Recent Advances in Asymmetric Catalysis Using p-Block Elements.

The development of new methods for enantioselective reactions that generate stereogenic centres within molecules are a cornerstone of organic synthesis. Typically, metal catalysts bearing chiral ligands as well as chiral organocatalysts have been employed for the enantioselective synthesis of organic compounds. In this review, we highlight the recent advances in main group catalysis for enantioselective reactions using the p-block elements (boron, aluminium, phosphorus, bismuth) as a complementary and sustainable approach to generate chiral molecules. Several of these catalysts benefit in terms of high abundance, low toxicity, high selectivity, and excellent reactivity. This minireview summarises the utilisation of chiral p-block element catalysts for asymmetric reactions to generate value-added compounds.

Reactivity of a series of triaryl borates, B(OArx)3, in hydroboration catalysis.

In this paper, we compare the reactivity of a series of triaryl borates B(OArx)3 as catalysts for the hydroboration of alkenes and alkynes. It was observed that commercially available B(OPh)3 performed the poorest, whereas catalysts with o-F atoms appeared to perform much better.

Chemodivergent Chalcogenation of Aryl Alkynoates or N-Arylpropynamides Using 9-Mesityl-10-Methylacridinium Perchlorate Photocatalyst.

Herein we report a cascaded chalcogenation of aryl alkynoates or N-arylpropynamides using 9-mesityl-10-methylacridinium perchlorate as a visible light photocatalyst to obtain selectively either 3-sulfenylated/selenylated coumarins or spiro[4,5]trienones. In a radical initiated process, the spiro-cyclization reaction was favored due to the presence of a -OMe or -F substituent at the para position of the aryl group, which helped to stabilize the allylic radical intermediate formed during the reaction. Otherwise, 6-endo-trig cyclization led to 3-sulfenylated/selenylated coumarins. Overall, the new C-S/C-Se, C-C, and C═O bonds were formed in a single step. The Stern-Volmer quenching study, EPR experiments, light ON-OFF experiments, radical trapping experiments, etc., helped to understand the radical-based mechanism.

B(3,4,5-F3H2C6)3 Lewis acid-catalysed C3-allylation of indoles.

Herein we report the B(3,4,5-F3H2C6)3-catalysed C3-allylation of indoles using allylic esters. 25 examples of C3-allylated products are presented in up to 97% yield. The mechanism for the reaction was explored using detailed Density Functional Theory (DFT) studies.

3-Arylsulfonylquinolines from N-Propargylamines via Cascaded Oxidative Sulfonylation Using DABSO.

We report a cascaded oxidative sulfonylation of N-propargylamine via a three-component coupling reaction using DABCO·(SO2)2 (DABSO). 3-Arylsulfonylquinolines were obtained by mixing diazonium tetrafluoroborate, N-propargylamine, and DABSO under argon atmosphere in dichloroethane (DCE) for 1 h. In a radical pathway, DABSO was utilized as the sulfone source and an oxidant in this radical-mediated cascaded reaction.

t BuOLi-promoted terminal alkyne functionalizations by aliphatic thiols and alcohols.

Selective radical addition to terminal alkynes is always a difficult task to achieve because it gives a mixture of stereo- and regioisomers. Herein we describe the selective addition of aliphatic thiols or alcohols to N-phenylpropiolamides (terminal alkynes) using lithium tert-butoxide (tBuOLi) in ethanol as a promoter. Mechanistically, it has been shown that the reaction proceeded through the generation of a thiyl radical intermediate, and the amide group in N-phenylpropiolamide could help in the activation of the alkyne, which led to thioacetalization via the formation of a (Z)-selective anti-Markovnikov vinyl sulfide. The (Z)-selectivity during the formation of vinyl sulfides was controlled by an intramolecular sulfur⋯oxygen interaction.

Reported Catalytic Hydrofunctionalizations that Proceed in the Absence of Catalysts: The Importance of Control Experiments.

The enlarged landscape of catalysis lies in the heart of chemistry. As the journey has set a milestone in organic synthesis, its darker side has not entered into the limelight. Studies disclose that the reported reactions by using catalysts were also attainable in the absence of catalysts in many cases. This article presents a literature collection that includes the significance of control experiments in hydrofunctionalization reactions. Systematic analysis reveals that the catalysts are ambiguous and might be unessential in chemical reactions enlisted here.

Chlorinative Cyclization of Aryl Alkynoates Using NCS and 9-Mesityl-10-methylacridinium Perchlorate Photocatalyst.

In a chlorinative cyclization, Mes-Acr-MeClO4 acted as a visible-light photocatalyst to obtain 3-chlorocoumarins from aryl alkynoates and N-chlorosuccinimide (NCS). The radical initiated reaction proceeded in a cascading manner via Cl- addition to alkynoates. Next, 5-exo-trig spirocyclization and subsequent 1,2-ester migration led to the formation of C-C and C-Cl bonds.

Disulfide metathesis via sulfur⋯iodine interaction and photoswitchability.

The idea of constitutional dynamic chemistry (CDC) and dynamic combinatorial chemistry (DCC) is widespread in the literature using the chemistry of disulfides. The synthesis of unsymmetrical diaryl disulfides is challenging due to the presence of a weak S-S bond. We report herein the synthesis of unsymmetrical diaryl disulfides from two symmetrical disulfides via a cross-metathesis reaction which was controlled by a weak sulfur⋯iodine (S⋯I) interaction. The unsymmetrical disulfides were stable in acetonitrile solution in the presence of N-iodosuccinimide (NIS), and found to be reversibly photoswitchable to the symmetrical disulfides under visible light irradiation.

Sulfuroxygen interaction-controlled (Z)-selective anti-Markovnikov vinyl sulfides.

The sulfur oxygen (SO) interaction was used herein to obtain (Z)-selective anti-Markovnikov vinyl sulfides from the addition of thiyl radicals to terminal alkynes. DFT calculations predicted that SO interaction originated from the delocalization of the lone-pair of the carbonyl oxygen to the adjacent σ* orbital of the S atom of C-S.

Metal-free C-S coupling of thiols and disulfides.

In organic synthesis, transition-metal and photoredox-catalysis-based reaction systems are emerging trends for the construction of C-S bonds. Many review articles have recently appeared in this field; however, we present herein an overview of metal-free C-S coupling reactions using thiols or disulfides as sulfur surrogates. The oxidants we have considered include peroxides, tert-butyl nitrite (TBN), DDQ, iodine reagents, and molecular oxygen. In addition, selective electrochemical oxidative transformations are also covered with mechanistic details.

Dithioacetalization or thioetherification of benzyl alcohols using 9-mesityl-10-methylacridinium perchlorate photocatalyst.

We report herein the use of 9-mesityl-10-methylacridinium perchlorate as the visible-light photocatalyst for dithioacetalization or thioetherification of benzyl alcohols in one pot using aerial dioxygen as a terminal oxidant. EPR analysis and Stern-Volmer quenching studies helped to rationalize the single electron transfer (SET) mechanism.

Noncovalent Interactions in C-S Bond Formation Reactions.

Organosulfur compounds are omnipresent in many drugs, natural products, and functional materials; therefore, methodologies of C-S bond formation reactions are desirable in synthesis. The recent trend to control many chemical reactions by cooperative multiple weak interactions have emerged as one of the popular topics in supramolecular catalysis. Herein, we have made a collection of literature which utilizes multiple noncovalent interactions like H-bonding, solvent bonding, S-H···π, C-H···π, π-π stacking, charge-transfer complexation, etc. toward aryl C-S bond-formation reactions.

(Z)-Selective anti-Markovnikov or Markovnikov thiol-yne-click reactions of an internal alkyne by amide hydrogen bond control.

Herein, we show exclusive control of stereo and regioselective thiol-yne click (TYC) reactions of internal alkynes via amide hydrogen bond control. By exploiting appropriate hydrogen bonding interactions like N-HS, N-HN and C-HO, either (Z)-selective anti-Markovnikov or Markovnikov products could be obtained for an internal alkyne, exclusively.