Cyclic Boric Esters

Table 5 summarizes the reactions of isoprene with aromatic aldehydes and unsaturated aldehydes. Salicylaldehyde provides the expected product as a cyclic boric ester derivative and shows apparently lower stereoselectivity, giving a mixture of 1,3-anti and 1,3-syn isomers in a ratio of 6 1 (run 1, Table 5). 2-Furfural reacts as usual and provides a 1,3-anti isomer as a single diastereomer in good yield (run 2). Unsaturated aldehydes, irrespective of their substitution patterns, undergo homoallylation selectively with excellent 1,3-anti selectivity, the geometry of the double bond of the starting aldehydes remaining intact (runs 3-5). 1,2-Addition to unsaturated aldehyde takes place selectively and no 1,4-addition is observed.

The assignment of the stereochemistry of both compounds based on NMR spectral data and their different behaviour on thin layer plates (silica gel), impregnated with boric acid112. Only isomer 205 is able to form a cyclic boric ester, therefore 205 runs faster on impregnated plates than 206. 

The general formula for boric acid esters is B(OR)3. The lower molecular weight esters such as methyl, ethyl, and phenyl are most commonly referred to as methyl borate [121 -43-7], ethyl borate [150-46-9], and phenyl borate [1095-03-0], respectively. Some of the most common boric acid esters used in industrial applications are Us ted in Table 1. The nomenclature in the boric acid ester series can be confusing. The IUPAC committee on boron chemistry has suggested using thalkoxy- 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 literature as boron alkoxides and aryloxides. Cyclic boric acid esters, which are trimeric derivatives of metaboric acid (HB02), are known as boroxines (1). 

Cyclic boric acid esters derived from triethanolamine (Figure 9.11) or diethanolamine can be stabilized toward hydrolysis by an intramolecular, boron-nitrogen coordination bonding. The blockage of the empty-orbital on the boron atom can alleviate hydrolysis. This effect has been used to prepare... 

The cyclic boron esters (99) resulted from the three-component reaction between benzil (96), urea (97), and boric acid (Scheme 21). The stereoisomeric products arose as a consequence of boric acid addition to the intermediate cyclic amides (98) <89PHA110>. Carbon-13 NMR spectroscopy and mass spectrometry are consistent with structure (99). 

B,0-Heterocyclics. o-Phenylene chlorohoronate heated with 2.2 moles n-hutane-thiol at 150-160° for 4 hrs. S-n-hutyl o-phenylene thiohorate. Y 79%.—These are the first reported examples of monothiohoric acid esters. A thio as well as a suhst. amino group attached to a horon atom is replaceable by an alkoxy group but not the reverse. F. e., also cyclic boric acid amide esters, s. W. Gerrard, M. F. Lappert, and B. A. Mountfield, Soc. 1959, 1529. 

A. I. Gren and V. V. Kuznetcov, Chemistry of Cyclic Esters Boric Acids. Naukova Dumka, Kiev, 1988. 

Boric acid and borax form cyclic esters with polyi vinyl alcohol). 

Inorganic Esters. Boric acid and borax form cyclic esters with poly(vinyl alcohol) (85—100). The reaction is markedly sensitive to pH, boric acid concentration, and the cation-to-boron ratio. An insoluble gel is formed at pH above 4.5—5.0 ... 

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. 

Alkyl metaborate esters in combination with organic hydroperoxides are reported to be active for epoxidation [182]. The borate esters are cyclic and have general formula (ROBO)3, where R is an alkyl group like cyclohexyl (CeHn). They are six-membered ring compoimds formed from O-B units with a pendant RO, and are 1 1 dehydro adducts of alcohol and boric acid. 

Synonyms Calcium diborogluconate D-Gluconic acid cyclic 4,5-ester with boric acid calcium salt... 

CAS 283-56-7 EINECS/ELINCS 206-003-5 Synonyms Boric acid, tris (2-aminoethyi) ester Ethanol, 2,2, 2"-nitrilotri-, cyclic borate 2,2, 2"-Nitriloethyl borate Triethanolamine borate... 

The solid electrolyte interface formation process can be influenced by the use of electrolyte additives. Cyclic carbonates containing a vinyl group, such as vinylene carbonate, halogenated cyclic carbonates, such as fluoroethylene carbonate, and difluoroethylene carbonate have been used as electrolyte additives. Other additives are sUyl esters such as sultones, and esters of phosphoric and boric acid. 

Dihydric alcohols readily yield cyclic ethers under esterification conditions and the usual catalysts are not effective. The use of boric acid as a catalyst allows one to prepare esters of mono- or dibasic acids. For example, esters of 1,4-butanediol and 2,5-hexanediol can be prepared by this method. 

---