Cyclic boroxine

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. 

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)."... 

The matter of purity is of real importance for SPC and should therefore be addressed. As boron-based functional groups, free boronic acid or one of the many cyclic boronic esters are used. The latter esters may hydrolyze to the acids during polymerization, which then enter the normal cross-coupling or follow an independent mecha-nism.f " Boronic acids always contain some water. Otherwise they are partially or completely condensed to cyclic boroxines. This water content has to be precisely... 

Boronic acids readily dehydrate at moderate temperatures (or over P4O10 at room temperature) to give trimeric cyclic anhydrides known as trialkyl(aryl)boroxines ... 

Nitrone derivatives of 2-fonny phenylboronie acid have been prepared, and these exist in cyclic, trisubstituted boroxin fonn (35a-d) <83JOM247>. Interestingly, treatment with a 1,2-dio converts them into monomers with a rather unique 1,3-zwitterion structure (36a-d),... 

Cyclotriboroxanes (RBO)3 (also known as boroxines) are formally anhydrides of the corresponding boronic acids RB(OH)2 only mild heating is required to convert these dibasic acids into a wide range of cyclic trimeric anhydrides [eqn (9.18)]. This dehydration process can be adapted for the synthesis of B3O3 rings with different aryl groups on the boron atoms by using two or more arylboronic acids in the appropriate stoichiometric ratio. 

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). 

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.)... 

Boronic acids readily undergo self-condensation to give cyclic anhydrides, and either these trisubstituted boroxins (11), or the acids themselves, react spontaneously, but not necessarily completely, in solution, with suitable diols, to give cyclic boronates 12 (see Scheme 1),... 

Heating boronic acids or treatment with P4Oi0 at room temperature results in loss of H20 to form boroxines, a much-studied class of cyclic organoboranes. 

Eq. 2), and no cyclic siloxane or boroxine products could be isolated in either case. This suggested that the formation of B-O-Si bonds is favoured over Si-O-Si or B-O-B bonds when B-OH and Si-OH species react and led us to consider the possibility of systematic syntheses of three-dimensional borosilicate cages with the connector units shown below. Clearly, at least two, three-connector units are required for the formation of three-dimensional cages such as.3, 4, and 5 above. 

The mechanism we have proposed to explain boronic acid-catalyzed amidation is depicted in Sch. 2. Arylboronic acid usually contains different amounts of cyclic tri-meric anhydrides (boroxines). The rate-determining step is the generation of ArB (OCOR)(OH). 

Compounds RB(0H)2 polymerize to cyclic trimers only, in contrast to R2Si(OH)2 which give polymeric linear siloxanes compare the formation of only one cyclic metaborate ion, 8306 . The formation of 6-membered rings is a prominent feature of boron chemistry, as in boroxine (p. 862) and the numerous substituted... 

Boroxines have trimeric, cyclic structures with planar rings of alternating boron and oxygen atoms. The alkyl groups are also in the plane of the ring. Boron can also be incorporated into numerous carbon ring systems. 

Trimeric cyclic condensation products of boronic acids - the boroxines - can be prepared and isolated. Their use instead of free boronic acids may help to overcome... 

Scheme 10.2 depicts the proposed mechanism. In general, arylboronic acids contain varying amounts of cyclic trimeric anhydrides (boroxines). 

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