The success of 2-benzothienyl and 4-vinylphenylboronic acids demonstrates the effectiveness of this process 3o and 3p 20 , We also examined the use of alkenylboronic acids with this procedure and found that it is suitable for one-pot, multi-component synthesis of the organoborons, whereas styrylboronic acid exhibits much better yield than non-stabilized alkenyl species 3q - 3t. More interestingly, vinylboronic acid which has very limited shelf life also tolerated the conditions. There are no rearranged products during the group migration from boron to boron and the study for single crystals of 3f, 3h and 3o , revealing pseudo-tetrahedral geometry around the boron centre with the N , O -bidentate ligand, confirms this observation see Supplementary Information.
Meanwhile, conventional synthesis heavily depends on the use of diarylborinic acids or triarylboranes. But these protocols suffer from the disadvantages associated with poor accessibility and stability. Moreover, the preparation of these boron reagents requires the use of highly reactive organolithium or Grignard reagents. This hampers the functional group compatibility thereby reducing the scope for the functionalization of peripheral groups attached to the boron atom. In view of these limitations, our finding will be a great advantage to fulfil this goal.
Organic group migration between boronic acids in the presence of 8-hydroxyquinoline.
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In order to prove the feasibility that allows rapid access to important structural motifs, we performed the base-promoted diarylborination with various N , O -bidentate ligands Fig. Substrates ranging from primary to tertiary alcohols 4a - 4c and phenol 4d worked well. Picolinic acid 4e and proline 4f also tolerated. Regarding the amine part of the molecules, secondary and tertiary amines worked well, while pyridine-containing substrates exhibited the best performance.
On the other hand, primary amines like ethanolamine failed. We thus revealed that one-pot synthesis of four-coordinate organoboron compounds can be extended to a wide range of N,O -bidentate ligands, preferentially producing five- and six-membered chelate rings. However, an attempted formation of the four-membered chelate ring from 2-hydroxypyridine was not observed. The method is viable to aryl, heteroaryl, and even alkenylboronic acids with an electronically and sterically devised fashion, thus could be used to generate a number of multi-functionalized organoboron compounds as optoelectronic and photovoltaic materials with high values.
One-pot synthesis of organoborons from various N,O -bidentate ligands. Conditions: N,O -ligand 1. In order to understand the unusual behaviour of boron to boron migration between boronic acids, several control experiments were investigated into the factors contributing to this unusual observation. As observed previously, the reaction of 8-hydroxyquinoline 1 with 9 equivalent of phenylboronic acid 2a gave 0. However, we could not observe the formation of 5 without the base.
When we used 1-naphthol instead of 8-hydroxyquinoline, we realized that a chelation effect is viable in this transformation Fig. These results confirm the role of the ligand in this transformation. In every case, excess boronic acid was recovered as its trimeric anhydride 2b after the reaction. Although a number of possible explanations could be advanced for the present observation, these results suggest that current transformation is more likely due to the disproportionation of boronic acid facilitated by the presence of chelate ligand under basic conditions.
As previously observed, there was no reaction when pinacol or MIDA boronates were employed as boronic acid equivalents entries 14—15, Fig. Considering thermodynamics of boronic acids, this indicates that dimeric anhydride or oligomeric aggregate might be involved in this transformation 22 — Furthermore, the addition of TEMPO revealed no significant difference and this reinforced an anionic pathway to be more convincing Fig.
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All reactions were performed with K 3 PO 4 3. An aliquot from the reaction mixtures was taken out for analysis in a time-dependent manner. The formation of the intermediate I is more feasible, similar to the formation of cyclic boronates from diols and polyols 22 — However, I could not be isolable. Based on their mass spectral molecular weights, we could assign possible boron intermediates corresponding to I , II , III and the product 3a , respectively Fig.
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This observation was further substantiated by the 11 B NMR experiments with a time interval, which distinctively revealing a new signal centered at 2. Upon the gradual consumption of II , the two peaks at Mechanistic considerations. Even with the experimental proof that the proposed intermediates do exist in the reaction mixtures, it's now a question of how to make organic group transfer from boron to boron and where it begins. From the spectroscopic data, we strongly believe the formation of the anionic boron complex is essential at the initial stage.
The formation of the product 3a was observed upon the gradual consumption of II. This migration event of II presumably ended up with the formation of trialkoxyboronate species. The observation of the borate intermediate III in the mass spectrum may partially answer the above question. After the event, III tends to rapidly hydrolyze to boric acid and borinic acid 5 , which readily couples with 8-hydroxyquinoline 1 to generate the product 3a. Diketone-based organoboron compounds are highly utilised as imaging agents for early detection of various human diseases 13 — Moreover, this protocol was successfully applied to curcumin 6f and also a cyclic diketone 6g.
We next searched for the extended scope to dipyyromethene derivatives, in which boron dipyyromethene BODIPY dyes and their derivatives are an area of intense interest in terms of high quantum yields and molecular extinction coefficients for strong fluorescence 28 , Examination also proved that it could be applicable to 2-pyrrolylpyridine and 2-pyridylindole 7a and 7b as well as BODIPY derivatives 7c - d resulting in good yields.
Reaction scope for organoborons from 1,3-diketones. Conditions: 1,3-diketone 1.
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Reaction scope for organoborons from N,N -bidentate ligands. Conditions: N,N -ligand 1.
In summary, we observed the first organic group migration from boron to boron between boronic acids. This migration provides an unprecedented approach to four-coordinate organoboron compounds simply employing boronic acids in the presence of bidentate ligands under basic conditions. The results suggest that the process possibly depends on the ligand-promoted disproportionation of boronic acids by the action of the base.
The reaction uses readily available starting materials, tolerates a variety of functional groups and operates under convenient conditions. Indeed the products are available from a series of N,O -, N,N - and O,O -bidentate ligands upon assembly with ranging from aryl, heteroaryl and even alkenylboronic acids.
Moreover, this approach allows the introduction of functional handles such as halides, amines and carbonyl groups that can be utilised for further elaborations. Current approaches for the synthesis of four-coordinate organoborons are extremely limited particularly in peripheral modifications onto boron cores.
In this respect, we firmly believe that this synthetic route can create more value for such luminescent materials due to their potential applications in material and biomedical sciences. The detailed experiments to obtain a deeper understanding of its mechanism and further applications are currently underway. All solvents and reagents were purchased from commercial sources and used as received without further purification, unless otherwise stated. Reactions were monitored by thin layer chromatography carried out on S-2 0. Merck silica gel plates 60F using UV light as the visualizing agent and an acidic mixture of anisaldehyde or a ninhydrin solution in ethanol and heat as developing agents.
Merck silica gel 60, particle size 0. All yields were calculated from isolated products. Melting points were recorded on Electrothermal IA apparatus and are uncorrected. Chemical shifts are reported in ppm and coupling constants are in Hertz Hz. We thank G. All authors contributed to designing, analyzing and discussing the experiments. Electronic supplementary material. Supplementary information accompanies this paper at doi Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
National Center for Biotechnology Information , U. Sci Rep. Published online Mar Venkata S. Author information Article notes Copyright and License information Disclaimer. Kee-In Lee, Email: rk. Corresponding author. Received Aug 31; Accepted Feb This work is licensed under a Creative Commons Attribution 4. Associated Data Supplementary Materials One-pot synthesis of four-coordinate boron III complexes by the ligand-promoted organic group migration between boronic acids. Abstract Multidisciplinary applications of four-coordinate boron III complexes make them very attractive and challenging research field in chemistry, biology and material sciences.
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