Chemistry schemes

Although redox processes are sometimes observed in metathetical reactions with metal halides, the pyramidal dianion [Te(NtBu)3] has a rich coordination chemistry (Scheme 10.8). For example, the reaction... 

The steps involved in automated oligonucleotide synthesis illustrate the current use of protective groups in phosphate chemistry (Scheme 1). Oligonucleotide synthesis involves the protection and deprotection of the 5 -OH, the amino groups on adenine, guanine, and cytosine, and -OH groups on phosphorus. 

A chemistry scheme of work based on the syllabus requirements was developed for the year in collaboration with all teachers who would be teaching the subject. The content to be covered in each of the topics was detailed on a weekly basis. Each of the topics had to be covered within a stipulated time frame followed by a cotmnon topic test that was to be administered on a particular week. In general, students were only required to learn and remember the chemical equations as well as the ionic equations, where relevant, with no specific emphasis on understanding the triplet relationship. 

Works developed by Navarro and coworkers for the palladium and stabilized ylides complexes have been extended to the platinum chemistry (Scheme 25) [97,98]. 

Whereas the original Moffat-Pfitzner oxidation employs dicyclohexylcarbodiimide to convert DMSO into the reactive intermediate DMSO species 1297, which oxidizes primary or secondary alcohols via 1298 and 1299 to the carbonyl compounds and dicyclohexylurea [78-80], subsequent versions of the Moffat-Pfitzner oxidation used other reagents such as S03/pyridine [80a, 83] or oxalyl chloride [81-83] to avoid the formation of dicyclohexylurea, which is often difficult to remove. The so-called Swern oxidation, a version of the Moffat-Pfitzner oxidation employing DMSO/oxalyl chloride at -60°C in CH2CI2 and generating Me2SCl2 1277 with formation of CO/CO2, has become a standard reaction in preparative organic chemistry (Scheme 8.31). 

With some transition-metal complexes, the ligand is not only an ancillary ligand. Similar to the transition-metal, it takes directly part in the hydrogen transfer process. Such ligand-metal bifunctional hydrogenation catalysis is dramatically changing the face of reduction chemistry (Scheme 9) (for reviews of ligand-metal bifunctional catalysis, see [32, 37 0]). 

There were two potential starting materials for Trost s Mo chemistry (Scheme 2.7). The first approach utilized the commercially available 3-fluorocinnamic acid (4). However, reduction of 4 did not proceed well with various reducing agents and provided the desired allylic alcohol 28 in only mediocre yield. Alternatively, 3-fluorobenzaldehyde (29) was used as the starting material. Vinyl Grignard addi-... 

Diaza compound 669 condensed with bromobutyryl chloride under classical basic conditions gives bicyclic derivative 670. This compound can be further functionalized, for example, via classical enolate chemistry (Scheme 110)... 

Carbenes are defined as molecular species with formally divalent and two-coordinate carbon atoms bearing various substituents X and Y and a lone pair of electrons. While the simple representatives are of low stability (such as CH2) and may only appear as short-lived reaction intermediates or in adducts with electron donors, some cyclic systems can be readily isolated. This is particularly true for many of the A-heterocyclic carbenes (NHCs), which are now widely applied as ligands to metals ( Wanzlick-Arduengo carbenes ). Such carbenes based on imidazol and benzimidazol have become the working horses in this branch of organogold chemistry (Scheme 54). 

In 2001, Sarko and coworkers disclosed the synthesis of an 800-membered solution-phase library of substituted prolines based on multicomponent chemistry (Scheme 6.187) [349]. The process involved microwave irradiation of an a-amino ester with 1.1 equivalents of an aldehyde in 1,2-dichloroethane or N,N-dimethyl-formamide at 180 °C for 2 min. After cooling, 0.8 equivalents of a maleimide dipo-larophile was added to the solution of the imine, and the mixture was subjected to microwave irradiation at 180 °C for a further 5 min. This produced the desired products in good yields and purities, as determined by HPLC, after scavenging excess aldehyde with polymer-supported sulfonylhydrazide resin. Analysis of each compound by LC-MS verified its purity and identity, thus indicating that a high quality library had been produced. 

A number of points are clear. First, in all cases the major expense of laser photons is the hardware, not the energy (even at Austin prices). Secondly, the intrinsically greater efficiency of the lower-energy lasers, especially the economic attractiveness of the CO2 laser, is evident. One can easily understand why laser chemistry schemes based upon multiphoton infrared absorption attract so much effort. Thirdly, on a per-unit-time basis the ion laser is more than twice as expensive to operate than even the rather exotic excimer laser. This is because of the inherent energetic inefficiency of the rare-gas plasma as a gain medium and because of the extrinsic, and hideous, expense of ion laser plasma tubes (and their poor reliability). 

Several notable interfacial chemistry schemes can strengthen the adhesion between the bodies involved in transfer printing. Figure 13.3 illustrates one such reaction in which silanol groups on the surfaces of adjoined bodies... 

A remarkable series of CH functionalizations has been described whereby the regiochemical outcome of the reaction is determined by the catalyst employed. Directed and nondirected C-H functionalizations on 2-phenylimi-dazole were observed. This orthogonal approach is excellent for introducing diversity and may have applications in library generation in areas including medicinal chemistry (Scheme 18). 

As seen in the vanadium chemistry (Schemes 46 and 47), activation of isocyanides is one of the key mechanisms in organotantalum chemistry. Scheme 55 generalizes the examples where organotantalum chemistry of isocyanides leads to the formation of products. Although these products remain coordinated to tantalum, these reactions provide insight into the potential for applications to organic synthesis. Methyltantalum species allow formation of... 

Denmark pursued intramolecular alkyne hydrosilylation in the context of generating stereodefined vinylsilanes for cross-coupling chemistry (Scheme 21). Cyclic siloxanes from platinum-catalyzed hydrosilylation were used in a coupling reaction, affording good yields with a variety of aryl iodides.84 The three steps are mutually compatible and can be carried out as a one-pot hydro-arylation of propargylic alcohols. The isomeric trans-exo-dig addition was also achieved. Despite the fact that many catalysts for terminal alkyne hydrosilylation react poorly with internal alkynes, the group found that ruthenium(n) chloride arene complexes—which provide complete selectivity for trans-... 

Overman et al. <1996JA9062> and Kibayashi et al. <2002JOC5517> have utilized the same alkynyl intermediate 222 as a substrate in their chemistry (Scheme 47). While the first group uses a nucleophilic attack of the triple bond on the iminium cation 223, the second group performs a regioselective hydrostannylation, followed by Pd-catalyzed carbonylation, to provide the precursor 224 to the cyclization step. 

Open-chain OXT from a ketose. Recently Leconte et al. reported the formation of an open-chain oxazoline-2-thione (OXT) in low yield from D-fructose based on hydroxyketone chemistry (Scheme 30).30... 

Aromaticity as a Cornerstone of Heterocyclic Chemistry Scheme 1. Monocyclic Aromatic Compounds... 

Many of the enone substrates used in polyamino acid-catalysed epoxidation reactions can be made via a simple aldol condensation, which leads directly to the desired enone after in situ dehydration. Enones that cannot be synthesised by the above route may often be synthesised using standard Wittig chemistry, (Scheme 6). The above methods of substrate synthesis provide compounds with a variety of groups R and enabling the incorporation of both aliphatic and aromatic moieties into the enone structure. 

The asymmetric center in fragment 3 was efficiently installed using Kiyooka s chiral Lewis acid promoted Mukaiyama aldol chemistry (Scheme 2.86). Commer-... 

For the synthesis of alkenes, the Wittig and Horner-Wadsworth-Emmons reactions have become important tools. Triphenylphosphine that is used in the Wittig reaction can be immobilized either on the polymer or can be used in solution for solid-phase chemistry (Scheme 3.18). The Horner-Wadsworth-Emmons reaction for example is used for the synthesis of aldehyde building blocks [261]. 

In 1918, a decade after he successfully established the correct structure of quinine, Rabe, in collaboration with Kindler, reported a three-step conversion of d -quinotoxine into quinine, reversing the Pasteur chemistry (Scheme 2). The lack of experimental details in this publication would become a major issue in the StorkAVoodward controversy almost a century later. Nevertheless, Rabe and Kindler would reinvestigate a sample left over from their 1918 experiments several years later, and would identify and isolate quinine (again) together with quiitidine, epiquinidine and epiquinine. ... 

In the case of ethane, this mechanism cannot occur since the resulting metal-ethyl intermediate does not display any alkyl group in the P-position. Consequently, with tantalum hydride(s), 3, which cleave ethane, another process must take place, involving only one carbon atom at a time. Among various reasonable possibilities, we assume a carbene deinsertion from a tantalum-ethyl species because the reverse step is known in organometallic chemistry (Scheme 3.4) [22]. Note that this reverse step has been postulated as the key step in Fischer-Tropsch synthesis [23]. 

Similar chemistry been used by Faigl and Schlosser in an elegant and simple synthesis of ibuprofen 632 using only superbase chemistry (Scheme 245). Starting with para-xylene 630, two successive metallations and alkylations give 631, which is once more metallated at the less hindered benzylic site and carbonated to give ibuprofen 632. 

The lithiation of y-chloro acetal 175 with lithium and a catalytic amount of naphthalene (4%) allowed the preparation of the intermediate 176, which can be considered as a masked lithium homoenolate, and was used for the preparation of the hydroxy ketone 179 through the hydroxy acetal 177 and dithiane 178 using known chemistry (Scheme 62)" . 

In the following the importance of differentiation in local symmetry of phosphorus li nds will be demonstrated by an example out of phosphorus organic chemistry and complex chemistry (Scheme 2.4-1). Typical of both systems is that C3-symmetry reduction is twofold, leading to local different symmetry. The importance of this twofold differentiation for the orientation in space (e.g. optical induction) should be considered. 

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