Research involving group 2 metals, especially metal precursors Ca(II), Sr(II), and Ba(II) has also flourished for several catalytic applications.46-49 Over the past few years, our research group has developed a variety of Catalysts efficient alkali metal and alkaline earth functionalization of unsaturated substrates.50-51 This paper will provide a recent cumulative report on the various hydroelementation reaction strategies promoted by a variety of magnesium metal precursors over the decade of last. The landmark contribution to the development of hydrofunctionalization reactions by new magnesium metal catalysts was taken care of by the research groups of Hill, Harder and Sadow and this field of research is still developing. An alkyl magnesium complex derived from β-diketiminato 1 was demonstrated by Hill and co-workers as the potent precatalyst for the reduction of aliphatic and aromatic aldehydes and ketones.
However, catalytic hydroboration of pyridine derivatives in stoichiometric amount in the presence of tetranuclear magnesium cluster could not be transformed to 1,2-regioselectivity. In parallel, Lin and his team demonstrated a unique metal-organic framework supported magnesium alkyl complex, namely TPHN-MOF (TPHN = 4,4'-bis-(carboxyphenyl)-2-nitro-1,1'-biphenyl)-supported Mg alkyl complex 4 as a single-site solid catalyst for the hydroboration of C=O and C=N derivatives as well as the hydroamination of aminoalkanes.56b TPHN-MOF was previously derived from the solvothermal reaction of ZrCl4 and H2TPHN in DMF and CF3COOH in excellent yield.56a Zr-MOF was further reacted with Me2Mg at room temperature in THF to prepare the desired catalyst TPHN-MOF-MgMe. High efficiency was shown by the catalyst for hydroboration of carbonyl compounds, imines and hydroamination of aminoalkenes in the presence of very low catalyst loading of 0.05-1 mol%.
A very low catalyst loading of 0.1 mol % Mg[N(SiMe3)2]2 performed the simple hydroboration of benzyl benzoate very smoothly. The complexes afforded the corresponding boronate esters from the hydroboration of a range of aromatic and aliphatic ketones, which is commendable.58 Mg(II) complexes 7–10 were obtained by the reaction between secondary amines and a Grignard reagent (Scheme 6). Complexes 7-10 showed remarkable catalytic performance compared to the reported Mg(II) catalyst 1 for ketone reduction.
Therefore, 0.1–1 mol% of 10 was subjected to the catalytic hydroboration of ketones, and quantitative yields of the products were successfully achieved at room temperature.
Scheme 8. Synthesis of Mg(II) catalyst 14 utilized for catalytic hydroboration of carbonyl compounds
Asymmetric magnesium–catalyzed hydroboration of ketones
Furthermore, the efficiency of the same catalyst was tested for the reduction of cyclic and linear carbonates. MgBu2-mediated reduction of several carbonates was carried out in the presence of 3.1 equiv of HBpin (Scheme 10).62 Predictably, double hydroboration was observed in challenging cyclic carbonates and excellent yields of diboronate esters were obtained. Notably, linear symmetric carbonates required higher reaction conditions and hydroboration selectively occurred alone to produce mono-boronate esters in 92–95% yield.
The proposed mechanism is displayed in Scheme 10 which shows that the active Mg-H catalyst is sequentially associated with three reduction cycles. The research group continued their study towards the development of MgBu2 as a versatile commercial catalyst system. 0.2–0.5 mol% of the catalyst aided chemoselective reduction of α,β-unsaturated ketones and excellent yields were achieved using pinacolborane under ambient conditions.63 In addition, successful reduction of a number of enones and propargylic ketones was achieved.
Computational studies to determine mechanical details have gained tremendous attention among many research groups. The trend of bond dissociation energies of Ae complexes was observed as Mg-H > Ca-H > Sr-H. Recently, MgBu2 has additionally been used for the hydroboration of linear and cyclic carbamates.65 3.5 equivalents of HBpin in the presence of 10 mol% MgBu2 under clean and mild reaction conditions afforded a library of N-methyl amines in good to excellent yields using as presented in scheme 12a.
Furthermore, incorporation of D with carbamates using DBpin was also performed since the corresponding N-trideuteromethylated products are valuable metabolic and pharmacokinetic substrates (Scheme 12b). Ma and co-workers also advanced their research with magnesium metal catalysts and generated highly efficient low-valent Mg(I) metal complexes for the catalytic hydroboration of several organic carbonates, esters, and CO2.66 1 mol% of supported β-diketiminato Mg(I) complex 15 having the same Mg–Mg bond as metal complexes 11–13 proved to be effective for the reduction of carbonates and esters with HBpin under regular conditions at room temperature (Scheme 13). The research group also applied the simple, low-cost and readily available Grignard reagent for the efficient reduction of carbonyl compounds.
The low loading of the MeMgI catalyst at room temp. and the neat state efficiently carried out hydroboration reactions of aldehydes and ketones within. 10–20 min reaction time.67 The chemoselectivity of the Grignard reagent toward the hydroboration of aldehydes over ketones was examined. Furthermore, DFT calculations also helped in the possible pathway for the reaction mechanism and thermodynamic and kinetic feasibility according to the experimental conditions.
Mg(I) catalyst 15 efficient for the reduction of organic carbonates and esters
Hill's research group previously published the hydroboration of carbonyl compounds that proceeded via heteroleptic Mg-H as the active intermediate.52 Easily accessible Mg-H. Catalytic hydrosilylation was also achieved with phenylsilane postulated by cleavage of the C≡O bond and formation of the magnesium formyl intermediate. A year later, Okuda and his team demonstrated the CO2-promoted hydroboration of a variety of unsaturated compounds including magnesium bis(hydridotriphenylborate) complex.71 Using 0.01–1 mol% of the complex [Mg(thf)6][ HBPh3]2 effectively catalyzed the reduction of ketones and pyridine, and the corresponding boronic ester and N-borylated 1,4-dihydropyridine were obtained.
The hydroboration reaction of ketones was performed in DMSO due to insolubility in THF, and a significant amount of DMSO reduction was noted. This motivated the researchers to investigate the catalytic deoxygenation of sulfoxides in the presence of [Mg(thf)6][HBPh3]2, and the corresponding sulfides were observed together with O(Bpin)2 and H2.
Hydroelementation reactions of alkynes
5 mol% catalyst 12 produced cis-alkenylboronate esters by applying a higher reaction temperature (Scheme 7).59 In 2019, the Rueping research group documented extensive studies on magnesium complexes adept at the reduction of various unsaturated compounds. Moreover, the same catalyst MgBu2 served selectively for hydroboration reactions towards the terminal and internal alkynes.74 DFT calculations yielded the most favored mechanistic route, which proved the formation of BuMgH from the reaction between MgBu2 and HBpin based on the relative free energies of the intermediates . (Scheme 19). The corresponding alkenylboronate esters were obtained in good yields and selectivity under preferred conditions.
In addition, higher-order variants of homoleptic triorganomagnesates were investigated, and the catalytic hydroamination of alkynes and styrenes using [(PMDETA)2K2Mg(CH2SiMe3)4] confirmed the best results.
Heterobimetallic catalysts mediated hydroamination reactions of diphenylacetylene and styrene
Hydroelementation reactions of nitriles
Reduction of the C≡N group of organic nitriles to primary amines has attracted researchers around the globe. Hydroboration of isonitriles by catalyst 1 and the most reliable mechanism for the synthesis of 1,2-diborylated amines.
Hydroboration of isonitriles by catalyst 1 and the most plausible mechanism for the synthesis of 1,2-diborylated amines
Mg(II)-catalyzed reduction of organic nitriles
Hydroelementation reactions of alkenes
Catalytic hydrosilylation and hydroboration of styrene substrates leading to Markovnikov products, hydroboration of carbodiimides and pyridine were efficiently performed using complex 19 (Scheme 24). The mechanistic route occurs via the introduction of styrene into catalyst 19, yielding the 1-phenylethyl derivative of the Mg(II) complex. As documented by the Hevia research group, catalytic hydroamination of styrenes was achieved in high yields, although with limitations in substrate size, in the presence of heterobimetallic [(PMDETA)2K2Mg(CH2SiMe3)4] catalyst (Scheme 20).75 Recently, Hill and colleagues efficiently applied dimeric β-diketiminato hydridomagnesium complex 2 and their derivatives reacting with C-C unsaturated substrates to form corresponding organomagnesium.
The rate-determining steps and the Mg-H/C=C insertion process have been calculated based on DFT studies.80 Scheme 25 gives an example of the alkene insertion in a dinuclear Mg(II)-H catalyst, in which magnesium cyclopentylmethyl derivative 22 is used. easily obtained by reacting 2 with 1,5-hexadiene. Catalytic hydrosilylation of terminal alkenes with PhSiH3 was achieved with Mg-H/C=C insertion, yielding anti-Markovnikov products.
Hydroelementation reactions of heterocumulenes
Studies revealed that Mg-H is the active intermediate formed when magnesium complex 1 was used for the hydroboration of carbodiimides. Conversely, when calcium and strontium analogs were used, the amide-metal intermediate generated by carbodiimide insertion into the metal hydride behaves as the catalytically active species. This difference in reaction mechanism can be attributed to the larger ionic radius of calcium and strontium relative to magnesium.82.
Mg(II)-catalyzed hydroboration and reduction of isocyanates
Miscellaneous
The Hevia research group published two magnesium complexes 28-29 and 1 derived from the β-ketimine ligand which is very effective for functionalizing fluoroarenes and promoting C-F bond activation processes.84 Metalation of fluorinated arenes was achieved in the presence of a equivalent amount of 28 within 1 h and at room temperature by inducing C–H bond activation. The metalated products were further used to undergo Negishi cross-coupling reactions using stoichiometric amounts of ZnCl2, 10 mol% Pd(PPh3)3, and two equivalents of iodobenzene (Scheme 28).
A most plausible mechanism for Mg-promoted C–F alkylation
Conclusion
Acknowledgments
Data availability statement
![Figure 4. The molecular structure of [Mg(HCO 2 BPh 3 ) 2 (thf) 4 ]. Hydrogen atoms are omitted for clarity](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10469083.0/18.918.194.720.120.429/figure-molecular-structure-hco-hydrogen-atoms-omitted-clarity.webp)
