Bond participations

Figure 3-14. Different levels of specification for a bond participating in a reaction.

Another line of evidence that bridging is important in the transition state for solvolysis has to do with substituent effects for groups placed at C-4, C-5, C-6, and C-7 on the norbomyl system. The solvolysis rate is most strongly affected by C-6 substituents, and the exo isomer is more sensitive to these substituents than is the endo isomer. This implies that the transition state for solvolysis is especially sensitive to C-6 substituents, as would be ejqiected if the C(l)—C(6) bond participates in solvolysis. ... 

Catalytic Hydrogenolysis of Carbon-Huonne Bonds it-Bond Participation Mechamsm Hudlicky, M J Fluorine Chem 44, 345-359 58 n o C) S ... 

In discussing nonclassical carbocations we must be careful to make the distinction between neighboring-group participation and the existence of nonclassical carbocations. ° If a nonclassical carbocation exists in any reaction, then an ion with electron delocalization, as shown in the above examples, is a discrete reaction intermediate. If a carbon-carbon double or single bond participates in the departure of the leaving group to form a carbocation, it may be that a nonclassical carbocation is involved, but there is no necessary relation. In any particular case, either or both of these possibilities can be taking place. 

A vast amount of work has been done on solvolysis of the 2-norbomyl system in an effort to determine whether the 1,6 bond participates and whether 52 is an intermediate. Most, although not all, chemists now accept the intermediacy of 52. 

For a competitive study of cyclopropyl versus double-bond participation, see Lambert,... 

Keywords Chemical orbital theory, Cw-stability, Cyclic conjugation. Disposition isomers. Diradicals, Donor-acceptor, Electron delocalization, Geminal bond participation, Inorganic heterocycles. Ring strain. Orbital phase. Orbital phase continuity. Polarization, Preferential branching. Reactivity, Selectivity, Stability, Tautomerism, Z-selectivity... 

Interactions of vicinal bonds have been extensively studied and are well known as hyperconjugation, resonance, and others [89-91], The a bonds vicinal to a reacting 71 bond have been proposed to participate in organic reactions and to control the selectivity [92, 93], Recently, we noticed the importance of the participation of the a bonds geminal to a reacting % bond (Scheme 35) [94] and have made extensive applications [95-102], Here, we present an orbital phase theory for the geminal bond participation and make a brief review. 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

H3SiO SiH3 Scheme 38 Geminal bond participation and reactivity... 

In 1987, more than a decade before our proposal of the geminal bond participation [94], ene reactions of allylsilanes with singlet oxygen was reported to afford the Z-isomers more than the -isomers of ally lie hydroperoxides (Scheme 40) [106],... 

The Z-selectivity is understood in terms of the geminal bond participation. The C-Si bond, which is more electron-donating than the C-H bond, occupies the Z-position. 

The unsaturated substituents with low-lying vacant orbitals were found [99] to favor the inward rotation more than expected from the geminal bond participation. 

The additional preference of the inward rotations is expected from vicinal bond participation or Rondan-Houk hypothesis. 

The gentinal bond participation [94-102] is expected to advance organic chemistry, bnt stiU remains unexplored, especially by experiments. 

Vinyl Cations by Multiple-Bond Participation in Soivolysis.229... 

Multiple-bond participation in solvolysis may be looked upon as a competition in a nucleophilic displacement between solvent and the tr electrons of the multiple bond or as an electrophilic addition of a carbonium ion, or carbonium ion like species, to the multiple bond. 

Formation of rearranged products in the solvolysis of homopropargyl systems need not involve triple-bond participation and vinyl cations in all instances. Ward and Sherman investigated the formolysis of 4-phenyl-1-butyn-l-yl brosylate, 57 (80). At 80°C in the presence of one equivalent of pyridine, they observed formation of phenyl cyclopropyl ketone, 58, and... 

When two equivalents of pyridine were added to the nmr sample and the probe heated to 80° C, the enol formate 61 decreased and phenyl cyclopropyl ketone 58 appeared at a rate approximately ten times faster than in the previous buffered system. The observation of intermediate 61 and the kinetic results, together with the observed induction periods, are consistent with the idea that some and perhaps all of the rearranged product ketone in the solvolysis of this system arises via double-bond participation in 61 rather than triple-bond participation and a vinyl cation (80). 

However, the observations of Ward and Sherman need not rule out triple-bond participation and vinyl cations in the systems studied by Hanack and co-workers (75-79). Presumably, the enol formate 61 itself arises via a transition state involving a rate-determining protonation and vinyl cation 62 (see previous section). A vinyl cation such as 62 with an adjacent phenyl group is considerably more stable and hence more accessible than a vinyl cation such as 63, stabilized only by a neighboring alkyl group. Hence, formation of enol formate 61 and its... 

Triple-bond participation was also observed in the formolysis of 64 (81). Tosylate 64 in formic acid at 60° in the presence of sodium formate gave a nearly quantitative yield of 5-methyl-4-hexen-3-one, 65. 

When the reaction was followed in an nmr probe, the appearance and disappearance of two intermediates 66 and 67 was observed along with the buildup of product 65. The rate of reaction of 64 at 75° in formic acid, k = 3xl0" sec, is six times faster than the rate of the corresponding saturated system, 2,2-dimethyl-1-pentyl tosylate, k = 5xl0" sec, under identical conditions. If the inductive rate retarding effect of the triple bond is taken into account, then the calculated rate enhancement resulting from triple-bond participation in the solvolysis of 64 is about 3000(81). The... 

Unlike triple-bond participation, no remote cumulative double-bond (allenic) participation in solvolysis has so far been observed or reported. A summary of triple-bond and allenic participation in solvolysis involving possible vinyl cations is given in Table VI. 

Stabilized vinyl cations also can be formed by allylic double-bond participation in vinyl halide solvolysis. Grob and co-workers (151) have investigated the solvolysis of a number of substituted 2-bromo-l,3-butadienes, 165. Bromodienes 165 solvolyzed via first-order rates in 80% aqueous ethanol... 

The reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

Considering ions of this type, the question arises, whether the P—S-bond participates in the resonance (I a) and (Ib) with a comparable degree to the PO bond ... 

Trapping reactions of benzoylmethyleneoxophosphorane 39 a with carbonyl compounds dispel any remaining doubts as to the existence of acylated phosphenes. Unlike the diphenylmethyleneoxophosphorane 9, whose P/C double bond participates in cycloadditions, compound 39 a acts as a hetero-1,3-diene and undergoes [4 + 2]-cycloaddition with aldehydes and ketones 10 I7,35> it may again be assumed that the reaction is a two-step process involving 55 as intermediate. 

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