Dihalogenocarbenes

Polyhalogenomethyltin and -lead compounds are of interest as possible sources of dihalogenocarbenes, CX2. Trimethyltin trihalogenoacetates,... 

Because of the presence of a lone pair and a vacant orbital, singlet carbenes are supposed to be able to react with both Lewis bases and acids. Transient electrophilic carbenes are known to react with Lewis bases to give normal ylides (Scheme 8.19). For example, carbene-pyridine adducts have been spectroscopically characterized and used as a proof for the formation of carbenes,and the reaction of transient dihalogenocarbenes with phosphines is even a preparative method for C-dihalogeno phosphorus ylides. Little is known about the reactivity of transient carbenes with Lewis acids. 

Following the pathway to diphosphiranes via the [2+1] cycloaddition between diphosphenes and dihalogenocarbenes Yoshifuji reacted diphosphene ( )-l with a mixture of carbon tetrachloride and -butyllithium. ( )-Diphosphirane 2 was obtained in 20% yield, while 64% of precursor ( )-l were recovered <2002CC3012> (Equation 1). 

Aziridine 12 was obtained as a by-product in the reaction of imine 13 and CF2 derived from perfluoropropylene oxide to give 14 (79IZV1826). It is unclear whether the reaction mechanism includes the addition of. CF2, the most electrophilic dihalogenocarbene, onto the C=N bond of imine 13. 

Thermal decomposition to a dihalogenocarbene from an organomercury precursor in the presence of substituted benzaldehydes and dimethyl acetylene dicarboxylate produces 2-halogeno-5-arylfuran-3,4-dicarboxylates 292 in isolated yields that range from 13 to 64%. These products appear to be the... 

Halogenoalkyltin compounds have also been prepared by insertion of an alkylene unit from a diazoalkane into SnCLj (e.g. equation 6-1),10 and Bu3SnCHCl2, Ph3SnCHCl2, and Bu3SnCHBr2 can be prepared by insertion of a dihalogenocarbene from NaOH/CHX3 into the SnH bond, under phase-transfer conditions (equation 6-2)." 12... 

The double bond of unsubstituted benzvalene is considered to be an electron-rich system which reacts with electron-deficient 1,3-dienes namely the Diels-Alder reaction is reverse electron demanding. Two Diels-Alder reactions of benzvalene are described in the following. Benzvalene does not react with usual 1,3-diene but with electron-deficient 1,3-dienes like 1,2,4,5-tetrazine (80) or hexachlorocyclopentadiene (81)74). Dihalogenocarbene also adds to the double bond (82) 75). The obtained... 

The special class of perhalogenomercury compounds, which may be used as sources of dihalogenocarbenes for transfer reactions to organic substrates, has been mentioned on page 285. A convenient preparation of such compounds48 is... 

Treatment of 2,4,4-trimethyI-l,3-dioxan under acid conditions gave 5,6-dihydro-2,4-dimethylpyran, but related dioxans also gave some 2,3-dihydro-pyrans. Dihalogenocarbenes, generated by phase-transfer catalysis, effect a regiospecific insertion reaction in 1,3-dioxans (317 R — H, alkyl, or aryl), giving (318). When the methylene-1,3-dioxan (319) is treated with s-butyl-... 

Cyclization of the dichlorosilane (381) has given a 30% yield of l,l-dichloro-2-methylsilacyclohexane (382). Silacyclohexadienes such as (383) reacts with dihalogenocarbenes to give adducts at either of the double-bonds, depending on the character of 1-Arsanaphthalene (384) has been synthesized from... 

Titanium tetrachloride lithium tetrahydridoaluminate Reductive generation of dihalogenocarbenes 1-Chloro-l-fluorocyclopropanes from ethylene derivs. 

The selectivity of addition of dihalogenocarbenes to steroidal and related model olefins has been examined. ... 

Steric factors are often of significance in the addition reactions of carbenes. Attempts to prepare the tetracycle (57) by dihalogenocarbene addition to (55 R = H) fail since bis-addition cannot be prevented. However, because of increased steric interference, (55 R = Br) only undergoes addition of dichloro- or dibromo-carbene to the... 

The cheletropic reactions of dihalogenocarbenes with norbomadienes have been the subject of further investigations. The main reactions involved are 1,2-addition, which can occur from the endo and exo directions, and homo-1,4-addition. For reaction with norbomadiene itself it has been found that the exo endo selectivity ratio increases som hat in the series difluoro-, fluorochloro-, dichloro-, and dibromo-carbene (respectively 1.2,1.8,4.0, and 2.0), whereas the selectivity ratio for the 1,2 1,4... 

The work of Seyferth s group on phenyl(trihalogenomethyl)mercury compounds as dihalogenocarbene precursors has been reviewed. "... 

The treatment of the cyclic vinyl ether (92) with di-iodomethane-zinc/copper couple does not lead to the corresponding oxabicycloheptane (93 X = H) since both the olefin and product cyclopropane are acid-sensitive. The only effective route to systems such as (93) must rely on dihalogenocarbene addition and subsequent... 

A convenient procedure reported for the preparation of 2,3-benzotropone involves the addition of a dihalogenocarbene to the ethyl enol ether of a-tetralone followed by Ag" -catalysed ring expansion and elimination. Bis(dibromocarbene) adducts of... 

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