Triethylboron

According to Brown, B(C 3115)3 may be prepared analogously to B(CH3)3 by dropwise addition ofann-bulyl ether solution of BF3 to a solution of CgHgMgBr in n-butyl ether. When the reaction is complete the product is distilled at 95°C under Ng. In Meerwein s procedure, ethyl ether is used instead of n-butyl ether, but in this case the ether and triethylboron are first distilled off together and then separated by fractional distillation under a nitrogen blanket. 

Other trialkylboron compoxmds may be synthesized by addition of BF3 to the ether solutions of the respective alkyl magnesium bromides. 

Colorless liquid. M.p. —38°C, b.p. 79.3°C. Combines with water, yielding CH3B(OH)2. The B and Oatoms form a symmetrical six-membered ring. 

The higher homologs of methylboroxine cannotbepreparedfrom B2O3 and the corresponding trialkylborons, because the latter decompose at elevated temperatures. Dehydration of the alkyl-boronic acids is preferable. 

A large number of organoboronic acids have been synthesized by the following procedure, given here for n-butylboronic acid as an example. 

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Cocatalysts, such as diethylzinc and triethylboron, can be used to alter the molecular-weight distribution of the polymer (89). The same effect can also be had by varying the transition metal in the catalyst chromium-based catalyst systems produce polyethylenes with intermediate or broad molecular-weight distributions, but titanium catalysts tend to give rather narrow molecular-weight distributions. 

Aziridine esters are a- and -amino acid derivatives at the same time. A characteristic reaction of a-amino acids is their reaction with triethylboron to give boroxazolidines. We showed that aziridinecarboxylic acids exhibit the expected behavior in their reaction with triethylboron, viz., that they form stable boroxazolidines 34 (Scheme 19) [29]. These boron heterocycles can be reconverted into the free amino acids by treatment with 8-hydroxyquinoline. 

Cyclization induced by mercuric ion is often used in multistep syntheses to form five- and six-membered hetereocyclic rings, as illustrated in Scheme 4.6. The reactions in Entries 1 to 3 involve acyclic reactants that cyclize to give exo-5 products. Entry 4 is an exo-6 cyclization. In Entries 1 and 2, the mercury is removed reductively, but in Entries 3 and 4 a hydroxy group is introduced in the presence of oxygen. Inclusion of triethylboron in the reduction has been found to improve yields (Entry l).113... 

This procedure was used in the synthesis of the cholesterol-reducing drug lescol.131 The diethylmethoxyboron can be prepared in situ from triethylboron and one equivalent of methanol. 

Allylation reactions can be initiated by triethylboron. This procedure has been found to give improved stereoselectivity in acyclic allylations.319... 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

In addition to the obvious uses of solubility parameters in predicting physical properties, it is also possible in some cases to study other types of intermolecular interactions. For example, the solubility parameter for triethylboron, (C2H5)3B, is 15.4h, whereas that for triethylaluminum (C2H5)3A1 is 23.7h. Triethylboron is known from other studies not to associate, whereas triethylaluminum exists as dimers. 

We have already mentioned that the mixed alkyl halide compounds of aluminum dimerize, and the solubility parameters shown in Table 12.1 for these compounds are consistent with that assessment. The solubility parameters for triethylboron and diethylzinc are 15.4 and 18.2 in J cm 3/2, respectively. These values are indicative of liquids that are not strongly associated, which is known to be the case. 

Another synthetic route to boryl nitronates (71k-m), which is, however, poorly developed (231), is based on the reaction of trialkylborons (exemplified by triethylboron) with a-nitroolefins (72a-c) proceeding by the 1,4-addition mechanism (Scheme 3.70, cf. Scheme 3.56). 

The nucleophiles that are used for synthetic purposes include water, alcohols, carboxylate ions, hydroperoxides, amines, and nitriles. After the addition step is complete, the mercury is usually reductively removed by sodium borohydride. The net result is the addition of hydrogen and the nucleophile to the alkene. The regioselectivity is excellent and is in the same sense as is observed for proton-initiated additions.16 Scheme 4.1 includes examples of these reactions. Electrophilic attack by mercuric ion can affect cyclization by intramolecular capture of a nucleophilic functional group, as illustrated by entries 9-11. Inclusion of triethylboron in the reduction has been found to improve yields (entry 9).17... 

We have also looked at a no-d case where we have cleaved R3B compounds, triethylboron, with carboxylic acids in an ethereal environment. Once again we have the possibility of coordinating a substance like a carboxylic acid with our boron compound in a six-membered ring,... 

SECOND-ORDER RATE COEFFICIENTS (l.mole-,.Sec ) FOR THE ACIDOLYSIS OF TRIETHYLBORON BY CARBOXYLIC ACIDS IN SOLVENT DIGLYME AT 31 °C, REACTION... 

Many electrophilic reagents are capable not only of electrophilic attack at the carbon atom of the C-M bond, but also of nucleophilic attack at the metal atom of the C-M bond. It is such nucleophilic attack that gives rise to the mechanisms SE2(cyclic) and SE2(co-ord). Dessy et al.12,13 have several times stressed the importance of internal nucleophilic assistance by the electrophilic reagent in reaction mechanisms of organometallic compounds, and it is the account by Dessy and coworkers13 of the acidolysis of triethylboron that is probably the classic report in this field. 

In solvent diglyme, the rate of acidolysis of triethylboron by carboxylic acids is found to be greater the weaker is the acid, there being a linear correlation between log k2 and the pK of the carboxylic acid. Thus nucleophilic coordination of the carboxylic acid to the boron atom must be an important feature of the reaction mechanism, and the acidolysis may therefore be denoted as following mechanism SE2(cyclic) or SE2(co-ord), perhaps through a transition state such as (I). ... 

The acidolysis of diethylcadmium by various nuclear-substituted benzyl alcohols in solvent ether is accelerated by electron-donating substituents in the electrophile and retarded by electron-withdrawing substituents14. As in the case of acidolysis of triethylboron, co-ordination of the electrophile to the metal atom... 

The reaction of triethylboron with oxygen is a convenient way to generate ethyl radicals at room temperature, or at lower temperatures. These conditions at room temperature in the presence of tributyltin hydride efficiently perform the Barton-McCombie deoxygenation reaction. Recently, we showed that the tin hydride could be replaced by diphenylsilane also at room temperature, for the deoxygenation of secondary alcohols.20 Many different thiocarbonyl derivatives can now be used for the Barton-McCombie reaction. Data were collected for R = Ph, />-F-C6H4, C6F5, C6C13H2 with various substrates (Scheme 2). The yields were excellent for all the secondary... 

No reaction occurs when triethylaluminum, aluminum chloride, stannic chloride, titanium tetrachloride, diethylzinc or triethylboron are used in lieu of an organoaluminum halide or sesquihalide. 

German workers independently found that 1,2- 1,3-, and some 1,4-diols can be converted into bis(diethylborinates) which, on heating to 200°, lose triethylboron to give cyclic ethylboronates. Otherwise, these boronates can be prepared directly from the diols by heating with triethylboron in the presence of diethylboryl pivalate, or by dismuta-tion from equimolar proportions of diols and their bis(diethylbori-... 

Addition to ot, -acetylenic esters and ketones (3, 108 6, 163-164). The addition of organocopper reagents to conjugated acetylenic carbonyl compounds is usually not stereospeciflc, although cis-addition predominates. Japanese chemists have found that the stereoselectivity in the reaction with alkylcopper reagents is markedly enhanced by use of the complex RCu BRj. Thus the reaction of dimethyl acetylenedicarboxylate with n-butylcopper complexed with tri-n-butylboron (or triethylboron) results in exclusive formation of the cis-adduct. In the absence of a trialkylborane the cis- and adducts are formed in the-ratio 85 15. 

The reaction of isoprene monoxide with a range of alcohol pronucleophiles in the presence of the ligand (3 mol%), Pd2dba3.CHCl3 (1 mol%) and triethylboron (1 mol%) gave the glycol monoethers in excellent yield and enantiomeric excess. The use of p-methoxybenzyl alcohol and 3-nonyl-3,4-epoxybut-1-ene afforded an intermediate that was converted into (—)-malyngolide (eq 8). ... 

Lithiumlithium triethylaluminum, sodium triethylboron, sodium triethanolamine borate,- potassium triethylboron and tri-n-butyltin cyclohexanone enolates have been successfully monoalkyl-ated. In Scheme 6 the behavior of the lithium enolate of cyclohexanone (11) and the lithium triethylaluminum enolate upon reaction with methyl iodide is compared. The latter enolate gives better results since no dimethylation products were detected, but clearly the cyclohexanone enolate (11) is much less prone to dialkylation than the cyclopentanone enolate (10). Scheme 6 also provides a comparison of the results of alkylation of the potassium enolate of cyclohexanone, where almost equal amounts of mono- and di-alkylation occurred, with the alkylation of the potassium tiiethylboron enolate where no polyalkylation occurred. The employment of more covalently bonded enolates offers an advantage in cyclohexanone monoalkylations but not nearly as much as in the cyclopentanone case. 

Regioselective alkylations at C-6 of 2-methylcyclohexanone have been accomplished via the alkylation of thermodynamically unstable trisubstituted lithium, lithium triethanolamine borate, potassium triethylboron, tri- -butyltin and benzyltrimethylammonium enolates (c/. 2). Alkylation is faster than equilibration for the more reactive alkylating agents. Although enolate equilibration has been shown to compete with butylation using n-butyl iodide under certain conditions, butylation of the enolate (2 M = Li) in liquid ammonia-THF gave a mixture of cis- and /ra/ij-2-methyl-6-butylcyclohexanone along with 2-methylcyclohexanone in an 83 17 ratio in 90% yield no 2,2-dialkylcyclohexanone was obtained in this reaction (Scheme 8). ... 

A rapid method for determining quantitatively the water content of a large variety of hydrated salts depends upon measuring the volume of ethane liberated when the compound is treated with triethylboron in the presence of small amounts of pivalic acid. One ethyl group is removed per hydrogen according to the equation 217... 

The involvement of a-and 7C-type dimeric radical-cations in one electron transfer to the aromatic sulfides (259) has been assessed. A laser-flash study of the behaviour of p-nitrobenzenethiol has shown that S-H fission is the dominant reaction with the formation of the corresponding thiyl radical. This occurs particularly when the irradiations are carried out in nonpolar solvents. The reactions encountered in polar solvents are different. Under these conditions the triplet state of p-nitrobenzenethiol is involved and this undergoes ready deprotonation. The amino acid derivatives (260) can be desulfurised by irradiation using triethylboron and triethylphosphite. ... 

Although triethylboron or triethylaluminum in diethyl ether catalyzes the 2,2-cyclodimerization of 3,3-dimethylcyclopropene (see Section 1.1.6.1.5.1.1.), addition of tripropylboron to 1-methylcyclopropene in 1 2 ratio gave, after oxidation, the cyclopropane 13 (50%). 

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