2-trimethylpropyl)borinates are relatively resistant to oxidation thus prolonged reaction times or higher temperatures are sometimes required to complete the reaction via the haloalkyne route. 

From these studies it may be concluded that boron is a borderline acceptor which becomes hard when surrounded by hard ligands, and it is rendered soft by soft substituents (e.g., H ). 3-(Methylthio)propylborane (7) is more stable than the borane-tetrahydrofuran complex (8) and it is distillable (33). The boron atom of the dihydroborepin (9) is internally coordinated by the amino nitrogen however, this chelation is removed by the addition of trityl fluoro-borate (34). This may indicate that the boron atom in the borinic esters (10) is harder, but not as hard as the trityl cation. 

J3-[(Z)-l-Bromo-l-alkenyl]borane derivatives offer potential for the conversion of 1-bromo-alkynes into ketones. Treatment of the adducts from the acetylene and dialkylboranes or alkylbromoboranes with sodium methoxide gives alkenyl-borinate or -boronate esters, either of which may be oxidized to the ketone (Scheme 14). When using dialkylboranes, thexylalkylboranes may be used with advantage. ... 

Chiral boronic and borinic esters form the subject of two papers by Brown and co-workers this year, in which asymmetric acyclic alcohols and ketones, respectively, are formed. In the first the borane (101) obtained from (—)-a-pinene and cw-but-2-ene is treated with acetaldehyde to eliminate a-pinene and give the (5)-(+)-boronic ester (102), which is subsequently converted, into (5)-(+)-butan-2-ol in >97% e.e. It is also worth noting that this sequence of reactions forms the basis of a convenient procedure for upgrading commercial... 

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