Reaction path insertion

The chemistry of metalated aziridines is far less developed than the chemistry of metalated epoxides, although from what is known [lb], it is obvious that their chemistry is similar. Like metalated epoxides, metalated aziridines can act as classical nucleophiles with a variety of electrophiles to give more highly substituted aziridines (Scheme 5.56, Path A). A small amount is known about how they can act as electrophiles with strong nucleophiles to undergo reductive alkylation (Path B), and undergo C-H insertion reactions (Path C). 

Figure 14. Classical trajectories for the H + H2(v = l,j = 0) reaction representing a 1-TS (a-d) and a 2-TS reaction path (e-h). Both trajectories lead to H2(v = 2,/ = 5,k = 0) products and the same scattering angle, 0 = 50°. (a-c) 1-TS trajectory in Cartesian coordinates. The positions of the atoms (Ha, solid circles Hb, open circles He, dotted circles) are plotted at constant time intervals of 4.1 fs on top of snapshots of the potential energy surface in a space-fixed frame centered at the reactant HbHc molecule. The location of the conical intersection is indicated by crosses (x). (d) 1-TS trajectory in hyperspherical coordinates (cf. Fig. 1) showing the different H - - H2 arrangements (open diamonds) at the same time intervals as panels (a-c) the potential energy contours are for a fixed hyperradius of p = 4.0 a.u. (e-h) As above for the 2-TS trajectory. Note that the 1-TS trajectory is deflected to the nearside (deflection angle 0 = +50°), whereas the 2-TS trajectory proceeds via an insertion mechanism and is deflected to the farside (0 = —50°).

Figure 9.3. Cartoon of a classic double cone conical intersection, showing the excited state reaction path and two ground state reaction paths. See color insert.

In Figure 9.10, this second principle appears to be violated since the reaction path appears to pass through the hyperline adiabatically. However, we emphasize—as indicated by the double-cone insert—that as one passes through the hyperline, decay takes place in the coordinates X X2 and in general their VB structure does not change. This idea is obviously easier to appreciate in Figure 9.9. We shall use both Figures 9.9 and 9.10 as models in subsequent discussions but the reader needs to remember the conceptual limitations. 

Under the catalytic action of Rh2(OAc)4, formation of a propargylic ether from a terminal alkyne (229, R1=H) is preferred as long as no steric hindrance by the adjacent group is felt162,218>. Otherwise, cyclopropenation may become the dominant reaction path [e.g. 229 (R1 = H, R2 = R3 = Me) and methyl diazoacetate 56% of cyclopropene, 36% of propargylic ether162)], in contrast to the situation with allylic alcohols, where O/H insertion is rather insensitive to steric influences. 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

FIGURE 5.6 (See color insert following page 302.) Momentum and coordinate space charge density profiles for the reaction path from HNC to HCN. 

Preliminary calculations of reaction paths have proved encouraging. Thus singlet carbene is predicted to insert into CH bonds, and to add to double bonds, by concerted processes involving no activation the critical geometries are as indicated in 32 and 33. The latter is of course that predicted by Skell56) and supported experimentally by ClossS7) it is also in accord with predictions based on considerations of orbital symmetry or Evans principle 31). The total lack of discrimination shown by carbene in reactions of this type also indicates that the activation energies must be zero or close to zero. 

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

The results showed that the radioactivity was retained in the 1-position to the extent of 99% in the liquid phase and 92% in the gas phase, thus supporting direct insertion as the predominant mechanism and ruling out a free-radical reaction such as (58) as the major reaction path. The redistribution that did occur, mainly in the gas phase, was attributed to abstraction of hydrogen by CH2 followed by free-radical reactions such as (58). 

Dimethylpropane91 (neopentane) is transformed to a mixture of ethyldi-methylcarboxonium ion (21) and dimethylmethylcarboxonium ion (15). At —78°C 21 is formed exclusively. At temperatures higher than -20°C, 15 becomes the predominant product. Formation of 21 can be best explained by a reaction path that involves insertion of protonated ozone into the C—H a bond, formation of the tert-amyl cation through rearrangement, and its quenching by ozone ... 

The catalytic dicarbonylation of ethylene to dimethyl succinate can be carried out in 90% conversion.94 High reaction temperatures and low carbon monoxide pressures can lead to unsaturated esters as a result of a faster -hydride elimination from the intermediate (23) than carbon monoxide insertion. This later reaction path has been termed oxidative carboxylation. 

Because neither hydrogen peroxide nor Magic Acid-S02C1F alone led to any reaction under the conditions employed, the reaction must be considered to proceed via electrophilic hydroxylation. Protonated hydrogen peroxide inserts into the C H bond of the alkane. A typical reaction path is as depicted in Scheme 5.60 for isobutane. 

Both Mg and transition metal complexes similarly undergo oxidative addition and insertion. Whereas the main reaction path of Grignard reagents is the insertion of a... 

Not only this Ni(0)-catalyzed reaction but also all reported allene dimer complexes, e.g., hexacarbonyl-p[l-3 l -3 -jj-(2,2 -biallyl)]diiron (Fe-Fe) (146), hexacarbonyl-/i-[l-3 l -3 -r)- (1,1 -diphenyl-2,2 -biallyl) ]diiron (Fe-Fe) (147), and di-/i-aeetato-/i-[l-3 l -3 -rj-(2,2,-biallyl) Jdipalladium (148) point to the formation of 2,2 -biallyl. A mononuclear Rh(I) complex containing this ligand was recently isolated (149). Accepting this biallyl formation, then the next step is the insertion to form the trimer ligand in complex IV. Thus the entire reaction paths leading to complexes I, II, and III may be depicted (Scheme 8). 

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