SNl-type process

In solution the situation is more complicated still, because the solvent can play a more or less active role in the reaction. For example, solvent turns out to be involved to some extent as a nucleophile in many SNl-type processes, where highly electrophilic intermediates are involved (Lowry and Richardson, 1987, pp. 335 ff.). Here a degree of bond making accompanies what had originally been regarded as uncomplicated rate-determining bond-breaking reactions. 

With respect to B this is an SNl-type process. If inversion occurs at B, it is likely that the first two steps are concerted, that a carbocation is not an intermediate, and that the process is SN2-Iike ... 

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... 

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. 

The importance of fluorinated organic componnds both in medicinal chemistry and biochemistry has resulted in much recent attention towards efficient carbon fluorine bond formation [30]. The reactions developed include a very successful electrophilic asymmetric mono-fluormation of 1,3-dicarbonyl compounds [31]. A nucleophilic variant was also investigated. In this context, the groups of Togni and Mezzetti have established that ruthenium Lewis acids could efficiently catalyze fluorination reactions [32]. In the presence of [Ru(l,2-bis(diphenylphosphino)ethane)2Cl][PF6] (8) (10 mol%), fert-butyl iodide reacted at room temperature with TIF (1.1 equiv.) to yield fert-butyl fluoride (84% yield). This reaction was extended successfully to a range of organic halides (Entries 1-3, Scheme 10.19). The use of the chiral complex [Ru((lS,2S)-N,N bis[2-diphenylphos-phino)benzylidene]diaminocydohexane))Cl][PF6] (9) showed modest chiral induction at the outset of the reaction (Entry 4, Scheme 10.17). The near-racemic mixture obtained at completion points to an SNl-type process in this nucleophilic halide... 

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... 

Although the Michaelis-Arbuzov reaction has been widely used for the preparation of phosphonates and the path shown in Scheme 10.50 has been generally accepted, it appears that if the alkyl group attached to oxygen is capable of leaving without the help of the nucleophile, some portion of the reaction may proceed by an SNl-type process. 

The dimethoxytrityl group was next removed, in an SNl-type process, from the 5 -hydroxyl with a trace of dichloroacetic acid in dichloromethane and the next deoxynucleoside was readied for addition (Equation 14.4). 

Reactions of vinyl derivatives under solvolytic conditions may occur via mechanisms other than a SNl-(El)-type process and yet lead to the same type of products. 

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... 

The nature of HCF operations makes a criticality event highly unlikely however, the HCF will process and store fissile materials in sufficient quantity to warrant a formal CSA. This section identifies the types of fissile materials that may be present within the facility, their forms and inventories, and the actual location and configuration where such materials are handled or stored. CSA s are prepared, reviewed, and approved in accordance with SNL ES H. Supplement GN470072 to evaluate criticality concerns and document the basis for operating limitations for all planned uses of fissile materials in the HCF. 

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