Backside attack, and the

Haloalkenes fail to undergo Sfjl reactions because the alkenyl carbocations produced through ionization of the carbon-halogen bond are too unstable. They fail to undergo Sn2 reactions because the planar geometry of the aikene does not allow backside attack, and the electron-rich nature of the aikene does not attract nucleophiles. 

When substitution occurs by an Sn2 mechanism, the nucleophile directly attacks the substrate, with the angle of approach being 180" to the C-L bond. This is called "backside attack," and the reaction proceeds with inversion of stereochemistry, the so-called "Walden inversion." The C-L bond is being broken concurrently with the formation of the C-Nu bond, so both the substrate, R-L, and the nucleophile are involved in the transition state of the rate-determining step. Reactions in which two reactants are involved in the transition state of the rate-determining step are termed bimolecular, and the rate of such processes depends on the concentration of the substrate and the nucleophile, as shown in Equation 14.5, where k2 is the second-order rate constant. 

Tertiary haloalkanes react through an Sjjl mechanism because the steric hindrance disfavors Sjj2 backside attack, and the attached alkyl groups stabilize a carbocation. 

The reaction shown in Eq. 12.67 gives complete inversion at the stereogenic center attached to Fe. This requires a backside attack, and the most logical way for this to occur is to start with an oxidative addition of the Br2, and then nucleophilic attack by bromide with Fe as the leaving group. The sequence of electrophilic addition of X2 to the metal followed by nucleophilic attack on the ligand is common for middle-to-late transition metals. Interestingly, when phenyl is in the 3-position, the reaction proceeds with retention. Retention is best explained by a double inversion, and the phenonium ion has been substantiated as the intermediate formed (Eq. 12.68). 

Because the substrate is a 3° haloalkane (which disfavors backside attack) and the solvent is polar protic (and stabilizes a carbocation), the ether is formed by the S jl mechanism. 

The one general exception to the rule that ethers don t typically undergo Sn2 reactions occurs with epoxides, the three-membered cyclic ethers that we saw in Section 7.8. Epoxides, because of the angle strain in the three-membered ring, are much more reactive than other ethers. They react with aqueous acid to give 1,2-diols, as we saw in Section 7.8, and they react readily with many other nucleophiles as well. Propene oxide, for instance, reacts with HC1 to give l-chloro-2-propanol by Snj2 backside attack on the less hindered primary carbon atom. We ll look at the process in more detail in Section 18.6. 

Identify the substitution pattern of the two epoxide carbon atoms—in this case, one carbon is secondary and one is primary. Then recall the guidelines for epoxide cleavages. An epoxide with only primary and secondary carbons usually undergoes cleavage by SN2-like attack of a nucleophile on the less hindered carbon, but an epoxide with a tertiary carbon atom usually undergoes cleavage by backside attack on the more hindered carbon. In this case, an S]sj2 cleavage of the primary C—O epoxide bond will occur. 

Furthermore, 48 solvolyzed 350 times faster than its endo isomer 51. Similar high exo/endo rate ratios have been found in many other [2.2.1] systems. These two results—(1) that solvolysis of an optically active exo isomer gave only racemic exo isomers and (2) the high exo/endo rate ratio—were interpreted by Winstein and Trifan as indicating that the 1,6 bond assists in the departure of the leaving group and that a nonclassical intermediate (52) is involved. They reasoned that solvolysis of the endo isomer 51 is not assisted by the 1,6 bond because it is not in a favorable position for backside attack, and that consequently solvolysis of 51 takes... 

C X bond, but not from B because only the has such an orbital. If the intermediate is in conformation B, the OR may leave (if X has a lone-pair orbital in the proper position) rather than X. This factor is called stereoelectwnic control Of course, there is free rotation in acyclic intermediates, and many conformations are possible, but some are preferred, and cleavage reactions may take place faster than rotation, so stereoelectronic control can be a factor in some situations. Much evidence has been presented for this concept. More generally, the term stereoelectronic effects refers to any case in which orbital position requirements affect the course of a reaction. The backside attack in the Sn2 mechanism is an example of a stereoelectronic effect. 

The above observations with respect to the reactivities of monomers IV-VI can be explained by postulating a direct interaction of the carbonyl and the ether functional groups with the propagating cationic center. This can occur by either an inter- or intramolecular process. As shown in equation 5, intramolecular backside attack by the ester carbonyl group of the d,l-trans IV isomers at either carbon of the protonated or alkylated epoxy group gives rise to bicyclic dioxacarbenium ions IX and X. 

Another ring-opening substitution reaction was observed for tricyclane 55 the attack occurred exclusively at the tatiary carbon, not at the quaternary one. The chiral isomer 56, of the symmetrical 55, has two 3 -4 bonds either of which may be the site of spin and charge, possibly in an equilibrium. Regardless of the actual structure of the radical cation, it appears that the attack of the nucleophile is less hindered at the carbon further removed from the dimethyl-substituted bridge (approach a). The isolated product 57 is optically active, and formed by backside attack on the less hindered carbon. ... 

The nucleophilic capture of tricyclane radical cations 115 " and 117 " supports the role of conventional steric hindrance 115 reacts at the 3° carbon ( 116 ), whereas the chiral isomer 117 + is captured by backside attack at the less hindered 3° carbon ( 118 ). " Both reactions are regio- and stereospecific and avoid attack at the neopentyl-type carbon (denoted by an asterisk). 

The nucleophilic O" group displaces the Cl atom (as Cl") by an intramolecular Sfj2-type process which requires a backside attack. In the irons isomer, the O and Cl are properly positioned for such a displacement, and the epoxide is formed. In the cis isomer, backside attack cannot occur and the epoxide is not formed. 

An elegant and clever experiment along these lines (Lieder and Brauman, 1974) confirms that the gas phase SN2 reaction proceeds by a predominant backside attack. Analysis of neutral products in the icr experiment shows that for reaction (27), backside attack, and consequently inversion at the reaction centre, occurs to the extent of 92 + 6%. For reaction (28) the same type of experiment indicates that inversion of configuration amounts to 87 + 7% of the reaction. 

The increased ionic freedom between the propagating polymer ion and its gegen ion occurs concurrently with increased space separation between the two ion species. The studies of Schuerch and co-workers and of Yoshino and co-workers (98) with deuterated acrylates and by Natta and co-workers (99) with sorbic esters show that this increased separation allows trans addition to mono olefins and 1,4 trans addition to conjugated dienes before complete loss of isotactic steric control at the end of the chain. The increased freedom between the propagating ion and the less closely associated gegen ion appears to result in a distortion of the cyclic transition state which permits backside attack at the beta position of the incoming acrylate monomer and 1,4 attack on the incoming sorbate monomer. 

The optically active Mo complexes 13a and 13b are configurationally stable28,29 Their optical rotations in solution remain constant over long periods of time. If, however, a trace of free R—(—)-a-phenyl ethyl isonitrile30,31 is added to solutions of 13a at room temperature, its optical rotation decreases within about one hour to values around 0 28,29). We explain this behavior by a backside attack of the free isonitrile on the optically active complex 13a according to Eq. (11). 

However, essentially only one of them—namely, B—undergoes a nucleophilic backside attack by the hydroxy group. The brominated tetrahydrofuran D is produced via the oxonium ion C. An analogous intramolecular backside reaction by the alcoholic OH group in the bromonium ion iso-B is energetically disfavored and hardly observed. The result is that the bromonium ion iso-B can revert to the starting material A, whereby the overall reaction takes place almost exclusively via the more reactive bromonium ion B. 

Unsymmetrically substituted C=C double bonds are hydrated according to the same mechanism (Figure 3.49). The regioselectivity is high, and the explanation for this is that the mer-curinium ion intermediate is distorted in the same way as the bromonium ion in Figure 3.44. The H20 preferentially breaks the stretched and therefore weakened Csec—Hg bond by a backside attack and does not affect the shorter and therefore more stable C. —Hg bond. 

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