Three-center transition state

Photolytically generated carbene, as mentioned above, undergoes a variety of undiscriminated addition and insertion reactions and is therefore of limited synthetic utility. The discovery (3) of the generation of carbenes by the zinc-copper couple, however, makes carbene addition to double bonds synthetically useful. The iodo-methylzinc iodide complex is believed to function by electrophilic addition to the double bond in a three-center transition state giving essentially cis addition. Use of the... 

Figure 6.38. Potential energy diagram for the hydrogenation of ethylene to the ethyl (C2H5) intermediate on a palladium(m) surface. The zero of energy has been set at that of an adsorbed H atom, (a) Situation at low coverage ethylene adsorbed in the relatively stable di-cr bonded mode, in which the two carbon atoms bind to two metal atoms. In the three-centered transition state, hydrogen and carbon bind to the same metal atom, which leads to a considerable increase in the energy...

However, our discussion so far applies to low coverages, as is usually the case in kinetic modeling. With highly covered surfaces another mechanism prevails, which offers an alternative to the energetically unfavorable three-centered transition state of Fig. 6.39(a). At higher coverages, ethylene and hydrogen are forced closer and... 

The thermal and photochemical reactions of Ir111 complexes (438) and (439), respectively, yield the same product species (440), R2 = Ph2, Me2, HPh.692 Kinetic studies indicate that the reaction involves the direct reductive transfer of the methyl to the phosphide via a three-centered transition state. The crystal structures of (438) and (439) (R2 = Ph2) are reported. 

Catalytic dehalogenation cycles with a binuclear reductive elimination step have not yet been reported, but many examples are known with a single metal. The RH product eliminates easily from d8 metals (Ni(II), Pd(II)) or d6 metals (Ru(II), Rh(III)) (Eq. (25)) [196]. This reaction is believed to go by a three-center transition state [193, 194, 212] ... 

The reductive elimination of formic add from RhH2(V-OCOH)(PH3)2 was theoretically investigated first by Dedieu and his collaborators [38]. A three-center transition state was optimised, in which the Rh-0 and Rh-H distances lengthened by about 0.2 A and 0.1 A, respectively, and the O-H distance was 1.25 A. The activation barrier was evaluated to be 23.6 kcal/mol with the MP2 method and 24.7 kcal/mol with the QCISD(T) method (Table 3). From these results, it was concluded that the reductive elimination was difficult. 

Cheng and coworkers have examined the photodissociation spectroscopy of MgCH4 in detail . The photofragmentation action spectrum has a broad featureless continuum ranging from 310 to 342 nm, with a maximum at 325 nm. In this region the channels observed are nonreactive (equation 36, ca 60%), H abstraction (equation 37, ca 1%) and CH3 abstraction (equation 38, ca 33%). Recent theoretical calculations on the C—H bond activation in MgCH4+ reveal that the formation of the insertion intermediate, CH3MgH, proceeds via a three-centered transition state . ... 

Mechanisms for oxidative additions vary according to the nature of X—Y. If X—Y is nonpolar, as in the case of H3. a concerted reaction leading to a three-centered transition state is most likely. 

The mechanism involving simple nitrogen-coordinated complexes also accounts for reactivities of certain sterically constrained systems. For instance, 3-(diethyamino)cyclohexene undergoes facile isomerization by the action of the BINAP-Rh catalyst (Scheme 18). The atomic arrangement of the substrate is ideal for the mechanism to involve a three-centered transition state for the C—H oxidative addition to produce the cyclometalated intermediate. The high reactivity of this cyclic substrate does not permit any other mechanisms that start from Rh-allylamine chelate complexes in which both the nitrogen and olefinic bond interact with the metallic center. On the other hand, fro/tt-3-(diethylamino)-4-isopropyl-l-methylcyclohexene is inert to the catalysis, because substantial I strain develops during the transition state of the C—H oxidative addition to Rh. 

It is worthwhile to note that in the reaction of the group 6B metal hexacarbonyls with the azide anion to provide isocyanatometallates, a concerted mechanism is proposed based on kinetic parameters which involves a three-centered transition state (7), and that the activation enthalpy is some 22.6 kJ lower for tungsten than for chromium (52). This reactivity sequence correlates with an increase in M-CO bond distances going from chromium to tungsten hexacarbonyl (53-55). 

Assume that the reaction proceeds via a three-center transition state with the atoms at the corners of a triangle. 

The orbital requirements for radical attack on any polyene are given in Table 6. If H3, HC2 and Cl8 (see Walsh diagram, Fig. 2) can be taken as models, then three-center transition states will be linear. If, however, cyclic transition states can be formed, HMO theory indicates a preference for them (Fig. 1). Unfortunately, attempted radical displacements have not been observed, simply because the radicals take other reaction paths (Pryor, 1966). The transition states may have been linear, but for abstraction from rather than displacement on carbon (Bujake et al., 1961). If the radical and molecule generated in these cases remain in... 

So effective was the collinear three-center transition state in clarifying the Walden inversion problem, that concerted backside attacks were proposed or considered for the Beckman rearrangement, isomerization of alkenes, etc. (Olson, 1933 Marvel, 1943), as well as substitution in alkenes (Gold, 1951 Ross et al., 1952). Since an appropriate arrangement of hybrid orbitals is available, some of the transition states did not... 

The actual mode of interaction between Co(III) and the alkane was not elucidated. It could involve electron transfer as described above or it may be an example of a general class of electrophilic substitutions at saturated carbon centers in which attack at a a bond occurs via a trigonal (three-center) transition state,3008 e.g.,... 

In view of the possibility that attack by ozone may proceed via oxygen insertion into the Si-H o-bond (state l), the known reaction of dichlorocarbene with the Si-H bond (32) was taken as a model for such insertion. This reaction is postulated to proceed via the three-center transition state 2, which is structurally analogous to 1. The isotope effect, ku/ki>, for insertion of dichlorocarbene into (n-C4H9)3SiH and (n-C4H9)SiD, determined in the same manner as for their ozonations, was found to be 1.23 (10). The large difference in the ku/kjy values... 

The migration of an alkyl ligand to a coordinated CO ligand in cis position is assisted by the polarization of CO, and proceeds through a three-center transition state ... 

To complete the alkene hydrogenation reaction sequence, the first hydrogen transfer must be followed by a second, which results in the reductive elimination of the alkane product. This proceeds through a three-centered transition state. The catalytic cycle is shown in Fig. 22-1 but the process is actually more complicated since the equilibria are dependent on phosphine, alkene, rhodium concentrations, temperature, and pressure. 

Rearrangement of the peroxynitrite to nitrate has been a long outstanding issue. A previous theoretical study suggested that this rearrangement proceeds via a three-centered transition state, which for HOONO corresponds to an activation energy of about 60 kcalmol [89-92]. However, the three-centered transition state is likely hindered by the substitute of a carbon chain to the hydrogen atom, and... 

However, intramolecular nucleophilic participation by the conjugate base during protonolysis of a C—Hg bond is questionable. A study of the acidolysis of the carbon-mercury bond in unsymmetrical di-alkylmercurials rather suggests that the reaction proceeds via a three-center transition state.In any case, substantial kinetic and stereochemical evidence has led to the idea that reaction occurs by a concerted, front side attack with a transition state that involves a pentacoordinate carbon center. In some cases unimolecular mechanisms, SeI, also have been observed. 

In insertion, as the name suggests, the carbene inserts itself between two atoms. Insertions have been observed into C—H, C—C, C—X, N—H, O—H, S—S, S—H, and M—C bonds, among others. The mechanism of the process is often concerted. A three-centered transition state is usually written for the concerted mechanism ... 

This general mechanistic scheme is widely accepted, but there are several different pathways possible for each of the three steps. For example, the reductive elimination could proceed via an Sn2 mechanism or via a concerted mechanism involving a three-center transition state. Many research activities have been devoted to the investigation of these detailed questions, which have been reviewed extensively by Stahl [3], while here only a summary is given (1) for the reductive elimination it could be concluded that an Sn2 mechanism is operative (2) the oxidation of RPt" to RPt does not occur by alkyl transfer, but by a two-electron transfer from RPt to Pt and (3) the electrophilic activation of the alkane is the most difficult part to investigate. At present none of proposed pathways shown in Scheme 5 could be discounted also, the experimental observation of H/D exchange in methane can be explained by both mechanisms. 

RE is the reverse of OA, whereby oxidation state, coordination number, and electron count all decrease, usually by two units. According to the principle of microscopic reversibility, the mechanistic pathways for RE are exactly the same as those for OA, only now in the reverse sense (this principle corresponds to the idea that the lowest pathway over a mountain chain must be the same regardless of the direction of travel). Equation 7.47 shows an example of RE from a platinum complex to give a silylalkyne." RE here likely goes through a concerted, three-centered transition state with both M-Si and M-C(alkynyl) bonds breaking and the new Si-C bond starting to form. 

The timing of the breakage of the M-C and M-H bonds and the formation of the C-H bond is of interest to chemists. Equation 7.61119 illustrates the commonly accepted and now-familiar pathway for RE to form C-H bonds, which first involves a three-centered transition state, followed by an iq2-C-H metal complex 42 and finally production of the free alkane and reduced metal. 

When R = allyl, OA seems to involve either an SN2 mechanism (inversion of configuration) or a mechanism that goes through a three-centered transition state (retention of configuration). Inversion is the stereochemical outcome in polar, coordinating solvents such as DMSO or acetonitrile, whereas retention is found with use of less polar and less coordinating solvents such as THF, benzene, and CH2C12.122... 

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