Nonsynchronous reaction

Cycloaddition reactions have been carried out with corresponding bases of Vilsmeier salts of pyrroles. Thus reaction of 153 with dimethyl acetylenedicar-boxylate gave pyrrolizines 154 and 3a-azaazulenes 155 in a nonsynchronous reaction. The structure of 3H-pyrrolizine 154a was proved by X-ray analysis.96 In a closely related reaction, pyrrolizinones 156 and 157 were obtained from the corresponding Vilsmeier salts with phenyl isocyanate.97 Reaction of 2-p-nitrobenzoylindolin-3-one (158) with methyl acetoacetate gave the benzopyrrolizinone (159).5 7... 

Calculations were performed with few of these systems, either for their formation or properties. Thus, PM3 calculations of the cycloaddition of pyridinc-A -oxide with an isocyanate were performed, with R = H for simplification. Two consecutive transition states were postulated, pointing out a nonsynchronous process for this reaction, via a... 

Essentially two mechanisms have been offered for polar cycloaddition. The first of these is a nonsynchronous or two-step mechanism which involves attack of the electrophilic cation on the nucleophile to produce a new cation, which undergoes cyclization by attack on suitably located TT-electrons or an unshared electron pair. This mechanism has been advocated at some time by seemingly all the major workers in the area with the exception of Fields et al. The alternative view, now held by Schmidt, is that the reaction is synchronous. In this section, an effort will be made to reconcile each of the two theories to some observations which have been made concerning polar cycloaddition. 

In spite of the known tendency of norbornene and related systems to undergo rearrangements of the Wagner-Meerwein type during eleetro-philic addition, no such rearrangement was observed when norbornene underwent cycloaddition with the acridizinium or the. A/ -methylenium benzamide cation. As Schmidt correctly pointed out, this lack of rearrangement is an argument for a concerted reaction. Alternatively, if the cycloaddition is nonsynchronous, the time interval between step 1 and step 2 must be very short. 

Theoretical work on the gas-phase hetero-Diels-Alder reaction of A -sulfinyl dienophiles was used to study both endo- and o-modes of cycloaddition for both (E)-29 and (Z)-30 dienophiles at the B3LYP/6-31G level (Scheme 2) <2000JOC3997>. In summary, these calculations have predicted that (1) the A -sulfinyl dienophiles prefer the (Z)-30 orientation over (E)-29 stereochemistry by 5-7 kcalmoP, (2) the transition state is concerted but nonsynchronous, and (3) an lYo-transition state with diene 31 is favored over the fvo-approach both kinetically and thermodynamically. 

On addition the stereochemistry at the C=C bond of the vinyl ether remains constant. This was established by labeling studies and by the use of cis and trans ethyl propenyl ether. From the similarity of the activation parameters, the reaction characteristics, and the stereoselectivity of the cycloaddition step to those of the reactions of the same complexes with 1,3-dienes or ynamines an associative concerted nonsynchronous process was deduced.224... 

Cycloaddition reactions of thioketene and isocyanic acid forming four-membered ring products have been studied by means of the ab initio RHF/6-31G method. The two alternative reactions are concerted but nonsynchronous, taking place through twisted or planar four-membered cyclic transition states. The activation barriers were calculated to be... 

Since the disclosures that the thermal dimerizations of acrolein and methyl vinyl ketone provide the 3,4-dihydro-2//-pyrans (1, 2) derived from 4ir and 2Tt participation of the a,3-unsaturated carbonyl compound in a Diels-Alder reaction, an extensive series of related observations have been detailed. This work has been the subject of several comprehensive reviews - - including the Desimoni and Tacco-ni extensive tabular compilation of work through 1974. Consequently, the prior reviews should be consulted for thorough treatments of the mechanism, scope, and applications of the [4 + 2] cycloaddition reactions of a,3-unsaturated carbonyl compounds. The [4 + 2] cycloaddition reactions of 1-oxa-1,3-butadienes with their 4-it participation in the Diels-Alder reaction exhibit predictable regioselectivity with the preferential or exclusive formation of 2-substituted 3,4-dihydro-2W-pyrans (equation 1). The exceptions to the predicted regioselectivity that have been observed involve the poorly matched [4 + 2] cycloaddition reaction of an electron-deficient l-oxa-l,3-butadiene with an electron-deficient dienophile, e.g. methyl crotonate or methacrolein. - Rigorous or simplified theoretical treatments of the [4 + 2] cycloaddition reaction of 1-oxa-1,3-butadienes predict the preferential formation of 2-substituted 3,4-dihy-dro-2f/-pyrans and accommodate the preferred endo approach of the reactants in which the carbon-carbon bond formation is more advanced than carbon-oxygen bond formation, i.e. a concerted but nonsynchronous [4 + 2] cycloaddition reaction. ... 

The discovery of the [47t -i- 2tu] cycloaddition reaction by Otto Diels (Nobel Prize, 1950) and Kurt Alder (Nobel Prize, 1950), a landmark in synthetic organic chemistry, permits the regio- and stereoselective preparation of both carbocyclic and heterocyclic ring systems. Its application can result simultaneously in an increase of (1) the number of rings, (2) the number of asymmetric centers, and (3) the number of functional groups. The reaction controls the relative stereochemistry at four contiguous centers. The Diels-Alder reaction can be depicted as a concerted -1- (suprafacial) cycloaddition. While depicted as a concerted process, the reaction has been proposed to proceed in a nonsynchronous manner via an unsymmetrical transition state. °... 

The elimination of acetic acid from the initially formed acylsulfonium salt in the reaction of sulfoxides (87) and (90a,b Scheme 19) is considered to involve a concerted nonsynchronous 2-type mechanism, rather than the stepwise process depicted in equations (22) and (23). - In the latter study, retention of... 

Similarly to the mechanism of the Claisen rearrangement, the Overman rearrangement is a suprafacial, concerted, nonsynchronous / 3,37-sigmatropic rearrangement. The reaction is irreversible, which is the result of the significant driving force associated with the formation of the amide functionality. The mechanism of the metal catalyzed reaction is believed to proceed via an iminomercuration-deoxymercuration sequence and it is only formally a [3,3]-sigmatropic shift. 

The reaction of ketenes with alkenes is assumed to occur via a concerted nonsynchronous mechanism, where the approach of the reacting partners is orthogonal. " As a consequence, the bulkier substituent of the ketene will end up on the sterically more crowded face of the cyclobutanone product. There are two descriptions that explain the experimental results 1) according to the Woodward-Hoffmann rules, the LUMO of the ketene reacts antarafacially with the HOMO of the alkene that reacts suprafacially " 2) the HOMO of the alkene forms a bond with the pz orbital of... 

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