Carbenes, cyclopropanes

In contrast to silylcarbenes, the analogous stannylcarbenes 2p are not stable, which explains why they have attracted little interest. Their instability is probably due to the long carbon-tin bond, which does not allow sufficient steric protection of the carbene center. Their reactivity seems to be quite similar to that of stable (phosphino)(silyl)carbenes Cyclopropanation reactions have been reported with methyl acrylate as well as coupling reactions with tert-butyl isonitrile.73... 

Furukawa-Simmons-Smith-Reakt.), 848 f. (Sawada-Sim-mons-Smith-Reakt.) E17a, 362f. (Ar—CH = M 4- En), 932 (3-C1 —2-Ar—propen/R2BH 4-OH-) E17b, 1279 (1-S02-Ar - 1-H) E18, 832 (En 4- Carben) E19b, 204/206 (En 4- Carben) Cyclopropan-[Pg.607]

Azoniaboratacyclopropanes 39 were prepared using the general carbene cyclopropanation route in Equation (23) <1999EJI255>. Owing to the reactivity of the aminoborane precursor, only those diazo compounds that can be isolated and purified are appropriate for the reaction. 

Of the various alkenes used to intercept these carbenes, cyclopropane derivatives are obtained only with 1,1-diphenyl-, l,l-bis(4-chlorophenyl)ethene and 2-methylpropenenitrile. For details of reactions with the first two alkenes, see Houben-Weyl, Vol. E19b, p 1622. 

This carbene insertion reaction has been used in a variety of syntheses, and is especially attractive when coupled with other synthetic techniques. Taber et al. used carbene cyclopropanation in several synthetic endeavors. In one example, the diazoketone was treated with bis-A-tert-butylsalicylaldiminato copper(II) [Cu(TBS)2, 388] to induce the carbene cyclopropanation reaction. The diazoketone was prepared by treating 386 with mesyl azide to give 387 in 82% yield, which was followed by treatment with the Cu(TBS)2 reagent to produce 389 in 80% yield in Taber and co-workers synthesis of (-i-)-isoneonepatelactone.308... 

Methylthio(chloro)carbene has been generated under phase transfer conditions from dichloromethyl methyl sulfide. This carbene cyclopropanates tetramethyl-ethylene in 43%yield, accompanied by a 24%yield of l,2-dichloro-l,2-Z /5-methyl-thioethylene [24]. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Furan and thiophene undergo addition reactions with carbenes. Thus cyclopropane derivatives are obtained from these heterocycles on copper(I) bromide-catalyzed reaction with diazomethane and light-promoted reaction with diazoacetic acid ester (Scheme 41). The copper-catalyzed reaction of pyrrole with diazoacetic acid ester, however, gives a 2-substituted product (Scheme 42). 

Two other fluorine-containing diazirines give similar results. Cyanofluorodiazirine (223) and fluoromethoxydiazirine (224) release nitrogen at 100 °C the carbenes formed can be trapped by tetrafluoroethylene to give cyclopropanes (65JHC371). Without a carbene trap alkenes are obtained. 

Cyclopropanations by carbenes from chlorodiazirines were observed in several cases, e.g. with the r-butyl compound. Cyclopropanation and stabilization by ring enlargement and by elimination compete in chlorocyclobutyldiazirine photolysis. 

In laser-impulse experiments with chlorophenyldiazirine the carbene could be observed by UV spectroscopy. On addition of defined amounts of alkene the rate of cyclopropanation was measured directly. The rate constants with various alkenes were (lO moF s ) 1-hexene, 1.3 ( )-2-pentene, 34 2-methyl-2-butene, 77 2,3-dimethyl-2-butene, 130 (80JA7576>. 

Cyclopent-2-en-l-one, 2-hydroxy-3-methyl-synthesis, 3, 693 Cyclopentenone, 4-methoxy-formation, 1, 423 Cyclopenthiazide as diuretic, 1, 174 Cyclopent[2,3-d]isoxazol-4-one structure, 6, 975 Cyclophane conformation, 2, 115 photoelectron spectroscopy, 2, 140 [2,2]Cyclophane conformation, 2, 115 Cyclophanes nomenclature, 1, 27 Cyclophosphamide as pharmaceutical, 1, 157 reviews, 1, 496 Cyclopiloselloidin synthesis, 3, 743 Cyclopolymerization heterocycle-forming, 1, 292-293 6H-Cyclopropa[5a,6a]pyrazolo[l,5-a]pyrimidine pyrazoles from, 5, 285 Cydopropabenzopyran synthesis, 3, 700 Cyclopropachromenes synthesis, 3, 671 Cyclopropa[c]dnnolines synthesis, 7, 597 Cyclopropanation by carbenes... 

Section 14.13 Carbenes aie species that contain a divalent carbon that is, a caibon with only two bonds. One of the chaiacteiistic reactions of caibenes is with alkenes to give cyclopropane derivatives. 

Interest in this reaction was revived when the relevance of a carbene mechanism was realized, particularly following the demonstration (cf. SectionI,B) of a similar ring expansion of indene to 2-chloro-naphthalene by dichlorocarbene via the cyclopropane adduct. Indeed, at this time Nakazaki suggested that these reactions occurred by the addition of dichlorocarbene to the indolyl anion and subsequent rearrangement to the indolenine and, with loss of chloride ion, to the quinoline [Eq. (12)]. The preference of dichlorocarbene for... 

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