Insertion cyclic transition state

In contrast to this, according to the T-Cat mechanism, the initiator forms an ester with the alkene which incorporates the next monomer molecule by means of an insertion reaction involving a 6-membered cyclic transition state. 

They envisaged the propagation as monomer insertion into the C-0 bond of the ester via a four- or six-membered cyclic transition state (II). 

The lack of isomerisation indicates that a non-ionic process is involved. The reaction does not continue to give a polymerisation of ethylene, because in contrast to the tertiary C-Cl bond which is broken, the primary C-Cl bond which is formed is too strong to be activated sufficiently by the A1C13. For the polymerisations involving an organic (monomer) halide and a metal halide, such as ZnBr2, it seems reasonable to represent the insertion also as occurring via a 6-membered cyclic transition state (II) ... 

At first glance, the insertion reaction appears to be the most mysterious. It does not seem reasonable to expect that the fairly bulky CH2 group can actually attack the electrons on the small C-H bond of a stable molecule to form what would have to be a cyclic transition state ... 

Many of the addition reactions of the silylenes 83-85 with molecules having multiple bonds and also many of their insertion reactions into activated (T-bonds are thought to proceed via cyclic transition states. For that reason these reactions are summarized in this section. It should be emphasized, however, that in only a few cases mechanistic investigations either by theoretical means or by experimental methods have been undertaken to substantiate the claim for a one-step cycloaddition. 

Singlet insertion by way of a three-centre cyclic transition state may not, in any case, be an invariable rule. The insertion of methylene into carbon-chlorine bonds (Franzen, 1959 Bradley and Ledwith, 1961) may... 

The insertion of an alkene into a M—H bond is considered a concerted intramolecular process involving a planar cyclic transition state, resulting in a cis addition to the C=C bond ... 

Direct concerted insertion is characteristic of singlet carbene and the three-center cyclic transition state 139 suggested by Doering and Prinzbach" has been supported by kinetic data" and theoretical studies." Singlet methylene inserts into C-H bonds of hydrocarbons in a statistical fashion" [Eq. (6.128)] ... 

It is generally assumed that olefin insertion into metal-carbon or metal-hydrogen bonds takes place by a concerted reaction path, that means through a more or less polar, cyclic transition state, with simultaneous bond breaking and bond making, e.g.-. 

A four-centered cyclic-transition state is involved in the propagation stage leading to the insertion of a monomer imit between the catalyst and the chain end, with subsequent regeneration of the cychc stmctnre. An alternative transition state, proposed by Cram and Kopecky, has a similar bnt more rigid stmctnre. 

The enantioselectivity-determining alkene insertion step involves a four-membered cyclic transition state, which adopts a square-planar geometry. When a chiral bidentate ligand, such as PyOx, is employed, there are four possible isomeric alkene insertion transition states (Table 2.2). 

Hydride to al)cene migration of NbH(CH2=CHR)Cp 2 in the presence of L (L = CNMe, CO) leads to the alkyls Nb(CH2CH2R)LCp 2, the reaction kinetics are interpretable in terms of concerted four centre cyclic transition states.- Alkynes with electron withdrawing substituents insert into the M-H bond of MH(CO)Cp2 resulting in the formation of the alkynyls M(CR=CHR)(C0)Cp2 (M =... 

The transition state in the above scheme differs from the cyclic titanium peroxo complex proposed earlier (217). In the earlier mechanism, any of the two peroxo oxygens in the Ti-O-O-H (bound end-on) could have been inserted into the C=C bond, and accordingly two isomers would be possible. They have never... 

For condensed-phase reactions, Kirmse (1964) has used SS as a criterion of cyclic (singlet) versus acyclic (triplet) transition states and/or intermediates in these cases. When insertion is stereospecific with retention, a cyclic species is assumed when insertion is unselective the acyclic species is assumed. We can, in fact, associate the singlet with a one-step, stereospecific process. On the other hand, one or more products could be formed from a triplet intermediate. 

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