Allylic 1,2-interaction

This type of allylic interaction between the equatorial methyl group and the methylene group adjacent to the nitrogen atom has been recently generali2ed by Johnson and Malhotra (7,7a) as the strain between substituents R and R in the cyclohexenyl-type system shown, as in (11), the dihedral angle between the substituents being 40°. The magnitude of... 

The stereochemical features of the reactions of racemic 1-substituted (Z)-2-butenyl-boronates 2 are considerably different from those of the 1-substituted 2-propenyl- and 1-substituted ( )-2-butenylboronates discussed above. Transition state 5 (see p 1470) is destabilized by allylic interactions between X and the (Z)-methyl substituent26, and consequently diastcrcomcr 10 is the major product via transition state 6 (sec the following table)4,15. 

The C-19 methylene protons appeared as a broad singlet at 8 2.72 and showed weak allylic interactions with the vinylic C-11 proton that appeared at 8 5.45 as a multiplet and itself showed vicinal coupling with the C-12 methylene protons. This trisubstituted olefinic bond when tailored into ring C of the skeleton gives rise to fragment II. 

Although the introduction of a substituent at both C-a and C-P may be expected to destabilize the transoid state of rearrangement due to additional 1,2-allylic interactions, the tendency to form an -double bond exclusively is retained in the synthesis of trisubstituted olefins as well. The first such report, shortly following the initial Evans report , was made by Grieco who achieved a completely stereospecific general synthesis of ( )-y-substituted methallyl alcohols, including the synthesis of racemic ( )-nuciferol (45, equation 24) . Subsequently, other examples of nearly or completely stereospecific syntheses of ( )-) , y-substituted allylic alcohols have also been pub-lished - " . On the other hand, in the synthesis of y,y-disubstituted allylic alcohols a diminished stereoselectivity has been observed. In this case, the /Z ratio depends on the... 

A possible explanation is that in the presence of the allyl group, the nitrogen electron pair is delocalized due to homo-allylic interaction of the n, n type (36)... 

The structure resembles the corresponding anthracene complex [203]. The ligand is bonded in an allylic fashion with Sm-N bonds in the range of Ln-N (r-bonds and Ln-C bonds at the long range end of allylic interactions (2.866(3), 2.877(2) A). In contrast the reaction of SmCpf with acridine again yielded a coupled product, which contains a similar allylic bonding pattern (Sm-C, 2.944(5), 2.970(5) A) [203]. 

Normally the less substituted double-bond isomer (la) is the more reactive since reaction at the more substituted position It engenders severe allylic interactions (A1,3-strain5) in the transition state. Further reaction therefore occurs preferentially to give the / ,/ -disubstituted enamine or iminium salt (3) and hence the a,a -disubstituted ketone (5) on hydrolysis. However, there are exceptions to this (see Sections III.B, V and VI.D). A recent development uses the observation6 that such steric interactions do not apply to secondary enamines (Section VIII) since a conformation (2) can be adopted... 

The classification in Fig. 2 contains other oversimplications. For example, the asymmetry of MP cations results in a disordered MP-TCNQ structure Such disorder leads to characteristic power laws in thermodynamical properties as illustrated most completely for quinolinium (TCNQ)2. Chemical disorder provides another possibility. Miller and Epstein have exploited phenazine doping " in segregated-stack alloys of the type P,(MP) jTCNQ for 0 g x 0.5. Phenazine doping of MjP-TCNQ, which has a mixed dimerized stack, restores segregated stacking and high conductivity in P(M2P)(TCNQ)2, a system that closely resembles MP-TCNQ. The MgP-TCNE complex in Section 4.4. probably shows yet another kind of disorder involving preferential TCNE-allyl interactions. 

To pursue the idea of specific TCNE-allyl interactions, we suppose in Fig. 19 that each TCNE has a strong contact (J) with one MP and a weaker one (J < J) with the other. This results in a dimerized stack if J and J alternate precisely when the unit cell is doubled and there is no reason for an enlarged thermal ellipsoid. A more promising hypothesis is that J and J do not alternate precisely, so that sometimes two TCNE form strong contacts with the same MgP, to give trimers in Fig. 19. MgP sites with two weak (J ) contacts are monomers. There are equal numbers of J and J, but now the distribution is random, subject to the constraint that no more than two successive J or J can occur. 

Overman, L.E., Lesuisse, I )., and Hashimoto, M., Importance of allylic interactions and stereoelec-tronic effects in dictating the steric course of the reaction of iminium ions with nucleophiles. An efficient total synthesis of ( )-gephyrotoxin, J. Am. Chem. Soc., 105, 5373. 1983. 

The foregoing results indicate that, in the case of the conformationally constrained systems 142a, 152a, and 161a, there is a marked preference for the oxidant to donate an oxygen atom to the sulfur moiety from the axial direction. Presumably, in the transition state leading to the equatorial sulfoxide, a significant allylic interaction between the oxidant and the... 

The conformational properties of dihydrothiazine oxides of types 239 and 242 have been examined by NMR spectroscopy. In the case of the (R)-oxides 239, there is a dramatic preference for the conformer 241 by contrast, the (S)-oxides 242 exist overwhelmingly as the conformer 243. These results illustrate that the conformer that possesses an axial 5-oxide is favored. The eonformer 243 possesses an imfavorable 1,3-diaxial interaction between the oxide group and R, and the conformer 241 is destabilized by an allylic interaction between R and R. Evidently, these interactions are less severe than the allylic interaction between the oxide and methoxy-carbonyl groups which would be present in the conformers 244 and 240. 

The entrance into the catalytic cycle from complex 5 may occur via a small equilibrium concentration of Ni-(la)-(MVN) complex6 (pathA, Schemes) and/or via oxidative addition of HCN to generate the species Ni-[la]-HCN, 7 (pathB). In either event, formation of the hydridoalkene complex Ni-[1]-(MVN)(H)(CN), 8, occurs and is followed by an insertion reaction to produce the (ri -benzyl)nickel cyanide intermediate 9. Although this allyl-type species has not been directly detected, the exclusive formation of the branched nitrile supports its intermediacy. Analogous intermediates have been postulated in the hydrocyanation of 1,3-butadiene with NilPlO-o-tolylljjj or Ni[P(OEt)3]4 and in the hydrocyanation of styrene with Ni[P(0-p-tolyl)3]4. Examples of other nick-el-benzyl complexes exhibiting similar allylic interactions in the solution and solid state are also known. 

The factors affecting which side of the double bond is attacked ought to be the same as in the Felkin-Anh mle, but with the conformation of the starting material affected by the fact that it is a C=C double bond and not a C=0 567 The presence of a substituent on the double bond cis to the stereogenic centre, even when it is only a hydrogen atom, increases the A1,3 allylic interaction with the medium-sized group 5.147. 

When the medium-sized group is a methyl group, and when the substituent cis to it is a hydrogen atom, as in the trans alkene -5.146, it appears that the Felkin-Anh conformation for the transition structure 5.147, with the medium-sized group inside, is still the lowest in energy in spite of the A1,3 allylic interaction, and attack... 

The reaction of endocyclic enamines with a,p-unsaturated ketones to afford cu-fused hydroindolones or hydroquinolones constitutes a complementary and highly useful annulation sequence developed extensively by Stevens and coworicers, see the reaction of (5) to give (6) in Scheme 7. ° The importance of stereoelectronic effects is highlighted in the reaction of (7) with methyl vinyl ketone, which provided only the alkylated product (8) and none of the expected ci5-hydroindolone (9)." The failure of intermediate (8) to cyclize in this case was attributed to nonbonded interactions between the aryl group and the side chain. This destabilizing allylic interaction s disfavors formation of conformer (10), the intermediate required for antiperiplanar addition of the enol nucleophile (Scheme 8). Cyclization via the alternate conformation would require a double boat-like transition state. 

The conformational properties of dihydrothiazine dioxides of type 250 have been examined by NMR spectroscopy.24 In general, N-unsubstituted derivatives, e.g., 260a-c, exist as a 1 1-3 1 mixture of the conformers 263 and 264. By contrast, N-substituted compounds, e.g. 261b and 262, favor the conformer 264. The conformer 263 is destabilized by an allylic interaction between R1 and R2, whereas the conformer 264 possesses an unfavorable 1,3-diaxial interaction between an oxide group and R1. Evidently, the latter interaction is less severe than the former when R1 and R2 are groups other than hydrogen atoms. 

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