Boron alkoxides

The general formula for boric acid esters is B(OR)2. The lower molecular weight esters such as methyl, ethyl, and phenyl are most commonly referred to as methyl borate [121 -43-7] ethyl borate [130-46-9J, and phenyl borate [1095-03-0] respectively. Some of the most common boric acid esters used in industrial appHcations are Hsted in Table 1. The nomenclature in the boric acid ester series can be confusing. The lUPAC committee on boron chemistry has suggested using trialkoxy- and triaryloxyboranes (5) for compounds usually referred to as boric acid esters, trialkyl (or aryl) borates, trialkyl (or aryl) orthoborates, alkyl (or aryl) borates, alkyl (or aryl) orthoborates, and in the older Hterature as boron alkoxides and aryloxides. CycHc boric acid esters, which are trimeric derivatives of metaboric acid (HBO2), are known as boroxines (1). 

Ebelman and Bouquet prepared the first examples of boric acid esters in 1846 from boron trichloride and alcohols. Literature reviews of this subject are available. B The general class of boric acid esters includes the more common orthoboric acid based trialkoxy- and triaryloxyboranes, B(0R)3 (1), and also the cyclic boroxins, (ROBO)3, which are based on metaboric acid (2). The boranes can be simple trialkoxyboranes, cyclic diol derivatives, or more complex trigonal and tetrahedral derivatives of polyhydric alcohols. Nomenclature is confusing in boric acid ester chemistry. Many trialkoxy- and triaryloxyboranes such as methyl, ethyl, and phenyl are commonly referred to simply as methyl, ethyl, and phenyl borates. The lUPAC boron nomenclature committee has recommended the use of trialkoxy- and triaryloxyboranes for these compounds, but they are referred to in the literature as boric acid esters, trialkoxy and triaryloxy borates, trialkyl and triaryl borates or orthoborates, and boron alkoxides and aryloxides. The lUPAC nomenclature will be used in this review except for relatively common compounds such as methyl borate. Boroxins are also referred to as metaborates and more commonly as boroxines. Boroxin is preferred by the lUPAC nomenclature committee and will be used in this review. 

S.3.2.4. from Boron Alkoxides with More-Polar Organometalllcs... 

The reaction of boron alkoxides with organometalllcs to form B—C bonds is the transmetallation process ... 

Although symmetrical dialkylborinic acids can be obtained from boron alkoxides or boronic esters and organolithium or RMgX reagents " , these compounds usually are more conveniently synthesized, either by partial hydrolysis of trialkylboranes , or by hydroboration of alkenes with monohalogenoboranes followed by hydrolysis (see also refs. 1-8, 5.3.2.4 and ref. 2, 5.3.2.4.1). Diarylborinic acids are prepared by the reaction of 2 mol of ArMgX with trialkoxyborane at low The products are... 

The interaction of boron alkoxides with cellulose and 0-methylcellulose has been investigated. An unsaturated cellulose, namely, 5,6-cel-lulosene (25) and its acetic ester have been prepared by the thermal decomposition of an allylxanthate and benzylxanthate of cellulose. ... 

Using miscellaneous boron sources, such as boron alkoxides, boron halides, or aminoboranes, Funayama et al. modified various hydropolysilazanes. Depending on the respective boron component used, different elimination reactions take place, producing polysilazane chains with different boron-containing groups attached [79-82]. 

More recent advances in intermolecular [3+2] reductive cycloadditions have involved combinations of enals or enoates with alkynes (Scheme 3-34).l 2 l The initially developed cycloadditions of enals and alkynes likely proceeds by initial formation of a metallacyclic enolate derivative, followed by enolate protonation and addition of the vinyl nickel unit to the resulting carbonyl to produce the boron alkoxide of the observed cyclopentenol product (Scheme 3-35). The analogous transformation with enoates may also proceed by this mechanism, depicted below by the sequence of initial generation of metallacycle 20, followed by enolate protonation to form 21 en route to product generation. Alternatively, the collapse of the metallacycle 20 to a ketene intermediate 22 may occur in the enoate variant. The precise pathway followed likely depends on whether protic or aprotic media are used. 

Boron alkoxides form the oxide or boric acid when reacted with excess watCT. Oxide formation can be written as... 

For most metal alkoxides, hydrolysis with excess water yields insoluble oxide or hydroxide precipitates that are useless for further polymerization reactions [Eqs. (5.21)-(5.23)]. However, if small amounts of water are added slowly to a sufficiently dilute solution, it is possible to form polymerizable molecular species from these alkoxides also. For example, when a dilute solution of boron alkoxide in alcohol is exposed to water, soluble transient molecular species such as B(0R)2(0H) and B(OR)(OH)2, representing various degrees of hydrolysis, form initially, e.g.. 

In the light of the above factors, the alcoholysis reactions of a few metal alkoxides may be briefly sutmnarized. In the alcoholysis reactions of boron alkoxides with primary alcohols, Mehrotra and Srivastava observed that reactions proceed to completion conveniently, but with tertiary alcohols, (tert-butyl alcohol) mono-alkoxy di-tert-butoxide was the final product. The non-replaceabiUty of the last alkoxy group with tert-butyl alcohol was ascribed to steric factors. It was observed that the reaction was fast in the beginning but slowed down at the later stages after the formation of the mixed alkoxide. Presumably steric hindrance of the mixed alkoxide B(OR)(OBu )2 prevented the close approach of another molecule of ter/-butyl alcohol. 

As stated earlier, the method has been extended for the preparation of alkoxides and mixed alkoxides of a large number of metals. However, Mehrotra and Srivastava made the interesting observation that boron alkoxides do not appear to undergo ttans-esterification reactions at all, although they do undergo alcoholysis reactions. 

In spite of a considerable amount of work, the mechanism of the thermal decomposition of boron alkoxides is not yet fully understood it probably occurs via a carbonium ion species as the thermal decomposition of boron tris(tetrahydrofurfuroxide) yields 2,3-dihydropyran which is also obtained by vapour-phase dehydration of tetrahydro-furfinyl alcohol." Similarly, decomposition of free ter/-butyl methylcarbinol has been found to yield the same product as that of its corresponding boron alkoxide derivative " this led Dupuy and King" to assume that the thermal decomposition of boron alkoxides might proceed via an acid catalysed mechanism rather than via a pyrolytic ds-elimination mechanism. 

The thermochemistry of boron alkoxides was studied by Chamley et al., which led to an estimate of the average B-O bond dissociation energies for boron alkoxides in the range of 110 5kcalmol these results are consistent with the order B-F > B-N > B-O > B-Cl, reported earlier by Sidgwick." In another study " the reported B-O bond dissociation energy D(B-OR) of B(OMe)3, B(OEt>3, B(OPr">3, were 118.0, 117.7, 119.0 2kcalmor, respectively. 

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