Kazuhiko Takai

Takai attended Kyoto University, receiving his Bachelor of Engineering degree in 1977 and Master of Engineering degree in 1979 before pursuing a doctoral degree. Working under Hitosi Nozaki, Takai developed a method for the methylenation of enolizable ketones and aldehydes using geminal dizinc species.^[3]

After focusing on the role of organoaluminium compounds in cross-couplings,^[4] sigmatropic rearrangements,^[5] and ketone synthesis,^[6] Takai left the doctoral program in 1981 and accepted an assistant professor position in Nozaki's lab. Following completion of his thesis in 1983, Takai joined Clayton Heathcock's lab at the University of California, Berkeley, where he focused on the diastereoselective synthesis of pentane-1,2,3,4-tetraols.^[7]

Takai would return to Kyoto and continue working as an assistant professor with Nozaki until his retirement in 1985, where he continued as an assistant professor under Kiichiro Utimoto until 1994. Takai moved to Okayama University to accept an associate professor position, gaining full professorship in 1998. Professor Takai continued his research at Okayama until his retirement in 2022.

While working as an assistant professor for Nozaki, Takai published several reports describing reactions between unsaturated organohalides and aldehydes in the presence of chromium(II) chloride.^[8] While the seminal report boasted mild reaction conditions and broad functional group tolerance, reproducibility issues hampered wide-scale application. The culprit behind the reproducibility issues was outlined in simultaneous publications from Takai^[9] and Yoshito Kishi,^[10] where they highlighted the importance of catalytic nickel for successful couplings. Several batches of commercial CrCl₂ revealed the presence of trace nickel impurities in lower quality samples. It is now accepted that the reaction proceeds via the initial formation of an organonickel species which undergoes transmetallation to the active organochromium nucleophile. This mechanism should not be confused with the related Nozaki–Hiyama allylation, where allylchromium species add into an aldehyde without any nickel additives.^[11]

Owing to its mild conditions and broad functional-group tolerance, the NHTK reaction has found applications in several natural product syntheses. One particularly notable example is in Kishi's total synthesis of halichondrin B, where extensive application of the coupling was eventually adapted by Eisai into an industrial route for the FDA-approved anti-cancer drug eribulin.^[12]

In 1987, Takai reported the synthesis of enol ethers via the olefination of esters.^[13] Building on previous studies with Nozaki, these transformations were hypothesized to involve a geminal dizinc intermediate. However, variable yields and reaction times dissuaded wide application of the method, and Lombardo would soon report an alternative procedure that involved "aging" the organozinc intermediate for several days, in what is now known as the Lombardo methylenation.^[14] Analysis of several batches of commercial zinc revealed that zinc produced via the pyrometallurgical process contained trace impurities of lead, which significantly altered the outcome of the methylenation. Takai subsequently reported that the methylenation proceeded quickly and reproducibly when catalytic PbCl₂ was added to the reaction. It is believed that the mechanism involves formation of a α-iodozinc species which can rapidly transmetallate to the organolead, which then primes the system for a second oxidative addition into the remaining halide.^[15] Due to the similarities between Takai and Lombardo's conditions, this general transformation is often referred to as the Takai–Oshima–Lombardo methylenation.^[16]

While attempting to synthesize geminal trichromium species from iodoform, Takai observed formation of (E)-iodostyrene when benzaldehyde was reacted with CrCl₂ and iodoform.^[17] This observation led to the development of what is now known as the Takai–Utimoto olefination, where carbonyls can be converted to (E)-haloalkenes in the presence of CrCl₂ and the corresponding trihalide.^[8] Like the corresponding organozinc reactions, the mechanism was hypothesized to involve a geminal dichromium intermediate, but the existence of such species was not proven until publications from Takai in 2017 alongside Anwander and Werner in 2018.^[18][19] However, unlike earlier zinc reactions, the Takai olefination exhibits a strong preference for the (E)-haloalkene, most likely due to stereoelectronic effects directly prior to oxygen elimination.

The formed haloalkenes are versatile synthetic intermediates that can be subjected to one-pot reaction conditions to rapidly build complex systems. One example was reported by Takai, where aldehydes can be subjected to tandem Takai–Utimoto/NHTK conditions to synthesize homologated allylic alcohols in decent yields.^[8] Later publications also report substituting the third iodide for heteroatoms or alkanes, allowing for the convenient synthesis of diverse (E)-substituted alkenes.^[20][21]

In 1990, Takai and coworkers reported the partial reduction of alkynes to (Z)-olefins in the presence of TaCl₅ and zinc.^[22] While the initial report involved protodemetallation of the intermediate metallocyclopropene, Takai published a series of subsequent reports that demonstrated the versatility of the organotantalum intermediate. By varying which trapping electrophile is added, a wide variety of functionalities can be synthesized. Notable examples include the synthesis of 1,4-dienes,^[23] (E)-allylic hydrazines,^[24] polysubstituted furans,^[25] (E)-allylic alcohols,^[26] and 1-naphthols.^[27] While these reactions generally proceed with favorable yields, varying levels of regioselectivity have hindered widespread use.

Takai's group has also contributed to the development of group 7 catalysis, mainly focusing on organomanganese and organorhenium-based methods.^[28] Primarily focusing on bromotricarbonylrhenium(I) species, Takai demonstrated that these complexes are capable catalysts in a variety of transformations, highlighting their use as agents for C–H and C–C bond activation. Notably, these complexes were capable of forming stereodefined cyclopentenes,^[29] inserting terminal alkynes into β-ketoesters,^[30][31][32] enabling the deaminative annulation of imines to form subsitued indenes,^[33] and facilitating a variety of metal-mediated cycloadditions to form fused aromatic species.^[34][35][32]

While not catalytic, Takai and coworkers developed mild conditions for the activation of manganese powder, allowing for its application as a single electron reductant in several reactions. Despite possessing a stronger reduction potential than zinc,^[36] manganese metal was seldom used as a reductant due to its propensity of rapidly forming an oxide coating, and activated forms of the element were most commonly encountered as a Rieke metal.^[37] In 1996, Takai reported that commercial manganese was readily activated in the presence of trimethylsilyl chloride and catalytic PbCl₂.^[38] The activated manganese was a competent reductant and was capable of affecting Barbier allylations and Reformatsky-type reactions.

While Mn(0) is capable of reducing Pb(II), these reactions do not proceed solely in the presence of lead powder. The activated manganese was later shown to serve as a versatile reductant, capable of promoting 3-component couplings,^[39] cycloadditions,^[40] and cyclopropanations, among other reactions.^[41]