Methyl borine

Eisen (>f6-l-Methyl-borin)-(tj6-toluol)- (hexafluorphosphat) XIII/3c, 26... 

Methyl Borate (Trimethyl borate or Trimethoxy borine), (CH3.0)3B, mw 103.92, OB to C02 — 115.47, colorl liq, mp —29°, bp68°,d 0.92 g/cc at 20° vap d 3.59. Prepn other properties are given in Beil (Ref l). Sax (Ref 2) lists this compd as a dangerous fire hazard when exposed to heat or flame. It reacts with w or steam to produce toxic flammable vapors, and vigorously with oxid materials Refs 1) Beil 1, 287, (143) [275] 2) Sax... 

The allylation reaction has been extended to enantiomerically pure allylic boranes and borinates. For example, the 3-methyl-2-butenyl derivative of (Ipc)2BH reacts with aldehydes to give carbinols of greater than 90% e.e. in most cases.39... 

N. W. Flodin, and A. B. Burg Hydrides of Boron, XII, Symmetrical Dimethyldiborane and the Methyl Derivatives of Borine-Trimethyl-amine. J. Amer. chem. Soc. 61, 1078 (1939). 

Boron. Earlier papers reviewed the properties of the borabenzene anion and discussed its potential aromaticity.130 182 A few examples have been shown in Schemes 31 and 53. The borabenzene anion (borinate) 141, with R = phenyl or methyl, gives complexes with transition metals such as Co (142) and Mn (143) (Scheme 61).183 Aza- and polyaza-... 

Alkali metal 1-methyl- and 1-phenyl-borinates are also available from bis(borinato)cobalt complexes (see below) on treatment with sodium or potassium cyanide in an aprotic solvent like acetonitrile. Cobalt cyanide precipitates and the alkali borinate remains in solution. After addition of thallium(I) chloride to some complexes, thallium 1-methyl- or 1-phenyl-borinate could be isolated as pale yellow solids, the only main group borinates isolated hitherto. They are insoluble in most organic solvents but readily soluble in pyridine and DMSO. The solids are stable on treatment with water and aqueous potassium hydride, but are decomposed by acids <78JOM(153)265). 

Chiral ketones.3 Asymmetric hydroboration of a prochiral alkene with monoisocampheylborane followed by a second hydroboration of a nonprochiral alkene provides a chiral mixed trialkylborane. This product reacts with acetaldehyde with elimination of a-pinene to give a chiral borinic acid ester in 73-100% ee. Treatment of this intermediate with a,a-dichloromethyl methyl ether (2,120 5, 200-203) and lithium triethylcarboxide followed by oxidation results in an optically active ketone in 60-90% ee. 

Asymmetric hydroboration 2171 of prochiral alkenes with monoisopinocampheyl-borane in the molar ratio of 1 1, followed by a second hydroboration of non-prochiral alkenes with the intermediate dialkylboranes, provides the chiral mixed trialkylbo-ranes. Treatment of these trialkylboranes with acetaldehyde results in the selective, facile elimination of the 3-pinanyl group, providing the corresponding chiral borinic acid esters with high enantiomeric purities. The reaction of these intermediates with base and dichloromethyl methyl ether provides the chiral ketones (Eq. 130)2l8>. A simple synthesis of secondary homoallylic alcohols with excellent enantiomeric purities via B-allyldiisopinocampheylborane has been also reported 219),... 

Borin 1,5-Diallyl-3-methoxy-2-methyl-1,4,5,6-tetrahydro-XIII/3a, 276... 

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... 

Methyl and 9-[(trimethylsilylmethyl)]-9-BBN derivatives are easily synthesized by the reaction of the corresponding lithium reagents with 9-methoxy-9-BBN. Unfortunately, such derivatives are spontaneously flammable in air, making them particularly hazardous to handle for the purpose of isolation. However, selective oxidation with anhydrous triethylamine A -oxide converts them to air-stable borinate esters which are efficient reagents for the methylation [149, 150] of haloalkenes or the syntheses of allylic and propargylic silanes (Scheme 2-52) [151]. 

Chiral boronic esters react with organolithium reagents to form diorganylalkoxyboranes (borinic esters). Subsequent reaction with the anion of dichloromethyl methyl ether then yields chiral ketones by rearrangement of both of the groups on boron (Scheme 42). No racemization is observed in this sequence and alkyl-, aryl- or alkynyl-lithium reagents can be used. 

Enol borinates are also intermediates in the reaction of organoboranes with z,/5-unsaturated ketones and aldehydes. Thus the reaction of triethylborane and 3-methyl-3-butene-2-one (5) followed by addition of (2, in DMSO) affords the Mannich base (6) in 87% yield. 

The trans double bond in 208 is responsible for the anti relationship between the alcohol and the adjacent methyl group. In fact, in a study with a variety of aldehydes, the most abundant of the three minor isomers also has that anti relationship. If we were to have a cis double bond in the enol borinate instead then we would expect to observe syn relationships here and that is exactly what happens. That is 1,2-control. 

Reaction with trialkylboranes. A trialkylborane (1), readily available by hydro-boration of an alkene, undergoes fast 1,4-addition to methyl vinyl ketone to give an enol borinate ester (2), which on hydrolysis affords a methyl ketone (3).24... 

A 100-mL round-bottom flask containing a magnetic stirrer is capped with a rubber septum and charged with 15 mmol of borinate. 15 mL of diethyl ether arc added and the reaction is cooled to 0°C. 2.04 mL (22.5 mmol) of dichloromethyl methyl ether are added followed by 16.2 mL (30 mmol) of lithium lert-bu-toxide. The ice bath is removed and the mixture stirred at r.t. for 1 h. A slightly exothermic reaction develops and a white precipitate forms. The reaction mixture is cooled to 03C and 18 mL (45 mmol) of pH 8 phosphate buffer arc added, followed by 5.1 mL (45 mmol) of 30% hydrogen peroxide. The ice bath is removed and the mixture stirred for 12 h. The phases are separated, the aqueous phase extracted with two 15-mL portions of diethyl ether, the combined extracts washed with two 15-mL portions of water, then aq NaCl, dried over MgS04 and filtered. The volatiles are removed under reduced pressure and the residual oil is purified by bulb-to-bulb distillation. The chemical purity of the ketones is determined by GC using a 5% SP 2100 on Chromosorb W column and is usually >95%. The chiral ketones are further purified by preparative GC. For further examples, see Table 2. 

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