Chemistry and catalysis advances in organometallic chemistry and catalysis
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Scheme 26.5 (a, b) Examples of cooperative enamine addition into metal-activated electrophiles generated from pervalent iodine reagents. electrophilic iodonium intermediate that can be attacked by the enamine. The trifluoromethylated aldehyde results from reductive elimination of phenyl iodide. Diaryl- and arylvinyl-iodonium salts were also employed as electrophiles in arylation and vinylation reactions, although the mechanism proposed for these substrates involves oxidative addition of the iodonium salts to Cu(I) catalyst, followed by enamine coordination to copper and consequent enantioselective reductive elimination. Cooperative catalysis using chiral imidazolidinones and metal catalysts is not restricted to carbon electrophiles. In 2012, the MacMillan group [50] disclosed a general approach to undertake enantioselective α-oxidation of aldehydes with TEMPO by combining organocatalysis with copper catalysis (Scheme 26.5b).
The concept of SOMO catalysis has recently appeared in the literature and is based on the utilization of photocatalysts that can oxidize and reduce organic substrates via the formation of radical intermediates. Some metallic complexes behave like photocatalysts when exposed to weak visible light by accepting a photon that will populate the metal-to-ligand charge transfer excited state. In this state, they become strong reductants and are able to cleave carbon–halide bonds [51]. MacMillan’s group took advantage of this chemistry and, by merging it with enamine-based organocatalysis, was able to develop new synthetic methods to perform highly enantioselective α-alkylation of aldehydes (Scheme 26.6) [52–54]. Enamines, as electron-rich alkenes, couple efficiently with electron-deficient radicals through a one-electron pathway and, for this reason, the alkyl halides explored have electron-withdrawing substituents (acyl groups [53], aryl [54] and perfluorolalkyl [52] chains). As proposed by the authors, the reaction relies on the photocatalyst’s ability to reductively cleave the carbon– halide bonds giving a halide anion and a carbon-centered radical. Such radical will attack the transient enamine formed between the organocatalyst and the aldehyde, yielding α-amino radicals. The latter are then oxidized to the iminium moiety by the photocatalyst and, in the final stage, the iminium is hydrolyzed, freeing the organocatalyst and the alkylated aldehyde (Scheme 26.6). These synthetic methods are unreachable to organocatalysis because most organocatalysts that could be employed are secondary amines that would become alkylated in the presence of alkyl halides.
Iminium-based organocatalysis is somewhat less explored than enamine-based organocatalysis and has been mostly used in the activation of α,β-conjugated aldehydes and ketones. Therefore, this type of catalysis has unsurprisingly been the subject of a limited number of studies under the umbrella of the metal–organic cooperative catalysis concept. In 2011, the C´ordova group [55] reported the first enantioselective and chemoselective β-silyl addition to α,β-unsaturated aldehydes using copper salts and chiral pyrrolidine derivatives as catalysts. As proposed, the chiral secondary amine forms an iminium salt with 330 METAL–ORGANO MULTICATALYSIS: AN EMERGING CONCEPT O H
+ ICF
3 Br O R' Br R' O H R CF 3 O H R O R' O H R R' N H N O .TfOH t Bu N H N O Bn .TfOH
Ru(bpy) 3 Cl 2 0.5 mol%
2,6-lutidine 20 mol% Up to 93% up to 99% e.e. O H R + N H N O .TfOH t Bu Ir(ppy) 2 (dtbbpy)PF 6 0.5 mol%
2,6-lutidine, −20 °C
26 W fluorescent light 20 mol%
Up to 86% up to 99% e.e. O H
+ fac-Ir(ppy) 3 0.5 mol%
2,6-lutidine, −20°C
26 W fluorescent light 20 mol%
up to 91% up to 93% e.e. N N
Bn R N N O Bn R Ar N N O Bn R Ar NH N O Bn Proposed mechanism: fac-Ir(ppy) 3 + fac-Ir(ppy) 3
3 Ar
Ar Ar O H R H 2 O O H R Ar H 2 O Scheme 26.6 Alkylation of aldehydes using SOMO catalysis and enamine catalysis. O H
R=aryl, alkyl + Me
2 PhSi-B(pin) N H
Ph OTMS
25 mol% CuCl 10 mol% KO t
4-NO 2 -C 6 H 4 CO 2 H 10 mol% O H R Si Ph 65 −80 %
up to 94% e.e. N H X N X R Me 2 PhSiCu(I)L + Bu t O-B(pin)
Me 2 PhSi-B(pin) + Cu(I)L KO t Bu N R CuSiMe 2 Ph N X R Cu(I)L PhMe 2 Si H 2 O,- Cu(I)L X
Enantioselective conjugated addition of silanes into α,β-unsaturated aldehydes catalyzed using copper and chiral iminium catalysts. the
α,β-unsaturated aldehydes, increasing their electrophilicity and providing a chiral environment for the silane-conjugated addition. Protonation of the organocuprate and iminium hydrolysis delivers the free saturated aldehyde (Scheme 26.7). 26.4 N-HETEROCYCLIC CARBENES AS ORGANOCATALYSTS NHCs are probably best known as ligands for metal catalysis, but they are also a class of nucleophilic carbon-centered bases that have been extensively used in organocatalysis [20]. Although NHCs display strong basic properties [56–58], they are rarely used in this role; their classic mode of action involves nucleophilic addition to the carbonyl group of an aldehyde followed by proton migration to what is known as a Breslow intermediate; recently, it has also been proposed that the direct NHC/carbonyl adduct may be the main intermediate in some reactions [59]. In Breslow intermediates, the formerly electrophilic carbonyl carbon becomes a strong nucleophile, resulting in polarity reversal (umpolung) [60]. If the aldehyde is conjugated to a double bond, the β atom is also rendered nucleophilic, resulting in homoenolate chemistry. Moreover, Breslow intermediates are very electron rich and prone to oxidation, even by atmospheric O 2 [61, 62]. Other nucleophilic catalysts, most notably phosphines [60, 63, 64], share with NHCs the property of stabilizing adjacent negative charges, resulting in umpolung type mechanisms; however, the totally different bonding, basicity, and nucleophilicity properties N-HETEROCYCLIC CARBENES AS ORGANOCATALYSTS 331 N N OH H R'' R' R N N OH R'' N N R' R + O R'' H
Electrophiles
H − H If R'' vinyl N N OH R'''
Electrophiles Acyl vinyl anion equivalent or homoenolate or If R'' alkyl or aryl Scheme 26.8 Reactivity umpolung of aldehydes in the presence of N-heterocyclic carbenes. between NHCs and phosphines seldom result in similar organocatalytic activity (Scheme 26.8). The dual role of NHCs as ligands and catalysts motivated their use for cooperative catalysis, aiming to access new structural scaffolds and/or improve the selectivity of known reactions [11, 65]. Scheidt’s group demonstrated that NHC-catalyzed homoenolate chemistry of cinnamaldehydes can be improved when using Lewis acids as cocatalysts. In their pioneering work, it was found that the addition of Mg(O
Bu)
2 to the NHC-catalyzed synthesis of γ -lactams from cinnamaldehydes and N-benzoyl hydrazones improved the reaction yield and selectivity (Scheme 26.9) [66]. Interestingly, Cu(II), La(III), and Zn(II) triflates or Ti(IV) alkoxides inhibited the catalysis, while Mg(II) halides or triflates appeared to have no impact over this transformation. These observations indicate that the catalysis strongly depends on the Lewis acidity of the cocatalyst, which was tuned by changing the metal and its counterion. Furthermore, mechanistic studies on the role of Mg(O t Bu)
2 unveiled an interesting inverse first-order kinetics that was explained on the basis of negative interactions with the organic base for higher Lewis acid loadings. To rationalize these observations, activation of the hydrazone by double coordination onto the magnesium was hypothesized, lowering the electrophile’s LUMO orbital and favoring the attack by the conjugated Breslow intermediate (Scheme 26.9). This cooperative strategy was applied to the synthesis of 1,3,4-trisubstituted cyclopentenes using chalcones as electrophiles. The addition of catalytic amounts of Ti(O i Pr) 4 resulted in the exclusive formation of the cis diastereomer, which constituted a complete reversion of the selectivity compared to the results obtained in the exclusively organocatalyzed reaction (Scheme 26.10) [67]. To further prove the titanium complex involvement in the catalysis, γ -butyrolactones were synthesized in moderate enantioselectivities using a nonchiral NHC organocatalyst and chiral titanium alkoxides via enantioselective homodimerization of cinnamates. Relatively to the synthesis of 1,3,4-trisubstituted cyclopentenes, the authors at the time suggested that the titanium catalyst played a double role in the organocatalytic cycle by first stabilizing the conjugated Breslow intermediate as homoenolate and then by organizing the two substrates into a rigid transition state that provides exclusively cis cyclopentenes upon 1,4-conjugated addition to the chalcone. Domingo’s group performed density functional Ph O
+ O NH p-tol N H CO 2 Et 5 mol% TBD 10 mol% THF, 60 ° C N O Ph EtO 2 C H N p-tol O No Lewis acid 31% yield, 6:1 d.r., 90% e.e.
N O N N R BF 4 R=2,6-Et 2 C 6 H 4 N N N R OH Ph δ− O N H p-tol N H CO 2 Et Mg Oalkyl
Oalkyl + HOMO LUMO Organocatalysis Lewis acid catalysis cooperative catalysis Scheme 26.9 Cooperative effect of magnesium alkoxides in the synthesis of γ -lactams catalyzed by NHCs. Ar 1 O H + Ar 2 Ar 3 O (10 mol%) DBU (15 mol%) Ti(O
i Pr)
4 (20 mol%) CH 2
2 Isopropanol (20 mol%) N O
N R BF 4 R=2,6-Et
2 C 6 H 4 Ar 3 Ar 2 Ar 3 50 −82% 20 : 1 dr cis 98 −99% e.e. N N N R O Ph Ti(O i Pr)
4 O Ph Ph Homoenolate stabilization Reorganization in a rigid conformation Scheme 26.10 Cooperative effect of titanium alkoxides in the synthesis of cyclopentenes catalyzed by NHCs. 332 METAL–ORGANO MULTICATALYSIS: AN EMERGING CONCEPT G O O X' G O + OH X' Fe(OTf)
2 /SIPrCl
(20 mol%) KO t Bu, dioxane, air G = aryl or styryl, X' = H, EWG or EDG, 51 −90%
G = Cy, X' = H, 32% EWG-electron withdrawing group EDG-electron donating group N N Ar' Ar'
N N Ar' Ar' O G N N Ar' Ar' O G HO G O O G O Fe/O 2 Ar'=2,4- i PrC
6 H 3 Proposed mechanism Scheme 26.11 Esterification of aldehydes based in cooperative NHC and iron catalysis. R 1
H R 1 = Ar, styryl; R 2 = Ar, alkyl + R 2 CF 3 O N O N N Mes BF 4 (20 mol%) Sc(OTf) 3 (10 mol%) Mg(OTf) 2 (10 mol%) K 2 CO 3 (50 mol%) quinone (1 equiv) THF
O R 2 O R 1 CF 3 52 −82%, 60−94% e.e. no metals 21% e.e. Scheme 26.12 Cooperative effect of scandium and magnesium triflates in the synthesis of β-unsaturated lactones catalyzed by NHCs. theory (DFT) calculations on this system and corroborated the mechanism originally proposed by Scheidt et al. They also shown that stronger Lewis acids such as Zn(OTf) 2 form more stable complexes with the imidazolidene NHCs than Ti(O i Pr)
4 [68]. This observation offers a plausible explanation for the negative impact of Zn(OTf) 2 on the catalysis; however, it must be taken with due caution because triazolidene NHCs were employed as organocatalysts. The Breslow intermediates are quite unstable toward oxidative conditions, providing acyl imidazolinium species that can acylate several nucleophiles or suffer 1,4-conjugated additions. Gois et al. disclosed the oxidative esterification of aldehydes using an iron/NHC catalytic system to prepare benzoic and cinnamic esters using phenols as nucleophiles (Scheme 26.11) [69]. The possibility of cooperative catalysis between the NHC and Fe/oxygen was advanced. Steric hindrance limits conjugated additions and homoenolate pathways for cinnamaldehydes with two β-carbon substituents, offering the possibility to deprotonate their γ -carbon in oxidized Breslow intermediates. Chi et al. [70] shown that the resulting nucleophiles only undergo highly enantioselective additions to trifluoromethyl ketones, yielding α,β- unsaturated lactones in the presence of a chiral NHC and a mixture of scandium and magnesium triflates (Scheme 26.12). The authors suggested that the Lewis acid organizes the electrophile closer to the nucleophile and the organocatalyst in a way that amplified chiral induction toward the remote γ -carbons.
General acid catalysis is surely one of the best known catalytic processes in organic chemistry. Protonation enhances the electrophilic character of a molecule, leading to increased reactivity. Moreover, the formation of tight ion pairs or hydrogen- bonded complexes between a protonated reactant and the conjugate base of chiral acids has been a widely exploited method for enantioselective synthesis [71, 72]. In the context of metal–organo multicatalysis, protic acids offer a definite advantage over other types of organocatalysts because of their reduced tendency to inactivate the metal cocatalyst by coordination. In fact, unlike most other common organocatalysts such as NHCs or proline-like amines, the conjugate bases of these acids are not very strongly coordinating, allowing the metal center to maintain, or even display, enhanced reactivity [73]. Among all Brønsted acids, phosphoric acids derived from BINOL stand out as a class of powerful organocatalysts, activating electrophiles via protonation and providing chiral environments suitable for highly enantioselective addition of nucleophiles [74]. This unique family of organocatalysts became quite useful to activate aldehydes, ketones, and imines toward additions of metal-stabilized nucleophiles. Beller et al. [75] used (S)-TRIP to induce excellent levels of enantioselectivities in the hydrogenation of imines using Kn¨olkers complex (a simple achiral iron hydrogenation catalyst). As proposed, the phosphoric acid protonates the imine substrate, which immediately accepts a hydride from the iron complex
BR Ø NSTED ACIDS AS ORGANOCATALYSTS 333 Fe O O P O O TMS
TMS OH OC OC H HN R 3 R 1 Fe TMS TMS O CO CO O O P O O H N H R 2 H R 3 R 2 R 1 N R 3 R 1 R 2 H N H R 3 R 2 R 1 H 2 Cooperative asymmetric hydrogenation Imine activation chiral environment R R R R R=2,4,6- i Pr 3 C 6 H 2 R 3 = Ar' R 2 = Ar, R1 = Me, Et > 67%, 80 −98% e.e. R 2
1 = Me > 69%, 67 −83% e.e. (S)-TRIP Scheme 26.13 Phosphoric acid effect in the activation of imines toward hydrogenation using Kn¨olkers complex. A N
B + Rh
2 (OAc)
4 [Rh]
A B NuH [Rh] A B HNu X C D Delayed H-shift A B Nu C D XH + Rh
2 (OAc)
4 No electrophile A B Nu H + Rh
2 (OAc)
4 Multicomponent reaction Formal Nu −H insertion A B HNu Ar 3 Ar 1 R 2 O NHAr 2 R 1 O 2 C up to 98% yield up to > 99 : 1 d.r. up to > 99% e.e. O O P O OH R R R = 9-phenanthrenyl O O
O OH R R R = SiPh
3 up to 93% yield up to >99 : 1 d.r. up to 99% e.e. Ar 1
3 NHCO
2 R 2 NHAr 2 R 1 O 2 C CO 2 R 1 NHBoc R 2 up to 99% yield up to 95% e.e. O O P O OH R R R = 2-naphthyl up to 93% yield up to >99 : 1 d.r. up to 99% e.e. Ar 1 NHAr 2 R 2 N R 1 O R 3 Multicomponent reactions Oxygen ylide Nitrogen ylide
SE Ar O O P O O HN Ar 2 H Ar 1 Ar 1 COAr
3 OR 1 NHAr 2 H O O P O OH R R R = 4-CF 3 C 6 H 4 up to 88% yield up to >97 : 3 d.r. up to 98% e.e. Oxygen ylide Proposed pathway O O P O O H OMe
O Rh 2 L 4 N H H Boc Proposed pathway Organocatalyst Scheme 26.14 Phosphoric acid effect in the activation of imines toward the addition of oxygen and nitrogen ylides generated in diazo compounds by rhodium(II) catalysis. (Scheme 26.13). On the basis of this strategy, aryl and alkyl imines were converted to the respective amines with excellent enantioselectivity. Regarding carbon nucleophiles, phosphoric acids have been applied as organocatalysts in multicomponent reactions between diazo compounds, alcohols, or amines, and aldehydes, imines, or Michael acceptors [76]. Diazo compounds can be converted into the respective metallocarbenes in the presence of dirhodium (II) carboxylates complexes [77]. Such intermediates can suffer a nucleophilic attack from alcohols or amines generating oxygen or nitrogen ylides that may undergo a proton shift, furnishing the respective O–H or N–H insertion products (insertion pathway, Scheme 26.14). Although it has been documented that such ylides have the ability to attack activated ketones [78], aldehydes [79], or imines [80] due to a delayed hydrogen-shift, initial attempts to induce high levels of asymmetry using chiral dirhodium(II) catalysts failed [78]. High levels of enantioselectivity were only obtained when the groups of Hu, Gong, and Doyle [76, 80–83] introduced chiral phosphoric acids to activate the electrophiles and provide a more pronounced chiral environment for the ylide nucleophilic attack (Scheme 26.14).
334 METAL–ORGANO MULTICATALYSIS: AN EMERGING CONCEPT L n
O O P O O Proposed pathway OH Pd(OAc)
2 (5 mol%) organocat. (10 mol%) BQ 2 equiv. toluene, 60 ° C
40 −78% yield up to 10 : 1 d.r. up to 98% e.e. R 1
2 R 1 R 2 O O P O OH R R R = 2,4,6-iPr 3 C 6 H 2
O H
Phosphoric acid effect in the activation of cyclobutanols toward palladium-catalyzed migratory ring expansion. Phosphoric acids have also been applied as the chiral inductor in combination with palladium (II) catalysts in the migratory ring expansion of cyclobutanols to yield spirocyclicindenes. This transformation is believed to proceed via enantioselective allylic C–H activation followed by semi-pinacol ring expansion to the vicinal π-allylpalladium intermediate (Scheme 26.15) [84]. 26.6 RELAY AND SEQUENTIAL CATALYSIS 26.6.1 Lewis and Brønsted Bases as Catalysts As aforementioned, amines in general are arguably the most diverse and important class of organocatalysts. They display a range of catalytic mechanisms, including simple general base catalysis [85], nucleophilic catalysis [86–89], and processes involving iminium and enamine intermediates [90–93]. Although hindered (weakly nucleophilic) amines may be used if a simple basic catalyst is required, most catalytically interesting amines also have a strong affinity for metal coordination. This property has to be taken into account when designing metal–organo multicatalytic systems, as it can result in total deactivation of the metallic partner, especially considering the high loadings of amine catalysts that are often used.
In terms of relay and sequential metal–organo multicatalytic systems, these catalysts are most commonly used for 1,2 (aldol type) or 1,4 (Michael type) additions to suitable electrophiles. The most pervasive approach takes advantage of some metal-catalyzed process to generate a substrate for the organocatalyst. The examples illustrated in Scheme 26.16 show how chemical [94] or photochemical [95] metal-catalyzed oxidation of amines generates imine species that then undergo organocatalyzed 1,2 addition. It should be noted that, even though chiral proline was used in the second example, only residual enantioselectivity was observed. The first two examples are typical enamine organocatalysis; the third example [96] probably involves a Baylis–Hillman type mechanism (nucleophilic catalysis). Rhodium-catalyzed decarboxylative hydroformylation has been used to generate a substrate for a Knoevenagel condensation with malonic acid. This interesting sequence results in two carbon homologation of carboxylic acids (Scheme 26.17) [97]. The 1,2 addition step may also be the first in the reaction sequence, followed by a metal-catalyzed reaction, for example, Au-catalyzed addition to alkynes (Scheme 26.18). In the first example in Scheme 26.18 above, the authors reported that, despite the compatibility of the catalysts, they had to be added sequentially, to avoid an undesired cooperative process that resulted in the polymerization of the alkyne starting material [98]. In the second case, it was found that both catalysts were deactivated owing to their strong mutual affinity, again requiring sequential addition [99]. Organocatalytic 1,4 additions have also been widely explored as a setup reaction for a metal-catalyzed cyclization with an alkyne. The considerable amount of work in recent literature requires that a selection be made; Scheme 26.19 presents where the first reaction is either a C–C [37, 42, 100, 101] (Michael), N–C [40] (aza-Michael), or O–C [41] (oxa-Michael) 1,4 addition; otherwise, they are quite similar, with good enantioselectivity and moderate to good yields (Scheme 26.19). Particularly noteworthy is that a rather complex catalytic mixture was essential to obtain good results for the aza-Michael example, owing to the difficult balance between the reactivity of the organocatalyst and the metal catalyst. It should be kept in mind that sequential systems where the second step is metal catalyzed are often more challenging in terms of optimization, because the organocatalyst already present may act as a ligand and deactivate the metal center. As already mentioned in the cooperative catalysis section, the organocatalyst also intervenes in the alkyne addition step, activating the aldehyde group via enamine formation.
RELAY AND SEQUENTIAL CATALYSIS 335 Cu(OAc)
2 (10 mol%), pyrrolidine acid (30 mol%) TBHP, acetone, 25 ° C
− 77% Via:
N Ar Ru(bpy) 3 (PF
6 ) 2 (1 mol%), L-proline (10 mol%) MeCN, light, O 2 (air)
Via: + R O N Ar O R N Ar Yield 47 − 95% Ar
N OR O + O Ar HN OR O O Ar N OR O N Ar EWG
+ Cu(OTf)
2 (10 mol%), quinine (20 mol%) O2 (1 bar), Na 2 SO
, 4Å MS, CH 2 Cl
, rt N Ar EWG yield 47 − 81% e.e. 85
− 99% Via:
N Ar
Metal redox/organocatalytic addition relay processes.
Bu CO 2 H CH 2 Cl 2 ,rt Rh(acac)(CO) 2 (0.5 mol%) ligand (5 mol%), CO/H 2 1 bar n Bu CHO n Bu CO 2 H pyrrolidine (1 mol%) pyridine (200 mol%) N N NH 2 O NH 2 PPh 2 Ligand:
Yield 67% CH 2 (CO 2 H) 2 Scheme 26.17 Sequential hydroformylation/Knoevenagel two carbon homologation of 2,3 unsaturated acids. 1) KO t
2) AuCl 3 (5 mol%) MeOH, 70 ° C Yield 21 − 86%
Via: + R 1 N R 2 O 2 N R 3 N R 2 R 1 R 3 NH R 2 R 1 R 3 NO 2 2) PPh
3 AuNTf
2 (5 mol%), TsOH (10 mol%), rt Yield 45 − 93% e.e. 58
− 88% via:
+ R 1 N Boc
NC R 2 CN N CN NC R 1 Boc R 2 NH NC NC R 1 Boc R 2 1) organocatalyst (10 mol%), −60 °C
CHCl 3 N H N N H N CF 3 F 3 C S O Organocatalyst: Scheme 26.18 Sequential 1,2 organocatalytic addition/gold alkyne addition processes. A wider variety of reactions has been reported if the metal-catalyzed process is the first, followed by the organocatalytic reaction. Recent sequential metal catalysis/organocatalytic 1,4 additions involve allylic oxidation [102] and double bond migration [103] metal-catalyzed processes (Scheme 26.20). In the second example in Scheme 26.20, although the yields are moderate at best, it is noteworthy that both the metal and the organocatalyst are chiral, and each controls the conformation of a different asymmetric center.
336 METAL–ORGANO MULTICATALYSIS: AN EMERGING CONCEPT Via: +
− 83% e.e. 73
− 99% Organocatalyst (20 mol%) H 2
NaOAc (100 mol%) Organocatalyst: R 1
TsHN R 2 R 3 N Ts R 1 CHO R 3 R 2 N H OTMS
Ph Ph DMAP (10 mol%) PdCl 2 (5 mol%) toluene, 0 °C N Ts R 1 CHO R 3 R 2 Via: + Yield 40 − 77% e.e. 90
− 99% Organocatalyst (20 mol%) PhCO 2
Organocatalyst: R CHO HO O R CHO N H OTES Ph Ph PdCl 2 (5 mol%) CHCl 3 or THF, 4 ° C O R CHO
Via: + yield 48 − 86% e.e. 93
− 98% Organocatalyst (20 mol%) Pd(PPh 3
4 (5 mol%) Organocatalyst: R 1 CHO R 1 CHO N H OTMS Ph Ph MeCN, rt R 1 CHO CO 2 R 2 R 2 O 2 C R 2 O 2 C CO 2 R 2 R 2 O 2 C CO 2 R 2 Scheme 26.19 Relay 1,4 organocatalytic addition/palladium alkyne addition processes. + Yield 66 − 89% e.e. 92
− 95% Organocatalyst (10 mol%) N( n
4 RuO
4 (7 mol%) Organocatalyst: R Br N H OTMS Ph Ph NMO (1.5 equiv), CH 2 Cl 2 , rt
CO 2 Et EtO 2 C OH R O EtO 2 C CO 2 Et R 2 OH R 1 P Ir N O Ph t Bu
Bu Metal catalyst, 5 mol% B(Ar
F ) 4 H 2 , THF, rt R 2 O R 1 Organocatalyst, 20 mol% SO 2 Ph SO 2 Ph R 2 O R 1 CH(SO 2 Ph) 2 Metal catalyst: Organocatalyst: N H OTMS Ph Ph R O R N Br EtO 2 C CO 2 Et RuO 4 Organocatalyst yield 28 − 66%
e.e. 75 − 99%
d.e. 67 − 96%
Scheme 26.20 Metal catalysis/organocatalytic 1,4 addition sequences. 26.7 N-HETEROCYCLIC CARBENES AS ORGANOCATALYSTS Sequential reactions involving metal and NHC catalysis are quite scarce in the literature; NHCs are powerful ligands that coordinate many metals, resulting in altered activities for both partners, so it may be difficult to create balanced catalytic systems. Fortunately, the few examples available cover several NHC mechanistic pathways, including the benzoin condensation [104], homoenolate addition [105], and NHC-catalyzed oxidation [106, 107] (Scheme 26.21).
BR Ø NSTED ACIDS AS ORGANOCATALYSTS 337 + AgOTf (5 mol%) IPr·HCl (5 mol%) Cs 2 CO 3 (25 mol%) R 3 CHO THF/DCE, 50 °C Yield 46 − 83% SIMes (10 mol%) Pd(OAc) 2
O 2 , Na 2 CO 3 xylene, 130 °C Ligand: + Organocatalyst (5 − 10 mol%) [Pd(allyl)Cl] 2 (0.5
− 2.5 mol%) ligand (1.25 − 7 mol%) Organocatalyst: t AmylOH, DBU, rt or 55 °C ArCHO OAc 2 Ar Ar O OH N S Mes N O PPh 2 Yield 32 − 89% e.e. < 26% R 2 N NHTs R 1 R 2 N NHTs R 1 R 3 CO 2 Me R 2 N NTs R 1 Via: R 3 NHC OH + Homoenolate equivalent MeOH
Yield 53 − 94%
ArCH 2 OH + OH R O R Ar O ArCHO
Via: Scheme 26.21 NHC/metal relay catalytic processes. The metal processes involved in these reactions are Pd-catalyzed allylation, Ag activation of alkynes, and Pd benzylic oxidation. In the last example, it is interesting to observe that the metal oxidizes the alcohol to aldehyde, while the NHC oxidizes the aldehyde to an activated acyl moiety, resulting in esterification.
As already stated, the use of protic acids in the context of metal–organo multicatalysis offers a definite advantage over other types of organocatalysts, because of their reduced tendency to inactivate the metal cocatalyst by coordination [73]. Hydride transfer from Hantzsch esters or similar NADH-like molecules is certainly one of the most interesting reactions catalyzed by Brønsted acids [108]. Imines, enamines, or similar substrates are protonated to iminium-reactive intermediates that are then reduced by the hydride donor. Metal-catalyzed processes that can generate these substrates may be combined with the reduction process, yielding a multicatalytic system. For example, intra- [109] or intermolecular [110] gold-catalyzed hydroamination of alkynes generates suitable imines or enamines (Scheme 26.22). R 2
2 R 1 R 2 N H R 1 Ph 3 PAuMe (5 mol%), Hantzsch ester, protic acid (15 mol%) Toluene, 25 °C Yield 82 − 100%, e.e. 87
− 99% Via:
R 2 N H R 1 Acid: O P O O OH Ar Ar Ar = 9-phenanthrenyl NH 2 R 1 R 2 N H R 1 t Bu 2 (o-diphenyl)PAuOTf (1 - 2 mol%), Hantzsch ester, protic acid (5
− 10 mol%) Benzene, 40 °C Yield 54 − 98%, e.e. 83
− 96% Via:
R 2 N H R 1 Acid: O P O O OH Ar Ar Ar = 2,4,6-tri-isopropyl-phenyl + R 2
Gold-catalyzed alkyne addition/acid-catalyzed hydride transfer relay processes.
338 METAL–ORGANO MULTICATALYSIS: AN EMERGING CONCEPT O NH
R 1 R 2 N H R 1 Mg(OTf) 2 (10 mol%), Hantzsch ester, protic acid (10 mol%) Toluene, 35 °C Yield 55 − 91%, e.e. 90
− 97% Via:
Acid: O P O O OH Ar Ar Ar = 9-anthracenyl H R 2 O + OEt O O OEt R 2 N R 1 O OEt Scheme 26.23 Cooperatively catalyzed cyclisation/acid-catalyzed hydride transfer relay system. N O
protic acid (5 mol%) Toluene, 40 − 60 °C Yield 75 − 97%, e.e. 80
− 94% Acid:
O P O O OH Ar Ar Ar = 9-phenathryl + R
R 2 O N O R 1 N O R 1 O R 2 Alkene cross methathesis Friedel-crafts Ru O
MesN Cl Cl SO 2 NMe 2 Hoveyda-Grubbs: O R
Scheme 26.24 Cross-methathesis/acid-catalyzed Friedel–Crafts relay system. N H
2 Pd(OCOCF
3 ) 2 (2 mol%), ligand (2.2 mol%), TsOH (100 mol%) CH 2 Cl 2 /CF 3 CH 2 OH 2 : 1, H 2 (40 bar), 50 ° C Yield 63 − 93%,
e.e. 41 − 91%
Ligand: PPh
2 PPh
2 + R 1 Friedel
−Crafts R 3 CHO N H R 2 R 1 R 3 N H R 2 R 1 R 3 HO N H R 2 R 1 R 3 Dehydration Hydrogenation Scheme 26.25 Brønsted acid/metal relay system where the enantioselectivity is determined by the metal ligand. This method allows the reduction of the imine functionality even if included within an aromatic ring. In a recent example [111], a quinoline is formed by a Friedlander reaction, and subsequently reduced by hydride transfer from a Hantzsch ester to a chiral tetrahydroquinoline (Scheme 26.23). This is an interesting case in which the authors have shown that the first reaction step is cocatalyzed by the Lewis acid and the protic acid, therefore being simultaneously an example of cooperative and relay catalysis. Brønsted acids can also catalyze a wide range of C–C-forming reactions [72] that can be combined with metal-catalyzed processes in ingenious ways. For example, Ru-catalyzed cross-methathesis [112] has been used to generate substrates for acid-catalyzed Friedel–Crafts reactions (Scheme 26.24). The Brønsted acid is not always responsible for the chiral induction step; as shown in Scheme 26.25, a Friedel–Crafts product can be the substrate for an enantioselective Pd-catalyzed hydrogenation [113]. Gold-catalyzed addition of N–H and O–H to a triple bond has proven to be a rich source of protic acid-based multicatalytic systems. The setup for hydrogen transfer from Hantzsch esters has already been discussed; other recent examples of Au/protic acid systems include combinations with well-known acid-catalyzed reactions such as the Fischer indole synthesis [114], Povarov reaction [115], and Diels–Alder reaction[116] (Scheme 26.26). Some of the examples that use TsOH suffer from high catalyst loadings. It is unclear if such high amount of catalyst is actually needed or if the authors were not concerned because of the cheapness of the reagent. REFERENCES 339 N H R2 Ph 3 PAuNTf 2 (2 mol%), TsOH·H 2 O (110 mol%) Toluene, 100 °C Yield 64 − 94% + R1 N H NH 2 R 1 R 2 N H N R 1 R 2 Via: LAuMe (10 mol%), acid (15 mol%) CH 2 Cl 2 , rt Yield 30 − 74%,
e.e. 85 − 98%,
d.r. 2:1 − 6:1
+ NH 2 NHCbz R 2 R 1 R 3 CHO +
N NHCbz
R 3 R 2 R 1 Acid: O P O O OH Ar Ar Ar = 9-anthracenyl Condensation (acid cat.) Alkyne addition (Au cat.) N H
R 3 R 2 R 1 N R 3 R 2 R 1 Povarov
(acid cat.) NHCbz
L: P
Bu 2
6 ] (6
mol%), acid (15 mol%) PhF, rt
Yield 67 − 98%,
e.e. 87 − 96%
+ Si R 3 Acid:
O P O O HN Ar Ar Ar = 1-pyrenyl OH R 4 R 4 R 2 R 1 O O Si R 3 O R 4 R 4 R 2 R 1 Via: Si R 3 O R 4 R 4 R 2 R1 O O Tf L: as above Scheme 26.26 Relay systems involving alkyne addition and Brønsted acid catalysis other than hydride transfer. 26.9 CONCLUSION As shown herein, the combination of metal catalysts and organocatalysts in a multicatalyzed approach is developing into a powerful strategy to synthesize complex molecules. The coexistence in one pot of a metal catalyst and an organocatalyst offers the possibility to activate both reactants in a synergistic or stepwise manner and this tactic may be explored to improve the reaction efficiency. Considering the current state of the art, and the available metal catalysts and organocatalysts still unstudied, it is clear that this emerging area is still in its infancy and exciting results still await discovery. 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