Abstract:Anhand von Modellen wird gezeigt und erklart, in welchen Fallen bei der Deprotonierung vonCyclopropancarbonsaure-methylestern Esterkondensation eintritt und in welchen Alkylierung des Esterenolats moglich ist. Unter bestimmten strukturellen Voraussetzungen (sterische und elektronische Effekte) sind die entsprechenden 1-alkylierten Derivate in hoher Ausbeute und mit maRiger bis guter Diastereoselektivitat zuganglich. Die Isomerenverhaltnisse werden bei 2-alkylund 2-arylsubstituierten Cyclopropancarbonestern hau… Show more
“…For example, α-methylstyrene yields the cyclopropanes 3 and 4 in a 3:2 ratio. They were characterized by 1 H- and 13 C-NMR, as detailed in the supporting information. …”
Methylrhenium trioxide (CH 3 ReO 3 or MTO) catalyzes several classes of reactions of ethyl diazoacetate, EDA. It is the first high valent oxo complex for carbene transfer. Under mild conditions and in the absence of other substrates, EDA was converted to a 9:1 mixture of diethyl maleate and diethyl fumarate. In the presence of alcohols, R-alkoxy ethyl acetates were obtained in good yield. The yields dropped for the larger and more branched alcohols, the balance of material being diethyl maleate and fumarate. An electron-donating group in the para position of phenols favors the formation of R-phenoxy ethyl acetates. The use of EDA to form R-thio ethyl acetates and N-substituted glycine ethyl esters, on the other hand, is hardly affected by the size or structure of the parent thiol or amine, with all of these reactions proceeding in high yield. MTO-catalyzed cycloaddition reactions occur between EDA and aromatic imines, olefins, and carbonyl compounds. Three-membered ring products are formed: aziridines, cyclopropanes, and epoxides, respectively. The reactions favor the formation of trans products, and provide a convenient route for the preparation of aziridines. Intermediate carbenoid and nitrenoid species have been proposed.In the presence of an oxygen source such as an epoxide, ethyl diazoacetate and azibenzil are converted to an oxalic acid monoethyl ester and to benzil; at the same time the epoxide was converted to an olefin. These results provide further support for the proposed intermediate, a cyclic species containing Re, O, and CHCO 2 Et (or, occasionally, CPhC(O)Ph) in a three-membered ring.
“…For example, α-methylstyrene yields the cyclopropanes 3 and 4 in a 3:2 ratio. They were characterized by 1 H- and 13 C-NMR, as detailed in the supporting information. …”
Methylrhenium trioxide (CH 3 ReO 3 or MTO) catalyzes several classes of reactions of ethyl diazoacetate, EDA. It is the first high valent oxo complex for carbene transfer. Under mild conditions and in the absence of other substrates, EDA was converted to a 9:1 mixture of diethyl maleate and diethyl fumarate. In the presence of alcohols, R-alkoxy ethyl acetates were obtained in good yield. The yields dropped for the larger and more branched alcohols, the balance of material being diethyl maleate and fumarate. An electron-donating group in the para position of phenols favors the formation of R-phenoxy ethyl acetates. The use of EDA to form R-thio ethyl acetates and N-substituted glycine ethyl esters, on the other hand, is hardly affected by the size or structure of the parent thiol or amine, with all of these reactions proceeding in high yield. MTO-catalyzed cycloaddition reactions occur between EDA and aromatic imines, olefins, and carbonyl compounds. Three-membered ring products are formed: aziridines, cyclopropanes, and epoxides, respectively. The reactions favor the formation of trans products, and provide a convenient route for the preparation of aziridines. Intermediate carbenoid and nitrenoid species have been proposed.In the presence of an oxygen source such as an epoxide, ethyl diazoacetate and azibenzil are converted to an oxalic acid monoethyl ester and to benzil; at the same time the epoxide was converted to an olefin. These results provide further support for the proposed intermediate, a cyclic species containing Re, O, and CHCO 2 Et (or, occasionally, CPhC(O)Ph) in a three-membered ring.
“…The synthesis of the required reductive alkylation substrate was somewhat interesting (Scheme ). We began by preparing tert -butyl cyclopropanecarboxylate ( 30 ) in 76% yield from the commercially available acid chloride 29 . Alkylation of the lithium enolate derived from 30 , with bromoethane, gave 31 in 95% yield.…”
Free radical cyclization of 4 and 7 gave the expected cyclization-reduction products (5 and 8) along with considerable amounts of products derived from a cyclization-atom transfer-secondary cyclization process (6 and 9). Two approaches to avoiding these unexpected products were explored. Use of a cyclopropylcarbinyl fragmentation avoided the secondary cyclization reaction (25 or 43 --> 26 or 44), whereas use of an allene as a radical acceptor avoided the atom-transfer reaction altogether (49 --> 52).
The metal‐catalyzed decomposition of diazo compounds in the presence of alkenes is a well‐established reaction. Since the original
Organic Reactions
review on the reaction of ethyl diazoacetate with alkenes and aromatic compounds in 1970, several new developments have revolutionized this area of chemistry. Most notably, major advances have been made in catalyst design such that highly chemoselective, diastereoselective and enantioselective carbenoid transformations can now be achieved. Furthermore, it has been recognized that a wide array of carbenoid structures can be utilized in this chemistry, leading to a broad range of synthetic applications.
This chapter comprises coverage of the metal‐catalyzed intermolecular cyclopropanations of diazo compounds containing at least one adjacent electron‐withdrawing group. The coverage of diazoacetate chemistry will be limited to material since 1970 because the previous
Organic Reactions
review covers the earlier literature. The alkene component is limited to alkenes, dienes, furans, and pyrroles because these are the systems that have resulted in the greatest developments since the 1970 review. Metal‐carbenoid intermediates derived from diazo compounds undergo a variety of useful reactions, including cyclopropanation, insertion, and ylide formation. In recent years several excellent reviews have appeared on various aspects of this chemistry. Three recent reviews have focused on asymmetric intermolecular cyclopropanations. Several books and reviews on carbenoid chemistry have major sections on intermolecular cyclopropanations. Because of the historical central prominence of carbenoids derived from diazoacetates, most reviews have tended to focus on this class of carbenoids. In this chapter, a comparison is presented of the chemical differences that exist among the major classes of carbenoids that contain adjacent electron‐withdrawing groups. The extensive nature of the topic precludes coverage of related reactions such as the metal‐catalyzed decomposition of diazoalkanes, phenyldiazoalkanes, or vinyldiazoalkanes that lack adjacent electron‐withdrawing functionality. Other cyclopropanation reactions such as the Simmons‐Smith reaction, photochemical or thermal decomposition of diazo compounds in the presence of alkenes, and cyclopropanation using stoichiometric metal carbenes are not covered.
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