Synthesis and utility of N-boryl and N-silyl enamines derived from the hydroboration and hydrosilylation of N-heteroarenes and N-conjugated compounds

Catalytic hydroboration and hydrosilylation have emerged as promising strategies for the reduction of unsaturated hydrocarbons and carbonyl compounds, as well as for the dearomatization of N-heteroarenes. Various catalysts have been employed in these processes to achieve the formation of reduced products via distinct reaction pathways and intermediates. Among these intermediates, N-silyl enamines and N-boryl enamines, which are derived from hydrosilylation and hydroboration, are commonly underestimated in this reduction process. Because these versatile intermediates have recently been utilized in situ as nucleophilic reagents or dipolarophiles for the synthesis of diverse molecules, an expeditious review of the synthesis and utilization of N-silyl and N-boryl enamines is crucial. In this review, we comprehensively discuss a wide range of hydrosilylation and hydroboration catalysts used for the synthesis of N-silyl and N-boryl enamines. These catalysts include main-group metals (e.g., Mg and Zn), transition metals (e.g., Rh, Ru, and Ir), earth-abundant metals (e.g., Fe, Co, and Ni), and non-metal catalysts (including P, B, and organocatalysts). Furthermore, we highlight recent research efforts that have leveraged these versatile intermediates for the synthesis of intriguing molecules, offering insights into future directions for these invaluable building blocks.


Introduction
Enamine is a functional moiety found in a variety of natural products, bioactive molecules, and pharmaceuticals (Burgess et al., 1991;Goldmann and Stoltefuss, 1991;Gordeev et al., 1998;Edafiogho et al., 2007;Sashidhara et al., 2009;Poulsen, 2021).Within the realm of synthetic organic chemistry, enamines serve as versatile intermediates (Fu et al., 2019;Wang et al., 2023) and catalysts (Notz et al., 2004;Mukherjee et al., 2007;Zou et al., 2018) to facilitate the synthesis of diverse chemical structures, including natural products and heterocyclic compounds (Cheng et al., 2004;Hanessian and Chattopadhyay, 2014;Wang, 2015).Traditionally, enamines are prepared through condensation reactions between secondary amines and carbonyl compounds, α,β-elimination of amides, or reductive acylation of ketoximes (Mengya, 2018).However, these conventional methods give low conversion or exhibit narrow functional group tolerance due to their harsh reaction conditions (Dehli et al., 2005).Consequently, numerous alternative methods have been developed, including the amination of alkenes or alkynes (Beller et al., 2002;Ahmed et al., 2003), methylenation of amides (Shen and Porco, 2000), dehydrogenation of tertiary amines (Zhang et al., 2003), cross-coupling of amines and alkenyl bromides (Barluenga et al., 2004), and N-formylation of amines with CO 2 and PhSiH 3 to synthesize linear enamines (Nakaoka et al., 2023).Furthermore, the cobalt-catalyzed hydrogen transfer of amines (Bolig and Brookhart, 2007) and palladium-catalyzed intramolecular amination of alkenes (Jiang et al., 2017) are utilized to synthesize cyclic enamines.In addition, the hydroboration and hydrosilylation of N-heteroarenes and conjugated imine or nitrile compounds are also used to obtain enamine derivatives, particularly borylated and silylated enamines.In contrast to traditional reductive methods such as metal hydride reduction and hydrogenation, the use of silanes and boranes as reducing agents enables milder reaction conditions, high chemoand regioselectivity, and compatibility with diverse functional groups (Park and Chang, 2017).Consequently, hydroboration and hydrosilylation have emerged as alternatives to the use of H 2 , which is a common method in catalytic reduction chemistry (Marciniec, 2005;Arai and Ohkuma, 2008).Moreover, hydroboration and hydrosilylation demonstrate catalytic versatility, as they can be facilitated by various metal-based complexes, metalloids, and non-metal compounds.The initial steps, depending on the activity of the catalyst, may involve the activation of the B-H bond in HBpin and the Si-H bond in silanes, or coordination with heteroatom centers in the reactant substrates.However, the general catalytic mechanisms can be categorized into outer-sphere and inner-sphere pathways (Park and Brookhart, 2010;Iglesias et al., 2014;Corey, 2016;Lipke et al., 2017), which determine the hydride attack pathway through the formation of distinct intermediates.The chemical reactivities, selectivities, and primary mechanistic insights of these hydroboration and hydrosilylation reactions have been extensively discussed in reviews focusing on the hydroelementation of alkene (Du and Huang, 2017), alkyne (Saptal et al., 2020), nitrile (Das et al., 2022), as well as dearomatization of unactivated N-heteroarenes (Park and Chang, 2017;Park, 2020;Escolano et al., 2024).A borane-catalyzed double hydrosilylation for the formation of sp 3 C-Si bonds (Park, 2019) and single hydroelementation reaction of N-heteroarenes were also reviewed (Park, 2024) (Figure 1A).However, a review focusing on the synthesis and utility of N-boryl and N-silyl enamines has not yet been reviewed.Thus, in this review, we first outline the formation of N-boryl and N-silyl enamines via the 1,4-reduction of quinolines, conjugated nitriles/aldimines, 1,2-reduction of isoquinoline, and both 1,2-and 1,4-reduction of pyridines.Then, we summarized their subsequent applications in nucleophilic additions and Diels-Alder reactions (Figure 1B).
2 Hydroboration in the synthesis of N-boryl enamines

Alkali-and alkaline-earth-metalcatalyzed hydroboration of N-heteroarenes
The magnesium-catalyzed hydroboration of pyridines and isoquinolines has evolved from mononuclear structures (Mg I) to the dinuclear β-diketiminate magnesium hydrides (Mg II).Recently, a phosphinimino-amido magnesium complex (Mg III) has also been reported (Figure 2A).While the Mg I and Mg II systems exhibited temperature-dependent selectivity between 1,2-and 1,4hydroboration (Arrowsmith et al., 2011;Intemann et al., 2014), the Mg III catalyst showed highly selective 1,2-hydroboration (Liu et al., 2020).Furthermore, the formation of both regioisomers under Mg I and Mg II conditions indicated that the magnesium hydride complex was not a key step in these mechanisms.Instead, the generation of an intermediate (Int 1, Figure 2) through the reaction of the [Mg]-1,2-reduced intermediate with HBpin directly transfers a hydride from boron to the 2-or 4-position of the pyridine ligand (Arrowsmith et al., 2011;Intemann et al., 2014).Moreover, density functional theory (DFT) calculations using the Mg III catalytic system showed that the dearomatization process resulted in a reasonable energy barrier for the 1,2-reduced intermediate, whereas 1,4-dearomatization presented considerable challenges because of its high energy demand (Liu et al., 2020).
Recently, Zhang et al. and Chang et al. reported the potassiumcatalyzed hydroboration of N-heteroarenes (Figure 2B).Both catalytic conditions resulted in the highly selective 1,4hydroboration of pyridines (Liu et al., 2019;Jeong et al., 2019).However, the KH catalytic systems reported by Zhang exhibited excellent 1,2-selectivity with quinoline substrates.The use of THF solvents and elevated reaction temperatures enhanced the 1,4selectivity in the hydroboration of pyridines.This selectivity is due to the formation of a 1,4-reduced intermediate through the thermodynamic isomerization of the 1,2-reduced intermediate (Liu et al., 2019).Conversely, the 1,4-regioselectivity in the KO t Bupromoted hydroboration in Chang's study originated from an outer-sphere mechanism.KO t Bu and HBpin react to form various borohydride species as active hydrides in the presence of BH 3 .These active hydride species then generate 1,4-reduced intermediates via nucleophilic hydride attack on the pyridine-BH 3 adducts.Subsequent hydride transfer generates N-borylated-1,4-dihydropyridines, while liberating BH 3 through σ-bond metathesis (Jeong et al., 2019).
Kinetic and mechanistic investigations of the Ni(acac) 2 /PCyp 3 catalytic system revealed the formation of bis(heteroarene) complexes as the first step (Tamang et al., 2018).In contrast, the (IPr)CuFp catalytic system formed (IPr)CuH as an active hydride species.Pyridine is then activated to form a pyridyl cation.N-borylated-1,4-dihydropyridines are produced through the interaction of an active hydride species at the 4-position of the pyridyl cation (Int 2, Figure 3) (Yu et al., 2020).Additionally, mechanistic studies of [LMn(CO) 2 ] showed that the 1,2-adduct was kinetically favorable, whereas the 1,4-adduct was more stable and was generated via a thermodynamic process.The unusual 1,4regioselectivity was derived from the decreased free-energy barrier for the 1,4-hydroboration compared to that for the 1,2hydroboration.This could be achieved by using a 1methylimidazole-based pincer Mn catalyst featuring cooperative C-H•••N and π•••π noncovalent interactions between the 1methylimidazole moiety and the quinoline substrate (Int 3, Figure 3) (Wang et al., 2023).
In accordance with the inner-sphere pathway, the hydroboration mechanism of the zinc alkyl complexes LZnEt and Mn (hmds) 2 involves the formation of metal hydride species through the interaction of metal catalysts with HBpin in the initial step (Wang et al., 2020;Ghosh and Jacobi von Wangelin, 2021).The insertion of a hydride into C=N in N-heteroarenes (Int 6, Int 7 in Figure 4) generates a 1,2-reduced intermediate complex that undergoes metathesis with HBpin to form N-boryldihydropyridines.
Differently from conventional catalytic mechanisms, (IPr)CuFp activates the B-H bond in HBpin, resulting in the generation of hydride species, whereas the iron-thiolate catalyst coordinates with N-heteroarenes and the thiolate interacts with HBpin.This interaction further facilitates hydride transfer from HBpin to the C2 position of the N-heteroarene ligand (Int 4, Figure 4) (Zhang et al., 2017).Additionally, the catalytic reaction of Cp*Ni(1,2-Ph 2 PC 6 H 4 O) is initiated by the dissociation of the B-H bond via the activation of HBpin and the nickel (II) complex, forming a nickel (II) hydride with an oxygen-stabilized boron moiety.Subsequently, nickel hydride is transferred to the C2-position of pyridine, which coordinates with the boron atom of the ligand (Int 5, Figure 4) (Liu et al., 2019).Eventually, N-borylated-1,2-dihydropyridine is generated through cleavage of the O-B bond.Similarly, the proposed catalytic cycle of (PPh 3 ) 3 CoH(N 2 )/( Me CNC) starts with the reaction of the in-situ-formed cobalt hydride [( Me CNC)CoH] with HBpin.This reaction generates [( Me CNC)Co(Bpin)] and simultaneously liberates H 2 without a hydride shift to C=N in pyridines.Int 8 (Figure 4) was then formed, as [( Me CNC) Co(Bpin)] was coordinated with pyridine.Subsequent transfer of the Bpin moiety, followed by the reductive elimination of Int 8, resulted in the formation of the N-boryl enamine product (Meher et al., 2023).

Metalloid and non-metal catalyzed 1,4hydroboration
Recently, the borane Ar F 2 BMe, NHC-parent silyliumylidene cation complex [(IMe) 2 SiH]I, and N-heterocyclic germylene LSi(NAr) 2 GeOTf (L = PhC(N t Bu) 2 and Ar = 2,6-i Pr 2 C 6 H 3 ) were utilized for the metalloid-catalyzed hydroboration of pyridines (Figure 4).These catalysts demonstrated highly selective 1,4-hydroboration of various substituted pyridines.Notably, in the borane (Ar F 2 BMe) catalytic system, 3-substituted pyridines with electron-donating groups decreased the reactivity and electronwithdrawing groups accelerated the reactivity with excellent chemoselectivity (Fan et al., 2015).Pyridines bearing C=O and CN groups were tolerated under [(IMe) 2 SiH]I catalyst conditions (Leong et al., 2019).Moreover, LSi(NAr) 2 GeOTf exhibited notable efficiency in the 1,4-hydroboration of 3-substituted pyridines bearing both electron-donating and electron-withdrawing groups (Hu et al., 2021).In the Ar F 2 BMe and LSi(NAr) 2 GeOTf catalytic systems, the mechanism involved an ionic hydroboration.The formation of intermediate Int 9 (Figure 5) and the [ArF 2 B(H)Me] anion hydride, as well as that of Int 11 (Figure 5) and the germylene hydride, plays a crucial role.1,4-Reduction occurs via hydride transfer from the [Ar F 2 B(H)Me] anion or germylene hydride to the 4-position of the pyridine in Int 9 or Int 11, resulting in the formation of N-boryl 1,4-dihydropyridine.Notably, the steric effects of the silaamidinate ligand or bulky borohydride may determine the 1,4-selectivity when the hydride attacks the pyridine moiety (Fan et al., 2015;Hu et al., 2021).In contrast, the catalytic mechanism of [(IMe) 2 SiH]I is initiated by the HBpin-activating silylium catalyst, which facilitates a nucleophilic attack at the C4-position of the pyridines to form the hydroborated intermediate Int 10 (Figure 5).Subsequently, Int 10 underwent hydride transfer from the borane moiety to the C4position, replacing the C-Si bond and leading to the regeneration of the catalyst and formation of N-boryl-1,4-dihydropyridines (Leong et al., 2019).
In 2018, two distinct non-metal catalytic systems were reported for the 1,4-hydroboration of pyridine.Chong and Kinjo's group utilized N-heterocyclic phosphenium triflates (NHP-OTf) as catalysts and demonstrated their regioselectivity and tolerance to electron-donating groups (Rao et al., 2018).In contrast, Speed et al. employed a neutral diazaphospholene catalyst that exhibited superior selectivity towards certain electron-deficient pyridines, and the reaction operated well in solvents with low polarity, such as benzene-d 6 or diethyl ether, in which Kinjo's procedure failed (Hynes et al., 2018).The reactions with both cationic and neutral phosphor catalysts resulted in high yields and chemoselectivities.The reaction mechanism for phosphenium triflates (NHP-OTf) was initiated by activation of the B-H bond of HBpin with pyridine and a phosphonium triflate catalyst, leading to the formation of the Py-Bpin-OTf complex.This intermediate complex then coordinates with a second pyridine molecule to afford the boronium [(Py) 2 •Bpin]OTf (Int 12, Figure 6).Subsequently, one of the activated pyridine moieties in Int 12 undergoes reduction via hydride transfer from NHP-H, resulting in the formation of either N-boryl 1,2-dihydropyridine or N-boryl 1,4-dihydropyridine, while simultaneously regenerating the phosphonium catalyst (Rao et al., 2018).However, the neutral diazaphospholene catalyst facilitated the interaction of pyridine with H-Bpin, resulting in the formation of a pyridinium borate complex (Int 13, Figure 6), which was not formed during Kinjo's catalytic cycle, along with diazaphospholene hydride.Subsequently, this Int 13 complex undergoes hydride transfer from the diazaphospholene hydride to the 4-position of the pyridine moiety, yielding a dihydropyridyl borate complex paired with a phosphenium cation.Eventually, the transfer of the hydride from the borate complex to the phosphonium cation regenerates the diazaphospholene hydride and releases N-borylated-1,4dihydropyridine (Hynes et al., 2018).Non-metal-catalyzed 1,4-hydroboration.

Noble d-block transition-metalcatalyzed-hydroboration
AgSbF 6 has proven to be an effective catalyst for the hydroboration of various unsaturated functionalities, including isocyanates, pyridines, and quinolines (Pandey et al., 2022).However, only pyridine substrates gave N-boryl enamines via 1,4-hydroboration.Good-to-excellent yields of the 1,4hydroborated products were obtained from 3-substituted pyridines.In addition, the hydroboration of 3,5-disubstituted pyridines was more efficient for electron-withdrawing than electron-donating substituents, but 2-substituted pyridines failed to undergo hydroboration.In contrast to previous reports, control experiments revealed that silver-salt-catalyzed hydroboration is a radical-mediated process.
The 1,2-selective hydroboration of pyridines using noble d-block transition-metal catalysts involved two distinct catalytic systems (Figure 7).One system utilized the iridium catalyst [Ir (cod)py][SZO], which was synthesized from (cod)IrCl(py) and [Me 3 Si][SZO 300 ] as part of Conley's research (Rodriguez and Conley, 2022), whereas the other employed the rhodium catalyst [RhCl(cod)] 2 developed by Ohmura and Suginome's group (Oshima et al., 2012).A comparison of the reactivities of pyridines with the Rh and Ir catalysts revealed notable differences.With the rhodium catalyst, various pyridines exhibited high-to-moderate yields of the corresponding N-borylated 1,2-dihydropyridines with good selectivity (Oshima et al., 2012), while with an iridium catalyst, the pyridines tended to yield mixtures of the 1,2-and 1,4hydroboration products, or showed a preference for one of the products based on the substituent pattern (Rodriguez and Conley, 2022).In the rhodium-catalyzed hydroboration, the key intermediate is Int 14 (Figure 7), which was formed through the oxidative addition of the B-H bond of HBpin to Rh(I), along with pyridine coordination.Pyridine insertion into the Rh-H bond at the 1,2-positions led to the formation of boryl rhodium amide, and the subsequent reductive elimination yielded the N-boryl enamine and regenerated Rh(I) (Oshima et al., 2012).Further research is needed to understand the mechanistic details of the hydroboration catalyzed by [Ir (cod)py][SZO], which likely proceeds via an inner-sphere pathway involving an iridium boryl hydride intermediate or the interaction of the pyridyl nitrogen with the Ir-BPin species (Rodriguez and Conley, 2022).

f-block transition-metal-catalyzedhydroboration
Efficient f-block transition-metal catalysts have recently emerged for the 1,2-regioselective hydroboration of pyridines.Notable examples include organolanthanide [Cp* 2 LaH] 2 and thorium complexes such as thorium methyl (C 5 Me 5 ) 2 ThMe 2 and thorium hydride [(C 5 Me 5 ) 2 Th(H) (μ-H)] 2 complexes (Figure 8).These catalysts demonstrated similar reactivity profiles and high 1,2regioselectivity for pyridine substrates.Specifically, orthosubstituted pyridines exhibited negligible activity under both catalytic conditions because of steric hindrance at the 2-position, whereas meta-and para-functionalized pyridines yielded N-boryl dihydropyridines in good yields and excellent selectivities.However, it was observed that the conjugated substituents could not be tolerated under thorium catalysis conditions (Dudnik et al., 2014;Liu et al., 2018).Furthermore, the catalytic mechanism underlying these transformations closely resembles that of other metalcatalyzed hydroboration reactions.It proceeds via an innersphere pathway, with the metal hydride species playing a pivotal role in the 1,2-addition of La-H or Th-H to the C=N bond of the coordinated pyridine, leading to the formation of a dihydropyridine complex intermediate.Subsequently, these intermediates undergo σ-bond metathesis with another HBPin molecule to afford N-borylated dihydropyridines (Dudnik et al., 2014;Liu et al., 2018).
3 Hydrosilylation in synthesis of N-silyl enamines 3.1 Earth-abundant-metal-catalyzed 1,2hydrosilylation of N-heteroarenes Harrod and Samuel's research on titanocene-catalyzed hydrosilylation began in 1998 and was further explored in a subsequent report in 2001.Their results revealed that Cp 2 TiMe 2 effectively promoted the hydrosilylation of pyridine, yielding high amounts of N-silyl-tetrahydropyridine with MePhSiH 2 , but was ineffective for hydrosilylation with PhSiH 3 or Ph 2 SiH 2 (Hao et al., 1998).In contrast, changing the ligand to Cp* 2 TiMe 2 led to successful hydrosilylation with PhSiH 3 , whereas using PhMeSiH 2 instead of PhSiH 3 resulted in a significantly slower reaction rate than that with Cp 2 TiMe 2 (Harrod et al., 2001).The mechanism of titanocene-catalyzed hydrosilylation involves the interaction of Cp 2 TiMe 2 with silane and pyridine to form a hydride complex (Int 17, Figure 9).The formation of Int 17 was not observed directly in the reaction, but was supported by model stoichiometric reactions.Moreover, the hydride complex Int 17 is believed to be the key intermediate in the dearomatization of pyridine, followed by the insertion of the Ti-H bond into the N=C bond of pyridine to form a 1,2-dihydropyridine complex (Hao et al., 1998;Harrod et al., 2001).Eventually, the 1,2-dihydropyridine complex underwent σ- bond metathesis with silane to produce the N-silyl-1,2dihydropyridine product and regenerate Cp 2 TiH.
In 2015, Harder et al. embarked on an investigation into the reactivity of the calcium hydride complex (DIPP-nacnac-CaH•THF) 2 ([Ca]-H) based on their prior research on hydroboration catalyzed by magnesium hydride complexes (Intemann et al., 2015).Their initial observations revealed that [Ca]-H exhibited higher reactivity and 1,2reduction selectivity in the dearomatization of pyridine than the magnesium hydride complex, with 1,2 to 1,4 isomerization upon increasing the temperature (Arrowsmith et al., 2011;Intemann et al., 2014).The formation of the calcium 1,2-DHP complex revealed the catalytic potential of [Ca]-H for hydroboration and hydrosilylation.However, unlike magnesium hydride, [Ca]-H was inactive in the hydroboration of pyridine owing to the predominant formation of B 2 (pin) 3 rather than the desired hydroboration product, although it proved to be efficient in hydrosilylation.Pyridines and isoquinolines were effectively reduced to afford excellent product yields.Furthermore, the mechanism was elucidated through stoichiometric experiments, initiating the catalytic cycle from the active calcium hydride species.Subsequently, the [Ca]-H species reacts with pyridine to yield the [Ca]-1,2-DHP complex (Int 18, Figure 9), which then reacts with silanes to form an ion pair containing hypervalent silicon species (Intemann et al., 2015).Ultimately, hydride transfer to the cationic calcium species occurs, leading to the release of the N-silyl enamine product and regeneration of the active [Ca]-H complex.

Borane-catalyzed 1,4-hydrosilylation
Chang et al. recently reported a B(C 6 F 5 ) 3 -catalyzed hydrosilylation with a broad substrate scope that encompasses the dearomatization of N-heteroarenes, such as quinoline, isoquinoline, and pyridine, as well as the reduction of conjugated nitriles and imines.
Based on experimental studies and density functional theory (DFT) calculations of the reaction mechanism, the formation of N-silyl enamine intermediates in borane-catalyzed hydrosilylation was concluded to occur via an ionic mechanism (Figure 8) (Gandhamsetty et al., 2014;Gandhamsetty et al., 2015a;Gandhamsetty et al., 2015b;Kim et al., 2018).Initially, B(C 6 F 5 ) 3 coordinates to the N center to establish a stable resting species.These species exist in equilibrium with both the free reactants and the active complex (C 6 F 5 ) 3 B•HSiR 3 .This active complex facilitates the transfer of its silylium cation to the reactants, leading to the formation of iminium salt Int 19 (Figure 10) or Int 20 (Figure 10) along with the borohydride anion [(C 6 F 5 ) 3 BH] − .Subsequently, the borohydride anion transfers the hydride to the C 4 site of Int 19 or Int 20, forming cyclic N-silyl enamines.

Transition-metal-catalyzed hydrosilylation
Because of their similar ionic mechanisms, various cationic Ru complexes, including [Cp( i Pr 3 P)Ru(NCCH 3 ) 2 ] + (Ru I), [Cp(phen) Ru(NCCH 3 ) 2 ] + (Ru II), and coordinatively unsaturated Ru II thiolate (Ru III), exhibit 1,4-selectivity in the hydrosilylation of N-heteroarenes (Figure 11).Ru I and Ru II display good conversions with 3-and 5-substituted pyridines, whereas 2-, 4-, and 6-substituents were ineffective (Gutsulyak et al., 2011;Lee et al., 2013).Conversely, Ru III reduces various N-heteroarenes, including pyridines, isoquinolines, and quinolines, resulting in high regioselectivity and chemoselectivity.Notably, 4-substituted pyridines react effectively to provide high yields of N-silyl-1,4-dihydropyridines (Königs et al., 2013).The proposed mechanism for the formation of Ru I and Ru II complexes begins with the formation of cationic silane complexes from Ru via nitrile dissociation and silane coordination.Cationic silane complexes facilitate the transfer of a silyl cation to the pyridine substrate to form Int 21 (Figure 11) and a reactive Ru hydride species, which then induces hydride transfer to the 4-position to yield the N-silyl enamine product and regenerate the active catalyst complex (Gutsulyak et al., 2011;Lee et al., 2013).In addition, the mechanism for the Ru III complex starts with the formation of a cationic silicon-sulfur intermediate via the activation of silane by the heterolytic cleavage of the Si-H bond.Subsequently, the cationic silicon-sulfur intermediate transferred the cationic silicon to the nitrogen center of pyridine, forming the N-silylpyridinium intermediate (Int 22, Figure 11) and neutral Ru hydride.Eventually, the neutral Ru hydride attacked the C4-position in the N-silyl pyridinium intermediate to generate the N-silyl enamine products (Bähr and Oestreich, 2018).
In addition to 1,4-hydrosilylation, transition-metal-catalyzed 1,2hydrosilylation was demonstrated using a metathesis-active ruthenium complex (Ru IV) and an iridium catalyst ([Ir (coe) 2 Cl] 2 ).These catalysts were versatile, functioned effectively to N-heteroarenes, and displayed excellent tolerance to various functional groups (Jeong et al., 2016;Ma and Nolan, 2023).The mechanisms of both the catalytic systems follow an inner-sphere path.In the iridium complex mechanism, the initial step involves the generation of two isomeric iridium olefin adducts via a reaction between [Ir (coe) 2 Cl] 2 and Et 2 SiH 2 .These adducts undergo ligand exchange with N-heteroarenes to form the bimetallic species (Int 23, Figure 12), which undergoes intramolecular insertion of an Ir-H bond into the C=N bond of the N-heteroarene ligand, generating a 1,2dihydropyridine intermediate.Subsequently, the 1,2-dihydropyridine intermediate produces the N-silyl enamine through reductive elimination (Jeong et al., 2016).In the Ru complex mechanism, the Ru complex transforms into the activated species via PCy 3 dissociation and ligand exchange.The N-heteroarene coordinates with the ruthenium species (Int 24, Figure 12), forming a 1,2reduced intermediate via selective hydride transfer at the 2position.Eventually, the 1,2-reduced intermediate undergoes σbond metathesis with another silane molecule, resulting in the release of N-silyl enamine (Ma and Nolan, 2023).

Transition-metal catalysis of both hydroboration and hydrosilylation
In 2017, Lin et al. reported a zirconium framework, Zr III H-BTC, that exhibited high activity and 1,4-selectivity in the dearomatization of pyridines and quinolines using HBpin and triethoxysilane (Figure 13).This selectivity was attributed to the bridging oxo/ carboxylate ligands and the site-isolation effect of the MOF, which stabilized the coordinatively unsaturated Zr III H centers (Ji et al., 2017).Moreover, 1,2-selective hydroboration and hydrosilylation of N-heteroarenes were achieved under β-diketiminate-supported dimeric zinc hydride [LZnH] 2 conditions by Nembena in 2023 (Sahoo et al., 2023).A large range of 3-and 4-substituted pyridines were transformed into the 1,2-hydroborated products in excellent yields.Similar to other 1,2-selective systems, 2-substituted pyridines failed to produce the reductive products.Interestingly, the hydrosilylation of pyridines and isoquinolines in this catalytic system produced bis-hydrosilylated products in quantitative yield.Similar to other metal-hydride-catalyzed hydroboration and hydrosilylation reactions, the reaction mechanism involves a 1,3-hydride transfer to furnish zinc amide intermediates.Further addition of HBpin or a hydrosilane to the amide intermediates produced selective 1,2hydroborylated and hydrosilylated products.
5 Applications of N-boryl and N-silyl enamines in organic synthesis N-Boryl and N-silyl enamines serve as versatile nucleophilic motifs that can react with a broad range of electrophilic reagents.
Recent synthetic methodologies have predominantly focused on the functionalization of the C 3 -position of borylated and silylated enamines through nucleophilic attack on external electrophiles, as well as the facilitation of the construction of complex molecular scaffolds via cycloaddition reactions involving the C=C moiety and various dipoles (Figure 14).Remarkably, the in situ utilization of N-boryl and N-silyl enamines enables efficient one-pot tandem reactions.N-boryl and N-silyl enamines have similar reactivities; the choice of one over the other is dictated by the specific cleavage pathways of their respective silyl and boryl groups in the final product, which may involve rearomatization, carbonylation, or facile N-H bond formation under acidic conditions.
In addition to their application in cycloaddition reactions, cyclic N-silyl enamines serve as nucleophilic motifs for various chemical transformations.Crudden et al. reported the synthesis of γ-aminoalcohols 12e9 derived from quinoline via dearomatization under borenium-catalyzed hydrosilylation, resulting in the formation of N-silyl-1,4-dihydroquinolines (Clarke et al., 2021).These N-silyl-1,4-dihydroquinolines were then added to aldehyde 12e, followed by reduction using NaBH 4 and subsequent deprotection of the silyl group to yield tetrahydroquinoline derivatives in modest-to-good yields.Furthermore, these N-silyl enamines derived from the borane-catalyzed hydrosilylation of Nheteroarenes have also been employed for the production of 3trifluoromethylated compounds 12f9 and 12f99 via electrophilic trifluoromethylation with Togni reagent I (12f) (Muta et al., 2022).Remarkably, research conducted by Stoltz et al. utilized N-silyl enamines derived from iridium(I)-catalyzed dearomative 1,2hydrosilylation as nucleophiles in a subsequent palladiumcatalyzed asymmetric allylic alkylation with cinnamyl methyl carbonate 12g, leading to the formation of C3-substituted tetrahydropyridine products 12g9 and 12g99 (Greßies et al., 2023).The desired products were isolated in moderate yields with excellent enantioselectivity following carbonylation with acetyl chloride.Moreover, bisalkylated tetrahydropyridines 12g99 were obtained in the presence of benzoic acid and an excess of the allyl carbonate substrate.The formation of the bisalkylated products arose from the tautomerization of the imine intermediate, resulting in an enamine that could participate in additional alkylation.
Similarly, Sakai et al. developed a Yb(OTf) 3 -catalyzed cyclization process utilizing the nucleophilicity of linear N-silyl enamines to synthesize quinolinone and tetrahydroquinoline derivatives from endione and enone precursors 12h (Sakai et al., 2006).This process yielded the desired cyclic products 12h9 and 12h99 in good yields; the quinolinone derivatives were further transformed into substituted quinolines upon treatment with NBS, AIBN, and p-toluenesulfonic acid in methanol.Joung et al. reported the use of linear N-silyl enamines in [3 + 2]

Summary
This review described the formation of N-boryl and N-silyl enamines through the hydroboration and hydrosilylation of a range of conjugated systems employing a variety of catalysts.The hydrides for the reduction process originate from in situ formed catalysts, the dissociation of B-H or Si-H bonds, or coordination of HBPin or silane as ligands of the catalyst.The selectivity between 1,2-and 1,4-addition arises from the formation of distinct intermediates.Specifically, active metal hydrides induce an intramolecular 1,3-hydride shift in the metal-substrate complex, leading to the formation of 1,2-adducts.Bulky active hydrides or sterically hindered ligands promote a hydride shift to the less-hindered C4-position.Isomerization from kinetic 1,2-adducts to thermodynamically stable 1,4-adducts can also afford 1,4adducts.As an application of these N-boryl and N-silyl enamines, the utilization of the various functional N-heterocyclic structures obtained from the diverse regioselective reductions of N-boryl and N-silyl enamines described above facilitates the development of onepot or tandem procedures for subsequent chemical transformations of the versatile N-boryl and N-silyl enamines.Frontiers in Chemistry frontiersin.org16 Cao and Joung 10.3389/fchem.2024.1414328 FIGURE1(A) Previous reviews on hydroelementation of pi-bond system.(B) This review: Syntheses of N-boryl and N-silyl enamines using hydroboration and hydrosilylation.