Ligand-controlled divergent formation of alkenyl- or allylboronates catalyzed by Pd, and synthetic applications

Abstract

The use of different ligands allows the preparation of either allyl-or alkenylboronates by Pd-catalyzed borylation of allylic carbonates containing alkyne groups. Unprecedented borylative cyclisation to alkenylboronates takes place with PCy 3 . The difficult dissociation of NHC ligands allows borylation of carbonates in the presence of alkynes. Oxidation, regioselective Suzuki coupling, as well as Au-catalyzed cycloisomerisation of boronates illustrate the potential synthetic applications of these reactions.Boronates are important synthetic reagents. They are stable and compatible with most functional groups and are environmentally friendly, tolerating oxygen and water. A wide variety of boronates can be used as nucleophilic partners in Suzuki cross-coupling reactions and allylboronates are reactive towards aldehydes and imines, just to cite the most common uses of these compounds. 1 For this reason, the development of methods for the preparation of boronates, avoiding the use of highly nucleophilic and basic reagents is of main interest. On the other hand, enynes constitute versatile starting materials for the formation of carbo and heterocycles, 2 skeletal rearrangements, 3 and alkoxy-cyclisation, 4 among other reactions. During the last few years, Pd-catalyzed borylative cyclisation reactions of polyunsaturated species have been developed by our group 5-9 and Bäckvall's group, 10 with the aim of preparing cyclized boronates. Our seminal work described an unprecedented preparation of homoallylic boronates starting with 1,6-enynes, in which esters substituting the allylic position were tolerated (Scheme 1, top). 5,11 In our previous studies, addition of ligands precluded the incorporation of the boron substituent. Now, we have found conditions in which the choice of the appropriate ligand allows control of the borylation of alkyne-containing allylcarbonates, affording simple Miyaura coupling to allylboronates, or unprecedented formation of alkenylboronates by borylative cyclisation, a method which is complementary to alkyne hydroboration and the usual reactions of strongly basic Li and Mg nucleophiles with B(OMe) 3 .Thus, we started our study by reacting enynes containing an allylic carbonate group (1). In our previous work mentioned above, 5 we reported that reactions of acetates or other ester derivatives, under ligandless conditions, took place without activation of the C-O bond, leading to homoallylic boronates containing the unreacted ester group (Scheme 1, top). Reactions in the presence of phosphines led to simple cycloisomerisation without incorporation of the boryl group.In contrast, reaction of carbonates 1a-l with B 2 (pin) 2 in the presence of Pd(OAc) 2 and the IMes ligand (formed in situ by deprotonation of the imidazolium chloride with t-BuOK) in xylene, afforded the corresponding allylboronates (Table 1). The reaction is fast (less than 30 min) and takes place under optimum conditions (50 1C) in the absence of added base, as usual for carbonates. A variety of starting compounds showing different ...

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

“…Reductive elimination subsequently affords the borylative cyclization product. A similar intramolecular borylative cyclization reaction was observed for allenyl ketones, 2-alkynylaryl isocyanates, a 2,3-dienylbutadienamine, 2-alkynylanilines, enynes, ,, enediynes, , allenynes, , enallenes, − 2-alkenylphenylisocyanides, 1,7-enynes, enone diones, cyclohexadienone-containing enynes, and dienynes (Scheme ). A recent publication by Bäckvall et al should be noted as chiral phosphoric acids were used as a cocatalyst with Pd­(OAc) 2 to help facilitate the enantioselective cyclization taking place, which generates borylated carbocycles in high enantiomeric excess (Scheme b) …”

Borylative Cyclization Reactions

“…More recently we focused on modulating the reactivity of the 1,6‐enynes by changing the already tested acetate at the allylic position for a carbonate group . As mentioned above, the presence of an allyl ester was compatible with the reaction conditions, and no formation of allyl‐Pd derivatives was observed.…”

Divergent g-C3N4-catalyzed Reactions of Quinoxalin-2(1H)-ones with N-Aryl Glycines under Visible Light: Solvent-Controlled Hydroaminomethylation and Annulation

“…Divergent synthesis is of great significance in organic synthetic chemistry , owing to the rapid acquisition of structurally diverse chemical scaffolds from the same starting materials, − which has also been considered as a highly attractive synthetic strategy for the synthesis of heterocyclic compounds. , Generally, different products could be generated from the identical starting materials through the variation of functional groups, − catalysts, solvents, − additives, − and ligands to switch the reaction pathways. This strategy provides an indispensable route to build a molecular library with maximum scaffold diversity. − Therefore, the divergent synthesis of various scaffolds from easily available starting materials is urgently pursued and expected.…”