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

should also play a role in determining the stereochemistry (vide supra218).

of (2S,4R)-4-hydroproline from (S)-O-benzylglycidol219. Stereocontrolled 5(O) ,n-endo- Trig iodoetherification has provided an entry to trans-2,5-disubstituted tetrahydrofuran derivatives220. On treatment with iodine, trichloroimidates of primary ˛-allenic alcohols are converted into oxazolines with high stereoselectivity221.

Electronic control has been found to dominate completely over conformational control in the iodo-lactonization of 1,6-heptadiene-4-carboxylic acids 136 and 137 under kinetic conditions (i.e. the addition is methallyl-selective)222. Conformational factors controlling C relative asymmetric induction favour formation of a trans-Cˇ Me C0 CH2I product from 136, while 137 gives mainly the cis-product. In contrast, all three stereoisomers of 138 (anti,syn; anti,anti ; syn,syn) undergo kinetic iodo-lactonization favouring a cis-Cˇ Me C CH2I relationship. The selectivity was rationalized on the basis of conformational control minimizing gauche interactions. Thus, conformational control which clearly differentiates C and C0 in 138 (147:1) presumably favours methallyl cyclization (C0 ) in 136, but does not disfavour methallyl cyclization (C0 ) in 137222.

The stereoselectivity of iodolactonization of 2-substituted 4-pentenoic acids (with OH, NHTs or CH2OH group at C-2) by NIS or I2 can be increased in the presence

of (PriO)4Ti223. In contrast, the stereochemistry of analogous haloetherification of 2- hydroxymethyl-4-penten-1-ol is reversed by the addition of (PriO)4Ti223.

A highly enantioselective iodolactonization has been achieved through face and diastereotopic-group differentiation (for the principle, see elsewhere224,225) using (2R,5R)- bis(methoxymethyl)pyrrolidine 139 ! 140)226, sultam227 or other moieties228 as chiral auxiliaries. All four isomers of 3-hydroxy-4-methyl- -butyrolactone have been synthesized by stereoselective iodolactonization and/or epoxidation229. An elegant, NBS-induced lactonization of bicyclo[3.2.0]hept-3-en-6-ones 141 has been reported (Scheme 2)230.

Iodolactonization of the allylic carbonate 143 has been found to proceed with high stereoand regio-selectivity to produce iodocarbonate 144. Steering by Ph (i.e. the primary coordination of the electrophile to Ph) has been suggested to account for this result231.

On treatment with NIS, acetoxycyclohex-2-ene has been found to react via 5(O) ,n- exo-Trig participation with no migration of the acyl group232. To the reviewer’s surprise, the authors claimed this behaviour to be unprecedented. This claim is not justified since a number of analogous electrophilic additions, where the acyl did not move, had been reported much earlier197. Furthermore, there are examples where the acyl does move, at least partially, giving a mixture of the two possible products197.

The 5(O) ,n-endo-Trig ring closure has been reported for the bromination and iodination of several allenic derivatives233 237. ˛-Carbamoyloxy allenes undergo iodination with 5(N)n-exo-Trig participation238.

The regioselectivity of the oxirane oxygen transannular participation on iodination of the epoxyolefin 145, followed by ring opening of the corresponding oxonium ions 146 and 147, has been rationalized by means of MNDO calculations161. These findings have been utilized in model studies towards the synthesis of polyether toxins containing a tetrahydrofuran ring239,240.

Neighbouring group participation has been employed to control the regioselectivity of HOBr addition across a double bond in the crucial step of the synthesis of strophanthidin. Two molecules of the reagent were used in order to introduce two hydroxy groups in one step (148 ! 149)241.

Kinetic studies of the transannular iodination of the bicylic dienes 150 (R D H or Me) show that the addition is first order in 150 and second order in I2. The rate is governed by electrostatic and electron-donor parameters of the solvent242. The reaction of brexadiene 151 with electrophiles (Br2, I2, CF3CO2D and CD3CO2D) proceeds with an initial exo- attack followed either by direct reaction with a nucleophile or by Wagner Meerwein rearrangements to give mixtures of products243. Addition of hypobromous acid (generated from NBS) to 152 afforded a mixture of two allylic bromides and two products of - particpation by the aromatic ring244.

H

H R

A detailed study of the iodocyclization of a series of unsaturated hydroxy acids has demonstrated that a ground-state conformational analysis can serve as a reliable indicator of the relative reactivities of various conformations. Thus, while 153 undergoes exclusive iodolactonisation, 154 gives the corresponding iodotetrahydrofuran as the sole product, in full agreement with the MM2 prediction224.

Participation of the pyridine nitrogen in electrophilic additions has been reported245. The cyclization of Z and E isomers 155 and 156 mediated by various electrophiles (MCPBA, PhSeX, NBS, I2) turned out to be stereospecific; the diastereoselectivity varies with the nature of the electrophilic reagent246.

While 2-azabicyclo[2.2.2]hept-5-en-3-one 157 has been found to undergo bromination and bromofluorination with participation by the amide nitrogen followed by rearrangement (158 ! 159 ! 160)247, phenylselenenylation results in a simple addition to afford 162247. This difference has been rationalized as follows247: the bulky PhSeC finds it difficult to approach the double bond from the more hindered face (even if this might have been boosted by neighbouring group participation) so that a normal addition results (via 161). By contrast, the reversible formation113 of the bromonium ions allows the equilibrium of two diastereoisomeric ions to be established. The less populated, sterically more crowded ion 158 is siphoned off248 by fast quenching due to the neighbouring group participation247. However, 157 is known to be exo-dihydroxylated with MnO4 or OsO4249, i.e. from the ‘more’ sterically hindered side, so that this issue is likely to be more complex.

The addition of Cl2, Br2 or ICl to 2-endo-3-exo-bis (phenylsulphonyl)norborn-5-ene 163

Regiochemistry of the electrophilic cyclization of 166 has been studied. Interestingly, 6(S)-endo-Trig cyclization seems to be more favoured than the expected 5(S)-exo-Trig

5-endo-Trig cyclization is in all cases favoured over the 4(S)-exo-Trig closure; the same trend has been observed for Dig cyclization of the corresponding alkynes251. Further examples of the 5(S)-endo-Trig cyclizations have been reported for o-thiostyrenes252.

IV. ELECTROPHILIC CHALCOGENS

Addition of PhSCl and related reagents is believed to proceed via cyclic intermediates. The first isolated phenythiiranium ion 169 has been prepared from AdDAd on reaction with PhSCl and CF3SO3Me (r.t., 10 min) and characterized by single-crystal X-ray crystallography and 1H and 13C NMR spectra253. The dimensions of the thiiranium ring are as follows: (C S) range 1.909(3) 1.937 (3) A;˚ (C C D 1.500 A;˚ (C S C D 46.1°; (C C S) range 66.3(2) 67.9(2)°. The phenyl ring is approximately orthogonal to the plane of the thiiranium ring, but nearly coplanar with the ring C S bonds.

CF3 SO3 −

Ph

S

+

(169)

The relative involvement of the cyclic (thiiranium or phenylselenenium) and open species in the addition reactions has been investigated by NMR. These studies have revealed dramatic substituent effects on the relative equilibrium stabilities of ions 170 172

(R1X D MeS, PhS, PhSe, C6F5Se, OH, Br; R2 D Me, CH2Cl; R3 D H, Br, Me), that have been generated in superacids from RXCl and acenaphthylene derivatives254. In agreement with MINDO/3 calculations, the relative contribution of 172 falls with increasing atom

number of X, electron-donating character of R2 and electron-accepting character of R3254 . Kinetic studies of the addition of 2,4-dinitrobenzenesulphenyl chloride to cyclohexene in the presence of LiClO4 have been interpreted in terms of an ion-pair mechanism. A similar conclusion has been arrived at for addition of (SCN)2 to cyclohexene and

ring-substituted styrenes, RC6H4CHDCH2 (R D H, 4-Me, 4-Cl, 3-Cl)255.

Kinetics of the addition of 4-RC6H4SCl (R D MeO, H, Cl) to the ring-substituted styrenes and ˛-methylstyrenes 4-R0 C6H4C(R00 )DCH2 (R0 D MeO, H; R00 D H, Me) indicate that the reactivity of the electrophile is governed by the stabilization of partial positive charge in the transition state256,257. The transition states for the addition to ˛-methylstyrenes occur later on the reaction coordinate rather than in the addition to styrenes as a result of differences in localization energy257. The relative reactivities of RC6H4CHDCH2 (R D 4-MeO, 4-Me, H, 4-Cl, 4-NO2 and 3-NO2) towards PhSCl are not affected by the solvent258.

HO

MeO

Cl

(175)

SCHEME 3

Irreversible acetoxyselenenylation of terminal and disubstituted olefins has been achieved on addition of PhSeBr in an acetate-buffered solution. Styrenes afford only Markovnikov adducts, while simple terminal olefins and olefins containing an allylic oxygen substituent (RCO2 or ArO group) furnish 50 80% of the anti-Markovnikov isomer. The product mixture can be isomerized to contain 90 97% of the Markovnikov product by a catalytic amount 6 41%) of BF3.Et2O in CHCl3259.

Excess of PhSeCl and prolonged reaction time are required to convert cholesterol (173) in MeOH first to 174 (62% yield), which is gradually transformed into the deselenenylated product 175 (27%; Scheme 3)260. Similar reactivity has been reported for ˛-substituted styrenes261 and vinyl halides262.

Kinetically controlled additions of PhSeCl, PhSeBr, PhSeOAc, 2-NO2C6H4SCl and 2,4-(NO2)2C6H3SCl to bicyclo[2.2.1]hept-5-en-2-one (27) proceed in an anti-fashion with complete stereoand regio-selectivity giving adducts 176 in which the electrophile occupies the exo-position, while the nucleophile is endo-orientated263 265. The results are in agreement with the predictions based on MO calculations, which suggest that a carbonyl group, homoconjugated with an electron-deficient centre, can act as an electron-donating remote substituent due to the favourable nCO $ C 1 C 2 $ pC 6 hyperconjugation

(177)263.

Tetraflurobenzobarrelene and its derivatives react with PhSX (X D Cl, Br) in AcOH, MeNO2 or CH2Cl2 to afford trans-ˇ-halosulphides. Structural rearrangement occurs when these reactions are carried out in systems such as AgSbF6/MeNO2 and [(MeS)2SMe]C [SbCl6] /CH2Cl2266.

The electrophilic anti-1,2-addition of the elements of MeSF to CDC has been achieved by a one-pot reaction of Me2SC -SMe BF4 and Et3Nž3HF with various types of alkenes267. Markovnikov products arise from unsymmetrically substituted olefins. The reaction of 2,6-norbornadiene proceeds with exclusive exo-attack on one double bond followed by participation of the second double bond to give rise to two isomeric 3,5- disubstituted nortricyclanes267. By contrast, no transannular -participation has been observed with 1,5-cyclooctadiene. The reaction is believed to occur via the corresponding thiiranium species267.

PhSeF, generated from Ph2Se2 and XeF2, adds to norbornene predominantly in an anti-fashion to afford the non-rearranged adducts, corresponding to exo- and endo- attack, respectively268. By contrast, PhSeF3 gives mainly the products of syn-exo-addition and a rearranged derivative in ca 7:1 ratio268.

Predominant anti-stereochemistry has been observed for the addition of TsSNR2/BF3.Et2O to olefinic substrates269. 40-Nitrobenzenesulphenanilide (ArNH SPh) reacts with HBr to generate in situ PhSBr, which attacks alkenes or alkynes in a regioand stereo-selective manner270.

1174 Pavel Kocovskˇy´

Oxidation of bis(4-methoxyphenyl)disulphide with ammonium peroxydisulphate generates a reagent that can be utilized to promote anti-stereospecific arylthioamidation in acetonitrile271. This procedure is also suitable for aryletherification and lactonization271. Other methods of generating electrophilic reagents from ArSSAr involve oxidation with (AcO)3Mn272 and anodic oxidation273.

Methoxyselenenylation of olefins can now be effected in one step by oxidation of PhSeSePh with (NH4)2S2O2 in MeOH274,275. The reaction is highly regioand stereo-

selective giving pure anti-products with Markovnikov orientation274. When the reaction is carried out in MeCN containing CF3CO2H and water, amidoselenenylation products are obtained275. Another method of generating PhSeC relies on the oxidation of PhSeSePh with p-nitrobenzenesulphonyl peroxide276.

Acid catalysis (AcOH) has been investigated for addition of ArSCl to cyclohexene. The acid anion has been found to compete with Cl in the formation of the final product277.

Treatment of olefins with dimethylthiomethylsulphonium salts and triphenylphosphine leads to the corresponding 2-methylthioalkylphosphonium salts278.

Stereocontrolled glycosylation of furanoid glycals with pyrimidine or purine bases has been accomplished via a Lewis acid-mediated sulphenylation279.

Enantiopure ditihiiranium salt 178 has been reported to transfer enantioselectively the MeSC group to trans-hex-3-ene to generate the corresponding thiiranium ion which, in turn, reacts with MeCN/H2O allowing the enantioselective synthesis of the vicinally disubstituted alkanes with up to 86% e.e.280.

Ring-closure reactions of olefins bearing a functional group can now be facilitated using PhSeX/Ag(I)281, PhSeCl/Tl(I)197 or PhSeSePh/(NH4)2S2O8 in MeOH, dioxane or MeCN282. Both exo-Trig and endo-Trig cyclizations have been reported197.

Cyclophenylselenenylation of aliphatic hydroxy olefins results in the formation of polysubstituted tetrahydrofurans with high stereoselectivity (114 ! 179)207,208, low temperature ( 78 °C) is generally recommended283. The 6(O)n-exo-Trig cyclization of 4-substituted 5-hexen-l-ols 180 with PhSeOTf has been found to produce preferentially the trans-isomer 181 for R D alkyl or Ph, whereas the cis-isomers 182 are favoured when R D OH, OR or R0 CO. The stereoselectivities have been rationalized by steric and electronic effects284. Phenylselenenyl halides have also been found to effect electrophilic cyclization of olefinic oximes285,286 (for discussion see the analogous mercuration286), generating reactive N-oxides.

Unhindered olefins have been found to react with NaHTe in refluxing EtOH to produce Markovnikov-like dialkyltellurides via an addition process, which is believed to occur through a radical mechanism287.

A method for acetoxytelluration of olefins has been developed using TeCl4 and AcOLi in AcOH at 80 °C. The reaction is highly anti-stereospecific and obeys the Markovnikov