SN displacement reactions

An extensive study (6) of Sn displacement reactions of allyl halides using silyl anions/anionoids has provided the following regioselective alternatives ... 

SN displacement reactions, 27-9 a-Selenocyclohexanones, 77 Senecioyl chloride, 33 Silmagnesiation, platinum-catalysed, 8 Silametallation of terminal alkynes, 7-9 Silver acetylide, 49 Silver trifluoroacctate, 42,127 Silyl cuprates, 7... 

KINETIC ISOTOPE EFFECTS Sn DISPLACEMENT REACTIONS SOLVOLYSIS S/N ratio,... 

In the first of these sequences, often called the Torgov-Smith synthesis, the initial step consists in condensation of a 2-alkyl-cyclopentane-l,3-dione with the allyl alcohol obtained from 6-methoxy-l-tetralone and vinylmagnesium chloride. Although this reaction at first sight resembles a classic SN displacement, the reaction is actually carried out with only a trace of base. 

Azaloxan (12) is an antidepressant agent. Its synthesis can be accomplished starting with the reaction of catechol (7) and 3,4-dibromobutyronitrile (obtained by addition of bromine to the olefin) to give l,4-benzodioxan-2-ylacetonitrile (8). A series of functional group transformations ensues [hydrolysis to the acid (9), reduction to the alcohol (10) and conversion to a tosylate (11)] culminating in an SN-2 displacement reaction on tosylate 11 with l-(4-piperidinyl)-2-imidazolidi-none to give azaloxan (12) [3]. 

Electroless deposition should not be confused with metal displacement reactions, which are often known as cementation or immersion plating processes. In the latter, the less noble metal dissolves and eventually becomes coated with a more noble metal, and the deposition process ceases. Coating thicknesses are usually < 1 pm, and tend to be less continuous than coatings obtained by other methods. A well-known example of an immersion plating process that has technological applications is the deposition of Sn on Cu [17] here a strong complexant for Cu(I), such as thiourea, forces the Cu(I)/Cu couple cathodic with respect to the Sn(II)/Sn couple, thereby increasing the thermodynamic stability in solution of thiourea-complexed Cu(I) relative to Sn(II). 

Unsaturated nitriles have been obtained from the SN. reaction of 3-chloroalk-1 -enes [22] using tetra-n-butylammonium iodide as the catalyst. Under the basic reaction conditions, isomerism occurs such that not only is the 1-cyanoalk-2-ene obtained, but also the conjugated 1-cyanoalk-l-ene. Surprisingly, when tetra-n-butylammonium chloride is used, direct SN displacement of the chloro group occurs, followed by isomerization, to give the 3-cyanoalk-2-ene. 

The important chemistry of alkyl halides, RX, includes the nucleophilic (SN) displacement and elimination (E) reactions discussed in Chapter 8. Recall that tertiary alkyl halides normally are reactive in ionization (SN1) reactions, whereas primary halides, and to a lesser extent secondary halides, are reactive in Sn2 reactions, which occur by a concerted mechanism with inversion of configuration (Sections 8-4 to 8-7). 

At lower temperatures the oxonium salt or the alkyl hydrogen sulfate may react by an SN displacement mechanism with excess alcohol in the reaction mixture, thereby forming a dialkyl ether. Although each step in the reaction is reversible, ether formation can be enhanced by distilling away the ether as fast as it forms. Diethyl ether is made commercially by this process ... 

As a consequence of the polarity of the amide group, the lower-molecular-weight amides are relatively high-melting and water-soluble, as compared to esters, amines, alcohols, and the like. The few that are liquids, such as 7V,7V-dimethylmethanamide and l-methyl-l-aza-2-cyclopentanone, have excellent solvent properties for both polar and nonpolar substances. Therefore they are good solvents for displacement reactions of the SN type (Table 8-5). 

One problem with this method is that the workup must be done carefully as die amine products tend to complex tenaciously with the aluminum salts formed from the LAH upon workup and thus are not recovered easily. There are standard workups which avoid these issues, but these should be followed carefully. Reduction of azides by catalytic reduction, phosphine or phosphite reagents, or Sn(II) chloride are all effective methods. The azides are also available from displacement reactions and give primary amines upon reduction. 

A pentacyclic diterpene 1 called dictyoxetane contains a most unusual subunit, a 2,7-dioxatricyclo[4.2.1.03>8 ]nonane. During model studies designed to provide access to this key subunit the bicyclic ether 2 was synthesised in the hope that Sn displacement would generate the unsaturated tricyclic oxetane. There was no reaction when 2 was treated with base. Reaction with a catalytic amount of p-toluenesulfonic acid in DMF at 75°C for 24 hours resulted only in formation of 4-methylacetophenone. The hydroxy mesylate 2 is also reported to decompose to 4-methylacetophenone on storage. 

The second reaction is an Sn displacement of a reasonable leaving group (chloride) by a rather b. S. Furniss etal., Vogel s texttx>ok eak nucleophile (acetate). The reaction is very slow unless catalysed by iodide - a better of ofgan/c chem/stry (5th edn),... 

The substitution reactions at the anomeric carbon usually proceed easily and via a SnI mechanism in most cases. However, it is not the case in the nucleophilic substitutions at nonanomeric sites. Because of the presence of vicinal electron-withdrawing substituents (OR or NHR) which strongly destabilize the intermediate carbocations, Sn2 displacement reactions instead of Sn 1 reactions are favored (O Scheme 1). 

Thermodynamic control of many insertion reactions is further demonstrated by (1) their reversibility and (2) the occurrence of various displacement reactions which are directed to the most stable products. Point (2) is neatly indicated by the contrasting behavior of the hard Si" or the soft Sn" reactions ... 

When a substituent with a good leaving ability is present in the 1,2,4-triazine ring at C-3, C-5, and/or C-6, nucleophilic substitution reactions can occur. Many SN reactions are described in the literature. In Sections IV,A-C, examples of displacement of one, two, and three nucleofugic groups are discussed. In this article, the displacement reactions are not exhaustingly reviewed. We show only the fundamental reaction features, their value, and scope for application in the synthesis of functionalized 1,2,4-triazine derivatives. As far as the mechanisms of nucleophilic sub-... 

---