Borate intermediates

Bis(pinacolato)diboron reacts with 1-halo-l-lithioalkenes, that is, alkylidene carbenoids, affording 1,1-diboryl-1-alkenes in good yields (Scheme 9).76 The reaction proceeds via formation of a borate intermediate, which is followed by 1,2-migration of the boryl group with elimination of the bromo group. 

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 borate intermediate was also coupled with aryl halides. When a ynyl group is present, coupling is followed by a tandem cyclization which leads to 2-[ t-(cycloalkylidene)alkyl]indoles. <95CC409>... 

Alternatively, a similar borate intermediate can be obtained by the reaction of organoborate with chloromethyl- or (dichloromethyl)lithiums (eq (22) and (23)). When a chiral diol ester reacts w ith (dichloromethyl)lithium, an optically active a-chloroalkylboronate is produced with an excellent enantiomeric excess [34] (eq (23)). The C-Cl bond is then displaced readily w ith alkyl, aryl, or 1-alkenyl nucleophiles with inversion of configuration. The procedure has been extensively used for the preparation of chiral boronates, which have been used in organic syntheses [30]. 

The first synthesis of an (a-haloalkyl)boronic ester [8], a free radical addition of a tetrahalomethane, was followed by mechanistic studies that indicated the potential for stereospecific alkylation with Grignard reagents via borate intermediates [9], if only there had been a way to obtain asymmetric examples. The discovery of the efficient reaction of (dichloromethyl)lithium with boronic esters to form (a-chloroalkyl)boron-ic esters by insertion of a CHCl group into the B-C bond opened a new opportunity [10]. Boronic esters of pinanediol, prepared from (+)-a-pinene by osmium tetroxide catalyzed oxidation, were soon found to undergo the insertion reaction with a strong asymmetric bias, with diastereomeric selectivities frequently in the 90-95% range [llj. It was subsequently found that anhydrous zinc chloride promotes the reaction and increases diastereoselectivity to as high as 99.5% in some cases [12]. 

New synthetic sequences based on four-co-ordinate borate intermediates continue to be discovered. The carbonylation of B-R-9-BBN, resulting ultimately in quantitative one-carbon homologation of R to primary alcohol or aldehyde, is preferentially and rapidly effected by K [(Pr 0)3BH], which is stable and does not give undesirable by-products on work-up. The lithiated indole of Scheme 6... 

Four co-ordinate lithium borates have again been used in several synthetic sequences, an example of which is the ketone synthesis shown in Scheme 12. Other reactions include the alkylation of borates RSBHC=CHR Li with benzo-l,3-dithiolium fluoroborate to give a,/3-unsaturated aldehydes, " addition of the same borates to a,/3-unsaturated esters to give 5-keto esters or y,5-unsaturated ester, and a synthesis of secondary alcohols that proceeds via a lithium borate intermediate. ... 

The homologation of phenylboronic esters of pinanediol with dichloromethyl-lithium proceeds via a lithium borate intermediate, leading to a synthesis of either the threo- or ery//tro-alcohols (70), depending on the enantiomer of pinanediol used in the second step. " ... 

Treatment of the borates with iodine leads to boron- C2 migration of an alkyl group[9]. This reaction has not been widely applied synthetically but it might be more applicable for introduction of branched alkyl groups than direct alkylation of an indol-2-yllithium intermediate. 

Aldehydes react with alkenylborates to give 1,3-diols upon oxidation of the intermediate (300). Alkynylborates ate transformed by epoxides into homoallyhc alcohols and alkenylborates into 1,4-diols (300,301). Carbon dioxide reacts with alkenylborates to yield catboxyhc acids (302). The scope of these transformations is further extended by the use of functionalized electrophiles and borates, often reacting with high stereoselectivity. For example, in the... 

P-Hydroxy acids lose water, especially in the presence of an acid catalyst, to give a,P-unsaturated acids, and frequendy P,y-unsaturated acids. P-Hydroxy acids do not form lactones readily because of the difficulty of four-membered ring formation. The simplest P-lactone, P-propiolactone, can be made from ketene and formaldehyde in the presence of methyl borate but not from P-hydroxypropionic acid. P-Propiolactone [57-57-8] is a usehil intermediate for organic synthesis but caution should be exercised when handling this lactone because it is a known carcinogen. 

Methyl borate is beheved to be the boric acid ester produced in the largest quantity, approximately 8600 metric tons per year (28). Most methyl borate is produced by Morton International and used captively to manufacture sodium borohydride [16940-66-2]. Methyl borate production was studied in detail during the 1950s and 1960s when this compound was proposed as a key intermediate for production of high energy fuels. Methyl borate is sold as either the pure compound or as the methanol azeotrope that consists of approximately a 1 1 molar ratio of methanol to methyl borate. 

Production of Sodium Borohydride. In the pulp and paper industry, sodium borohydride is used to generate sodium hydrosulfite (sodium dithionite), a bleaching agent, from sodium bisulfite. Methyl borate is used as an intermediate in the production of sodium borohydride (33). 

Fillers. Fillers are not commonly added to CR adhesives. Calcium carbonate or clay can be primarily added to reduce cost in high-solids CR mastics. Maximum bond strength is obtained using fillers with low particle size (lower than 5 [jim) and intermediate oil absorption (30 g/100 g filler). In general, fillers reduce the specific adhesion and cohesion strength of adhesive films. Although polychloroprene is inherently flame retardant, aluminium trihydrate, zinc borate, antimony trioxide or... 

Recent optimization studies reveal that the yield of 2-(2-propenyl)-1,3,2-dioxaborolane-4,5-di-carboxylate esters (i.e., the tartrate ester modified allylboronates) is improved by using triiso-propyl borate as the borylating agent1. The improved yields are directly related to the increased efficiency of the preparation of the intermediate allylboronic acid. 

To a —78 C solution of 23.1 mL (100 mmol) of triisopropyl borate and 8.15 mL (110 mmol) of 3-chloro-l-propene in 100 inL of dry THF is added dropwisc via a cannula over 0.5 h a solution of LDA (110 mmol prepared in 200 mL of THF from 110 mmol of diisopropylamine and 47.9 mL of 2.3 M butyllithium in hexane), This mixture is stirred for an additional 0.5 h at — 78 "C then a solution of 75.9 ntL of 2.9 M anhyd hydrogen chloride in diethyl ether is added and the mixture is allowed to warm to 25 °C. The mixture is concentrated in vacuo (20 Torr) and the residue extracted with three 100-mL portions of pentane, Filtration under nitrogen followed by distillation under reduced pressure provides 18.0 g (88%) of diisopropyl l-chloro-2-propenylboronate bp 95-96 "C/25 Torr. Transesterification of this intermediate with 1.3-propanediol provides the title compound in almost quantitative yield bp 110-112°C/20Torr. 

Arylboronic acids have traditionally been prepared via the addition of an organomagnesium or organolithium intermediate to a trialkyl borate. Subsequent acidic hydrolysis produces the free arylboronic acid. This limits the type of arylboronic acids one can access via this method, as many functional groups are not compatible with the conditions necessary to generate the required organometallic species, or these species may not be stable intermediates. 

Organoboranes undergo fragmentation if a good leaving group is present on the 8-carbon.102 The reactive intermediate is the tetrahedral borate formed by addition of hydroxide ion at boron. 

Trialkyl- Hydroperoxide Unstable intermediate Borate ester borane ion... 

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