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

Mercury(II) and other electrophilic reagents for the biomimetic cyclization of 1,5- dienes (e.g. geranylacetone) have been classified in four groups: (1) Lewis acids, such as SnCl4, SnBr4, (CF3CO2)2Sn, BF3.Et2O etc; (2) bromonium ions; (3) mercurinium ions;

(4) phenylselenonium ions. The effect of the choice of reagent upon the stereoselectivity has been discussed424.

Polar cyclization of diene hydroperoxide 304 has been successfully effected by Hg(NO3)2 to produce a 2:1 mixture of 1,2-dioxolane 305 and 1,2-dioxane 306. In contrast, a radical cyclization with (But OOCO)2 and O2 or N-iodosuccinimide, respectively, is more selective giving only one 1,2-dioxane425.

BrHg

O

OOH

O

(304)

(305)

HgBr

+

O

O

(306)

The oxime nitrogen in 307 has been employed as an internal nucleophile in mercuration of a double bond to generate nitrone 308, which instantaneously underwent the dipolar

O

HO

(314)

Intramolecular, thallium(III)-mediated cyclization of unsaturated alcohols 315 has been studied and, in certain instances, the organothalliated primary products 316 isolated and characterized by 1H NMR429. The subsequent displacement of Tl(OAc)2 in 316 by OAc has been interpreted as proceeding via SN2 or SNi pathway. However, in light of the previously reported evidence430,431 the reviewer is of the opinion that involvement of the neighbouring ether oxygen as a participating group is more likely.

R2

R2

OAc

R1 O

(317)

Methoxythallation of allenes, such as 318, has been found to occur at the more substituted double bond, and to be far more regioselective than methoxymercuration, giving predominantly the product with (AcO)2Tl attached to the central carbon 319432.

D. Lead

Lead(IV) in acidic media has been found to promote oxidative addition of Cl , CF3CO2 , AcO , MeSO3 and ClO4 to cyclohexene, 1-hexene and styrene433. Sonochemical switching from ionic to radical pathway in the reactions of styrene and trans-ˇ-methylstyrene with (AcO)4Pb has been observed434.

E. Palladium

An improved procedure for cyclopalladation-carbonylation (of e.g. 320) relies on the addition of CuCl and LiCl to the standard PdCl2 CuCl2 mixture. This indicates that both the Cu2C and CuC are required in sufficient concentrations to keep up the cascade of the catalytic cycle. This method is superior to the Hg(II)-mediated cyclization followed by transmetallation with Pd in CO/MeOH197. For application in the synthesis of polyether antibiotics, see elsewhere435.

HO

O

CO2 Me

Methyl glyoxylate adducts of N-BOC-protected allylic amines 321 have been utilized to construct a new C O bond by an intramolecular, Pd(II)-catalysed reaction436. In analogy, lactones 324a437 and cyclic ethers 324b438 can be prepared by the Pd(II)-catalysed cyclization of the suitable precursors 322a and 322b, respectively.

(321)

N-Tosyl carbamates, such as 325, derived from allylic alcohols, undergo the Pd(II)- catalysed cyclization to furnish oxazolidinones 326 under 1 atm of CO439. Analogous Pd(II)-catalysed N-cyclization of allenic N-tosylcarbamates has generated the corresponding vinylpalladium intermediate that could be further alkylated440.

The Pd(II)-catalysed cyclization of 283 exhibits lower stereoselectivity ( 43% d.e.) than its Ag(I)-mediated counterpart (up to 81% d.e.)413. A Pd(II)-catalysed 5-endo-Trig cyclization of 2-hydroxybut-3-enylamines 327 has been reported to occur with moderate to good yields. The OH group is essential for the cyclization441.

A stereospecific, Pd-catalysed, 5-exo-Trig ring closure reaction of 328 has been reported442,443.

The regioselectivity of the Pd(II)-catalysed hydrocarboxylation of styrene has been elucidated and two different mechanisms have been suggested to account for the differences

reacts with other alkenes having a low-lying LUMO (e.g. tetracyanoethylene and carvone) while no reaction occurs with cyclopentene and cyclohexene. Detailed examination of the reaction course by NMR led to the formulation of the mechanism, which includes the formation of (DIOP)Pd(H)CN as an intermediate, followed by the rate-limiting reaction of the latter complex with norbornene445.

Catalytic asymmetric hydrosilylation of terminal olefins has been developed, using palladium coordinated to the novel binaphthyl ligands (MOP). In all cases (MOPa d), the enantioselectivity is excellent ( 90% e.e.). The products can be converted into the corresponding secondary alcohols with retention of configuration446.

(MOP)

Intramolecular, Pd(II)-catalysed bis-silylation of a CDC bond proved to be highly

stereoselective447. The reaction is believed to proceed via a chair-like transition state 329; the product 330 can be oxidized to triol 331447.

Me2

(329)

The Heck reaction has now been reviewed448,449. Evidence for the formation of zerovalent palladium from (AcO)2Pd and Ph3P via a redox process has been provided450. This explains the origin of Pd(0) required for certain palladium-catalysed reactions in cases where Pd(II) is added to the reaction as the primary form of the Pd-catalyst. Thallium(I) has been found to accelerate the Heck-type cyclization-carbonylation451.

Intramolecular Heck reaction of organomercurial 332 has been used to prepare unsaturated lactone 333 by a non-traditional strategy452. Sequential, regiospecific C C and C N bond-forming reactions via a novel Heck-type coupling have been developed453.

The rate of the palladium-catalysed Heck-type phenylation of allylic alcohols has been found to be markedly enhanced by addition of tertiary amines454. Regioselectivity can be increased, in some cases, by adding Et4NCl or employing a Wilkinson Rh catalyst (rather than Pd)455,456. Another Heck-type reaction involves addition of arenediazonium tetrafluoroborates to ˛-silylstyrenes to give (E)-PhCHDCHAr. A BF4 -mediated syn-elimination of silicon and palladium has been suggested to account for the stereochemistry457.

AcO

( )n

N

R

(336)

O

OH

a

AcO

(337)

(338)

O

b

AcO

(339)

(a) (AcO)2 Pd, p-BQ, AcOH, Me2 CO, AcOLi

(b) (AcO)2 Pd, p-BQ, AcOH, Me2 CO, AcOLi, LiCl

R = Ts, COMe, CO2 Bn, CONHBn

SCHEME 9

F. Rhodium

Extremely high regioselectivity has been observed for hydroformylation of fluoroolefins RfCHDCH2, catalysed by group VIII transition metals. While a Co catalyst gives the normal product 345 on hydroformylation of 344, a Rh catalyst gives mostly the isomeric aldehyde 346470. In another study, hydroformylation of 1-hexene was catalysed by rhodium(I) with concomitant isomerization471.

CH O

The kinetics of the Rh4(CO)12-catalysed hydroformylation of 2-butenes are consistent with a mechanism involving fragmentation of the catalyst to the active monoand nonactive bi-nuclear Rh-complexes. Interaction of the monomeric HRh(CO)3 with alkene appears to be the rate-limiting step. Binuclear Rh-complexes, predominating in the reaction mixture, serve as a reserve for the active monomeric complexes472. Amine-directed, Rh(I)- mediated hydrocarbonylation has been reported (347 ! 348)473.

Me

(348)

Asymmetric, intramolecular hydrosilylation, catalysed by Rh(I) coordinated to chiral diphosphine complexes, such as chiraphos or BINAP, has been reported to give up to99% e.e. (349 ! 350)474 476.

The efficiency of the tail-to-tail dimerization of methyl acrylate has been further improved, based on a detailed study of the role of the Rh(III)-catalyst, the resting state and the mechanism of the catalyst deactivation477.

G. Osmium

The systematic study of the origins of high enantioselectivity in the osmium-catalysed dihydroxylation carried out by the Sharpless group has led to defining certain crucial features and ruled out other mechanistic proposals478 481. Thus, the dihydroxylation has been