Reaction modes

Consider now the reaction between butadiene and ethylene, where both 2-1-2 and 4-1-2 reaction modes are possible. The qualitative appearances of the butadiene HOMO and ethylene LUMO are given in Figure 15.4. The MO coefficients are given as a, b and c, where a > b > c. 

The ring closure of a diene to a cyclobutene can occur with rotation of the two termini in the same conrotatory) or opposite disrotatory) directions. For suitable substituted compounds, these two reaction modes lead to products with different stereochemistry. 

Obviously, the ionic liquid s ability to dissolve the ionic catalyst complex, in combination with low solvent nucleophilicity, opens up the possibility for biphasic processing. Furthermore it was found that the biphasic reaction mode in this specific reaction resulted in improved catalytic activity and selectivity and in enhanced catalyst lifetime. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

In 1965, Breslow and Chipman discovered that zinc or nickel ion complexes of (E)-2-pyridinecarbaldehyde oxime (5) are remarkably active catalyst for the hydrolysis of 8-acetoxyquinoline 5-sulfonate l2). Some years later, Sigman and Jorgensen showed that the zinc ion complex of N-(2-hydroxyethyl)ethylenediamine (3) is very active in the transesterification from p-nitrophenyl picolinate (7)13). In the latter case, noteworthy is a change of the reaction mode at the aminolysis in the absence of zinc ion to the alcoholysis in the presence of zinc ion. Thus, the zinc ion in the complex greatly enhances the nucleophilic activity of the hydroxy group of 3. In search for more powerful complexes for the release of p-nitrophenol from 7, we examined the activities of the metal ion complexes of ligand 2-72 14,15). 

Substrate Alkylating Agent Products Reaction Mode Isomers Ratio Yield (%) Ref... 

This reaction mode of alkynylcarbene complexes of type 23 undoubtedly provides the most convenient access to /J-amino-substituted a,/J-unsaturated Fischer carbene complexes 27 (X=NH2, NHR2, NR2). Fischer et al. reported the very first such addition of an amine to an alkynylcarbene complex of type 23 and observed a temperature-dependent competition between 1,4- and 1,2-addition [12]. In a later systematic study, de Meijere et al. found that in addition to the 1,4-addition products 30,1,2-addition-elimination (formal substitution)... 

Scheme 17 HOMO amplitude controls the selectivities of reaction modes...

In the photooxygenation of electron-rich olefins with allylic hydrogen atoms, ene reactivity usually dominates [96]. Nevertheless, other reactions become the preferred reaction mode. Inagaki et al. [92] attributed the exclusive [2h-2] cycloaddition... 

Suspension Polymerization. Water is the suspending phase. Inorganic salts and vigorous agitation prevent coalescence and agglomeration. The reaction mode is batch. The largest use of suspension polymerization is for the manufacture of expandable polystyrene beads. 

To this point the complexes considered have shared the coordination number six and approximate octahedral geometry. It has been argued that they also share the dissociative reaction mode. There are examples of reactions both with and without intermediates of reduced (that is, 5) coordination, but the insensitivity to entering ligands is a consistent feature. It will be interesting, shortly, to see if the dissociative pattern persists in more or less organometallic octahedral systems but first we shall give some attention to the non-labile square planar systems. 

Enzymes are proteins catalyzing all in vivo biological reactions. Enzymatic catalysis can also be utilized for in vitro reactions of not only natural substrates but some unnatural ones. Typical characteristics of enzyme catalysis are high catalytic activity, large rate acceleration of reactions under mild reaction conditions, high selectivities of substrates and reaction modes, and no formation of byproducts, in comparison with those of chemical catalysts. In the field of organic synthetic chemistry, enzymes have been powerful catalysts for stereo- and regioselective reactions to produce useful intermediates and end-products such as medicines and liquid crystals. ... 

P 40] A similar protocol was made for the 2x2 multi-reaction mode. Using a special script to guide flow over 700 or 1000 s, depending on the individual reaction, a residence time of 120 s was achieved for each reaction. The other features of the protocol are identical with ]P 39]. 

Figure 7.2 Variation of environmental impact potentials as a consequence of the change from macro-scaled batch (0%) to micro-reaction mode Vbatch = 60%, Vdonti = 88% four scenarios regarding the lifetime of the micro-structured devices (Conti wc 1 week, Conti Set 3 months, Conti Sc2 3 years, Conti Sc3 10 years).

This electrophilic substitution reaction is the most common reaction mode for aromatic compounds. Carbon-carbon bond formation via... 

So far as actual changes of mechanistic pathway with change of solvent are concerned, increase in solvent polarity and ion-solvating ability may (but not necessarily will) change the reaction mode from SN2— SN1. Transfer from hydroxylic to polar, non-protic solvents (e.g. DMSO) can, and often do, change the reaction mode from SN1 — Sn2 by enormously increasing the effectiveness of the nucleophile in the system. 

Diels-Alder reaction of the same or different partners via the transition states het-ero-(7-148)2 and homo-(7-148)2 furnished epoxyquinol A (7-149) and B (7-150) in 40% and 25 % yields, respectively. Remarkably, the reaction proceeds only via these two transition states, although there are 16 possible reaction modes. 

This reaction mode sheds new light on the mechanism of the Si-H activation reaction of the 16e complex [(7j5-H3CC5H4)Mn(CO)2 x THF] 6. 

Whereas pyrrole was reported not to give N/H insertion by ketocarbenoids, such a reaction mode does occur with imidazole Copper-catalyzed decomposition of ethyl diazoacetate at 80 °C in the presence of imidazole gives ethyl imidazol- 1-ylacetate exclusively (93 %) small amounts of a C-alkylated imidazole were obtained additionally under purely thermal conditions 244). N/H insertion also takes place at benzimidazole 245 a). The reaction is thought to begin with formation of an N3-ylide, followed by N1 - C proton transfer leading to the formal N/H insertion product. Diazomalonic raters behave analogously however, they suffer complete or partial dealkoxycarbonylation under the reaction conditions 244) (Scheme 34). N-alkylation of imidazole and benzimidazole by the carbenoids derived from co-diazoacetophenone and 2-(diazoacetyl)naphthalene has also been reported 245 b>. 

Efforts to realize an intramolecular version of the above reactions met with limited success when monocyclic 4-thio-substituted (3-lactams were used. Cu(acac)2-catalyzed decomposition of diazoketone 358 produced the epimeric carbapenams 359 a, b together with the oxapenam derivative 360 341 these compounds correspond to the C4/S insertion products obtained in intermolecular reactions. Oxapenams were obtained exclusively when the acrylate residue in 359 was replaced by an aryl or heteroaryl substituent 275 342). The different reaction mode of diazoketones 290a, b, which furnish mainly or exclusively carbonyl ylide rather than sulfur ylide derived products, has already been mentioned (Sect. 5.2). 

Hydride and 1,2-alkyl shifts represent the most common rearrangement reactions of carbenes and carbenoids. They may be of minor importance compared to inter-molecular or other intramolecular processes, but may also become the preferred reaction modes. Some recent examples for the latter situation are collected in Table 23 (Entries 1-10, 15 1,2-hydride shifts Entries 11-15 1,2-alkyl shifts). Particularly noteworthy is the synthesis of thiepins and oxepins (Entry 11) utilizing such rearrangements, as well as the transformations a-diazo-p-hydroxyester - P-ketoester (Entries 6, 7) and a-diazo-p-hydroxyketone -> P-diketone (Entry 8) which all occur under very mild conditions and generally in high yield. 

Iodorhodium(III) porphyrins generally lead to alkylrhodium(III) porphyrins (Scheme 42)398>. This is also true for the reaction with ethyl diazoacetate in the presence of HOAc or an alcohol, and the insertion product 412 (M = Rh) could not be detected, in contrast to the corresponding cobalt porphyrin. A mechanistic scheme, which includes the diverse reaction modes of metalloporphyrins towards diazo compounds, has been proposed by Callot 393,398). 

An alternative reaction mode may be encountered in which the primary cation radical picks up a nucleophile but then loses an electron and a proton to regain the aromatic furan state. Such processes simulate substitution... 

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