Addition of electrophiles

Addition of Electrophiles.—Cr + and CrCHCla + react in perchloric acid solution to give CrCH2Cl + and equal quantities of Cr + and CrCl +. At high acid concentrations rate law (1) is followed which suggests that the first and rate-determining step is 

Preliminary accounts of the reactions of HCl with complexes [Pt(C CH)2L2] (reported in Vol. 5, p. 386) have been superseded by a full paper. A feature of the reactions is the intermediacy of platinum-stabilized vinyl carbonium ions [Pt-(C+=CH2)] and another paper by the same authors is concerned with mechanistic studies of the formation and decomposition of alkoxycarbene complexes, reaction (2), which can also be rationalized in terms of a mechanism involving such species 

Isomerization and Exchange.— The rate of isomerization of cyclopropylmethyl-cobalamin (10)- (11) is virtually independent of the polarity of the solvent and is unaffected by oxygen. This excludes a mechanism involving polar intermediates and the rearrangement probably proceeds either via pathways involving homo-lytic or heterolytic Co—C bond fission or alternatively by a mechanism in which the Co—C bond remains intact. 

A further kinetic study of the vinyl rearrangement, in this case reaction (3), has been reported with a view to establishing the effect of solvent on the reaction rate. Except for aprotic solvents, reasonable correlations between log/c [/ = first-order rate constant for reaction (3)] and empirical solvent parameters were found. The slopes 

---

Chemical Properties. The chemistry of ketenes is dominated by the strongly electrophilic j/)-hybridi2ed carbon atom and alow energy lowest unoccupied molecular orbital (LUMO). Therefore, ketenes are especially prone to nucleophilic attack at Cl and to [2 + 2] cycloadditions. Less frequent reactions are the so-called ketene iasertion, a special case of addition to substances with strongly polarized or polarizable single bonds (37), and the addition of electrophiles at C2. For a review of addition reactions of ketenes see Reference 8. 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

Electrophile Addition. The addition of electrophilic (acidic) reagents HZ involves two steps the slow transfer of hydrogen ion from Z to the butylene to form a carbocation and, a rapid combination of the carbocation with the base Z. 

Fig. 6. FDP-aldolase-catalyzed addition of electrophiles (94) with DHAP (139—146). Representative R groups ia (94) are given as (a—j) (a) methyl, CH (b)...

Conjugated dienes undergo several reactions not observed for nonconjugated dienes. One is the 1,4-addition of electrophiles. When a conjugated diene is treated with an electrophile such as HCl, 1,2- and 1,4-addition products are formed. Both are formed from the same resonance-stabilized allylic carbocation intermediate and are produced in varying amounts depending on the reaction conditions. The L,2 adduct is usually formed faster and is said to be the product of kinetic control. The 1,4 adduct is usually more stable and is said to be the product of thermodynamic control. 

In favor of a dihydrogen intermediate, the following should be noted (i) the addition of HX molecules to OsHCl(CO)(P Pr3)2 is a useful synthetic route for dihydrogen compounds (vide infra), (ii) the electrophilic character of the dihydrogen complexes has been demonstrated,38 and (iii) the addition of electrophiles to metal alkynyl complexes is a general method of preparing vinylidene compounds.39... 

Addition of electrophiles to a coordinated benzenoid ligand is a characteristic reaction of 20-e complexes. [Co(C5H5BPh)2] in 59 is readily protonated at C-2, producing complex 79 (60) which alternatively can be obtained by hydride addition to cation 61 (69). Formal electrophilic addition is also observed in the reaction of [Co(C5H5BMe)2] in 58 with Phi which affords the two isomeric phenylation products 80 and 81 (60). 

Syntheses of the monooxaadamantane skeleton may be classified as follows (a) addition of electrophiles to bicyclononadiene (39) (b) ring closure of bicyclononadione (31) or its monomethylene analog (35) (c) dehydration of alcohols of the type 105 (d) radical cleavage of 2-ada-matanol (106) (e) miscellaneous. 

Benzotriazole-related methodology publications appeared in 2006. Reaction of 1-formylbenzotriazole with triphenylphosphine/carbon tetrachloride afforded l-(2,2-dichlorovinyl)benzotriazole, where lithiation followed addition of electrophiles gave a variety of functionalized M-(ethynyl)benzotriazoles <06T3794>. Novel mono- and symmetrical di-/V-hydroxy- and IV-aminoguanidines were readily prepared from the reaction of diverse hydroxylamines or hydrazines with reagent classes di(benzotriazol-l-yl)methanimine, (bis-benzotriazol-1 -y 1-methy lene)amines, benzotriazole-1 -carboxamidines, benzotriazole-1 -... 

Based on the data collected in this section, one must conclude that the addition of radicals to dienes is certainly rapid enough to compete against the typical chain-breaking processes and that especially the addition of electrophilic radicals to polyenes appears to bear significant potential. Terminally substituted polyenes are likely to be unsuitable for radical addition reactions due to their lower addition rates and to undesirable side reactions. 

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. 

Although the most characteristic reaction of the pyrrole nucleus is the predominant addition of electrophiles to the C-2 position, it is interesting to note that contrary to previous assumptions, sulfonation of pyrrole and its 1-methyl derivative with sulfur trioxide-pyridine complex affords mainly the 3-sulfonated pyrroles <00TL6605>. As Mizuno wisely points out, it is likely that some of the pyrrole-2-sulfonates reported previously are actually pyrrole-3-sulfonates. 

The addition of electrophiles at the oxygen atom could activate nitronates as C-electrophiles, analogously to that in the case of the carbonyl group (297). However, this aspect of reactivity of nitronates has remained unknown until very recent times. 

The tandem zirconocene-induced co-cyclization of dienes or enynes/insertion of allyl carbenoid/addition of electrophile is a powerful method for assembling organic structures. Two illustrations of its application are the synthesis of the dollabelane natural product acetoxyodontoschismenol 99 [57,62,63] and the one-pot construction of linear terpenoids 100 (Scheme 3.25) [59,64],... 

In contrast to the rich chemistry of alkoxy- and aryloxyallenes, synthetic applications of nitrogen-substituted allenes are much less developed. Lithiation at the C-l position followed by addition of electrophiles can also be applied to nitrogen-containing allenes [10]. Some representative examples with dimethyl sulfide and carbonyl compounds are depicted in Scheme 8.73 [147, 157]. a-Hydroxy-substituted (benzotriazo-le) allenes 272 are accessible in a one-pot procedure described by Katritzky and Verin, who generated allenyl anion 271 and trapped it with carbonyl compounds to furnish products 272 [147]. The subsequent cyclization of 272 leading to dihydro-furan derivative 273 was achieved under similar conditions to those already mentioned for oxygen-substituted allenes. 

Additions of electrophiles to allenylsilanes have been shown to proceed by an anti SE2 process (Eqs. 9.52 and 9.53) [59]. Reactions of enantiopure allenylsilanes yield propargylic adducts with <1% racemization. 

Weak nucleophile-electrophile interactions (and the donor-acceptor complexes) are considered precursors in aromatic electrophilic substitutions133 and in additions of electrophiles to C=C double bond of olefins the first step (the addition of the electrophile to an electron-rich substrate) is probably the same for both reactions. 

Addition of electrophilic mercury(II) salts to carbon-carbon double bonds in nucleophilic solvents (i.e. oxymercuration, solvomercuration etc.) is a well documented methodology in organic synthesis146. In these reactions a mercuric salt, usually the chloride or... 

The addition of electrophilic reagents to chiral a,/3-unsaturated sulfoxides is also accompanied by asymmetric induction. Stirling and Abbott (318,322) found that the addition of bromine to the optically active (.R)-vinyl-p-tolyl sulfoxide 319 yields a mixture of diastereo-meric a,/3-dibromosulfoxides 320. Oxidation of this mixture gives the optically active sulfone 321, with a center of chirality at the a-carbon atom only. The optical purity (32%) of this sulfone was estimated by comparing its specific rotation with that obtained as a result of oxidation of diastereomerically pure sulfoxide (/ )-320. The assignment of configuration at the a-carbon atom was based on the analysis of the polarizabilities of substituents. 

Electrophilic vinylidene complexes, capable of reacting with non-carbon nucleophiles to yield Fischer-type carbene complexes, can be obtained by addition of electrophiles to alkynyl complexes (Figure 2.11, Table 2.7, Entries 11, 12) [134,144]. 

Fig. 3.25. Generation of carbene complexes by addition of electrophiles to carbyne complexes [533,538,540,543],...

Of these potential approaches addition of electrophiles only has attained some relevance in the preparation of non-heteroatom-substituted carbene complexes. 

The addition of electrophiles like Me" [30] to the complexes 4 results in the formation of lla,b corresponding to the reaction in Eq. (11). Whereas complex 11a is not stable at room temperature and must be prepared and... 

Addition of electrophiles to diynyl complexes is expected to occur at either or C, the latter being favored if sterically demanding ligands shielding C and are present. The products are butatrienylidenes and the chemistry of these species is closely related to the chemistry of the related unsaturated carbene ligands (Section VIILB). " ... 

The addition of electrophilic agents to terminal nitride ligands can be applied to the synthesis of imido complexes. This has been demonstrated with triphenylcarbonium salts and compounds of the formula [Re NC(C6H5)3 X4] (X = C1, Br, F, NCS) have been studied by EPR spectroscopy and show similar spectroscopic features to their [Re NB(C6F5)3 X4] analogues. " ... 

One of the most general and useful reactions of alkenes and alkynes for synthetic purposes is the addition of electrophilic reagents. This chapter is restricted to reactions which proceed through polar intermediates or transition states. Several other classes of addition reactions are also of importance, and these are discussed elsewhere. Nucleophilic additions to electrophilic alkenes were covered in Chapter 1, and cycloadditions involving concerted mechanisms will be encountered in Chapter 6. Free-radical addition reactions are considered in Chapter 10. 

---