Reactions, complementary insertion

DNA synthesis is catalysed by DNA polymerases and requires the precursor dNTPs (dATP, dGTP, dCTP and dTTP, each of these existing as Mg2+ complexes), a template (i.e. the dsDNA being copied) and a primer (an initial deoxyribose 3 -OH to enable the reaction to insert the first new nucleotide). The reaction proceeds in a 5 to 3 direction, that is, at the end of the synthesis there is a vacant deoxyribose 3 -OH. The fidelity of the replication process is based on the incoming nucleotides base pairing with the correct base on the antiparallel template. DNA synthesis is semi-conservative (i.e. the newly synthesized strand partners its antiparallel complementary strand) and is bidirectional (because both original strands are replicated). 

Merlic et al. were the first to predict that exposing a dienylcarbene complex 126 to photolysis would lead to an ort/zo-substituted phenolic product 129 [74a]. This photochemical benzannulation reaction, which provides products complementary to the classical para-substituted phenol as benzannulation product, can be applied to (alkoxy- and aminocarbene)pentacarbonyl complexes [74]. A mechanism proposed for this photochemical reaction is shown in Scheme 54. Photo activation promotes CO insertion resulting in the chromium ketene in-... 

This photodriven benzannulation was used in the synthesis of indolocar-bazoles (Eq. 22) [96] and calphostins (Eq. 23) [97]. The thermal insertion of isonitriles into these same classes of carbenes provided a complementary approach to similar benzannulations [98-100]. Manganese alkoxycarbene complexes underwent both inter- [101] and intramolecular [102] photodriven benzannulation reactions with alkynes (Eqs. 24 and 25). 

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

The C-H insertion into alkanes described above is a surrogate of enolate alkylation (Figure 2). The two strategic reactions are complementary, because enolate alkylation is preferred at primary alkyl halides while the C-H activation tends to preferentially occur at tertiary C-H bonds. 

The sequential double migratory insertion of CO into acydic and cydic diorganozircono-cene complexes through acylzirconocene and ketone—zirconocene species provides a convenient procedure for preparing acyclic and cyclic ketones (Scheme 5.6) [8], Thus, the bi-cydic enones from enynes can be obtained through CO insertion into zirconacyclopen-tenes followed by a subsequent rearrangement (Scheme 5.7). The scope and limitations of this procedure have been described in detail elsewhere [8d]. This procedure provides a complementary version of the well-known Pauson Khand reaction [9]. 

Transposition of DNA, which is discussed in Chapter 27, Section D,4, may seem to be a rare and relatively unimportant event in our body cells. However, transposon DNA accounts for 35% or more of the human genome740 and apparently plays a major role in evolution. Like other transposons, the DNA sequences known as retrotransposons also move about within DNA. However, they use an indirect mechanism that involves synthesis of mRNA and reverse transcription.740 741 The reverse transcribed complementary DNA may be inserted back into the genome at new locations. The necessary chemical reactions parallel those involved in the replication of retroviruses (Fig. 28-23, 28-24). Retrotransposens, truncated retrotransposons, and related sequences constitute as much as 16% of the human genome.741... 

For example, photocyclization61,78 of compound 49, derived from the phenyl complex la by addition of cyclopentadiene, was shown to afford the 1,2-dioxy naphthalene 50 (Scheme 18).79 Thus, the regiochemistry of this type of ring annulation is complementary to that of the Dotz reaction, which yields 1,4-dioxy compounds. Photocyclization of aminocarbene complex 5180 involves insertion of carbon monoxide and leads to production of an 2-amino-l-oxy naphthalene derivative 52. Regio isomers, e.g., l-amino-2-oxy compounds 54, can be generated thermally by intermediate formation of a ketene imine 53 by addition of an isocyanide to compound 49 79,81,82 (Amino)alkynylcarbene complexes also give [4+2] cycloadducts, but they prove to be much less reactive than the corresponding alkoxycar-bene compounds (Scheme 18).42,83... 

The 0( D) +H2 reaction has been widely investigated both experimentally and theoretically and has become the prototype of insertion reactions [1, 10, 11, 12, 13, 14, 15, 16. 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28], The combination of high r( sohition experiments and high accuracy of the theoretical calculations has allowed remarkable improvement in understanding of the reaction dynamics of this system. Many experimental results are available such as differential cross section (DCS), product translational energ - distribution, excitation function and product rotational angular momentum polarization [15, 17, 18, 23]. Two complementary detailed experiments have recently been performed. 

The use of silyl ethers also provided a good glimpse into the steric effects of the C-H insertion [98], Relative rates were obtained for insertion a to the oxygen atom in silyl protected n-butanol. It was found that the reaction rate increased dramatically as the size of the silyl protecting group decreased, with a 100-fold rate difference between TBDPS and TMS. Complementary steric and electronic effects were observed with the tetralkoxy silane substrates [97,98], hi competition experiments, it was found that the carbenoid derived from diazo ester 99 reacted solely with tetraethoxy silane 123 to form product 126, and not the corresponding tetramethoxy or tetraisopropoxy derivatives 124 or 125 (Scheme 28). Thus, the secondary C-H bonds appear to possess the right balance between steric and electronic requirements for the insertion. 

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