Attack direct

If R is tertiary, RCOmay lose CO to give R, so that the alkylarene ArR is often a side product or even the main product. This kind of cleavage is much more likely with relatively unreactive substrates, where the acylium ion has time to break down. For example, pivaloyl chloride (McaCCOCl) gives the normal acyl product with anisole, but the alkyl product MesCPh with benzene. In the other mechanism an acyl cation is not involved, but the 1 1 complex attacks directly. 

Simpler chiral pyrrolidine thioethers, reported in 2004 by Skarzewski et al., proved to be effective ligands in the test reaction. The sense of the stereoinduction was in agreement with the nucleophilic attack directed at the allylic carbon located trans to the sulfur atom in the intermediate complex (Scheme 1.40). 

Haug M. and Brain P. (1978). Attack directed by groups of castrated male mice towards lactating and non-lactating intruders a urine-dependent phenomenon. Physiol Behav 21, 549-552. 

Grafting to create long chain branches employs well-known organic reactions to incorporate polymer chains that have reactive end groups. When there is no suitable reactive group on the backbone that can be attacked directly (as is the case with polymers such as polypropylene,... 

In general, Pd-catalyzed allylic substitutions with soft nucleophiles involve nudeophilic attack directly on the allyl unit, on the opposite face to that occupied by the metal. This is contrasted with the situation for hard nucleophiles where the initial attack occurs at the metal, with subsequent migration of the nudeo-phile to the allyl moiety - the addition to the allyl unit therefore occurring from the same face as the metal. Obviously, this has profound implications on the stereochemical outcome. 

It is worth mentioning that the photooxidation of porous silicon behaves differently [49]. Indeed, ETIR spectra show that there is a tremendous increase in vsi o, without a correspondingly large loss of vsi H peak intensity. The decrease of the vsi H band is offset by an increase in the vosi—h band, resulting in no net loss of hydride species on the surface during the course of the photooxidation reaction. These data apparently suggest that oxidation does not result in the removal of H atoms, implying that Si—Si bonds are attacked directly. 

Scheme 7.1 Proton attack directed at a metal or hydride center (schematically).

Nucleophilic reagents can attack the N—H hydrogen to yield an anion (15) which reacts readily with electrophiles at the nitrogen, oxygen and /3-carbon atoms (Scheme 3). Strong nucleophiles may attack directly at the carbonyl groups to afford (16,17) or in conjugate fashion to produce (18,19) these intermediates usually rearomatize to pyridines (see Chapter 2.06). Reactions of free radicals with pyridinones are not very numerous. 

One of the most characteristic types of ground-state reaction for alkenes is electrophilic addition, often involving a proton acid as addend or catalyst. In the excited state similar reactions can occur, with water, alcohols or carboxylic acids as commonly encountered addends. However, there is a variety of photochemical mechanisms according to the conditions or substrate used. In a few instances it is proposed that the electronically excited state is attacked directly by a proton from aqueous acid, for example when styrenes are converted to l-arylethanols (2.47 the rate constant for such attack is estimated to be eleven to fourteen orders of magnitude greater than that for attack on the ground state, and the orientation of addition is that expected on the basis of relativecarbonium ion stabilities (Markowni-kov addition). 

In general then, activation of C02 by coordination to the metal is apparently not required for insertion of C02 into a metal-amide or metal-alkox-ide bond. Instead, reaction between free ligand and C02 in solution may take place, or the C02 may attack directly the coordinated ligand, followed by a rearrangement to yield the product. 

The term substitution in an unrestricted sense is rather too broad to be useful in classification of radical reactions, since most of them result in replacement of one group by another. We have already seen typical examples of bond homolysis, in which a molecule dissociates to yield two radicals which combine with each other or with another molecule. We are primarily concerned in this section with those elementary reaction steps in which a radical attacks directly an atom of another molecule (Equation 9.64), displacing from the site of attack another group, and with the overall reaction schemes in which these elementary reactions occur. 

An alternative mechanism for the base-promoted reaction of silanes with silica has been described by Blitz et al.83 In this mechanism, the base attacks directly to the surface silanols. The bonded amine renders the silanol more nucleophilic which then attacks the silicon atom of an approaching silane, giving rise to a pentacoordinate intermediate. 

When the cyclopropene has a single carbomethoxy or aryl substitunt at the 3-position, the predominant stereoisomer of the enone obtained has the substituent cis-to the ketone 260 261). One possible explanation is the formation of a bicyclobutane by peracid attack from the side away from this substituent, followed by a stereocontrolled rearrangement. If the 3-substituent is hydroxymethyl, the reverse stereochemistry is observed in the enone, in agreement with a peracid attack directed by this group to the same face of the cyclopropene, followed again by rearrangement 262). 

There is a myriad of analytical solutions for steady-state conduction heat-transfer problems available in the literature. In this day of computers most of these solutions are of small utility, despite their exercise in mathematical facilities. This is not to say that we cannot use the results of past experience to anticipate answers to new problems. But, most of the time, the problem a person wants to solve can be attacked directly by numerical techniques, except when there is an easier way to do the job. As a summary, the following suggestions are offered ... 

The cyanide group is a typical group for promoting conjugate addition. It is possible for nucleophiles to attack directly at,the CN group but it is not very electrophilic so that these reactions tend to be thermodynamically controlled and attack is preferred in the conjugate position. 

On the other hand, lithium enolates derived from substituted endocyclic ketones have largely been exploited in the synthesis of steroids since the regioselectivity of their deprotonation can be controlled and high levels of 1,2- and 1,3-stereoselection occur9,418. The control is steric rather than electronic, with the attack directed to the less substituted ji-face of the enolate for conformationally rigid cyclopentanones, whereas stereoelectronic control becomes significant for the more flexible cyclohexanones. Finally, an asymmetric variant of the formation of a-branched ketones by hydration of camphor-derived alkynes followed by sequential alkylation with reactive alkyl halides of the resulting ketones was recently reported (Scheme 87)419. 

The more a carbonyl group is like that of protonated acrolein (Fig. 4.12), the more likely it is that all nucleophiles will attack directly at the carbonyl carbon atom. In agreement with this perception, and in contrast to its behaviour with methyl acrylate, ammonia reacts with acryloyl chloride at the carbonyl carbon atom to give acrylamide. 

Lead tetraacetate (LTA) and lead tetrakisfluoroacetate (LTFA) are ccmunon oxidants fw the introduction of the hydroxy group. It has been suggested that the oxidation proceeds via electrophilic attack directly onto oxygen, but this seems unlikely in view of the fact that aryllead species are well characterized compounds,and they are known to give the corresponding acetates on treatment with tii-fluoioacetic acid or acetic acid. Some examples of the use of LTA have already been described (see Sections 2.10.2.4 and 2.10.3). 

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