Methyl butene carbenes

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>. 

In pentane, the distribution of 1,3-insertion product 25 to 1,2-Me shift product 26 is 91 9. Upon addition of 2-methyl-1-butene, the yield of 25 smoothly decreases (to 19% with 4 M alkene), but the yield of 26 is unaffected 1 Moreover, correlation of addn/l,3-CH insertion (to 25) for 18 is nicely linear. The simplest interpretation is that 25 comes directly from carbene 18, whereas the 1,2-Me shift product 26 comes from the excited diazirine.27 Interestingly, thermolysis of 24 at 79°C produces 90% of 25 and 10% of 26, but now the yields of both products smoothly decrease in the presence of an alkene. In thermolysis the (electronically) excited diazirine is unavailable, both 25 and 26 stem from the carbene, and their formation is suppressed by the alkene s interception of the carbene. A pyridine ylide kinetic study gave the 1,3-CH insertion rate constant (18 - 25) as 9.3 x 10s s"1.27-47... 

The experimental evidence most often employed to decide between the singlet and triplet multiplicity of a carbene is [1+2] cycloaddition to cis- or trans-olefins. The olefins employed in these studies are mostly 2-butenes or 4-methyl-2-pentenes, but in a few cases other olefins were also utilized. 

Relative reactivities for the electrophilic carbenes CH3CCI and CCI2 appear in Table 7.3, ° in which they can be seen to react most rapidly with the most highly alkylated olefins, (CH3)2C=C(CH3)2, (CH3)2C=CHCH3, less rapidly with disubstituted frani-butene (designated as the standard alkene, and assigned fcrei = 1.00), and least rapidly with such electron-poor olefins as methyl acrylate or acrylonitrile. 

As early as 1964 Frey observed that the ratio of 1,1-dimethylcyclopropane and 2-methyl-2-butene, the products from intramolecular reactions of tert-butyl diazomethane, was strongly dependent on the method used to decompose the diazo compound (Table The response of the community of carbene chemists was... 

Carbene generation by C atom deoxygenation has been useful in answering questions concerning the intermediacy of free carbenes in certain systems. For example, ferf-butylcarbene (101) from several precursors gives 1,1-dimethylcyclo-propane (102) by C—H insertion, and 2-methyl-2-butene by C—C insertion. However, calculations ([QCISD(T)/6-31+G(2d,p)]// MP2/6-31G(d)) indicate that C—C insertion should not be competitive with C—H insertion in this carbene. 

The most convincing evidence for this mechanism is that in the reaction between isobutene-1-I4C and carbene the product 2-methyl-1-butene was labeled only in the 1 position.236 This rules out a free radical or other free intermediate such as a carbocation or carbanion. If 28 (or a corresponding ion) were an intermediate, resonance would ensure that some carbene attacked at the 1 position ... 

A careful search was made for methyl cyclopropane138 in the photolysis of n-butane and of t -butane with only negative results. The absence of methyl cyclopropane may be due to the great excitation in the carbene produced photochemically, although a better explanation may be that the carbenes produced have a much higher probability of isomerizing to butenes. 

Reaction of dichloro(2,2,Ar,A-tetramethyl-3-buten-l-amine)palladium-(II) with CH2N2 gives the carbene insertion product, a-chloro-A-(chloromethyl)-c/,c-(2,2,jV,7V-tetramethyl-3-buten-l-amine(palladium(II)) (56), together with analogous ethoxymethyl and methyl complexes (127). 

To draw each product, add the carbene carbon from either side of the alkene, and keep all substituents in their original orientations. The cis methyl groups in c/s-2-butene become cis substituents in the cyclopropane. Addition from either side of the alkene yields the same compound—an achiral meso compound that contains two stereogenic centers. 

Acyclic and cyclic allylic alcohols arc attacked by phase-transfer-generated dichlorocarbene with surprisingly high diastereofacial selectivity24,25. The formation of the major diastereo-mers is consistent with carbene delivery from the side on which the most stable conformer bears the hydroxy group. The unselective reaction of 3-methyl-3-buten-2-ol might be due to an unfavorable allylic 1,3-strain 26 in this conformer. 

PhCF and PhCCl appear to be electrophilic their selectivities resemble that of MeCCl in that all three carbenes are less reactive toward methyl acrylate and acrylonitrile than trans-butene. Moreover, PhCF and PhCCl certainly differ from ambiphilic MeOCCl, which is much more reactive toward the electron-... 

The most common application of carbenes in synthesis is in the formation of three-membered rings by addition to multiple bonds. This is a typical reaction of all carbenes that do not undergo intramolecular insertion. Generation of the carbene in the presence of an alkene gives a cyclopropane product. Addition of halocarbenes to alkenes is a stereospecific cis reaction, but this is not necessarily the case with all carbenes. Hence Z-2-butene 109 gives the cyclopropane 110, in which the two methyl groups remain cis to one another (4.86). The stereospecificity... 

The formation of the reactive intermediate, dichlorocarbene, is confirmed by die selective trapping of the carbene with 2,3-dimethyl-2-butene to form l,l-dichloro-2,2,3,3-tetramethylcyclopropane. In a similar fashion, die trichloromethyl radical is trapped by 2,3-dimethyl-2-butene to yield 2-methyl-2-trichloromethyl-1-butene. These trapped intermediates can be identified and quantified by gas chromatography/mass spectroscopy (GC/MS) techniques. 

A relay enyne RCM/CM reaction of dienyne 69 and 2-methyl-3-buten-2-ol was the key step in a total synthesis of the epoxyquinoid natural product (-1-)-panepophenanthrin (Scheme 2.27) [16ej. TTiis domino transformation, which is initiated by attack of the mthenium carbene at the terminal alkene followed by an RCM with release of 2,5-dihydrofuran, worked best with the phosphine-free... 

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