Enamides display a fine balance of stability and reactivity, which is now leading to their increasing use in organic synthesis. Enamides offer multiple opportunities for the inclusion of nitrogen based functionality into organic systems. Recent examples of these compounds as substrates are discussed in this article.

Enamides: valuable organic substrates.

Metathesis in the synthesis of aromatic compounds.

ed from in the transition state. This answers the question of both the retention and the inversion of stereochemistry for the simple exchange reaction giving back I (which it should be noted is highly disfavored because k2 ≫ k−1). Retention of configuration would occur by first noting that the potassium ion would also be coordinated to the same face as the abstracted hydrogen atom because it was originally coordinated to the oxygen of the base. A molecule of deuterated alcohol can then approach this same face through coordination to the potassium. Rotation of these ligands around potassium would result in the deuterated alcohol now hydrogen bonded to the allylic anion (IV-top), which following collapse would return the chiral alkene I-d with the same configuration. On the other hand, inversion of stereochemistry would occur via the deuterated solvent approaching from the face of the allylic anion opposite to that hydrogen bonded, and exchange with the alcohol giving (IV-bottom). This could collapse and give chiral alkene I-d with inversion of configuration. The E/Z ratio of 60 for product III-h is also important because this value is extremely high as compared to the equilibrium value of 4.2 for the diastereoisomers, and compares to other systems as well. For example, the isomerization of allylbenzene into 1-propenylbenzene has an E/Z ratio of 44 when using potassium t-butoxide in DMSO. The reason for this higher-than-equilibrium ratio is explained in Scheme 3. Two “conformations” of the chiral alkene I are suitable for deprotonation and will lead to either the E or the Z allylic anions as shown. The rate of “conformational rotation” must be many orders of magnitude greater than the rates for Scheme 2. Proposed Base-Mediated Reaction Mechanism Scheme 3. Interactions Leading to Observed E/Z Ratios Chemical Reviews Review DOI: 10.1021/acs.chemrev.5b00052 Chem. Rev. XXXX, XXX, XXX−XXX E deprotonation (kE and kZ), and thus the E/Z ratio in the product must be related to the kE/kZ ratio. The transition state leading to the Z-allylic anion must thus be higher in energy than the one leading to the E-allylic anion, presumably due to the disfavored 1,3-allylic interactions, which would make charge stabilization through conjugation more difficult. Cram has reported this in more detail, including an analysis of collapse ratios (i.e., ratio of protonation of either carbon on the allylic anion), the stereochemical stability of allylic and vinyl anions, and the kinetic and thermodynamic stabilities of olefinic products formed by protonation of allylic anions. 2.2. Transition Metal-Mediated Mechanisms The mechanisms for the transition metal-mediated isomerizations have in some cases been very carefully determined, while in many they are assumed from one of two generally accepted pathways (Schemes 4 and 5). The first involves a discrete transition metal−hydride active catalyst and the formation of a transition metal−alkyl intermediate, while the second involves the formation and collapse of a η-allyl hydride complex. Each of the mechanisms described below bases its premise on the fact that each step is reversible, and thus the reactions are under thermodynamic control at equilibrium. 2.2.1. Alkyl Mechanism. For this mechanism to occur, the transition metal catalyst must have both an empty 2e− coordination site (e.g., via dissociation of a ligand) and a metal hydride bond that is typically generated in situ under the reaction conditions (see Scheme 4). The catalyst first coordinates to the π-electrons of the alkene followed by an insertion reaction to give either a primary or a secondary metal−alkyl intermediate. The primary metal−alkyl is generally formed faster for many catalysts, but is a mechanistic dead-end returning the starting material through β-hydride elimination and is thus a nonproductive pathway. The secondary metal− alkyl intermediate can however produce either the Eor the Z1-propenylbenzene on β-hydride elimination, which is clearly thermodynamically favored because of the conjugation to the benzene ring. 2.2.2. Allyl Mechanism. The allyl mechanism, however, requires a transition metal capable of having two vacant coordination sites, and, more specifically, no metal−hydride should be present or the alkyl mechanism will take place (see Scheme 5). The first step involves coordination of the πelectrons of the allylbenzene to one of the transition metal’s vacant sites. This is followed by an oxidative addition reaction giving an η-allyl metal−hydride complex, which can collapse to either the starting material or the rearranged and thus more stable alkene. The η-complex may also rearrange to the ηcomplex as part of the reaction pathway. The difference between these two mechanisms can be determined through deuterium labeling with 32 and crossover experiments (Scheme 6). The allyl mechanism is entirely intramolecular and involves an effective 1,3-hydride shift as the only mechanistic pathway. Thus, in a crossover experiment, such as that in Scheme 6, the deuterium should (a) only be found at the 1and 3-positions of the allylic system, that is, 34, and (b) not be incorporated into the second nondeuterated substrate. In the case of the alkyl mechanism, products similar to those of the allyl mechanism may be detected in addition to (a) the nondeuterated substrate showing some deuterium incorporation and (b) the loss of deuterium and incorporation of hydrogen on the deuterated substrate. Furthermore, deuterium incorporation at the 2-position of the allylic system, 35, is also expected because the initial metal−hydride insertion reaction may have poor regioselectivity as already explained in Scheme 2. Of course it should be noted that these two general mechanisms are just that, and the specific reaction pathway for different transition metal catalysts will depend heavily on the transition metal, ligand, solvent, and substrate combinations. More detailed mechanistic data can be obtained from kinetic studies supplemented by state-of-the-art measurements, for example, nanosecond time-resolved IR, NMR, and DFT calculations. These studies have invariably revealed Scheme 4. Alkyl Mechanism M = transition metal; [L]n = bound ligand(s); [L]0 = dissociating ligand or vacant 2e− site. Scheme 5. Allyl Mechanism M = transition metal; [L]n = bound ligand(s); [L]0 = dissociating ligand or vacant 2e− site. Chemical Reviews Review DOI: 10.1021/acs.chemrev.5b00052 Chem. Rev. XXXX, XXX, XXX−XXX F mechanisms more complicated than those presented above, but at the same time generally holding true to them. One major mechanistic departure has been that proposed by Harvey and Lloyd-Jones in which they suggest a binuclear palladium complex being involved in the E/Z isomerization of alkenes. However, it should be noted that this is only applicable in some cases. More important is that the specific mechanisms themselves answer questions related to E/Z selectivity in these transition metal-mediated isomerizations. Simplistically, the reactions can be considered to be under thermodynamic control, and therefore the E/Z ratios will favor the E-isomer. The π-allyl mechanism has been linked, in a general sense, with higher E/Z ratios, but can by no means be used as proof for a particular mechanism. This is because the E/Z selectivity in the reaction is strongly governed not only by the thermodynamic stability of the E isomer (and intermediates leading to the E-isomer), but can be shifted through ligand and or kinetic control to high ratios of either the E or the Z isomers. The sections below detailing the specific examples in the literature will highlight these selectivities. 3. ISOMERIZATION METHODS − GENERAL Perusal of the literature quickly confirms that 2-propenylaryl isomerization reactions have been promoted by mainly two classes of methods, viz., the application of bases (section 4) or transition metal complexes (section 5). In this Review, these sections will thus be discussed separately, in addition to a final section describing miscellaneous allylaryl isomerizations (section 6). In each section, substrates of particular interest will be highlighted, with particular care being taken to convey important experimental data such as yields, the cis/trans ratios obtained, and any other relevant information. 4. BASE-MEDIATED ISOMERIZATIONS In general, base-initiated reactions require at least a stoichiometric amount of base to accomplish the isomerization of an allylbenzene. This method has seen much application in the past literature, and some general information has been collated in book chapters. 4.1. Hydroxide/Alkoxide Ion-Mediated Isomerizations This particular method involves the use of fairly harsh reaction conditions in that it generally comprises heating the substrate in a protic solution (ethanol, methanol, or n-butanol) of sodium or potassium hydroxide (giving rise to a mixture of the hydroxide and corresponding alkoxide under equilibrium conditions). Other examples include the use of hydroxide in DMSO. Of interest is that the KOH/ethylene glycolmediated isomerization of eugenol 5 has actually been incorporated into the curriculum of a teaching laboratory to demonstrate how the kinetics of a reaction can be studied using NMR spectroscopic, GC, and HPLC laboratory techniques. It should also be mentioned here that the importance of the potassium t-butoxide system means that a separate subsection will be dedicated to this method (see section 4.2). In the first section, examples of where hydroxide-mediated isomerization has been applied will be highlighted. This will be followed by methods in which the isomerization approach has been modified, albeit with additives (phase transfer), microwave heating, or different solvent systems. 4.1.1. Hydroxide/Alkoxide Ions in Alcohol (Methanol, Ethanol, n-Butanol) or without Solvent. It should be noted that the use of sodium ethoxide has been rigorously studied, showing that the isomerization of para-substi

Isomerization of allylbenzenes.

The use of metathesis, especially in the context of ring-closing metathesis (RCM) to form five- and six-membered rings, is widespread in organic chemistry today. However, there are surprisingly few examples of the reaction being used to form aromatic compounds. The central place of aromatic compounds in both medicinal chemistry and natural products synthesis, coupled with the efficiency and functional group tolerance of RCM catalysts, means that there is now an interesting opportunity to use RCM for the synthesis of arenes. Although the formation of an aromatic compound was viewed in many early examples as an undesirable degradation product, several rationally designed methods towards the preparation of aromatic compounds by RCM have recently been developed.

Ring‐Closing Metathesis as a Basis for the Construction of Aromatic Compounds

Alkoxycarbonyl radicals, formed from carbazates under air and iron catalysis, smoothly react with terminal alkenes to afford β-hydroxyesters, e.g. (III), (V) and (VII), in moderate yields.

Iron‐Catalyzed Oxidative Addition of Alkoxycarbonyl Radicals to Alkenes with Carbazates and Air

An indole-templated ring-closing metathesis or a 2-indolylacyl radical cyclization constitute the central steps of two alternative approaches developed to assemble the tricyclic ABC substructure of the indole alkaloid apparicine. From this key intermediate, an intramolecular vinyl halide Heck reaction accomplished the closure of the strained 1-azabicyclo[4.2.2]decane framework of the alkaloid with concomitant incorporation of the exocyclic alkylidene substituents.

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Total synthesis of the bridged indole alkaloid apparicine.

The dimethyltitanocene methylenation of N-acylamides derived from ortho-vinylanilines, ortho-allylaniline, and ortho-vinylbenzylamine provides the corresponding enamides, which upon exposure to the second generation Grubbs ruthenium catalyst give access to indoles, 1,4-dihydroquinolines, and 1,2-dihydroisoquinolines, respectively. This sequential protocol also allows the synthesis of dihydrobenzoazepines, although the ring-closing metathesis (RCM) step is complicated by the alkene isomerization processes. From certain substrates, the direct annulation is observed in the titanium-mediated step, which is likely to occur through an olefin metathesis−intramolecular olefination sequence.

Sequential N-Acylamide Methylenation−Enamide Ring-Closing Metathesis: Construction of Benzo-Fused Nitrogen Heterocycles

Sequential N-acylamide methylenation–enamide ring-closing metathesis: a synthetic entry to 1,4-dihydroquinolines

C-4 or C-12 ethyl substituted 1,5-methanoazocino[4,3-b]indoles, which constitute the tetracyclic framework of uleine alkaloids as well as the ABDE substructure of the Strychnos alkaloid family, have been synthesized by novel 6-exo and 6-endo cyclizations of selenoester-derived 2-indolylacyl radicals upon 5-ethyl-1,2,3,6- and 3-ethyl-1,2,5,6-tetrahydropyridines, respectively.

A new acyl radical-based route to the 1,5-methanoazocino[4,3-b]indole framework of uleine and Strychnos alkaloids.

Regioselective 7- and 8-endo cyclizations of selenoester derived 2-indolylacyl radicals upon amino tethered alkenes have been used to synthesize azepino[3,2-b]- and azocino[4,3-b]indoles, which are tricyclic subunits present in the indole alkaloids mersicarpine and apparicine, respectively.

Davinia García-Díaz

Papers

Total synthesis of the bridged indole alkaloid apparicine.

Sequential N-Acylamide Methylenation−Enamide Ring-Closing Metathesis: Construction of Benzo-Fused Nitrogen Heterocycles

Sequential N-acylamide methylenation–enamide ring-closing metathesis: a synthetic entry to 1,4-dihydroquinolines

A new acyl radical-based route to the 1,5-methanoazocino[4,3-b]indole framework of uleine and Strychnos alkaloids.

Novel 7-and 8-endo 2-indolylacyl radical cyclizations : Efficient construction of azepino-and azocinoindoles