Synthetic results

Removal of tert-AW Groups, Alkyl groups on a phenol nucleus can be removed selectively to produce a desired synthetic result. The oxidative coupling of phenol offers a good example. 2,6-Di-/ f2 -butylphenol can be coupled under oxidative conditions to... 

Before entering the discussion of the synthetic results, a brief description of the structure of Cp2TiCl and the mechanism of epoxide opening is given. These results serve as a guide for choosing a suitable catalyst for a given application. 

The existence of results that cannot be explained solely by the effect of rapid heating has led authors to postulate the existence of a so-called microwave effect . Hence, acceleration or changes in reactivity and selectivity could be explained by a specific radiation effect and not merely by a thermal effect. Several reviews have collected synthetic results that have been attributed to the microwave effect [5 b, 38, 39]. 

A characteristic feature of contemporary investigations in the held under consideration, is the interest in cycloaddition reactions of nitrile oxides with acetylenes in which properties of the C=C bond are modified by complex formation or by an adjacent metal or metalloid atom. The use of such compounds offers promising synthetic results. In particular, unlike the frequently unselec-tive reactions of 1,3-enynes with 1,3-dipoles, nitrile oxides add chemo-, regio-and stereoselectively to the free double bond of (l,3-enyne)Co2(CO)6 complexes to provide 5-alkynyl-2-oxazoline derivatives in moderate to excellent yield. For example, enyne 215 reacts with in situ generated PhCNO to give 80% yield of isoxazoline 216 (372). 

The optically active isoxazolidines obtained in these cycloaddition reactions can be easily transformed into biologically active 3 -amino acids, into j3-lactams and into important chiral building blocks such as y-amino alcohols. The multitude of synthetic results in these reactions is of course expected by the wide variety... 

However, it is necessary to formulate the most common features of the strategy developed by Prof. Denmark and summarize the most important synthetic results. 

The use of group 6 heteroatom stabilized carbene complexes (Fischer carbene complexes) in organic synthesis is relatively recent and, in spite of that, it has already produced impressive synthetic results [1]. These versatile organometallic reagents have an extensive chemistry, and they are probably one of the few systems that undergo cycloadditions of almost any kind. For instance, [1+2], [2+2], [3+2], [3+3], [4+1], [4+2], [4+3], [6+3]... 

In the laboratory experiments of Seyfried et al. (1998), naturally altered sea floor basalt (5 Li = +7.4) was reacted with Li-free alkali-chloride aqueous fluid at 350°C for 890 hours (initial fluid/solid mass ratio 2). Samples of the fluid were taken throughout the experiment, and showed initial rapid influx of isotopically heavy-enriched Li released by early-dissolving alteration minerals. However, with progressive reaction, isotopic composition of the fluid decreased and Li concentration reaehed apparent steady state. Although an equilibrium model applies best to the synthetic results, Rayleigh distillation was considered most likely to apply in hydrothermal reactions occurring in nature. 

It is noteworthy that in the case of methyl sulfones (ArSOgMe), the reaction proceeds with the homologation of the methyl to an -propyl group that is, the methylation is still highly selective toward the substitution of only two of the three methyl protons. Aside from the synthetic results, this observation is relevant from the mechanistic viewpoint, as will be clarified in the discussion of Scheme 4.8. 

Phase transfer catalysis (1,2) has become in recent years a widely used, well-established synthetic technique applied with advantage to a multitude of organic transformations. In addition to a steadily increasing number of reports in the primary literature, there are several reviews (3-6), comprehensive monographs (7-10) and an ACS Audio Course (1 ) which describe the phase transfer process and which provide extensive compilations of phase transfer agents and reaction types. While the list of applications and in many cases the synthetic results are impressive, phase transfer catalysts (PTCs) suffer some of the same disadvantages as more conventional hetero-and homogeneous catalysts — separation and... 

The synthetic results have been summarized In Table I. In most cases the displacement of trlflate anion occurred rapidly ( 30 min) at or below reflux temperature however. In two examples (see Table I, compotinds 4 and 13) an elimination reaction was found to predominate. 

One of the breakthroughs in the field was reported in 1983 by Mimoun and coworkers . On that occasion they reported the synthesis, characterization and radical reactivity of a class of vanadium peroxo complexes representative of which is the species V0(02)pic (HaOIa, 36. The oxidative ability of this complex has been tested with several aliphatic and aromatic hydrocarbons and the synthetic results obtained can be summarized as in Scheme 20. 

Numerous examples of solid/solid/liquid phase transfer catalysis are now known to be useful synthetically but have not been investigated mechanistically. Poly(ethylene glycol) immobilized on alumina and silica gel is active for reaction of solid potassium acetate with 1-bromobutane 184). Some of the best synthetic results with polymer supports are shown in Table 15. Often use of other solid salts or other catalysts gave poorer yields. It would be valuable to know for the design of future syntheses how these reactions depend on the partial solubility of the inorganic salts in the organic solvents and on the presence of trace amounts of water. 

Poly(lignin-g-(l-amidoethylene)). These nonionic molecules are small in size, readily adsorbed on silica surfaces, and prone to complex di- and tri-valent metal ions from aqueous solution (2,3). Synthetic results for several samples of poly(lignin-g-(l-amidoethylene)) are given in Table III. 

The salt crystallizes in violet-red to carmine-red prisms, depending on the crystal size. The solubility at room temperature is one part of the salt in fifteen parts of water. The aqueous solution has a basic reaction to litmus. Dilute acids form the diaquotetrammine salt, whereas concentrated acids form the diacido- or monoacidoaquotetram-mine complex. Treatment of the salt with dilute acid, then with an excess of hot dilute ammonia, forms the aquopen-tamminecobalt(III) series of salts. The salt is stable indefinitely, samples 40 years old having yielded satisfactory synthetic results. 

Since the absolute configuration of the angular methyl group is retained throughout the reactions discussed above, these results lead to the experimental and unambiguous determination that the absolute stereochemistry of (+)-halenaquinol and (+)-halenaquinone is 12bS. In addition, these synthetic results also proved that the absolute configurations of halenaquinol compounds theoretically determined were actually correct. 

A transformation will be regarded as a thermolysis if the halocyclopropane is heated either neat, in the gas phase or in an inert solvent. This solvent should neither serve as a nucleophile HY, nor have a high dielectric constant (to favor cation formation). If the solvent does have one or both of the aforementioned properties, the transformation will be regarded as a solvolysis. Although all these methods in principle obey the same general rules vide supra), their synthetic results are clearly distinct. Conditions in thermolysis are harsh. Since there is no other nucleophile present, the allylic cation can only be trapped by the halide ion that has left the cyclopropane. At the elevated temperature of thermolysis, the resulting allylic halides... 

Solvolyses in the absence or presence of an assisting base can lead to different synthetic results. Secondary acid-catalyzed rearrangements of kinetically preferred secondary or tertiary allylic alcohols or acetates to thermodynamically preferred primary allylic alcohols or acetates have been mentioned (vide supra). An assisting base binds the evolved acid HX. However, it may favor the formation of 1,3-dienes via elimination of HY. 

This paper describes the synthetic results and metal coordination properties of several new selective chelating polymers, linked via methyleneamino bridges to the macromolecu-lar network (partial results were previously presented at several conferences [9-11]). 

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