Non-concerted

The reactions of 1-azirines with ketenes and ketenimines represent non-concerted additions and are formally different from the additions to 47r-systems of dienes and 1,3-dipolar compounds (73JOC3466, 71CB2786). 

The effect of substitutents at the C3 and C6 positions of the azepine ring is much more dramatic in that they force the 1//-azepine into a competing [6 + 2] Tt-cydoaddilion at the Cl —Cl positions.6 1 In fact, at room temperature [6 + 2] cycloaddition by a kinetically controlled, non-concerted, ionic process appears to be dominant, since on treating a mixture of ethyl 3,6-dimethyl- and ethyl 2,5-dimethyl-l//-azepine-l-carboxylate with less than a molar equivalent of ethenetctracarbonitrile, only the [6 + 2] cycloadduct 10 of the 3,6-dimethyl-l//-azepine is formed. 

The rates of thermal decomposition of diacyl peroxides (36) are dependent on the substituents R. The rates of decomposition increase in the series where R is aryl-primary alkyKsecondary alkyKtertiary alkyl. This order has been variously proposed to reflect the stability of the radical (R ) formed on (i-scission of the acyloxy radical, the nucleophilicity of R, or the steric bulk of R. For peroxides with non-concerted decomposition mechanisms, it seems unlikely that the stability of R should by itself be an important factor. 

Initiators where the radical generating functions are sufficiently remote from each other break-down in a non-concerted fashion. Examples include the azoperoxide (68) and the bis-diazene (67).261 Their chemistry is often understandable in terms of the chemistry of analogous monofunctional initiators.260 This class also includes the dialkyl peroxyketaLs (see 3.3.2.4) and hydroperoxyketals (see 3.3.2.5). 

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

The formation of the c -annulated hexahydrothioxanthene (34) from thiophenol and hept-6-enals may also result from a [ +2] cycloaddition, although the minor stereoisomers probably arise from a cationic non-concerted cyclisation <96SL396>. 

Apart from their intrinsic interest, these electrocyclic reactions have considerable synthetic carbon-carbon bond-forming importance because of their rigid stereospecificity, which is much greater than in the vast majority of other, non-concerted reactions involving biradical or bipolar intermediates. 

The evidence is clearer in the case of the well-known tendency of singlet excited species to give products of concerted pericyclic reactions and of triplet excited species to give their non-concerted counterparts (for references see Refs. 12i,i58)i There, the rationalization based on the electron-repulsion argument is on firmer ground and is discussed in more detail below. Still, presently unexplained exceptions to the general tendency appear to exist.159)... 

Cp2Zr -induced alkene stereoisomerization via partially non-concerted dipolar mechanism. 

Dipolar species have been observed in the cycloaddition of polar intermediates. Thus cyclobutanes can be formed by non concerted processes involving zwitter ionic intermediates. The combination of an electron rich alkene (enamimes, enol ethers) and an alkene having electron withdrawing groups (nitro a cyano substituted alkenes) first gives a zwitter ion which can rotate about the newly formed bond before cyclization and gives both a cis and a trans adduct. 

A fundamentally different approach to the synthesis of 3-pyrrolines is evidenced in the annulation in Eq. 13.50 [58]. Ethyl 2,3-butadienoate 150 reacts with N-sulfony-limine 151 in the presence of triphenylphosphine under very mild conditions to give JV-protected 3-pyrroline 152 in 90% yield. The mechanism that has been postulated is related to that of the Baylis-Hillman reaction. Michael addition of triphenylphosphine to the allenyl ester generates a zwitterion that combines with the imine to give 153 in a non-concerted process. This is followed by ring closure, proton exchange and expulsion of triphenylphosphine to give 152. This annulation is successful only for aromatic or cinnamyl imines [59]. 

Subsequent studies on cyclopentadiene, in which the antarafacial concerted 1,4-addition is impossible because of interference between the antimony system and the methylene of cyclopentadiene, have however shown70 that both butadiene and cyclopentadiene react with SbCIs through a stepwise mechanism involving a carbenium ion intermediate. In agreement with a non-concerted mechanism are also the data related to the 1,4-addition... 

This finding confirms an opinion that, at least in some cases, diradicals such as 18 can be the actual intermediates in the non-concerted Cope rearrangement, so-called stepwise cyclization-then-cleavage mechanism. Berson and coworkers who previously excluded diyl intermediate in the acetylenic Cope rearrangement20 designed in their next work24... 

Bowen, R.D. Williams, D.H. Non-Concerted Unimolecular Reactions of Ions in the Gas-Phase the Importance of Ion-Dipole Interactions in Carbonium Ion... 

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

The Cope rearrangement of the highly strained diene (32) (Scheme 13) is shown to proceed by a non-concerted mechanism involving the diradical (33), which may be trapped by oxygen to give the peroxide (34). A full kinetic study confirms the intermediacy of the diradical. ... 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of 3-formylamino-l,2,4-oxadiazole and 3-hydroxy-iminomethyl-1,2,5-oxadiazole has been carried out7 The treatment has shown the participation of asymmetric transition states and non-concerted processes via symmetrical intermediates. A detailed ab initio and density functional study of the Boulton-Katritzky rearrangement of 4-nitrobenzofuroxan has indicated a one-step mechanism for the process. 

In contrast to thioethers 63 and 64, the corresponding sulfoxides (e.g., 66 and 69, respectively) cyclize in the course of an extremely mild thermolysis in DMF at 110°C [84H(22)467], Thereby, 2-(chloroallylsulfinyl)tropone 66 is transformed to thiopyranotropone 67 and its sulfoxide 68. Crossover deuterium labeling experiments confirm the reaction to be a radical, non-concerted, intermolecular process. 

Whether this condition can be fulfiUed depends on the electron count of the metal, and the stereochemistry of the elimination. For instance, in m-elimination from octahedral d , or square planar d , systems, metal ndipP -y ) acts as acceptor, and this should be a facile process ( e Fip. 1, 2). For /rans-elimination, on tiie other hand, the lowest empty orbital of correct symmetry is (n + l)p. Such elimination Kerns energetically less Ukely, unless a non-concerted pathway (such as successive anionic and cationic loss) is available. The same arguments apply, of course, to oxidative additions. It foUows that the many known cases of traits oxidative addition to square planar t/ systems are unlikely to take place by a concerted mechanism, and this conclusion is now generally accepted There are special complexities in reductive elimination from trigonal systems, and these are discussed furdier in Part III. 

Of the Ru(IV) complexes recorded here most are mono-oxo species which, despite the strong axial distortion brought about by the terminal oxo ligand, are probably all paramagnetic. Semi-empirical molecular orbital calculations (INDO/1) for epoxidations effected by oxo-Ru(IV) complexes have been reported (a non-concerted [1 h- 2] pathway was preferred) [642], [643] and for alcohol oxidations by octahedral species containing an Ru" (0) unit [644]. The reactivity of high oxidation-state polypyridyl complexes of osmium and Ru, with particular emphasis on Ru(IV) and Os(IV) oxo species, has been reviewed [43]. 

An important point, however, is that although the configurations of the reactants are preserved in the products (i.e. the additions are stereoselective), some cycloadditions, including those of ketenes to imines, occur more rapidly in polar rather than in non-polar solvents (Scheme 8.9). For such examples it may well be that the addition proceeds in a stepwise manner (non-concerted), and the collapse of a dipolar intermediate is so quick that the stereochemistry of the reacting species is not compromised. 

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