SNl-type mechanisms

Allylic nitro groups are readily displaced by nucleophiles via an SNl-type mechanism. Thus, nitro groups with heteroatoms at the OC- or P-positions (for example, a- or P-nitrosulfides) are expected to be cleaved in a similar way. In fact, the nitro group in a-nitrosulfides is replaced by nucleophiles in the presence of a Lewis acid31 or acetic acid.32 The nitro groups in the reaction of Eqs. 7.27 and 7.28 are cleanly replaced by CN, allyl, or PhS group on treatment with MejSiY (Y = CN, allyl) in the presence of SnCl4 or simple treatment with PhSH in AcOH. 

The SNl-type process occurs mostly when B is a tertiary atom or has one aryl group and at least one other alkyl or aryl group. In other cases, the SN2-type process is more likely. Inversion of configuration (indicating an SN2-type process) has been shown for a neopentyl substrate by the use of the chiral neopentyl-l-d alcohol.18 On the other hand, there is other evidence that neopentyl systems undergo rearrangement by a carbocation (SNl-type) mechanism.19... 

Evidence for the SNl-type mechanism is provided by fluorodehydroxylation of diastereomeric benzylic alcohols both diastercomers give the same mixture of products which implies that bond breaking precedes bond making in precursors that readily form carbenium ions. Evidence for an SN2 displacement is based on the results of fluorodehydroxylation of epimeric a-amino-/ -hydroxypropanoic acids which proceeds with predominant inversion of configuration.40... 

In 95% acetone/water the relative rates of hydrolysis are 0.545 (4-MeO), 0.493 (4-Me) and 1.00 (H), indicating that 4-MeO benzoyl chloride is tending to react partly by an SN1 mechanism. In 95% acetone/water, 2 4 6 trimethyl-benzoyl chloride therefore reacts by an SNl-type mechanism and would not be expected to show an effect with added hydroxide ion. In 80% (v/v) acetone/ ethanol, the reaction is again SN2-type. 

Taking into account the lack of chemoselectivity and stereoselectivity of these silver ion-mediated reactions, it is reasonable to postulate an SNl-type mechanism for this process, leading to the formation of oxonium ion 14, despite the electron-withdrawing effect of the CF3 group (see Scheme 6.8). The stereochemical outcome should be ascribed to the steric or electronic preference of methanol addition to two possible diastereo-faces of the trifluoromethyl oxonium ion 14, in which the interactions are probably different from those in nonfluorinated DHA derivatives. In order to disfavor the formation of oxonium 14, the reaction was performed without silver salt in MeOH. While the reaction rate did not decrease significantly, only 23% of glycal 18 was obtained. Furthermore, the... 

In the nucleophilic substitution reactions of isopropyl />-substituted benzenesul-fonates with amines in acetonitrile an SNl-type mechanism was favoured. The alkaline hydrolyses of a range of ortho- and para- substituted phenyl tosylates in various solvents from pure water to 80% DMSO have been studied at different temperatures. The results showed that altering the concentration of hydroxide ion affected the activation entropy, especially for the or//jo-isomers. 

Another problem with the reaction of phenols with aziridines is the selectivity between O-alkylation vs C-alkylation. A recent report has identified that the use of (ArO)3B selects for C-alkylation <06OL2627>. Most of the examples reported in this paper showed less than 5% of the O-alkylation product. What is interesting about this report is the stereochemistry of the product. While the mechanism is not known, the product is formally an SNl type product. Generally less than 5% was the product of inversion of configuration (the Sn2 product). In addition to the A-tosyl, both the A-Cbz and A-Dpp aziridines gave excellent yields of aziridine-opened product. 

Acid hydrolysis of an octahedral metal ion complex is typically a dissociative or SNl-type reaction. In the case of base hydrolysis, reactions tend to display SN2-type reaction mechanisms, although others take place by what is termed an SnI-conjugate base mechanism. The latter involves attack by an electrophile to abstract a proton... 

Alkyl chloroformates, R0C(=0)C1, might be expected to hydrolyse rapidly by an SNl-type process, reminiscent of the hydrolysis of alkoxy ethers such as chloromethyl ether. Similarly, carbamates would be expected to react by a similar mechanism because of the possible stabilisation of the acylium ion formed, viz. 

Several triphenylvinyl sulphonates were also investigated (cases 33-37 of Table 7) and the reaction mechanism shown to be an SNl-type... 

Tyr-325 (b). After release of the reducing end moiety (ROH, c) the initial state of the active site is regenerated by proton transfer via a water chain ([H20] , d) and re-binding of polysiahc acid. In total, the stmctural data allow the conclusion that the endosialidase mechanism is an SNl-type reaction, i.e., the sialic acid stereochemistry is directly inverted into the p-anomer [106]. By contrast, exosialidases form a covalently bound intermediate which is released by a water molecule thus the a2,8-linked sialic acid residue is released as a-anomer tmderlying the mutarotation towards the energetically favored p-conformation. However, no crystal structure of a non-cleavable substrate is available as yet, which would allow an unambiguous elucidation of the endosialidase mechanism [106]. 

These effects were hidden in previous studies, which were restricted to hydrolysis in mixtures with aprotic solvents or in sulfuric acid where only one product is obtained. The experimental data indicated changes in reaction mechanism depending on water content and solvent polarity. In agreement with previous studies, they indicated the operation of two simultaneous reaction channels in the different media used, with an SNl-type reaction favoured in more aqueous media. In the methanolysis of p-substituted benzenesulfonyl chlorides in various solvent combinations, the mechanism appeared to consist of a mixture of the SnI and Sn2 reaction pathways, but in methanol-ethylene glycol mixture, the results favoured the Sn2 mechanism. ... 

The product exhibits a remarkable atropisomerism [61] determined by the rotation barrier around the bonds of quaternary carbon center. After optimization of the reaction conditions with different phosphorous triftamides and by varying the temperature, solvent, catalyst loading, and concentration, the bis indole 17 was obtained with a remarkable 72% ee. In the supposed mechanism, a stabilized vinylic carbenium indolyl intermediate (Figure 26.4) undergoes a Snl-type reaction by formation of a diastereoisomeric ion pair. 

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