Heterolytic cleavage of the

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

First, let us consider the formation of ions from covalently bound species, i.e., the heterolytic cleavage of the covalent (or partially covalent) bond. Charge separation under the influence of the solvent generates an ion pair in a process called ionization this ion pair may then separate into free ions in a dissociation step (Eq. 8-18). 

Compound 104 could not be obtained from 103, and a hypothesis about its formation considered the (homolytic or heterolytic) cleavage of the O—N bond (Scheme 43) (68TL2417). The sensitized reaction didnotgive adifferentresult the author supposed that the reaction involved the excited triplet state of the molecule. When the reaction was carried out in methanol, 104 was obtained in 8% yield... 

In certain cases, e.g. with Z = tert-butyl, the experimental findings may better be rationalized by an ion-pair mechanism rather than a radical-pair mechanism. A heterolytic cleavage of the N-R bond will lead to the ion-pair 4b, held together in a solvent cage ... 

The retrosynthetic operations that we have addressed thus far have not resulted in significant structural simplification. After all, intermediate 6 still possesses a linear fusion of four rings and six contiguous asymmetric carbon atoms. But, nevertheless, intermediate 6 could potentially be derived in one step from intermediate 8, a polyunsaturated monocyclic compound containing only one stereogenic center. Under conditions that would be conducive to a heterolytic cleavage of the C-OH bond in 8, it is conceivable that the resultant tertiary allylic carbonium ion 7 would participate in a... 

Potassium peroxodisulphate (K2S2Og) also oxidizes sulphoxides to sulphones in high yield, either by catalysis with silver(I) or copper(II) salts at room temperature85 or in pH 8 buffer at 60-80 °c86-88. The latter conditions have been the subject of a kinetic study, and of the five mechanisms suggested, one has been shown to fit the experimental data best. Thus, the reaction involves the heterolytic cleavage of the peroxodisulphate to sulphur... 

Heterolytic cleavage of the tin-carbon bond is reviewed in references (94-96). Cleavage by electrophiles (e.g, HgXj or halogen) is dominated by electrophilic attack at carbon, and cleavage by nucleophiles principally involves nucleophilic attack at tin. Much of the interest in these processes centers on the intermediate mechanisms that may exist between these extremes, in which electrophilic attack is accompanied by some nucleophilic assistance, and vice versa. Allylic, al-lenic, and propargylic compoimds show a special reactivity by a special (Se2 or SE2y) mechanism. 

The facile thermal isomerization (17) of norbornadiene derivatives [71]-[77] to cycloheptatrienes in polar solvents has been explained in terms of the initial heterolytic cleavage of the strained C(l)-C(7) bond (Hoffmann and Hauser, 1965 Lemal et al., 1966 Hoffmann, 1971, 1985 Lustgarten and Richey, 1974 Hoffmann et al., 1986 Bleasdale and Jones, 1993). The resulting zwitterion intermediates are stabilized by the cation-stabilizing groups attached to C(7) and the cyclohexadienyl-type delocalization of the negative charge. 

The Ir complexes 83 or [lr(lMes)Cl2Cp ], in the presence of NaOAc and excess of (Bcat), catalyse the diboration of styrene, at high conversions and selectivities for the diborated species, under mild conditions. Other terminal alkenes react similarly. The base is believed to assist the heterolytic cleavage of the (cat)B-B(cat) bond and the formation of Ir-B(cat) species, without the need of B-B oxidative addition [66]. 

The enantioselective P-borylation of a,P-unsaturated esters with (Bpin) was studied in the presence of various [CuCl(NHC)] or [Cu(MeCN)(NHC)] (NHC = chiral imidazol-2-ylidene or imidazolidin-2-ylidene) complexes. The reaction proceeds by heterolytic cleavage of the B-B bond of the (Bpin), followed by formation of Cu-boryl complexes which insert across the C=C bond of the unsaturated ester. Best yields and ee were observed with complex 144, featuring a non-C2 symmetric NHC ligand (Scheme 2.31) [114]. 

The photocolored form is assumed to form via the heterolytic cleavage of the C—O bond. Studies on the thermal fading kinetics have shown 2,50... 

A comparison of the rate constant for photoisomerization of the unsubstituted 3-phenyl derivative (kT = 3 x 1010 sec-1) to that of the 3-(p-methoxy phenyl) derivative (kr = 1.5 x 1010 sec-1) indicates that the presence of the p-methoxy groups imparts no special stability to the intermediate responsible for isomerization even though cleavage of a cyclopropane bond is predominant. Clearly these results are inconsistent with an intermediate possessing electron-poor or electron-rich species such as would be obtained from heterolytic cleavage of the cyclopropane. On the other hand, the results are consistent with a biradical species as intermediate. Further evidence consistent with this conclusion was obtained in a study of trans-3-p-cyanophenyl-/ra w-2-phenyl-1 -benzoylcyclopropane,<82)... 

It involves heterolytic cleavage of the C-Cl bond and there is no other bonds are formed in this step. 

It is generally accepted that photolysis of R3SnX (R = alkyl group X = halogen) results in the homolytic cleavage of the Sn—X bond. However, under certain conditions, for example in polar solvents such as EtOH which can act as a Lewis base, the photochemistry can switch to heterolytic cleavage of the Sn—X bond followed by formation of solvent adducts such as R3(X)Sn(Sol) (Sol = solvent)17. 

Additional results of the enhancement in phenol conversion (to dihydroxy benzenes) and oxidation of allyl alcohol (to glycidol and allylic oxidation products) catalyzed by TS-1 in various solvents are illustrated in Fig. 46. In solvents with high dielectric constants, the heterolytic cleavage of the 0-0 bond... 

A tentative mechanism involving the heterolytic cleavage of the 0-0 bond along with electron transfer from the alkene to the electrophilic oxygen of the Ti(02H) complex is shown in Scheme 27. 

In the envisaged titanium oxo complex, the Ti atom is side-bound to the peroxy moiety (02H), consistent with all the spectroscopic results mentioned in Section III in Scheme 27, between the two O atoms that are side-bound to Ti4+, the O atom attached to both the Ti and H atoms is expected to be more electrophilic than the O atom attached to only the Ti atom and is likely to be the site of nucleophilic attack by the alkene double bond. The formation of the Ti-OH group (and not the titanyl, Ti=0, as proposed by Khouw et al. (221)) after the epoxidation and its subsequent condensation with Si-OH to regenerate the Ti-O-Si links had been observed (Section III.B) by FTIR spectroscopy by Lin and Frei (133). Because this is a concerted heterolytic cleavage of the 0-0 bond, high epoxide selectivity and retention of stereochemistry may be expected, as indeed has been observed experimentally (204). 

Phosphorylated derivatives of /3-nitroalcohols, upon exposure to Bu3SnH and AIBN, afford /3-(phosphatoxy)alkyl radicals. These radicals undergo heterolytic cleavage of the phosphate group to afford an alkene radical cation which is trapped intramolecularly in a tandem polar/radical crossover sequence. Derivative 37 (Scheme 13), through a 6-exol 5-exo overall cyclization, afforded the indolizidine derivative 38 as an equimolecular mixture of two diastereoisomers <2003JA7942, 2002OL2573>. 

Cationic polymerisations of vinyl derivatives proceeds by a polar or heterolytic cleavage of the double bond by the action of initiators which act as electron acceptors. Initiation is carried out by proton donation by acids or co-catalysts... 

Photosolvolysis of ( )-styryl iodonium salt 26 also gave product distributions quite different from those obtained in thermal solvolysis.16,23 Quantum yields for the formation of photoproducts formed via heterolytic cleavage of the vinylic C-I bond of 26 in methanol and TFE at 20 °C are given in eq 20.23... 

The term acid catalysis is often taken to mean proton catalysis ( specific acid catalysis ) in contrast to general acid catalysis. In this sense, acid-catalyzed hydrolysis begins with protonation of the carbonyl O-atom, which renders the carbonyl C-atom more susceptible to nucleophilic attack. The reaction continues as depicted in Fig. 7. l.a (Pathway a) with hydration of the car-bonium ion to form a tetrahedral intermediate. This is followed by acyl cleavage (heterolytic cleavage of the acyl-0 bond). Pathway b presents an mechanism that can be observed in the presence of concentrated inorganic acids, but which appears irrelevant to hydrolysis under physiological conditions. The same is true for another mechanism of alkyl cleavage not shown in Fig. 7.Fa. All mechanisms of proton-catalyzed ester hydrolysis are reversible. 

Another type of reaction was seen for dalvastatin (8.151), a prodrug that bears an unsaturated side chain. The hydrolysis of dalvastatin to the active acid competes with epimerization at C(6), the rate of the reaction being independent of pH above pH 2 [192], The mechanism is believed to be one of heterolytic cleavage of the C(6)-0 bond to generate a C-centered carbonium ion stabilized by the extended conjugated system characteristic of this compound. In the pH range 2 - 7, the rate of epimerization was found to be ca. 100 times faster than hydrolysis. Above pH 7, base catalysis accelerates hydrolysis, the rate of which increases ca. 100-fold between pH 7 and 9. These facts serve only to complicate the design of HMG-CoA reductase inhibitors and the interpretation of their pharmacokinetic behavior. 

The hydrolysis of diphenhydramine and analogues (11.24, Fig. 11.2) has been studied extensively [46 - 48], These compounds are essentially inert toward base-catalyzed hydrolysis, but do undergo proton-catalyzed hydrolysis, the mechanism of which is shown in Fig. 11.2. The reaction begins with protonation of the ether O-atom and continues with the irreversible heterolytic cleavage of the C-0 bond to produce the benzhydryl cation. This reaction is greatly facilitated by the weakening effect of the benzhydryl moiety on the adjacent C-0 bond. The benzhydryl cation itself is stabilized by resonance, which also explains why the reaction is facilitated. The last step is the for-... 

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