Synchronous bond formation

For reactions of A-acyliminium ions with alkenes and alkynes one has to distinguish between A-acyliminium ions locked in an s-trans conformation and those which (can) adopt an s-cis conformation. The former type reacts as a (nitrogen stabilized) carbocation with a C —C multiple bond. Although there are some exceptions, the intramolecular reaction of this type is regarded as an anti addition to the 7t-nucleophile, with (nearly) synchronous bond formation, the conformation of the transition state determining the product configuration. 

Cis-addition is the normal result of a synchronous bond formation in a n4s + n2s cycloaddition and needs no further comments. 

Similarly small rate factors were obtained for 1,3-dipolar cycloadditions between diphenyl diazomethane and dimethyl fumarate [131], 2,4,6-trimethylbenzenecarbonitrile oxide and tetracyanoethene or acrylonitrile [811], phenyl azide and enamines [133], diazomethane and aromatic anils [134], azomethine imines and dimethyl acetylenedi-carboxylate [134a], diazo dimethyl malonate and diethylaminopropyne [544] or N-(l-cyclohexenyl)pyrrolidine [545], and A-methyl-C-phenylnitrone and thioketones [812]. Huisgen has written comprehensive reviews on solvent polarity and rates of 1,3-dipolar cycloaddition reactions [541, 542]. The observed small solvent effects can be easily explained by the fact that the concerted, but non-synchronous, bond formation in the activated complex may lead to the destruction or creation of partial charges, connected... 

In contrast to strongly solvent-dependent [2- -2]cycloaddition reactions, which proceed through a 1,4-dipolar zwitterionic intermediate by a two-step mechanism or through a dipolar activated complex by a one-step mechanism cf. Section 5.3.2, and Eqs. (5-33) to (5-35) [92, 94-107], [2- -2]cycloadditions are also known that exhibit concerted, nearly synchronous bond formation without significant charge separation on activation in the transition state. An example is given in Eq. (5-47). Since the rate constant for this diphenylketene/styrene addition is practically independent of solvent polarity [140], it can be classed as concerted. 

Nucleophilic substitution (5 ) reactions of saturated aliphatic compounds may be either associative or dissociative and the majority lie between the limits set by iSnI reactions, in which the rate-determining step is heterolysis of the bond to the leaving group, and typical 5 2 reactions with fully synchronous bond-formation and bond-rupture. Nl-like reactions represent an intermediate case and are characterized by a greater extent of bond-rupture than bond-formation. Hence, in aliphatic 5 reactions the rate-limiting process involves some degree of prior or concurrent bond-rupture. 

There have been several computational studies of the peroxy acid-alkene reaction. The proposed spiro TS has been supported in these studies for alkenes that do not present insurmountable steric barriers. The spiro TS has been found for ethene (B3LYP/6-31G ), propene and 2-methylpropene (QCISD/6-31G ), and 2,3-dimethylbutene and norbornene (B3LYP/6-311- -G(c , These computational studies also correctly predict the effect of substiments on the and account for these effects in terms of less synchronous bond formation. This is illustrated by the calculated geometries and a(B3LYP/6-31G ) of the TS for ethene, propene, methoxyethene, 1,3-butadiene, and cyanoethene, as shown in Eigure 5.3. Note that the TSs become somewhat unsymmetrical with ERG substituents, as in propene, methoxyethene, and butadiene. The TS for acrylonitrile with an EWG substituent is even more unsymmetrical and has a considerably shorter C(3)- O bond, which reflects the electronic influence of the cyano group. In this asynchronous TS, the nucleophilic character of the peroxidic oxygen toward the (3-carbon is important. Note also that the is increased considerably by the EWG. 

Cycloaddition reactions of heterocumulenes have been much used in the synthesis of / -sultams. The two general methods, emphasizing different synchronous bond formations, are outlined in equations 198 and 199. 

Mechanism (24) shows significant formal analogies to the 8 2-mechanism in electrophilic aromatic substitution. It has sometimes been denoted as 2-mechanism. This nomenclature is, however, misleading as S 2 was originally introduced by Ingold for that type of aliphatic nucleophilic substitution in which synchronous bond-formation and bond-breaking occur in a single step. 

Concerted but non-synchronous bond formation, Alder ene reaction 61 —, Diels-Alder reaction 1, 48 Concertedness of Alder ene reaction 61... 

Whether the formation of alkene 3 proceeds directly from alkoxide 4 or via a penta-coordinated silicon-species 6, is not rigorously known. In certain cases—e.g. for /3-hydroxydisilanes (R = SiMes) that were investigated by Hrudlik et al —the experimental findings suggest that formation of the carbon-carbon bond is synchronous to formation of the silicon-oxygen bond ... 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

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 ... 

The evidence for perfect synchronization between bond cleavage, bond formation and positive charge delocalization was obtained for the proton transfer from hydronium ion to substituted a-methoxystyrenes ArC(OMe)=CH264. 

Structurally related dienols and acyclic trienols, when reacted in fluorosulfuric acid, give tricyclic ether derivatives in kinetically controlled cyclization.810,811 The stereospecific product formation is rationalized by synchronous internal anti-addition via chair-like conformations of the protonated cyclohexene ring, resulting in ring closure with equatorial C-C bond formation and concomitant internal nucleophilic termination by anti-addition of the OH group [Eq. (5.294)]. Z/E isomerization may be competitive with cyclization. 

Competitive experiments with 2H-, 13C- and 180-labelled chorismate derivatives and three different chorismate mutase enzymes have shown that in all catalysed and non-catalysed Claisen rearrangements a non-synchronous, concerted, pericyclic transition state is involved, with C-O bond cleavage considerably in advance of C-C bond formation. Some evidence has suggested that the ionic active site of the enzymes may polarize the transition state more than occurs in solution. Similar findings apply to the retro-ene fragmentation of chorismate to 4-hydroxybenzoate.17... 

Electrolysis of 1,2-dibromofumaric and 1,2-dibromomaleic acid or the diesters produces a 90-100% yield of acetylenedicarboxylic acid or diester 302). For vie-dihalides with no radical or anion stabilizing group in the a position an E2B like elimination mechanism is strongly indicated, /.e., 2e-transfer and double bond formation in a synchronous process (Eq. (148)). 

The stereospecific cis-addition of diboron tetrachloride to alkynes and alkenes (37) may be interpreted as an interaction of the empty 7r-orbitals of the boron atoms with the 7r-orbital of the organic species. According to this picture, boron-boron bond breaking would lag behind boron-carbon bond formation. The transition state is a 4 + 2 Hiickel aromatic ( .=0), and thermal addition is allowed. If bond making and breaking were synchronous, this four-center reaction would be more like the <7-77 exchange reactions, which we shall discuss later. With regard to (37), there is a discrepant case in which an apparent trans addition of diboron tetrachloride to cyclopentadiene has been found (Saha et al., 1967). 

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