Iron- and Cobalt-Catalyzed Asymmetric Hydrofunctionalization of Alkenes and Alkynes

Abstract

Conspectus Transition metal catalyzed asymmetric hydrofunctionalization of readily available unsaturated hydrocarbons presents one of the most straightforward and atom-economic protocols to access valuable optically active products. For decades, noble transition metal catalysts have laid the cornerstone in this field, on account of their superior reactivity and selectivity. In recent years, from an economical and sustainable standpoint, first-row, earth-abundant transition metals have received considerable attention, due to their high natural reserves, affordable costs, and low toxicity. Meanwhile, the earth-abundant metal catalyzed hydrofunctionalization reactions have also gained much interest and been investigated gradually. However, since chiral ligand libraries for earth-abundant transition-metal catalysis are limited to date, the development of highly enantioselective versions remains a significant challenge. This Account summarizes our recent efforts in developing suitable chiral ligands for iron and cobalt catalysts and their applications in the highly enantioselective hydrofunctionalization reactions (hydroboration and hydrosilylation) of alkenes and alkynes. In ligand design, we envisioned that chiral unsymmetric NNN-tridentate (UNT) ligand scaffolds could promote these enantioselective transformations with earth-abundant metals. Therefore, several types of chiral UNT ligands were designed and prepared in our laboratory, utilizing readily available natural amino acids as chiral sources. In the very beginning, chiral oxazoline iminopyridine (OIP) ligands were proposed and investigated through the rational combination of nitrogen-containing ligand scaffolds. After a systematic survey of the ligand effects, the imine moiety in the rigid OIP ligands was replaced by a conformationally more flexible amine unit, leading to the construction of reactive oxazoline aminoisopropylpyridine (OAP) ligands. Subsequently, imidazoline iminopyridine (IIP) and thiazoline iminopyridine (TIP) ligands were prepared by altering the oxygen atom of oxazoline with nitrogen and sulfur linkers, respectively. To further expand the chiral ligand library, other tridentate ligands containing a twisted pincer, anionic, and nonrigid backbone were also designed and synthesized, including iminophenyl oxazolinyl phenylamine (IPOPA) and imidazoline phenyl picolinamide (ImPPA). The efficacy of these chiral UNT ligands for asymmetric induction in iron and cobalt catalysis has been demonstrated through asymmetric hydrofunctionalization of alkenes and asymmetric sequential hydrofunctionalization of alkynes, which exhibit excellent reactivity as well as high chemo-, regio-, and stereoselectivity with broad functional group tolerance. Notably, highly regio- and enantioselective hydrofunctionalization of challenging substrates, such as 1,1-disubstituted aryl alkenes and terminal aliphatic alkenes, was also achieved. Furthermore, the development of asymmetric sequential isomerization/hydroboration of internal alkenes and sequential hydrofunctionaliz...

Cobalt‐Catalyzed Asymmetric Alkylation of (Hetero)Arenes with Styrenes

“…Over the past few years, transition-metal-catalyzed asymmetric hydrometalation reaction, represented by CuH, NiH and CoH catalysis, has emerged as a robust and practical synthetic platform for construction of enantioenriched CÀ C and carbon-heteroatom bonds. [12] Such transformation is typically achieved via in situ generated chiral alkyl metal intermediates, originated from MH migratory insertion into alkenes, serving as surrogates of pre-prepared organometallics to be intercepted by various electrophiles (Scheme 1d). A diverse range of carbon electrophiles including aldehydes, [13] ketones, [14] imines, [15] allyl/alkyl electrophiles, [16] etc [17] have been implemented to enantioselective hydrocarbofunctionalization for chiral CÀ C bond formation.…”

Regio‐controllable Cobalt‐Catalyzed Sequential Hydrosilylation/Hydroboration of Arylacetylenes

“…Earth-abundant metals possess several advantages such as high reserves,l ow toxicity,a nd affordable costs. [16] Our groupsr esearch interests focus on earth-abundant-metal catalysis.I nspired by our previous studies [17] and others works, [2e] here,w es ystematically developed ar egio-controllable cobalt-catalyzed sequential hydrosilylation/hydroboration of arylacetylenes (Scheme 1c).…”

Copper-catalyzed regio- and stereo-selective hydrosilylation of terminal allenes to access (E)-allylsilanes

“…It is worth to mention that copper is an earth abundant transition metal, which makes it an ideal candidate to develop transformations in sustainable chemistry. However, compared to relatively more extensively studied Ni, Co, Fe-catalyzed hydrosilylation reactions 2 , 40 – 42 , copper catalysis has been rarely used in hydrosilylation of unsaturated carbon carbon bonds 24 , 43 – 49 . Herein, we report a copper-catalyzed hydrosilylation of allenes which affords linear ( E )-allylsilanes with excellent regio- and stereoselectivity.…”