Carbene dimethoxy

In Section 1.2, we indicated that the nucleophilic properties of dimethoxy-carbene could be put to synthetic use for example in the synthesis of heterocyclic compounds by reactions with isocyanates and isothiocyanates. [23,24] It is worth noting that analogous chemistry is being elaborated by Rigby s group forthebis(alkylthio)carbenes e.g., ( -PrS)2C. [109] Obviously, the nucleophilic properties of (RS)2C parallel those of (MeO)2C. Moreover, further manipulation of the alkythio groups in the products of (RS)2C reactions opens substantial synthetic opportunities. [109]... 

In a three component reaction, the zwitterionic intermediate resulted from the addition of a dimethoxy carbene to dimethyl acetylenedicarboxylate was trapped by an aldehyde to provide 2,5-dihydrofurans in good yields <01TL2043>. 

Dimethoxy carbene has been shown to behave as a carbonyl 1,1-dipole equivalent in [ 1 -E 4] cycloadditions with vinyl isocyanates also yielding functionalized adducts. However, the oxadiazole precursor requires higher thermolysis temperatures and hydrolysis of the acetals in the resultant adducts can be difficult for acid-sensitive systems. 

Cycloaddition with Vinyl Isocyanate. When a large excess of 2,5-dihydro-2,2-dimethyl-5,5-bis(propylthio)-1,3,4-oxadiazole carbene precursor is added rapidly in refluxing benzene to a vinyl isocyanate, two equivalents of carbene add to the substrate prior to ring closure, leading to a six-membered adduct (eq 3). However, no carbene insertion into the amide N-H bond occurs. This phenomenon is not observed with the dimethoxy carbene. ... 

Furans. Reaction of a,a-dimethoxy ketones with 1 affords a dihydrofuran (2) presumably via a carbene (a) that inserts intramolecularly into a C—H of an adjacent methoxy group. The reaction often results directly in a furan, since the elimination of methanol from 2 is facile. 

Thus for instance, in the sensitization experiments carried out with ethoxy-carbonyl-carbene, dimethoxy-carbonylcarbene, and carbene A, the light capture was generally more than 80% 35> Jn the case of phenyl-cyclopentadienyhdenes the nonstereospecificity clearly depends on the fight capture by the sensitizer... 

The bimolecular rate constant for the reaction of DPC with butadiene is determined to be 6.5 X 10 M s . Isoprene can be employed as a selective trap for triplet carbenes. Styrene is also shown to be an efficient trap for triplet carbene. (E)-p-Deutero-a-methylstyrene (89) is a very convenient reagent to diagnose the mult-plicity of the reacting carbene because it reacts with both singlet and triplet carbenes with different stereochemical outcomes. The stereochemistry of the adduct cyclopropane (90) can be easily judged by NMR (Scheme 9.28). For example, BA (22) reacts with styrene with total loss of stereochemistry, while in the reaction with dimethoxy FL (23a), the expected cyclopropane is obtained with complete retention of stereochemistry. The rate constants for the additions are (1.2 0.2) x 10 and... 

The silylcarbene CHSi(OMe)3 was generated by low-temperature photolysis at 345 nm of the related diazomethane, and was characterized. Photolysis of the silyldiazomethane at 280 nm in the cold led to l,l-dimethoxy-l-sila-2-oxetane, arising from insertion of the carbene into one of the methoxy groups134. 

Irradiation (A>295nm, Ar, 10 K) of matrix-isolated (trimethoxysilyl)carbene produced l,l-dimethoxy-l,2-siloxe-tane which was identified by IR spectroscopy in comparison with ab initio calculations at the RHF/6-31G(d,p) level of theory. The most intense IR absorption was observed at 1104 cm-1 <19960M736>. Similarly, vacuum pyrolysis-matrix isolation Fourier transform infrared (FTIR) and DFT studies of 3,3-dimethyl-3-germa-6-oxabicyclo[3.1.0]-hexane indicated the transient formation of dimethylgermoxetane <1998OM5041>. 

Ethen l,2-Dichlor-l,2-dimethoxy-E19b, 1660 (Carben-Dimerisier.)... 

Cyclopropan l,l-Dimethoxy-2-(trichlor-ethenyl)- E19b, 688 (Carben + En)... 

Cyclopropan 2,2-Dimethoxy-l-ethoxy-carbonyl- E19b, 1141 (En + Carben)... 

Benzocy clobuten-< l-spiro-2)-l, 3-dioxolane 3-Methoxy-2-oxo- -E17f, 1333 (Oxo - O.O-acetal) Bicyclo 3.2.0 heptan 1,5-Dimeth-oxycarbonyl- IV/5a. 393 f. Bicyclo[4.1.0 heptan 7,7-Dimethoxy-carbonyl- E19b, 1049 (Carben + En)... 

Bernsteinsaure (Dimethoxy-methy-len)- -diethylester E19b, 1674 [Carben 4 En (1,2-H-shift)]... 

The thermal decomposition of [dimethoxy(phenyl)methyl]triphenylsilane leads to the generation of methoxy(phenyl)carbene which reacts with dimethyl /ran -butenedioate ° (Houben-Weyl, Vol. E19b, p 1642) to give 18. 

Thermal decomposition of 7,7-dialkoxynorbornadiene derivatives in the presence of an alkene requires rather high temperatures ( 125°C) l,2,3,4-tetrachloro-7,7-dimethoxy-5-phenylbicyclo[2.2.1 ]hepta-2,5-diene is the most often applied source of dimethoxycarbene. Electrophilic alkenes such as )- and (Z)-but-2-enedioates and cinnamates, styrene derivatives, " 1-phenylbuta-l,3-diene etc. undergo addition of this carbene across the double bond to give 1,1-dimethoxycyclopropanes 2. 

The stereochemistry of the cyclopropanation reactions is difficult to predict. (Dimethoxy-phosphoryl)carbene reacts stereospecifically with ( )-l,2-diphenylethene to give 3-dimethoxy-phosphoryl-/ra .v-l,2-diphenylcyclopropane. In contrast, photolysis of (dimethoxyphosphor-yl)(phenyl)diazomethane in the presence of (Z)-but-2-ene gave a mixture of diastereomeric m-isomers, (la,2, 3 )-2 and (la,2a,3a)-2, along with the /ranj-isomer, (la,2ot,3/ )-2, in 15, 3 and 5% yield, respectively. Reaction with ( )-but-2-ene afforded only two of the possible cyclopropanes, namely (la,2, 3 )-2, and (Ia,2a,3/S)-2. ... 

CARBENES, GENERATION Chloral. Dibromofluoromethane. Dimethoxy-methyl)trimethoxysilane. Dimethyl diazomethylphosphonate. S -Dimethyl-N-nitroso-2-oxazolidone. Ethylidene iodide-Diethylzinc. Ethyl trichloroacetate. Phenyl(dibromochloro methyl) mercury. Phenyl(dihalocarbomethoxymethyl)mer-cury. Phenyl(trifluoromethyl)mercury. Phenyl(trihalomethyl)mercury. Sodium chlorodiiluoroacetate. Zinc powder. 

Patenden et al. [46] synthesized heterocyclic analogues of pulvinones 52-55 and of permethylated pulvinic acids 56, 57 which were found in higher fungi (Scheme 21). All compounds possessing lactonic structures were synthesized in the Horner-Wittig reaction using the corresponding heteroaryl aldehydes and 4-dimethoxy-phosphorylated lactone 58. The latter was obtained as the main product in the reaction between substituted maleic anhydrides 59 and sodium dimethyl phosphite via the postulated carbene. 

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