Nucleophilic capture

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

FIGURE 22 5 The diazo mum ion generated by treatment of a primary al kylamine with nitrous acid loses nitrogen to give a car bocation The isolated prod ucts are derived from the carbocation and include in this example alkenes (by loss of a proton) and an al cohol (nucleophilic capture by water)... 

Allenes react with other typical electrophiles such as the halogens and mercuric ion. In systems where bridged-ion intermediates would be expected, nucleophilic capture generally occurs at the allylic position. This pattern is revealed, for example, in the products of solvent capture in halogen additions and by the structures of mercuration products. ... 

A number of examples of nucleophilic capture of n-complexes have also been uncovered in the study of nitration of alkylated benzenes in acetic acid. For example, nitration of 3 at 0°C leads to formation of 4 with acetate serving as the nucleophile ... 

A similar intramolecular nucleophilic capture of an allylic sulfenate generated thermally from the corresponding sulfoxide was also reported for the facile transformation of the azetidinone 61 into a new 3-acetylthio-2-thiacephem ring system 62 (equation 29)127. 

The addition reactions discussed in Sections 4.1.1 and 4.1.2 are initiated by the interaction of a proton with the alkene. Electron density is drawn toward the proton and this causes nucleophilic attack on the double bond. The role of the electrophile can also be played by metal cations, and the mercuric ion is the electrophile in several synthetically valuable procedures.13 The most commonly used reagent is mercuric acetate, but the trifluoroacetate, trifluoromethanesulfonate, or nitrate salts are more reactive and preferable in some applications. A general mechanism depicts a mercurinium ion as an intermediate.14 Such species can be detected by physical measurements when alkenes react with mercuric ions in nonnucleophilic solvents.15 The cation may be predominantly bridged or open, depending on the structure of the particular alkene. The addition is completed by attack of a nucleophile at the more-substituted carbon. The nucleophilic capture is usually the rate- and product-controlling step.13,16... 

From a synthetic point of view, the participation of water in brominations, leading to bromohydrins, is the most important example of nucleophilic capture of the intermediate by solvent. To favor introduction of water, it is desirable to keep the concentration... 

Internal nucleophilic capture of seleniranium ion is governed by general principles similar to those of other electrophilic cyclizations.96 The stereochemistry of cyclization can usually be predicted on the basis of a cyclic TS with favored pseudoequatorial orientation of the substituents. 

A deuterium-labeling study of a reaction of this type demonstrated syn stereoselectivity in both the oxypalladation and P-elimination, which indicates that the cyclization occurs by internal migration, rather than by an anti nucleophilic capture.113 This particular system also gives products from double-bond migration that occurs by reversible Pd(II)-D addition-elimination. 

Polyene Cyclization. Perhaps the most synthetically useful of the carbo-cation alkylation reactions is the cyclization of polyenes having two or more double bonds positioned in such a way that successive bond-forming steps can occur. This process, called polyene cyclization, has proven to be an effective way of making polycyclic compounds containing six-membered and, in some cases, five-membered rings. The reaction proceeds through an electrophilic attack and requires that the double bonds that participate in the cyclization be properly positioned. For example, compound 1 is converted quantitatively to 2 on treatment with formic acid. The reaction is initiated by protonation and ionization of the allylic alcohol and is terminated by nucleophilic capture of the cyclized secondary carbocation. 

Carbonyl-ene reactions can be carried out in combination with other kinds of reactions. Mixed acetate acetals of y,8-enols, which can be prepared from the corresponding acetate esters, undergo cyclization with nucleophilic capture. When SnBr4 is used for cyclization, the 4-substituent is bromine, whereas BF3 in acetic acid gives... 

Photolysis in MeOD indicates that all three of the above products arise via initial protonation of the double bond to form the 2-phenyl-2-norbomyl cation, which then undergoes either nucleophilic capture by solvent, hydride... 

Extrapolation of the data to zero time suggests that the endo acetates (65) and (67) are produced in amounts as great as or greater than the exo isomer. Solvolysis studies of the bicyclo[3.1.0]hex-2-en-6-yl cation reveal that nucleophilic capture occurs preferentially from the exo side to give (66) rather than (67). Similarly, solvolysis of cation (63) leads to exo product (64) in at least a 90% yield. Photolysis of benzene in deuteriophosphoric acid results in (68), in which all the deuterium is incorporated into the 6-endo position ... 

The vinblastine alkaloids are important chemotherapeutic materials, and are well known for their antitumor properties. A two-electron oxidation of alkaloid cantha-ranthine (125) followed by nucleophilic capture of the resulting imminium ion (126) by Cio of vindoline (127), afforded (128) and provided access to this important class of materials (Scheme 29) [56]. 

Once the methoxy group has been installed and nucleophilic capture of the intermediate has occurred, the product (132) can be treated with an enol ether (e.g. 133) and titanium tetrachloride to affect C-C bond formation adjacent to nitrogen. This sequence served nicely in syntheses of both indolizidine alkaloids elaeokanine A (135) and C (136). 

The table shows the effect on product ratio of ultrasonic irradiation (Kerry Pulsatron cleaning bath 35 kHz 50 W) during electrolysis. Here there is only 8% of the bicyclohexyl dimeric one-electron product, with approximately 41 % of the two-electron product from nucleophilic capture of the intermediate carbocation. The preponderance of cyclohexene (32 %) over cyclohexane (> 3 %) shows its formation is by proton loss from the carbocation intermediate, since free-radical routes to cyclohexene (i. e. hydrogen atom abstraction) also produce cyclohexane in equal if not greater amounts... 

Radical cations as well as cations are good electrophiles dependent on steric and electronic effects and, therefore, potent targets for nucleophilic capture. 

Concerning their structure and reactions, organic radical cations have been the focus of much interest. Among bimolecular reactions, the addition to alkenes and their nucleophilic capture by alcohols, which lead to C—C and C—O bond formation, respectively have been investigated in detail. Unimolecular reactions like geometric isomerization and several other rearrangements have also attracted attention. 

NUCLEOPHILIC CAPTURE OF CYCLOPROPANE RADICAL CATIONS CORRELATIONS BETWEEN RADICAL CATION STRUCTURE AND REACTIVITY... 

However, electron transfer-induced photoreactions in the presence of nucleophiles have attracted by far the greatest attention a rich variety of cyclopropane systems have been subjected to these reaction conditions. We will consider several factors that may affect the structure of the radical cations as well as the stereo- and regiochemistry of their nucleophilic capture. Factors to be considCTcd include (1) the spin and charge density distribution in the cyclopropane radical cation (the educt) (2) the spin density distribution in the free-radical product (3) the extent of... 

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