Silver-catalyzed cycloadditions

Scheme 6.7 shows some other examples of enantioselective catalysts. Entry 1 illustrates the use of a Co(III) complex, with the chirality derived from the diamine ligand. Entry 2 is a silver-catalyzed cycloaddition involving generation of an azome-thine ylide. The ferrocenylphosphine groups provide a chiral environment by coordination of the catalytic Ag+ ion. Entries 3 and 4 show typical Lewis acid catalysts in reactions in which nitrones are the electrophilic component. 

The [3+2] cycloadditions are the most prolific of all the silver-catalyzed cycloadditions. The unique affinity of silver for imines has facilitated the development of highly efficient and enantioselective cycloadditions of azomethine ylides to alkenes. Judicious choice of reaction conditions is crucial in achieving high yields for different substitution patterns. 

The chemistry of sulfoxide 57 has been studied in the Diels-Alder reaction with dienes.29 Ruano et al. studied its reaction with iminoesters 43a, c-d under a variety of conditions (Scheme 2.16).30 Silver-catalyzed cycloaddition atO°C or room temperature afforded 1 1 mixtures of diastereomers 58 and 59 in variable yields. In contrast, deprotonation in the absence of silver using LiHMDS at -78 °C afforded product 58 in 98 2 dr and 89-93% yield.30... 

A range of thiazoles with various combinations of Ri, R2, and R3 substituents was synthesized using this method. The silver-catalyzed cycloaddition of propargylic alcohol with thioamide was proposed to proceed through the intermediacy of a propargylic cation or the corresponding allenyl cation and its subsequent reaction with the nucleophilic sulfiir of thioamide, followed by a 5-exo-dig attack by nitrogen. 

Gao M, He C, Chen H, Bai R, Cheng B, Lei A (2013) Synthesis of pyrroles by click reaction silver-catalyzed cycloaddition of terminal alkynes with isocyanides. Angew Chem Int Ed 52 6958-6961... 

Discovery and Development of the Silver-Catalyzed [3+2] Cycloaddition of Azomethine Ylides... 

The scope of the silver-catalyzed [3+2] cycloaddition is not limited to amino acid-derived iminoesters. Grigg has reported the use of aminophosphane-derived azomethine ylides in the cycloaddition to afford the corresponding phosphonate-substituted pyrrolidines (Scheme 2.36).62 The diastereoselectivity of these cycloadditions is not inferior to those of standard amino acid-derived imines. 

As a part of a program directed toward the synthesis of the potent topisomerase I inhibitors, the lamellarins (e.g., 153 and 154), Porco has reported the silver triflate-catalyzed tandem cycloisomerization-azomethine ylide cycloaddition of 155 (Scheme 2.42).75 The postulated mechanism of this intriguing and highly efficient process is shown in Scheme 2.43. Silver-catalyzed addition of the imine nitrogen to the alkyne results, on subsequent deprotonation, in the formation of an azomethine ylide 160. This ylide participates in [3+2] cycloaddition with the alkyne component leading to formation of a dehydropyrrole 161. Finally, oxidation by adventitious oxygen leads to formation of the product 162. 

Saito reported the extension of Billups tandem silver-catalyzed ring-opening cycloaddition reaction methodology to addition to imines (Scheme 2.45).77 Naphtho- /) cyclopropene 167 added to substituted aryl imines 168 at room temperature in the... 

Scheme 7.46. Scope of silver-catalyzed silacarbonyl ylide cycloaddition.

Silver compounds are versatile catalysts for various cycloaddition reactions, including [2 + 1]-, [2 + 2]-, [3 + 2]-, and [4 + 2]-cycloadditions. An example for the silver-catalyzed formation of three-membered rings by [2+ l]-cycloaddi-tion is the silacyclopropanation reaction of mono- and disubstituted alkenes by silylene transfer from the cyclohexene silacyclopropane 432 that was reported recently by Woerpel et /.355,355a (Scheme 127). The reaction tolerates a number of functionalities in the substrate (OBn, OSiR3, BuTlC, etc.,) and is stereospecific with regard to the cisjtrans... 

An example for a silver-catalyzed [2 + 2]-cycloaddition was reported by Kozmin and co-workers.359 Treatment of siloxyalkynes 438 with acceptor-substituted alkenes in the presence of 5 mol% of AgNTf2 afforded the cyclobutenes 439 in good yields (Scheme 130). The use of cyclohex-2-enone allowed the preparation of the corresponding... 

Enantioselective silver-catalyzed aza-Diels-Alder reactions have also been described. Whereas the use of BINAP as the chiral ligand gave only unsatisfactory enantioselectivities,367,367a Hoveyda etal 6 used the peptidic phosphine 373 which has already been used for silver-catalyzed asymmetric Mannich reactions (see Scheme 108) in the efficient Ag(i)-catalyzed cycloaddition of arylimines 449 with Danishefsky s diene (Scheme 135). After acidic work-up, the chiral pyridones 450 were obtained with excellent yields and enantioselectivities. The presence of isopropanol as a proton source is essential for obtaining high conversions and stereoselectivities. Similar to the Mannich reactions, the cycloaddition is not affected by the presence of air or the use of undistilled THF. 

A silver-catalyzed reaction of methyl ortfoi-fiarmylphenylpropargylate with alkenes led to a tandem 1,3-dipolar cycloaddition/cyclopropanation producing a series of polycyclic compounds with tetrahydrofuran moieties (140L4412). 

Sansano has reported a highly efficient silver-catalyzed enantioselective domino imine formation/l,3-dipolar cycloaddition reaction [54]. As shown in Scheme 11.35, benzaldehyde (92a) and N-methylmaleimide (158) or 3-pyridinecarbaldehyde (92b) and disulfone 161, glycine methyl ester hydrochloride (159), triethylamine (1.05equiv), and (S)-BlNAP-AgSbFg (5mol%) were mixed together in toluene, and the resulting mixture was allowed to react at room temperature. The outcomes... 

Scheme 11.35 Silver-catalyzed, enantioselective domino imine formation/l,3-dipolar cycloaddition reaction [54].

Georg and coworkers reported a C=C bond cycloaddition with a ketene to provide six-membered enaminones, under mild conditions (Scheme 92) (2010JA15512). From this intramolecular cycloaddition of a ketene created from diazoketone 289, employing a silver-catalyzed Wolff rearrangement, a six-membered enaminones such as 290 was obtained. 

McNulty J, Keskar K, Vemula R (2011) The first well-defined silver(l)-complex-catalyzed cycloaddition of azides onto terminal alkynes at room temperature. Chem Eur J 17 14727-14730... 

The other recent examples of asymmetric syntheses involving Danishefsky s diene focused on hetero Diels-Alder reactions. Shibasaki and Feng separately reported asymmetric reactions with carbonyl dienophiles. Shibasaki demonstrated successful asymmetric reactions of ketones using a chiral Cu(I)-Walphos catalyst. Feng used a chiral A,iV -dioxide/In(OTf)3 catalyst in asymmetric cycloaddition reactions of aldehydes. Imine dienophiles are also amenable to asymmetric Diels-Alder reactions with Danishefsky s diene. Wulff reported enantioselective reactions using a VAPOL-B(OPh)3 catalyst system, while Snapper and Hoveyda disclosed silver-catalyzed enantioselective aza Diels-Alder reactions. ... 

The reaction of isocyanide 42 and phenylacetylene (43) afforded pyrrole 44 under silver-catalyzed conditions. The pyrroles are formed regioselec-tively, in high yields, and with broad functional group tolerance. This represents the first example of the transition metal-catalyzed cycloaddition of isocyanides with unactivated terminal alkynes (13AG(I)6953). A similar system for the synthesis of substituted pyrroles was reported by Lei (13AG(1)6958). 

Ketenes generated in situ by dehydrochlorination react with isoqui-noline-N-oxides generated in situ from silver catalyzed reaction of 2-alkynylbenzaldoximes, and give an initial [3 -f- 2] cycloaddition followed by rearrangement to 161 and then to the product 162 (Eqn (4.101)). ° ... 

Density functional theory (DFT) calculations (at the B3LYP/LANL2DZ level of theory) account completely for the regio-and stereochemical result of the silver-catalyzed azomethine ylides cycloaddition onto C70 (Fig. 34.6). A stepwise mechanism indicates that the origins of the observed selectivity are determined by the first step of the cycloaddition at the site, regio, and enantio levels. 

Figure 34.6 Blockage of the (si,re) face of the 1,3-dipole in the silver-catalyzed azomethine ylides cycloaddition onto Cjg, accounted by DFT calculations.

Xie H, Zhu J, ChenZ, Li S, Wu Y (2010) Diastereoselective silver-catalyzed 1,3-dipolar cycloaddition of azomethine ylides with fluorinated imine. J Oig Chem 75 7468-7471... 

Cycloaddition. A silver-catalyzed [2 + 2] cycloaddition has been described. Although AgOTf is not the most efficient silver salt, it promotes the formation of functionalized cyclobutenes from siloxy alkynes and unsaturated ketones in reasonable yields... 

Shen T, Wang T, Qin C, Jiao N (2013) Silver-catalyzed nitrogenation of alkynes a direct approach to nitriles through C=C bond cleavage. Angew Chem Int Ed 52(26) 6677-6680 Kadaba PK (1990) TriazoUnes XX. Vinyl azides as dipolarophiles in 1,3-dipolar cycloadditions intermolecular cycloaddition of hydrazoic acid and a-styryl azide to give a tetrazole. Synlett 6 349-351... 

The Huisgen azide-alkyne 1,3-dipolar cycloaddition (AAC) [75] and later developed Cu-catalyzed azide-alkyne cycloadditions (CuAAC) [76-80], which are the most commonly utilized methods for the synthesis of N-substituted 1,2,3-triazoles, will not covered here. Very recently, Bi and co-workers [81] developed an elegant silver-catalyzed tandem hydroazidation/alkyne-azide cycloaddition of diynes with TMSN3 providing an easy access to 1,5-fused 1,2,3-triazole frameworks [82, 83]. 

A selectivity divergence between phosphite and NHC ligands was also reported in the gold-catalyzed cycloaddition of allenedienes [eqn (4.8)]. With the IPr ligand on the putative active species LAu, which is generated in situ by anion metathesis using a silver salt, the major product obtained from substrate 123 is the formal [4C+3C] cycloaddition product 124. On the... 

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