Electrophihc Metalation

Studies involving the specific exchange mechanisms," as well as theoretical calculations" " of alkene metathesis reactions, generally agree that the mechaiusm begins with jt-coordination of the alkene to the electrophihc metal as shown for a general diene in Scheme 5. 

Dysprosium(III) catalysis in organic synthesis (Friedel—Crafts alkylation, Mannich-type, Diels—Alder, and Povarov reactions) 12T2015. Enamides and related functional groups as nucleophihc components in ring-forming processes catalyzed by electrophihc metal salts 12SL814. 

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

Phosphinidenes differ from carbenes because of the additional lone pair. This lone pair enables interactions with, e.g., a transition metal group for increased stability, while maintaining carbene-hke behavior. These terminal /] -complexed phosphinidenes differ from the p2-> fi3-> and p4-complexes, which are not part of this survey. Phosphinidenes that are stabilized by a transition metal group also relate to carbene complexes. A distinction in Fischer and Schrock-type complexes has been advanced to distinguish phosphinidene complexes with nucleophilic properties from those that are electrophiHc [ 13 ]. In this survey we address this topic in more detail. 

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

Ir(OH)(cod)]2 catalyzed a formal [3+2] cycloaddition of 2-formylphenylboronic acid and 1,3-dienes (Scheme 11.41) [50]. The transmetaUation of boronic acid with iridium would yield aryliridium, where the carbonyl group coordinates to the metal. An electrophihc attack of the diene terminus to formyl carbon would then... 

Due to their high electrophihcity, isocyanates are useful starting materials for the synthesis of heterocycles and C-C bond formation using carbon nucleophiles. A key feature of these reactions is the requirement for more than one equivalent of base. Additionally, a few examples of transition metal-catalyzed homologation of the central carbon of the heterocumulene moiety have been reported. 

Hydrogen bonding to substrates such as carbonyl compounds, imines, etc., results in electrophilic activation toward nucleophilic attack (Scheme 3.1). Thus, hydrogen bonding represents a third mode of electrophihc activation, besides substrate coordination to, for example, a metal-based Lewis acid or iminium ion formation (Scheme 3.1). Typical hydrogen bond donors such as (thio)ureas are therefore often referred to as pseudo-Lewis-acids. ... 

Those N-heteroq clic carbenes show a surprising stabihty which was of interest to organometallic chemists who started to explore the metal complexes of these new ligands. The first examples of this class had been synthesized as early as 1968 by Wanzhck [11] and Ofele [12], only four years after the first Fischer-fype carbene complex was synthesized [2,3] and six years before the first report of a Schrock-fype carbene complex [13]. But at that time their potential as hgands had not been explored. Once the N-heterocychc ligands are attached to a metal they show a completely different reaction pattern compared to the electrophihc Fischer- and nucleophilic Schrock-fype carbene complexes. 

Once the oxidative-addition reaction of dioxygen to metal d -ions has occurred, the essentially electrophihc dioxygen becomes a nucleophilic peroxide ligand. Since the oxidation of substrates is associated with electron transfer from the substrate to the oxidant, i.e. in this case the dioxygen adduct, effective oxygenations require a further activation to transform the nucleophihc peroxide into an electrophihc species prior to the oxygen transfer. 

A second way to activate the peroxide ligand for oxygen transfer to C = C bonds could be via homolytic 0-0 cleavage of the peroxide by coordination to a second metal center. From this point of view, the peroxide ligand is converted into the [M(/z-0)]2 moiety in which the oxide bridges are now electrophihc and then each oxygen atom from the M2(/z-oxo)2 core is transferred to a close C = C bond in a cis position (Scheme 8). As this conclusion is based on only two reported examples, it should be taken as a working hypothesis. 

General Characteristics. In accordance with the high tendency for formation of Co(CO)4 (2 ) and the acidity of (3), the former componnd behaves in many respects as a psendohalide see Pseudohalide). Relative rates of electrophilic snbstitntion with Mel and MeOTs classify (2 ) as a hard and relatively weak nncleophile see Hard Soft Acids and Bases). Compounds of the type RCo(CO)4 are prepared by two different methods reaction of an electrophihc halide, snlfate, or tosylate (RX) with (2 ), or reaction of HCo(CO)4 with alkenes. Simple alkyl derivatives have low stability nevertheless, many studies have been conducted in which species have been identified as RCo(CO)4 intermediates. Apart from alkyl and aryl derivatives, for example MeCo(CO)4 and PhCo(CO)4, (2 ) forms derivatives with main group residues R3E and R2E and with the main group metals Zn, Cd, and Hg as well as with transition metals, as shown by examples in Scheme 4. Co E bonding in these compounds appears to be predominantly covalent. The bond distances can be long, and consequently, the coordination number of Co can be high. 

In summary, the potential utility of the metal catalyzed hydroboration reaction is vast. Future work will focus on its applicability in the synthesis of biologically active compounds where conventional electrophihc additions have proven ineffective. Convenience and mild conditions required for these reactions provide the organic chemist with a valuable synthetic tool. 

A complete survey of polar coupling reactions might include all types of organic ligands, n- and generally useful synthesis methodology based on polar activation by transition metals is nucleophile addition to... 

Nucleophilic Addition at the CO. The formation of the metal acyl [ReCp(C0)2 C(0)Me ] (99) is a classic example of acylmetallate formation. The site of electrophihc alkylation of (99) is dependent on the hardness of the electrophile used. In fact, its reaction with the oxonium [MesOJ+gives the expected Fischer carbene [ReCp(CO)2 C(OMe)Me ], but the softer electrophile Mel attacks the Re, forming [ReCpMe(C0)2 C(0)Me ] (100) (Scheme 23). 

Considerable attention was given to the stereochemistry for the alkylation of metal enolates of y-butyrolactones during the past 1980 s decade. It is well recognized that electrophihc attack on the enolates of -substituted y-butyrolactones is controlled exclusively by the -substituent leading to the trans addition products . However, Iwasaki and coworkers reported the reverse diastereofacial differentiation in the alkylation of the enolates of a, S-dibenzyl-y-butyrolactones. These authors proposed that the factor controlling the selectivity in this case was allylic strain. Also, y-substituted y-lactones give stereoselective trans alkylation . ... 

Most of the information available on this topic is derived from works on the reactions of alkali metal salts (sodium or potassium) of dialkyl cyanomethylphosphonates with sp or sp electrophihc center. Thus, in the presence of a further equivalent of sodium hydride, sodium dialkyl cyanomethylphosphonates add to carbon disulfide in I I2O at room temperature to give alkenedithiolates, which are further characterized by alkylation or oxidation. For example, reaction with methyl iodide provides dialkyl 1-cyano-2,2-/ /.v(methylthio)vinylphosphonates in 21—40% yields (Scheme 6.16),-° whereas treatment with acetyl chloride in refluxing THF leads to l,3-dithietane-2,4-diylidene/ /.v(cyanorncthylphos-phonate) in low to fair yields (10-54%, Scheme 6.16). -... 

On the other hand, the promoting effect of potasaum doping in selective oxidation reactions is well known. So, addition to metal oxide catalysts, such as VjOs/TiOj and MoOs/TiOj increases the selectivity to oxidehydrogenation (OXD) of propane [28]. It has been shown that potassium addition brings about a decrease in acidity, lowers sur ce potential (work fimcdon) and hinders the formation of electrophihc O spedes, vsdiich are the responsible for total combustion. Thus, the reduction of total converdon as well as the increase in the selectivity to OXD products when the potassium contort increases could be interpreted as due to the modifications of these properties induced by the presence of potasshim... 

---