Diols borate esters from

Myers et al. reported that partially dehydrated APB is an effective intumescent flame retardant in thermoplastic polyurethane.77 APB at 5-10 phr loading in TPU can provide 7- to 10-fold improvement in burn-through test. It is believed that in the temperature range of 230°C-450°C, the dehydrated APB and its released boric oxide/boric acid may react with the diol and/or isocyanate, the decomposed fragments from TPU, to produce a highly cross-linked borate ester and possibly boron-nitrogen polymer that can reduce the rate of formation of flammable volatiles and result in intumescent char. 

The results of numerous studies have established that RG-II is present in the primary wall predominantly as a dimer that is cross-linked by a 1 2 borate-diol ester [11, 12]. A single borate ester cross-links two of the four apiosyl residues present in the dimer ([52] see Figure 5B). In vitro studies have shown that in the presence of boric acid and certain cations, two RG-II monomers rapidly self-assemble to form a dimer [52]. Moreover, the structure of RG-II itself may determine the location of the borate ester, since the location of the cross-link is the same in naturally occurring and in vitro-formed dimers. Thus, RG-II is the first example of a plant cell wall pectic polysaccharide that self-assembles to form structurally identical dimers [52]. The specificity and cation-dependence of this cross-linking suggest that there are distinct structural requirements for dimer formation and this may explain why the structure of RG-II is highly conserved in vascular plants [12, 53]. It is not known whether dimer formation in planta results from spontaneous self-assembly or is an enzymically catalyzed process. 

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. 

Reaction of a C2-symmetrical diol ester of a (dihalomethyl)boronic acid with an alkyllithium or Grignard reagent will yield the same borate anion 2 as that from the corresponding alkylboronic ester 1 with a (dihalomethyl)lithium. 

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