C-Alkylation reductive

Note The C-alkylation of quinoxaline has been done by addition (with or without subsequent aromatization see also preceding subsection), by reductive alkylation, or by homolytic alkylation. 

During the course of the development of our group s alkylation/reductive decyanation strategy, a very reliable method for distinguishing between syn-and anfz-l,3-diols was discovered [17,18]. The acetonide methyl groups reliably display diagnostic C-NMR chemical shifts, allowing for stereochemistry to be determined simply by inspection (Fig. 1). Evans later extended the C-NMR chemical correlation to polypropionate chains [19,20]. 

Some sugar residues in bacterial polysaccharides are etherified with lactic acid. The biosynthesis of these involves C)-alkylation, by reaction with enol-pyruvate phosphate, to an enol ether (34) of pyruvic acid, followed by reduction to the (R) or (5) form of the lactic acid ether (35). The enol ether may also react in a different manner, giving a cyclic acetal (36) of pyruvic acid. 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

Other liquid-ligand two-phase reactions mediated by polyethers include anion promoted C-alkylations, oxidations, and (borohydride) reductions. In such cases, the organic substrate and a catalytic amount of polyether in an organic phase are shaken with a saturated aqueous solution of the required anionic reagent. 

Thus to date, virtually all studies of C-C reductive elimination to form alkanes from Pt(IV) have found that these reactions proceed via five-coordinate intermediates. Only very recently have stable examples of Pt(IV) alkyl hydrides been synthesized (53-69). Detailed studies of C-H reductive elimination to form alkanes from these related complexes have identified similar five-coordinate intermediates on the reaction pathway (see following section). 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

Subsequent reduction of the alkylated cyanomethylphosphonates produces p-aminoethylphosphonates. Diethyl aminomethylphosphonates are C-alkylated with a range of agents via the initial formation of the imines [73],... 

Of great synthetic potential are demetallations with simultaneous formation of C-Sn bonds (Figure 2.20) [137,146,230,281,282]. These reactions presumably proceed via a heterobimetallic intermediate containing an Sn-M-C group. Reductive elimination of the metal M from this intermediate leads to formation of the carbon-tin bond. The resulting alkyl- or vinylstannanes are valuable synthetic intermediates. 

Keywords Alcohols Alkenes Asymmetric transfer hydrogenation C-alkylation Imines Ketones W-aUcylation Oxidation Reduction Transfer hydrogenation... 

In closely related experiments it was shown that sp C—H activation takes place reversibly within the coordinahon sphere of the electron-rich Ir(I)-diphosphine complex 58 (Scheme 6.9) to form an alkyl-amino-hydrido derivative 57 reminiscent of the CCM intermediate 24 the solid-state structure of 57 is shown in Figure 6.13 [40]. It appears that C—H activation only takes place after coordination of the amine function to the Ir(I) center (complex 58, NMR characterized). Amine coordination allows to break the chloro bridge of 59 and to augment the electron density of the metal center, thus favoring oxidative addihon of the C—H bond. Most importantly, the microscopic reverse of this C—H activation process (i.e. C—H reductive elimination) models the final step of the CCM cycle (see Scheme 6.1) indeed, the reaction of Scheme 6.10 is cleanly reversible at 373 K. 

Problem 18.8 Prepare ethylamine by (a) Gabriel synthesis, (6) alkyl halide amination, (c) nitrile reduction, (d) reductive amination, (e) Hofmann degradation. ... 

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