REDUCTIVE COUPLING table

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

A variety of such ternary catalytic systems has been developed for diastereoselective carbon-carbon bond formations (Table). A Cp-substituted vanadium catalyst is superior to the unsubstituted one,3 whereas a reduced species generated from VOCl3 and a co-reductant is an excellent catalyst for the reductive coupling of aromatic aldehydes.4 A trinuclear complex derived from Cp2TiCl2 and MgBr2 is similarly effective for /-selective pinacol coupling.5 The observed /-selectivity may be explained by minimization of steric effects through anti-orientation of the bulky substituents in the intermediate. 

Et2Zn also participates in the reductive coupling as a formal hydride source. Results for the Ni-catalyzed, Et2Zn-promoted homoallylation of carbonyl compounds with isoprene are summarized in Table 7 [30]. Et2Zn is so reactive that for the reaction with reactive aromatic aldehydes it causes direct ethylation of aldehydes, and the yields of homoallylation are diminished (runs 1 and 2). Unsaturated aldehydes seem to be subject to the Michael addition of Et2Zn. Accordingly, for the reaction with cinnamaldehyde, none of the expected homoallylation product is produced instead, the 1,4-addition product of Et2Zn, 3-phenylpentanal is produced exclusively (run 3). 

Another advantage of the Et2Zn-Ni catalysis over the Et3B-Ni catalysis is that only the former can promote the reductive coupling of 1,3-cyclohexadiene with aldehydes (c.f., run 8, Table 3). For example, under the Et2Zn-Ni catalysis, 1,3-cyclohexadiene smoothly reacts with benzaldehyde at room temperature and provides 53 in 61% isolated yield (Scheme 13). Curiously, however, the product 53 is not the expected homoallylation product, but an allylation product. 

Table III. Reductive Coupling of Carbonyls with Zinc and Trimethylchlorosilane...

TABLE 22. Polyenes through reductive coupling with zinc... 

Reductive Cross-Coupling of Nitrones Recently, reductive coupling of nitrones with various cyclic and acyclic ketones has been carried out electrochem-ically with a tin electrode in 2-propanol (527-529). The reaction mechanism is supposed to include the initial formation of a ketyl radical anion (294), resulting from a single electron transfer (SET) process, with its successive addition to the C=N nitrone bond (Scheme 2.112) (Table 2.9). 

Table 22.5 Catalytic reductive coupling of 1,3-cyclohexadiene with alkyl, aryl and heteroaryl a-ketoaldehydes.

Table 22.8 Regioselective reductive coupling of non-symmetric 1,3-diynes to various a-ketoaldehydes.

Table 22.9 Reductive coupling of 1,3-enynes with ethyl (N-tert-butanesulfinyl)iminoacetate.

The reduction potentials of some redox couples are given in Table 7.1. The potentials listed are written as if they are for reduction couples, with any reactions written as oxidation couples with potential values of the same magnitude but opposite sign. 

Table 22 Cathodic perturbations of first ligand centred reduction couple observed on addition of various anions."...

TABLE 5.8. Corey s reductive coupling of carbonyl compounds... 

Only one structure of Cyg has a closed-shell configuration. It possesses Dy symmetry and is therefore chiral [48]. Its two enantiomers, C76 and ent-Cyg are normally isolated as a 1 1 racemic mixture. Four reversible reduction couples corresponding to 0/—1, —lj—2, —21—2), and - ij-A can be observed by CV in PhCN, DCM, and ODCB (see Table 5) [37, 59]. Six evenly spaced waves have been detected in a 4 1 (v/v) PhMe/MeCN mixture at — 15°C, but the sixth reduction is observed at the limit of the solvent potential window and appears to be irreversible... 

A The HiO MT) couple has a high positive reduction potential ( Table 6.I4) and because the O ICO potential is considerably smaller, hydrogen peroxide is unstable with respect to disproportionation ... 

All C60 adducts have low-lying LUMOs that can easily be populated by electrochemical methods. For C60 itself, six reduction couples have been observed by cyclic voltammetry (CV) or square-wave voltammetry (SWV), and as many as four reduction couples have been found for many organometallics (9,84). Most of the studies have been performed in thf or acetonitrile at lower temperatures, which increases the size of the potential window. Table VII lists the half-wave potentials for some metal complexes, and Fig. 7 shows the cyclic voltammogram for [Co(NO)(PPh3)2(i72-C60)]. 

The first tantalum nitrene was obtained in 1959 by thermolysis of [Ta(NEt2)]5-288 This class of compounds is presently accessible by several routes, including hydrogen abstraction from the mono- or di-alkylamides, reaction of metallacarbenes with organic imines, oxidation of low valent species by organic azides, or reductive coupling of nitriles (Table 13). The tantalum derivatives are usually stabler than those of niobium. 

Continuation of the head-to-tail addition of five-carbon units to geranyl (or neryl) pyrophosphate can proceed in the same way to farnesyl pyrophosphate and so to gutta-percha (or natural rubber). At some stage, a new process must be involved because, although many isoprenoid compounds are head-to-tail type polymers of isoprene, others, such as squalene, lycopene, and /3- and y-carotene (Table 30-1), are formed differently. Squalene, for example, has a structure formed from head-to-head reductive coupling of two farnesyl pyrophosphates ... 

The numerical value of an electrode potential depends on the nature of the particular chemicals, the temperature, and on the concentrations of the various members of the couple. For the purposes of reference, half-cell potentials are taken at the standard states of all chemicals. Standard state is defined as 1 atm pressure of each gas (the difference between 1 bar and 1 atm is insignificant for the purposes of this chapter), the pure substance of each liquid or solid, and 1 molar concentrations for every nongaseous solute appearing in the balanced half-cell reaction. Reference potentials determined with these parameters are called standard electrode potentials and, since they are represented as reduction reactions (Table 19-1), they are more often than not referred to as standard reduction potentials (E°). E° is also used to represent the standard potential, calculated from the standard reduction potentials, for the whole cell. Some values in Table 19-1 may not be in complete agreement with some sources, but are used for the calculations in this book. 

Sulopenem (CP-70429 see Tables 1 and 7) has been prepared via this reaction as the key step (G=0/C=S reductive coupling). The total synthesis utilizes L-aspartic acid to generate the chiral precursor 78 of the C-2 side chain, a modified chiron 76 (X = C1) to improve the preparation of the trithiocarbonate intermediate 79, a chemoselective oxalofluoride-based azetidinone N-acylation to give 80 (a procedure that avoids sulfoxide O-acylation), and mild final deprotection conditions of hydroxyl and carboxyl functions. In particular, the chloroallyl ester 81 has been selected, owing to its smooth cleavage by a palladium-mediated transesterification procedure (Scheme 42) <1992JOC4352>. 

A list of redox potentials for the first ( E°), second (2E°), and third (3E°) electron reduction couples for different polyimide films is provided in Table I. The potentials were measured by cyclic voltammetry of polyimide films on an electrode surface and represent the average of the cathodic and anodic peak potential for the respective redox couple. 

---