Cyclopropane alkanal 2354 Transformation

Oxa-l -silabicyclo[ . 1,0 alkanes (n = 3 111 n = 4 113) were the only products isolated from the photochemical, thermal or transition-metal catalyzed decomposition of (alkenyloxysilyl)diazoacetates 110 and 112, respectively (equation 28)62. The results indicate that intramolecular cyclopropanation is possible via both a carbene and a carbenoid pathway. The efficiency of this transformation depends on the particular system and on the mode of decomposition, but the copper triflate catalyzed reaction is always more efficient than the photochemical route. For the thermally induced cyclopropanation 112 —> 113, a two-step noncarbene pathway at the high reaction temperature appears as an alternative, namely intramolecular cycloaddition of the diazo dipole to the olefinic bond followed by extrusion of N2 from the pyrazoline intermediate. A direct hint to this reaction mode is the formation of 3-methoxycarbonyl-4-methyl-l-oxa-2-sila-3-cyclopentenes instead of cyclopropanes 111 in the thermolysis of 110. 

The catalyst causes a classical carbenium ion to be formed by acid catalyzed activation reactions. The classical carbenium ion is transformed into the key intermediate which can be described as a protonated cyclopropane structure. After some rearrangements cracking occurs. The formation of branched paraffins is very fortunate since branched paraffins have high octane numbers and the isobutane produced can be used in alkylation. The preferred products are those of which the formation proceeds via tertiary carbenium ions. Carbenium ions can also be generated by intermolecular hydride transfer reactions between alkane and carbenium ions that are not able to form tertiary carbenium ions (see Chapter 4, Section 4.4). Under more severe conditions lower paraffins can also be cracked. 

The pentacyclic alkene (172) is photochemically reactive and is readily transformed (0°C, quartz filter, MeCN solution) into the hexacyclic alkane (173) in high yield. The reaction is a conventional [2+2] cycloaddition between the olefinic rr-bond and the cr-bond of the cyclopropane ring. Compound (174) reacts analogously, yielding (175). 

The word carbene derives from the name given to free, disubstituted carbon compounds with general structure 3 (X and Y same as in l).2 Because the central carbon atom does not possess an octet of electrons, free carbenes are electron deficient and extremely reactive. They are so reactive that some carbenes insert themselves into normally inert alkane C-H bonds (equation 10.1) or react with alkenes to form cyclopropanes (equation 10.2), a synthetically useful transformation. 

Transition metal catalyzed decomposition of unsaturated a-diazo ketones or a-diazo esters is a powerful method for the synthesis of certain 2-oxobicyclo[n.l.0]alkanes. In contrast to the thermal (see Section 1.2.1.2.4.2.6.1.) and photochemical (see Section I.2.I.2.4.2.6.2.) methods, which have only been applied successfully in a few cases, the carbenoid version has been extensively utilized for the construction of simple or highly substituted bicyclic, tricyclic or higher systems of predictable stereochemistry (for reviews, see refs 2, 82, 320). Several of the cyclopropanes so obtained have been transformed further into natural products with diverse molecular skeletons. As examples and procedures have already been presented in Houben-Weyl, Vol. E19b, ppl088ffand 1271 ff, only some important aspects concerning the scope and limitation of the method as well as recent developments concerning its stereochemistry will be discussed here. 

Two reviews address the reactions of bare transition metal atoms and ions with hydrocarbons in the gas phase and the reactions of monosubstituted alkanes with bare transition metal ions. 2 A study of the reactivity of ground-state, neutral transition metal atoms from the left hand side of the 4d series (Y through Mo) shows that they are unreactive towards linear alkanes but that they will react with cyclopropane and alkenes.3 Atomic metal cations form a 1 1 adduct with tribenzocyclotriyne in a Fourier-transform ion cyclotron resonance spectrometer. Reaction of molecular oxygen with M(C2H4) results in ligand exchange to M02 for the early first row transition metals. Activation of the O—O bond and product formation is observed for ccnnplexes of Sc+,Ti+andV+.5... 

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