Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1135 122

I. INTRODUCTION

This chapter represents an update to the previous two editions, published in 19771 and 19892, and covers the literature of the period 1989 1994 with some references to 1995 papers. It deals mainly with electrophilic additions across the CDC, CDSi and SiDSi bonds and includes both theoretical (ab initio calculations, orbital approach, molecular modelling etc.) and experimental aspects. Particular attention is paid to mechanistic studies, facial selectivity and neighbouring group participation. Synthetic utilization of electrophilic addition is discussed only if including substantial mechanistic insight; purely synthetic work is not covered. Aside from the classical reactions, such as hydration, bromination etc., newly included material comprises aziridination (Section VI), attack at CDC bond by an electron-deficient carbon (Section VII) and those electrophilic reactions which utilize a transition or non-transition metal as the electrophile (Section VIII).

A. Reviews

During the coverage period of this chapter, relevant reviews have appeared on the following topics: the principle of hard and soft acids and bases in addition to unsymmetrical alkenes3; why does norbornene show selective reactivity on the exo-face4, structure and reactivity of strained olefins having non-planar CDC bonds5; additions to bridgehead olefins and enones6; utilization of stepwise AdE reactions in designing organic syntheses7; preparative aspects of addition of electrophiles such as (halogen)C , NOC , NO2C , SO3HC , RSC , RCOC and others in the presence of nucleophiles (H2O, ROH, R2O, RCO2H, RCN, RSCN etc.)8, the role of bromonium ions and ˇ-bromocarbocations in olefin bromination9; reactions of N-chloroamines and N-haloamides with unsaturated compounds10; novel brominating agents11, application of [hydroxy(organosulphonyl)iodo]arenes in organic synthesis, including electrophilic additions12; diastereofacially differentiating electrophilic additions to chiral bis-allylic diols13; stereocontrolled cyclofunctionalizations of double bonds through heterocyclic intermediates arising by electrophile-mediated ring-closure reactions14; polyolefinic cyclization via bromination15; chemistry of the thiiranium ions16; additions of dithio-acids to unsaturated compounds17, kinetics and mechanism of C C bond formation by addition of carbenium ions to alkenes18; control of electrophilicity in aliphatic Friedel-Crafts reactions by Lewis acids19; and the role of charge in synthetically important cationic cyclizations20.

Increasing importance of the reactions employing organometallics as reagents or catalysts is reflected by the appearance of the reviews on: palladium(II)-catalysed reactions of olefins with oxygen nucleophiles21; new aspects of oxypalladation of alkenes22; palladium catalysis for intramolecular addition of hydroxy, amino and carboxylic groups23; new developments in the palladium-catalysed 1,4-additions to conjugated dienes (Backvall¨ reaction)24; newly developed asymmetric arylation of olefins (Heck reaction)25; nucleophilic addition reactions of cationic iron- -alkyne and related complexes26; and enantioselective cis-hydroxylation27.

The author of this chapter has summarized his work in the area of electrophilic additions and application of transition and non-transition metals in organic chemistry in a personal account28.

B. General Aspects of Electrophilic Additions

Correlations of ionization potentials (IP) versus relative reactivities of a variety of alkenes towards bromination, oxymercuration and hydroboration clearly show that the reaction rate decreases as the IP is increased29; the transition states of the ratedetermining steps of oxymercuration and hydroboration are similar, but different from that of bromination29.

Remote electronic control of the -diastereofacial selectivity of electrophilic additions has been demonstrated with 7-methylenenorbornanes30 and 7-isopropylidene benzonorbornenes31,32 (1a 1c; Table 1). Whereas the -face stereoselection is moderate in the case of additions proceeding through cyclic transition states (epoxidation and hydroboration), it is significantly enhanced in the case of the more polar addition (oxymercuration). Generally, electron-withdrawing groups tend to favour syn-addition, while anti-attack is dominant for the compounds with electron-donating groups30. The results have been rationalized by electrostatic interactions31 and by the Cieplak33,34 hyperconjugative model30 in which the stabilizing interaction between the electron-rich antiperiplanar bond and the developing Ł orbital lowers the transition state energy.

Different electrophilic reagents have been founds to have very different profiles in reactions with benzonorbornenes2: syn addition is highly favoured by strong electrophiles, such as: CCl2, R C OC (R D Me, H) and ButO(Cl)HC , whereas weaker (MCPBA, NBS, O2, ButOCl) or transvestial35 (OsO4, MnO4 ) electrophiles, which all bear lone-pair electrons, exhibit a preference for anti attack32.

Hyperconjugation appears to be the dominant factor governing the diastereoselectivity of the hydrochlorination of 5-substituted 2-methyleneadamantanes 3 (Table 2)36. However, the product distribution for epoxidation suggests that the stereochemical course of electrophilic additions not mediated by carbocations is most likely regulated by direct field effects36. Note that, unlike in the previous reactions, the facial selectivity in this case reflects the preference for the nucleophilic attack on the corresponding carbocation.

Distortion of olefin -orbitals in dibenzobicyclo[2.2.2]octatrienes 4 has been detected37. Thus, nitro and fluoro groups give large to moderate bias with preferred syn attack (with

TABLE 1. Electrophilic additions to 1

X

respect to the substituent), whereas MeO exhibits a negligible bias (Table 3). These effects have been interpreted in terms of desymmetrization of -lobes of the olefin orbitals arising from non-equivalent interaction rather than from an electron-donating or electronwithdrawing effect37.

Partial ergot alkaloid substrates of 5 and related, conformationally fixed styrenes 6 8 have been found to undergo electrophilic additions (epoxidation, HOBr addition and

syn anti

dihydroxylation) with a level of stereoselectivity, which cannot be rationalized by steric control (Table 4) but is consistent with electrophilic attack to minimize torsional strain38.

Both experimental and theoretical studies of the electrophilic additions to vinylic sulphoxides have demonstrated that the -facial stereoselection can be rationalized by the transition state 939.

The possibility of simultaneously finding an electron-hole and an electron pair in a-system substituted by an electron-withdrawing group (NO2) and/or electron-donating group (NH2) has been examined with cis- and trans-isomers of 10 (push pull olefins) and for non-vicinal positions40.

II.PROTON AS AN ELECTROPHILE

A.Hydration, Addition of ROH and Related Reactions

Data indicative of the relative basicities of CDC and C C bonds and relative solvation energies for protonation processes have been obtained from measurements of hydration rates of RCHDCH2 and RC CH (R D H, Me, But ) in aqueous H2SO441. Enthalpies of hydration of a series of acyclic olefins producing tertiary alcohols have been determined42,43.

A study of acid-catalysed hydration of norbornenes and nortricyclanes gave ˛-values for the protonation which correlate well with those for the solvolyses of 2-norbornyl tosylates. This indicates that the first formed cations for both reactions are of a similar character44.

The activation parameters and solvent deuterium isotope effects for acid-catalysed hydrations (HClO4) of 1-methylcycloalkenes 11 14 and methylenecycloalkanes 15 18 agree with the rate-determining protonation of the double bond. The small Gibbs energy difference between the transition states for hydration of 11 and 15 (1.2 kJ mol 1) contrasts with a large difference in the case of 12 and 16 (11.8 kJ mol 1). The origin of the latter was attributed to the changes of conformation during the protonation of 1245. A carbocationlike transition state is assumed for the HClO4-catalysed hydration of cyclopentene and the mechanism has been formulated as an Ad-SE2 reaction46. The HClO4-catalysed hydration of 14 (at 25 °C) is reversible, whereas the reaction of 18 has been found to be essentially irreversible47.

Investigation of the reactivity of alkenes 19 21 in CF3CO2H (neat and buffered with CF3CO2K) and in a CF3CO2H MeCN mixture (3:1) shows that 20 and 21, which form a tertiary cation, react 6.6 ð 104 and 5.8 ð 104 times faster than 19 (CF3CO2H MeCN, 25 °C). The rates in CF3CO2D gave KIEs of 6.8 (19; 26.5 °C), ca 5 (20; 18 °C) and 3.9 (21; 18 °C). The isomeric trifluroacetates 22 and 24 are formed from 19 in the same ratio (ca 53:47) in CF3CO2H and CF3CO2D, which indicates that 19 reacts entirely by a carbocation mechanism with no measurable contribution from a molecular addition. The 22:24 ratio is close to that observed in the solvolysis of 2348.

The hydration rates of isobutylene in concentrated aqueous solutions of heteropolyacids (HPA) such as H3PMo12O40 and H3PW12O40 have been found to be about 10 times higher than those in aqueous mineral acids. This acceleration was attributed to better solubility of isobutylene in concentrated HPA and stronger acidity of concentrated aqueous HPA, as

revealed by measurement of the Hammett acidity functions49 54. The substrate selectivity is remarkable: competitive hydration of an isobutylene/1-butene mixture below 80 °C exhibits up to 99.9% preference for the formation of But OH50.

The relative reactivity, solvent isotope effect kH/kD and activation parameters for the acid-catalysed hydration of allylic alcohols CH2DCR CH2OH (R D H, Me) have been found to be similar to those for other alkenes. Whereas the results can be interpreted in terms of the conventional Ad-E2 mechanism, computed values for the life-time of possible carbocation intermediates suggest another feasible mechanism for CH2DCHCH2OH, according to which the nucleophilic attack by the solvent is concerted with protonation55,56.

Cobalt(II)(salen)2 complex has been found to catalyse the aerobic hydration of styrene in EtOH solution in the presence of Ph3P, giving the Markovnikov product. Kinetic studies indicate that the rate-determining step is H-abstraction from the solvent by the coordinated O2 ligand of (O2)Co(salen)(PPh3). A mechanism has been proposed for the remaining steps involving an oxymetalation of the CDC bond of styrene by HOO Co(salen)(PPh3)57.

In contrast to the ground-state hydration, the photoaddition of water and of several alcohols to the triplet excited states of m-nitrostyrenes affords the corresponding antiMarkovnikov products58.

Terpenoid alcohols, such as 25, are cyclized in superacids (FSO3H/SO2) under a mixture of kinetic and thermodynamic control. Intermediate oxonium species were identified by 13C NMR59.

HO

O

SeMe substituents do not stabilize the adjacent positive charge better than does the OMe group. This conclusion is supported by the ab initio calculation at the STO-3G and 3-21 levels67.

The kinetics and mechanism of the acid-catalysed hydration of dihydro-1,4-dioxin have been reinvestigated. The solvent isotope effect kHC /kDC D 2.2 indicates that the reaction proceeds by a rate-determining proton transfer from the catalyst to the substrate68 rather than by a pre-equilibrium mechanism.

A surface-mediated (SiO2 or Al2O3) addition of hydrazoic acid, generated in situ from Me3SiN3 and CF3SO3H, to 1-methylcyclohexene and 1,2-dimethylcyclohexene has been reported69. The reaction obeys the Markovnikov rule and is therefore believed to proceed via the initial protonation of the double bond to generate a carbocation. This mechanism is also supported by the observed non-stereospecificity69.

Silenes have been found to add alcohols in a non-stereoselective fashion, even if the bond rotation is prohibited by cyclic structures, as in 33. This is in disagreement with the simple two-step or a concerted four-centred mechanism70,71. On the other hand, disilenes (E), and (Z)-PhMeSiDSiMePh, generated photochemically in an argon matrix at 10 K, react with alcohols (EtOH, PriOH, and But OH) via a highly diastereoselective syn addition, presumably involving a four-membered intermediate71. Addition of alcohols to PhMeSiDSiMe2 gives PhMe(H)Si Si(OR)Me2 with high regioselectivity71.

Si

Me3 Si Me

(33)

B. Additions of Hydrogen Halides and Other Acids

Ethylene, HF and H3OC have been used as a model system in the ab initio closedshell SCF calculation of the acid-catalysed hydrogenation of olefins. While catalysis by HF exhibits bifunctional character, catalysis by H3OC proceeds via initial formation of a carbocation72.

Ab initio SCF calculations and statistical thermodynamic analysis of the addition of hydrogen halides HX and (HX)2 to ethylene indicate that for (HF)2 and (HCl)2 the termolecular transition states are hexacentric, whereas the bimolecular transition state of the HCl addition is bicentric rather than tricentric reported previously. The driving force for formation of the termolecular transition state in the HF addition appears to be the bonding between C and F. By contrast, in the HCl addition the driving force is the bonding between C and H, as generally accepted for electrophilic additions73.

Ab initio 3-21G study of the addition of HF to fluoroethylenes C2HnF 4 n with n D 0 4 has been performed, with geometry optimization of all charge-transfer complexes and transition states. The barriers so obtained are in fair agreement with experimental data74. Alteration of orbital configuration and hybridization changes in the addition of HBr to ethylene were illustrated in terms of orbital tilting on the basis of the concept of stabilizing and destabilizing second-order orbital interactions. The predominance of anti-stereochemistry has also been visualized quantitatively75.

A study of the solid-state hydrohalogenation (HCl, HBr) of 2-methyl-2-butene led to formulation of a mechanism involving the 2HXžC5H10 complex76.

Mixtures of gaseous HCl and 1,3-butadiene at 294 334 K and <1 atm of total pressure give mixtures of 3-chloro-1-butene and (E)- and (Z)-1-chloro-2-butene with the ratio of 1,2- to 1,4-addition products being approximately unity. Kinetic measurements in pyrex cells, using FT IR spectroscopy, revealed that surface catalysis is required and that the reaction most probably occurs between a multilayer of adsorbed HCl and gaseous or weakly adsorbed butadiene. This highly structured process is believed to proceed with nearly simultaneous proton and chloride transfer77,78.

The mechanism for the uncatalysed and HC -catalysed reactions of simple quinone methides with solvent and halide ions has been investigated. The observed differences in the isotope effects for addition of HX (X D Hal) and ROH are consistent with a stepwise mechanism for the HC -catalysed addition of solvent and concerted mechanism for the HC -catalysed reactions of halide ions79.

The observed deceleration of the rate of transannular cyclization of a series of olefinic substrates 34 > 36 > 37 > 35 has been rationalized by decreasing electron density on the C-atom undergoing protonation. Negative temperature coefficients for the HCl-catalysed transannular hydrochlorination are consistent with the formation of charge-transfer intermediates. An ion-pair mechanism has been proposed80.

Z

Z′

(35) Z = Z′ = C CH2

(36) Z = CH2 , Z′ = C CH2

Asymmetric hydrohalogenation of trans-2-butenoic acid has been achieved in a crystalline ˛-cyclodextrin complex using gaseous HBr at 20 °C and HCl at 0 °C. The products were formed with 58% and 64% e.e., respectively, and were of (S)-configuration81. This contrasts with the low enantioselectivity of halogenation attempted in the same paper (vide supra).

III. ELECTROPHILIC HALOGENS

A. Halogenation

The transition-state structures for fluorination, chlorination and bromination were obtained by ab initio MO calculation82. Chlorination and bromination were found to proceed via three-centred geometries (cyclic halonium ions) leading to anti-addition. In contrast, fluorination involves a four-centred transition state which is consistent with the observed syn-stereoselectivity82.

The geometries and relative energies of the singlet and triplet states of ions involved in electrophilic halogenations, namely halogen cations XC (X D F, Cl, Br), X3C (X D F, Cl) and hydrohalonium ions H2XC and HX2C (X D F, Cl) were also studied with ab initio MO calculations. The monoatomic halogen cations have triplet ground states, whereas most of the triatomic species have singlet ground states. The geometries have been optimized83.