Boroxine formation

Fig. 9 (a) Reversible synthesis of a bisiminoboronate-guanosine derivative from guanosine and a bisiminoboronic acid, (b) Dynamic polymeric networks based on reversible boroxine formation, (c) Formation of a macrocycle in a [4 + 4 + 2] condensation via simultaneous reversible formation of imine bonds and boronic esters... 

Figure 1.3 Continued) (n) imine exchange, boroxine formation, (v) transboroxoaromatic (o) hydrazone exchange, (p) oxime exchange, esterification, (w) reversibie resorcinoi and (q) nitrone exchange, (r) aikene metathesis, 1,4-butanediai condensation, (x) metai-...

Boroxine formation, through the cyclotrimerization of three boronic acid units, is another reversible reaction recently applied in dynamic covalent synthesis (Figure 1.3u). The forward reaction is entropicaUy driven by the release of water molecules upon condensation and is favored where electron-donating groups in the para position are used [42]. To date, this reaction has only been used in the designed thermodynamic synthesis of a Cj-symmetric [4]rotaxane 26, apparently under thermodynamic control (Scheme 1.12) [43]. However, it could be envisaged that... 

Scheme 1.12 Templated synthesis of a Cs-symmetric [4]rotaxane 26 generated through reversible boroxine formation [43].

Despite their Hmited structure determination capabilities, ultraviolet and infrared spectroscopy were determinant characterization techniques in the early days of boronic acid research [332]. Notable IR absorptions are the strong H-bonded OH stretch (3300-3200 cm ), and a very strong band attributed to B-O stretch (1380-1310 cm ). IRis particularly diagnostic of the presence of boronic anhydrides. Upon anhydride (boroxine) formation, the OH stretch disappears and a new strong absorption appears at 680-705 cm [68]. 

Boronic acids (69 and 70) (Fig. 45) with more than one boronic acid functionality are known to form a polymer system on thermolysis through the elimination of water.93 Specifically, they form a boroxine (a boron ring system) glass that could lead to high char formation on burning. Tour and co-workers have reported the synthesis of several aromatic boronic acids and the preparation of their blends with acrylonitrile-butadiene-styrene (ABS) and polycarbonate (PC) resins. When the materials were tested for bum resistance using the UL-94 flame test, the bum times for the ABS samples were found to exceed 5 minutes, thereby showing unusual resistance to consumption by fire.94... 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

SCHEME 9.2 Formation of boroxine frompara-diboronic acid. 

The boroxines could then be subjected to Suzuki coupling with aryl, vinyl, or benzyl halides. Suzuki coupling with tri- -butylstannyl chloride also gave the tri- -butylstannyl-substituted thiophenes 221. These can form the starting materials for further transformations. The carbonylative coupling with halides resulted in the formation of ketones tin-lithium exchange followed by reaction with electrophiles led to a host of other useful products (Scheme 65). 

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 reaction is not "clean." Hydroperoxide decomposition yields aldehyde and ketone. Moreover, at other than quite low conversion, further oxidation leads to scission of carbon-carbon bonds and formation of acids [63], However, if a boric-acid ester or boroxine is added, secondary alcohol can be obtained in good yield (see Example 5.5 in Section 5.4). 

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... 

The most prevalent boron-oxygen oligomeric systems are based on the six-membered (8—0)3 r ttg> formally a derivative of boroxine, (HBO)3. These compounds are synthesized directly, the tendency to ring formation usually precluding the isolation of any intermediate products. 

The solvothermal conditions used to synthesize boroxine- and boronate-ester COFs (temperature, solvent, solvent-to-head-space ratio) have a strong effect on the product morphology [14,20] and it seems that particular solvents give rise to ordered crystalline materials whereas others do not. hi principle, this may be understood in terms of templating effects but could equally arise from differences in monomer solubility, for example, which in turn affects the rate of network formation. 

Scheme 28.7 Formation of periodic network structures (21 and 22) in solids using equilibrium boroxine and boronate formation with a monomer 23 and with monomers 23 and 24, respectively.

Reversible bond formation can, in principle, be used for the selfcorrection of product structures. For example, the formation of Schiff bases, boroxines and boronates has been applied to on-surface polymerizations, but has so far met with limited success. Linderoth and Gothelf employed trialdehyde 62 and diamine monomers 63 towards 2-D polymer synthesis on Au(lll) under UHV (Figure 28.28a) [134], but unfortunately obtained only branched or irregularly networked stmctures, a situation that was ascribed to the flexible monomer structure employed. In addition, UHV conditions caused an irreversible loss of water, the presence of which was essential to bring about the back-reaction. When Abel and coworkers synthesized boroxine/boronate networks on Ag(lll) under UHV [135] they were unable to achieve the expected periodic order this contrasted with the findings of Yaghi et al., who employed the solution approach (see Section 28.5.2). This difference was most hkely a consequence of irreversible bond formation. 

COF-1 Condensation reactions, boroxine ring formation 711 0.8 0.34 Crystalline... 

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