Synchronous substitution

The fundamental processes at atomic sites are junclive (associative, additive or complexative), disjunctive (dissociative, eliminative or decomplexative), or synchronous (substitutive, simultaneously junctive and disjunctive). The concept of junctivity/disjimctivity was discussed in Chapter 8. 

Let us consider briefly the material on synchronous substitution reactions. When an intermediate radical in the stepwise substitution decomposes very rapidly, it is rather difficult to establish the mechanism. 

Oxaziridines substituted in the 2-position with primary or secondary alkyl groups undergo decomposition at room temperature. In the course of some weeks, slow decomposition of undiluted compounds occurs, the pattern of which is analogous to that of acidic or alkaline N—O cleavage (Sections 5.08.3.1.3 and 4), Radical attack on a C—H bond in (109) effects N—O cleavage, probably synchronously (57JA5739). In the example presented here, methyl isobutyl ketone and ammonia were isolated after two hour s heating at 150 °C. 

The synchronous bimolecular mechanism for aromatic nucleophilic substitution involves unfavorable geometry (bonds made and broken are both in the plane of the ring and backside attack is not possible) and unfavorable energetics (one high-energy step is required... 

A simple approach for the formation of 2-substituted 3,4-dihydro-2H-pyrans, which are useful precursors for natural products such as optically active carbohydrates, is the catalytic enantioselective cycloaddition reaction of a,/ -unsaturated carbonyl compounds with electron-rich alkenes. This is an inverse electron-demand cycloaddition reaction which is controlled by a dominant interaction between the LUMO of the 1-oxa-1,3-butadiene and the HOMO of the alkene (Scheme 4.2, right). This is usually a concerted non-synchronous reaction with retention of the configuration of the die-nophile and results in normally high regioselectivity, which in the presence of Lewis acids is improved and, furthermore, also increases the reaction rate. 

Schlenk equilibrium 93 s-cis 7, 9, 26, 31, 35 sdyl-substituted 16 Simmons-Smith reaction 87 SnClj 309 SnCU 309 solid-phase 198 square bipyramidal 255 rr-stacking 8 stannyl-substituted 16 s-trans 7, 26 - acrolein 307 succinimide 227 sulfonamides 122 synchronicity 306... 

The pioneering work of Denney et ai19 on the synthetic utility of oxyphosphoranes has been thoroughly exploited by Evans et al. in demonstrating that diethoxytriphenylphosphorane promotes mild and efficient cyclodehydration of diols (e.g. 11) to cyclic ethers (e.g. 13) via the cyclic phosphorane (12)20>21. Simple 1,2-, 1,4-, and 1,5- diols afford good yields of the cyclic ethers but 1,3-propanediol and 1,6-hexandiol give mainly 3-ethoxy-l-pro-panol and 6-ethoxy-l-hexanol respectively whereas tri- and tetra-substituted 1,2-diols afford the relatively stable 1,3,2- diox-phospholanes. In some instances (e.g. 14), ketones (e.g. 16) are formed by a synchronous 1,2-hydride shift within (15). The synthetic utility has been extended to diethoxyphosphoranes supported on a polystyrene backbone22. 

The generalized Woodward-Hoffmann rule suggests that a synchronous addition of disulfonium dications at the double C=C bond of alkenes would be a thermally forbidden process and so would be hardly probable. Simulation of the frontal attack by ethylene on l,4-dithioniabicyclo[2.2.0]hexane 115 gave no optimal structure of an intermediate complex. On the other hand in the lateral approach of the reactants, orbital factors favor attack of the double bond by one of the sulfonium sulfur atoms of the dication. This pattern corresponds to SN2-like substitution at sulfur atom as depicted in Figure 5. Using such a reactant orientation, the structure of intermediate jc-complex was successfully optimized. The distances between the reaction centers in the complex, that is, between the carbon atoms of the ethylene fragment and the nearest sulfur atom of the dication, are 2.74 and 2.96 A, respectively. 

Another milder example of this is a standard Non-Synchronous Buck switcher IC. Every single typical applications diagram on the datasheet shows a Schottky diode, without perhaps explicitly stating as much. This is an example of an implied expectation on the part of the vendor—that you, the customer, won t miss the truly obvious. Yet there are many who think they have achieved some slender advantage in substituting an ultra-fast diode in its place. First read the Abs Max section of the EC tables carefully. Most vendors specify that the SW node should never be taken more than 0.4V below IC ground. That is because... 

The well-known Diels-Alder reaction [95,104-106] is a standard method for forming substituted cyclohexenes through the thermally allowed 4s + 2s cycloaddition of alkenes and dienes. In particular, the reaction between ethene and 1,3-butadiene to yield cyclohexene is the prototype of a Diels-Alder reaction (Scheme 28.4). It is now well recognized that this reaction takes place via a synchronous and concerted mechanism through an aromatic boatlike TS [105]. 

M. Quack, U. Schmitt, and M. A. Suhm, FTIR spectroscopy of hydrogen fluoride clusters in synchronously pulsed supersonic jets Isotopic isolation, substitution and 3 D condensation. Chem. Phys. Lett. 269, 29 38 (1997). 

The aqua ion Au(H20)4+ has not been characterized either in solution or in the solid state. Most of the substitution studies have involved the halide complexes AuXj and Au(NH3) (Ref. 319). A number of earUer generalizations have been confirmed. Rates are very sensitive to the nature of both entering and leaving ligands and bond formation and breaking are nearly synchronous. The double-humped energy profiles witnessed with Pd(II) and Pt(II) are not invoked the five-coordinate species resulting from an associative mechanism is the transition state ... 

There has been a study of the mechanism of the activation of carboxylic acids to peptide formation by chloro-s -triazines in combination with tertiary amines. The first step, exemplified in Scheme 2 by the reaction of 2-chloro-4,6-disubstituted-l,3,5-triazines (18) with A -methylmorpholine, is formation of a quaternary triazinylammonium salt (20). Here there is NMR evidence for the formation at —50°C of the intermediate (19), showing that the substitution involves the two-step SnAt mechanism rather than a synchronous pathway. The subsequent reaction of (20) with a carboxylic acid yields the 2-acyloxy derivative (21), which carries an excellent leaving group for the amide-forming step. ... 

An extensive review of the hetero-Diels-Alder reactions of 1-oxabuta-1,3-dienes has been published. Ab initio calculations of the Diels-Alder reactions of prop-2-enethial with a number of dienophiles show that the transition states of all the reactions are similar and synchronous.Thio- and seleno-carbonyl compounds behave as superdienophiles in Diels-Alder reactions with cyclic and aryl-, methyl-, or methoxy-substituted open-chain buta-1,3-dienes.The intramolecular hetero-Diels-Alder reactions of 4-benzylidine-3-oxo[l,3]oxathiolan-5-ones (100) produce cycloadducts (101) and (102) in high yield and excellent endo/exo-selectivity (Scheme 39). A density functional theoretical study of the hetero-Diels-Alder reaction between butadiene and acrolein indicates that the endo s-cis is the most stable transition structure in both catalysed and uncatalysed reactions.The formation and use of amino acid-derived chiral acylnitroso hetero-Diels-Alder reactions in organic synthesis has been reviewed. The 4 + 2-cycloadditions of A-acylthioformamides as dienophiles have been reviewed. ... 

The basic classification of nucleophilic substitutions is founded on the consideration that when a new metal complex is formed through the breaking of a coordination bond with the first ligand (or water) and the formation of a new coordination bond with the second ligand, the rupture and formation of the two bonds can occur to a greater or lesser extent in a synchronons manner. When the mpture and the formation of the bonds occur in a synchronous way, the mechanism is called substitution nucleophilic bimolecular (in symbols Sn2). On the other extreme, when the rupture of the first bond precedes the formation of the new one, the mechanism is called substitution nucleophilic unimolecular (in symbols SnI). Mechanisms Sn2 and SnI are only limiting cases, and an entire range of intermediate situations exists. 

The experimentally observed substituent effect on the triplet and singlet quantum yields in the complete series of methyl-substituted dioxetanes, as well as the predicted C—C and 0—0 bond strength for the four-membered peroxidic rings , have led to the hypothesis that a more concerted, almost synchronized, decomposition mechanism should lead to high excitation quantum yields (as in the case of tetramethyl-l,2-dioxetane), whereas the biradical pathway presumably leads to low quantum yields (as in the case of the unsubstituted 1,2-dioxetane)" . However, it appears that this criterion of concertedness is difficult to apply generally to structurally dissimilar dioxetane derivatives. 

Principle of nonpeifect synchronization (PNS), methyl substituted-benzyl carbinyl hypochlorite photolysis, 907 Probucol... 

---