Cycloadditions proposed mechanism

Scheme 2.1 Proposed mechanism for TMM [3+2 cycloaddition with an electron deficient olefin...

The diastereofacial selectivity of this asymmetric [3C+2S] process is explained following a model similar to that described in Sect. 2.6.4.4 for the reaction of chiral alkenylcarbene complexes and 1,3-dienes. Thus, the proposed mechanism that explains the stereochemistry observed assumes a [4+2] cycloaddition reaction between the chromadiene system and the C=N double bond of the imine. The necessary s-cis conformation of the complex makes the imine... 

Scheme 5.7 Proposed mechanism of [3+2] cycloaddition reaction of internal alkynes...

In accord with the proposed mechanism, copyrolyses of la or lb with 2,3-dimethyl-1,3-butadiene (DMB) or isoprene lead to silacyclopentene derivatives via a formal [4+1] cycloaddition of the silylenes (Scheme 2). The simultaneous existence of the silaethenes 2a/2b and the resulting silylenes 4a/4b in the gas phase is proven by the formation of the corresponding 1,3-disilacyclobutanes (5) and - in case of isoprene as the quenching partner - of the two isomeric silacyclohexenes 7 (Scheme 2) [2]. 

Photoproducts arising by a [3 + 2] cycloaddition of s-triazolo[4,3-b]-pyridazine (269) to alkenes have been described.222 Addition to cyclohexene, for example, led to the formation of adducts 270 and 271, and the proposed mechanism is outlined in Scheme 8. The reaction has been extended to include addition to cis- and rans-hex-3-ene,223 cyclooctene,224 and furan.225... 

In order to gain more insight into this proposed mechanism, Montgomery and co-workers tried to isolate the intermediate metallacycle. This effort has also led to the development of a new [2 + 2 + 2]-reaction.226 It has been found that the presence of bipyridine (bpy) or tetramethylethylenediamine (TMEDA) makes the isolation of the desired metallacycles possible, and these metallacycles are characterized by X-ray analysis (Scheme 56).227 Besides important mechanistic implications for enyne isomerizations or intramolecular [4 + 2]-cycloadditions,228 the TMEDA-stabilized seven-membered nickel enolates 224 have been further trapped in aldol reactions, opening an access to complex polycyclic compounds and notably triquinanes. Thus, up to three rings can be generated in the intramolecular version of the reaction, for example, spirocycle 223 was obtained in 49% yield as a single diastereomer from dialdehyde 222 (Scheme 56).229... 

Vinyl Fischer carbenes can be used as three-carbon components in Ni(0)-mediated and Rh(l)-catalyzed [3 + 2 + 21-reactions with alkynes (Schemes 48 and 49)142 and with allenes (Schemes 50 and 51).143 All three of the proposed mechanisms for the [3 + 2 + 2]-cycloadditions involve an initial carbene transfer from chromium to nickel or rhodium (Schemes 49, 52, and 53). As is seen from the products of the two [3 + 2 + 2]-reactions with 1,1-dimethylallene, although the nickel and rhodium carbenes 147G and 147K appear similar, the initial insertion of the allene occurs with opposite regioselectivity. 

Scheme 1.—Proposed Mechanism for Cycloaddition between 3,4,6-Tri-O-acetyl-D-glucal (1) and Acetone.

The proposed mechanism of the above cycloisomerizations are depicted in Scheme 11.30. The oxidative coupling of a metal to an enyne yields a bicyclic metaUacyclopentene, which is a common intermediate. The reductive elimination and subsequent retro-[2+2] cycloaddition gave vinylcyclopentene derivatives, while the two patterns of P-elimination and subsequent reductive eUmination gave cychc 1,3- and 1,4-dienes, respectively. The existence of a carbene complex intermediate might explain the isomerization of the olefinic moiety. 

The [3 -f 2] cycloaddition of aziridines and dipolarophiles, like dimethyl acetylenedicarboxylate or dimethyl fumarate and maleate, was investigated by Gaebert and Mattay. Via C—C and C—bond cleavage five-membered heterocycles are formed in moderate yields. The different product ratios dependent on the reaction conditions (PET/direct excitation/thermal reaction) gave insights to the reaction details and are summarized in the proposed mechanism (Scheme 52) [84],... 

The stereochemical characterization of the adduct 53 follows from its NMR spectrum and a comparison with that of the l-(2-thienyl) compound (54). The aSY-exo configuration for the adducts 51 and 52 is consistent with the NMR spectra (hydrogen atoms at C-2, C-3, C-5, and C-6 all equivalent), with the proposed mechanism of formation, and with the failure of the related tetramethyl ester to xmdergo N-acetylation even in very vigorous conditions. N-substituted derivatives of compounds such as 51-53 may be obtainable directly from similar dipolar cycloaddition reactions of mesoionic N-substituted oxazolium 5-oxides, although the formation of only the N-methyl derivative of (52) has so far been reported. ... 

While the fadhty and effidency of the [5+2] cycloaddition of oxygen-substituted VCPs could be attributed solely to the electronic contribution of the heteroatom substituent, it could also be a consequence of its conformational influence. Substitution of a VCP, particularly at the 1-position, has been shown to reduce the difference in energy between the s-cis (local minimum) and s-trans (global minimum) conformations through steric effects [50]. Based on the proposed mechanisms for the [5+2] cycloaddition, only the s-cis conformations, 190 and 192, can lead to productive reaction, so biasing intermediates toward this conformation could therefore accelerate the reaction (Scheme 13.16). 

The authors proposed mechanism for dimerization involves initial formation of metal vinylidene complex 9 via 1,2-H-migration. A second molecule of arylacetylene acts as a dienophUe in a formal [4 + 2] Diels-Alder cycloaddition with 9. A subsequent... 

The authors proposed mechanism, outlined in Scheme 9.19, was tested using a deuterium-labeling experiment. H-migration consistent with initial formation of a Pd-vinylidene was observed. The key intermediate of Buono s mechanism is a palladacyclobutane (125) resulting from [2 + 2]-cycloaddition. Direct C—C reductive elimination from intermediate 125 proceeds to give highly strained products (123), despite the apparent availability of a (l-hydride elimination pathway [39]. 

The cycloaddition of carbon dioxide to A,A-diethylaminophenylacetylene leads to aminopyran-4-ones (72TL1131). Initial cycloaddition to the ynamine probably forms (422) which rearranges to the ketene. A further cycloaddition, this time at the conjugated amide, leads to the pyran-4-one (Scheme 142). Supporting evidence for the proposed mechanism includes the formation of the pyranone from the ketene (423) (72TL1135). 

Using the proposed mechanism for the Diels-Alder reaction, explain why you would not expect a reactive dienophile to form [4 + 2] cycloaddition products... 

Scheme 6.23 Proposed mechanism for isocyanate formation via [2+2] cycloaddition of C02...

However, internal cycloaddition fails to give an adduct when an ethylenic moiety is linked to the ylid precursor. Even a styrene does not give any adduct and a diamine is obtained instead (note that in this instance zinc chloride was used instead of lithium fluoride). The same result is observed starting from the corresponding phenyl derivative, i.e., from /V-benzyl-/V-(methoxy methyl)trimethylsilylmethylamine. The proposed mechanism involves demethox-ylation under the action of the Lewis acid to form cation, K, which adds to the ylid to give the ethylenediamine framework.219... 

A possible mechanism for the PEG6000(NBu3Br)2-catalyzed cycloaddition of C02 with epoxides is proposed as shown in Scheme 5.2. The proposed mechanism involves the activation of propylene oxide (PO) by the ammonium cation (step I), the ring opening of the epoxide via nucleophilic attack of bromine at the least-hindered carbon (step II), and the insertion of C02 into the N-O bond (step III). Subsequent cyclization via an intramolecular nucleophilic attack (step IV) leads to the propylene carbonate (PC) and the regeneration of the catalyst. 

Scheme 5.2 Proposed mechanism for PEG6000(NBu3Br)2—catalyzed cycloaddition of C02 with epoxides. Reprinted from Ref. [10], with permission from Elsevier...

The proposed mechanism of cycloaddition reaction of C02 with epoxides catalyzed by BrTBDPEG150TBDBr is depicted in Scheme 5.6. 

Scheme 5.6 Proposed mechanism for the BrTBDPEG150TBDBr-catalyzed cycloaddition reaction of C02 with epoxides. Reproduced from Ref. [15] by permission of The Royal Society of Chemistry...

Scheme 16 Proposed mechanisms for thermal and catalytic cycloaddition reactions...

CR(Q(262)1017>. The nucleophilic reactivity of the oxygen atom has been observed in the acetylation by acetic anhydride of 2-aryl- and 2-heteryl-A2-thiazolin-4-ones (Scheme 136). 2-Alkoxy and 2-methyl derivatives of A2-thiazolin-4-one (196) react with OPCl3 to yield thiazolylphosphoric esters (197) which have insecticidal uses (Scheme 137). An example of the electrophilic reactivity of the C-4 atom is the easy formation of oxime and phenylhydrazone derivatives. 5-Aryl-A2-thiazolin-4-one (198) gives the 1,3-dipolar cycloaddition product (199) with methyl fumarate and methyl maleate (Scheme 138). Under similar conditions, treatment of (198) with dimethyl acetylenedicarboxylate (DMAD) yields a thiophene derivative (202) when R = Ph and a pyridone derivative (203) when R = H (Scheme 139). The proposed mechanism involves the formation of a mesoionic intermediate (200) which reacts in a cycloaddition with a second molecule of DMAD, yielding (201), the decomposition of which depends on the R substituent. 

The metal mediated synthesis of cyclopentenones via a [2 + 2+1] cycloaddition between an alkyne, an alkene and carbon monoxide has become commonly known as the Pauson-Khand (PK) reaction. This report will briefly summarise some of the major developments since its initial discovery including an intramolecular variant of the reaction, the progress made towards making the process catalytic and examples of how the reaction has been utilised. The proposed mechanism for the reaction and the factors that influence the product distribution will also be introduced. 

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