Diene-carbenoid addition

The tandem zirconocene-induced co-cyclization of dienes or enynes/insertion of allyl carbenoid/addition of electrophile is a powerful method for assembling organic structures. Two illustrations of its application are the synthesis of the dollabelane natural product acetoxyodontoschismenol 99 [57,62,63] and the one-pot construction of linear terpenoids 100 (Scheme 3.25) [59,64],... 

When phosphane-free nickel complexes, such as bis(cycloocta-l,5-diene)nickel(0) or te-tracarbonylnickel, are employed in the codimerization reaction of acrylic esters, the codimer arising from [2-1-1] addition to the electron-deficient double bond is the main product. The exo-isomer is the only product in these cyclopropanation reactions. This is opposite to the carbene and carbenoid addition reactions to alkenes catalyzed by copper complexes (see previous section) where the thermodynamically less favored e Jo-isomers are formed. This finding indicates that the reaction proceeds via organonickel intermediates rather than carbenoids or carbenes. The introduction of alkyl substituents in the /I-position of the electron-deficient alkenes favors isomerization and/or homo-cyclodimerization of the cyclopropenes. Thus, with methyl crotonate and 3,3-diphenylcyclopropene only 16% of the corresponding ethenylcyc-lopropane was obtained. Methyl 3,3-dimethylacrylate does not react at all with 3,3-dimethyl-cyclopropene, so that the methylester of tra 5-chrysanthemic acid cannot be prepared in this way. This reactivity pattern can be rationalized in terms of a different tendency of the alkenes to coordinate to nickel(O). This tendency decreases in the order un-, mono- < di-< tri- < tet-... 

The cyclopropane intermediate results from the rhodium-carbenoid addition to the less-hindered terminal double bond of the diene. This generates the cyclopropane in which both alkenes are cis related, which allows the subsequent Cope rearrangement of the divinyl cyclopropane (see Section 3.6.1). The conformation of the boat-shaped transition state for the rearrangement must have the methyl and acetate groups trans to one another. See W. R. Cantrell and H. M. L. Davies, J. Org. Chem., 56 (1991), 723. 

Condensatioii Reactions.—Little emphasis has been placed on the use of condensation reactions in cyclopropane synthesis during the past year. Dimethylsulphoxonium methylide has been used for cyclopropanation of the side-chain double bond in brassicasterol, a A steroidal diene, due to the failure of carbene and carbenoid additions. Michael addition of the ylide (136) to acrolein affords an isomeric mixture of cyclopropanes, whereas addition to acrylonitrile stereoselectively gives only one isomer, the identity of which has not been established. An analogous... 

Boron—nitrogen coordination polymers, with ferrocene, 6, 208 Boron nucleophiles, in conjugate additions asymmetric 1,4-additions, 10, 388 mechanisms, 10, 384 to nitroolehns, 10, 388 to a,/3-unsaturated amides, 10, 386 to a,/3-unsaturated esters, 10, 386 to a,/3-unsaturated ketones, 10, 384 Boron-silicon bonds, addition to alkenes, 10, 760 to alkynes, 10, 758 to allenes, 10, 760 to carbenoids, 10, 766 to 1,3-dienes, 10, 762 to isocyanides, 10, 765... 

Vinyl carbene intermediates can also be generated from the intermolecular addition of TMS diazomethane onto an alkyne component in the presence of Ni(cod)2 as a catalyst. If a diene moiety is also present, as in 1,6-enyne system 78, reaction of the intermediate nickel carbenoid with this partner gives birth to the fused 5,7-bicyclic system 79 in fair yields (Scheme 34). Several mechanistic scenarios are possible from generic precursor 78, including a metathesis-type sequence to generate a nickelacyclobutane 80, followed by... 

In enynes, both the double and the triple bond can compete for the carbenoid that is generated from alkyl diazoacetate under copper or rhodium catalysis. The chemoselectivity is sometimes not very pronounced, but but-l-en-3-yne is selectively attacked at the triple bond [methyl diazoacetate, RhjCOAc), 70% yield], whereas in 2-methylhexa-l,5-dien-3-yne only the double bonds accept the carbenoid (Table 12, entry 2). The vinylcyclopropene obtained by addition to the triple bond may dimerize to form a 3,6-dialkoxycarbonyltricyclo-[3.1.0.0 ]hexane.2 "... 

A route to specifically frans-fused a-methylenebutyrolactones, which seems to hold considerable promise, involves addition of the carbenoid derived from t-butyl diazopyruvate to a cyclic diene followed by Wittig reaction and... 

Addition of diazo compounds to metallic complexes allows the formation of metal carbenoid species which can react with unsaturated molecules to form new carbon-carbon bonds. The Cp RuCl(cod)-catalyzed addition of diazo compoimds to alkynes led to the selective synthesis of functional 1,3-dienes by the combination of two molecules of diazoaUcane and one molecule of alkyne [115,116] [Eqs. (53) and (54)]. The ruthenium carbene, generated from diazo compound, reacts with the C=C bond to produce vinylcarbene intermediate able to add a second molecule of diazo compotmd to generate dienes. The stereoselective formation of these conjugated dienes results from the selective creation of two C=C bonds, probably due to the possibility for (C5Me5)RuCl moiety to accomodate two cis carbene ligands. This reaction occurred with terminal or internal alkynes as well as 1,3-diynes [115] and was applied successfully to alkynylboronates [116]. 

---