Boroxines reactions with

The boroxines could then be subjected to Suzuki coupling with aryl, vinyl, or benzyl halides. Suzuki coupling with tri- -butylstannyl chloride also gave the tri- -butylstannyl-substituted thiophenes 221. These can form the starting materials for further transformations. The carbonylative coupling with halides resulted in the formation of ketones tin-lithium exchange followed by reaction with electrophiles led to a host of other useful products (Scheme 65). 

During these catalytic or stoichiometric processes, possible side reactions do occur which are shown in Scheme 4.3. These explain why the use of more than lequiv. of boronic acid is necessary. Furthermore, during the oxidation of copper(ll) to copper(lll), hydrogen peroxide is produced, which can decrease the yields of the reaction as a result of its reaction with the aryl boronic add. This may also explain why more than 1 equiv. of aryl boronic acid provides enhanced yields [10]. Moreover, the aryl boronic acid can form triaryl boroxines [11], and in doing so forms water, which can be removed from the reaction by molecular sieves. Evans and coworkers postulated that phenolic products would be formed as a result of the competitive arylation of water formed during the reaction process. 

B. 3-(4,4,5,5-Tetramethyl-[l,3,2]dioxaborolan-2-yl)pyridine. A 250-mL, one-necked, round-bottomed flask equipped with a magnetic stirbar and a Dean-Stark trap fitted with a condenser capped with a nitrogen inlet adaptor is charged with tris(3-pyridyl)boroxin-0.85 H20 (3.0 g, 9.1 mmol), pinacol (4.07 g, 34.4 mmol) (Note 6), and 120 mL of toluene. The solution is heated at reflux for 2.5 hr in a 120°C oil bath. The reaction is complete when the mixture changes from cloudy-white to clear. The solution is then concentrated under reduced pressure on a rotary evaporator to afford a solid residue. This solid is suspended in 15 mL of cyclohexane (Note 7) and the slurry is heated to 85°C, stirred at this temperature for 30 min, and then allowed to cool slowly to room temperature. The slurry is filtered, rinsed twice using the mother liquors, washed with 3 mL of cyclohexane, and dried under vacuum to afford 4.59 g (82%) of 3-pyridylboronic acid pinacol ester as a white solid (Note 8). 

In the asymmetric addition to alkenylphosphonate 33 (Eq. 2) [25], the yield is dependent on the amount of water present. The combination of boroxine and water (1 equiv. relative to boron) gave a high yield of the desired product 34, with 96% enantiomeric excess. The alkylphosphonate 34 can be used as a chiral building block for the synthesis of optically active alkenes, using a Horner-Emmons type of reaction. 

Trialkyl derivatives of boron, and in fact many other molecules such as boroxines with carbon-boron bonds, react readily with oxygen. The initial products are peroxy derivatives with BOOR bonds, which tend to react further to form borate esters. The ease of the initial reaction is shown by the fact that reported examples of vinyl polymerization induced by trialkyl borons require oxygen and are actually radical processes induced by the boron oxygen reaction or intermediate peroxides (7). 

Reaction 17 resembles the displacements on phosphorus discussed earlier, except that since trialkyl borons are electron deficient with only six electrons around boron, no valence-shell expansion is required. It appears to be a general reaction for boron and has been suggested by Matteson (15) to explain the conversion of trimethyl boroxine by tert-buty hypo-... 

Boroxines result from the condensation of boronic acids. RBfOH). The cyclic Irimeric anhydride of methylboronic acid is (IVfeBO),. Give a balanced equation for the reaction of (MeBO)j with water (See Brown, H. C. Cole, T. E. Organometallics 1985.4. 816.)... 

Researchers at Sepracor later disclosed the use of a new class of chiral oxazaborolidines derived from r/. v-aminoindanol in the enantioselective borane reduction of a-haloketones.6,7 The 5-hydrogen oxazaborolidine ligand 10 was prepared in situ from d,v-aminoindanol 1 and BH3 THF.8 Stock solutions of 5-methyl oxazaborolidine 11-16 were obtained by reaction of the corresponding N-alkyl aminoindanol with trimethyl boroxine.6,7 5-Methyl catalyst 11 was found to be more selective (94% ee at 0°C) than the 5-hydrogen catalyst 10 (89% ee at 0°C), and enantioselectivities with 11 increased at lower temperatures (96% ee at -20°C). The catalyst structure was modified by introduction of A-a I kyI substituents. As a general trend, reactivities and selectivities decreased as the steric bulk or the chelating ability of the A -alkyl substituent increased (Scheme 17.4). 

Many compounds with B-S-R groups have been synthesized, including thioborates or trialkyl- and tiaryl-thioborates, B(SR)3, the sulfur analogs of trialkoxy- and triaryloxybo-ranes, and trialkyl- and triarylborathiins, (SBR)3, the sulfur analogs of boroxins. Simple thioborates are usually prepared by reactions of boron halides with thiols (equation 13). 

Example 5.5. Oxidation of paraffins to secondary alcohols. Alcohols can be produced by oxidation of paraffins with air or oxygen at moderate temperatures (typically 120 to 180° C) in the presence of boric-acid esters or boroxines [16-18], These intercept the alkyl peroxide, the first oxidation product, preventing it from generating free radicals that would cause further degradation including scission of carbon-carbon bonds and produce aldehydes, ketones, and acids (see also Section 9.6.2). The peroxy borates so formed then are hydrolyzed to yield the alcohol. The carbon atoms at the chain ends are largely immune to oxidation, so the product consists predominantly of isomeric secondary alcohols. The reaction does not stop at... 

The fairly harsh conditions required to break the carbon-hydrogen bond in cyclohexane cause various side reactions, and the yield to the desired end products (based on cyclohexane converted) is only about 60 to 70%, even at low conversion. A higher yield could be obtained with added borate ester or boroxine (see Example 5.5 in Section 5.5), but this would require hydrolysis of the resulting cyclohexyl ester and is not practical in a process that calls for a dry product. 

The reducing power of diborane has been blunted by forming its adduct with dimethyl sulfide. This adduct, Me2S BH3, is stable and commercially available and therefore more attractive as a hydride reagent than diborane itselfNevertheless, the adduct still reduces carboxylic acids to alcohols which are isolated as cyclic boroxins (2 Scheme 1). In a one-pot reaction, carboxylic acids can be reduced to boroxins and then oxidized with pyridinium chlorochromate to the required aldehyde (Scheme I)."... 

Since the boroxines are readily available from B203 and trialkyl boranes or trialkoxy boranes, they are in effect reactions of boron oxide with trialkyl alanes. Gallium tribromide and triethylalane give triethylgallane in over 80% yield, but only the first alkyl group on aluminum is involved in alkylation (63, 65) ... 

Trifluorophosphine-borane has also been prepared in the gas phase reaction between boroxine and trifluorophosphine, but the method is somewhat hazardous and probably best carried out with small amounts of material 16). Kinetic studies indicate a first-order dependence on the... 

Access to 2,4-disubstituted furans is exemplified by the reaction of 4-phenyloxa-zole and trimethylsilylacetylene to give 3-(trimethylsilyl)furan 136 in 70% yield (Fig. 3.40). As before, metalation with r-butyllithium and quenching with benzyl bromide gave 2-benzyl-4-(trimethylsilyl)furan 137. Conversion of 137 to the corresponding boroxine and subsequent Suzuki coupling afforded a 4-substituted 2-benzylfuran 138. ... 

Phenylboronic acid (17) and its derivatives are widely used. Boronic acids are sometimes difficult to purify because they undergo cyclotrimerizadon with loss of water to form boroxines. On the other hand, organotrifluoroborate salts 18 are easily prepared, purified, and handled [23]. Aryltrifluoroborate salts 18 are prepared by the reaction of arylboronic acid with HF and base [24]. Alkenyltrifluorobo-rates 19 are prepared by hydroboration of 1-alkynes, followed by treatment with KHF2 [25]. 

Sodium phenolates can directly react with aiyl boroxines in the presence of Ni(COD)2, PCy3 and BEts to produce biaiyls (Scheme 14.23). In this reaction it is believed that a borate is formed first, and the borate undergoes... 

Reversible bond formation can, in principle, be used for the selfcorrection of product structures. For example, the formation of Schiff bases, boroxines and boronates has been applied to on-surface polymerizations, but has so far met with limited success. Linderoth and Gothelf employed trialdehyde 62 and diamine monomers 63 towards 2-D polymer synthesis on Au(lll) under UHV (Figure 28.28a) [134], but unfortunately obtained only branched or irregularly networked stmctures, a situation that was ascribed to the flexible monomer structure employed. In addition, UHV conditions caused an irreversible loss of water, the presence of which was essential to bring about the back-reaction. When Abel and coworkers synthesized boroxine/boronate networks on Ag(lll) under UHV [135] they were unable to achieve the expected periodic order this contrasted with the findings of Yaghi et al., who employed the solution approach (see Section 28.5.2). This difference was most hkely a consequence of irreversible bond formation. 

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