Cyclopropane moieties formation

The same authors have demonstrated that 1,3-diynes behave in predictable yet distinctive manners compared to simple enynes under electrophilic transition metal-mediated reaction conditions. This characteristic behaviour of 1,3-diynes is presumably caused by the slightly electron-withdrawing nature of the alkynyl substituent, which not only renders preferentially the formation of 5-exotype alkylidenes but also allows for the subsequent [l,3]-metallotropic shift. Several salient features of reactions with this functionality include the following (a) an acetate is more reactive than the tethered alkene as an initiator, generating [l,2]-acetate migrated alkylidene intermediate, whereas an alkene is a better terminator than an acetate/bromide to generate the cyclopropane moiety (b) allene products are not formed at all under current reaction conditions (c) 5-exo/6-endo-type alkylidene formation depends on the heteroatom substituent in the tether (d) facile metallotropic [1,3]-shift of the intermediate alkylidenes occurred whenever possible. 

The O-silyl derivatives of some tertiary bicyclic or tricyclic alcohols with a cyclopropane moiety upon treatment with IOB, followed by addition of tetrabutyl-ammonium fluoride, underwent oxidation with ring expansion and formation of a,/J-unsaturated ketones, for example [22] ... 

Synthesis of selenocyclopropanes has rarely been carried out by routes that result in formation of a bond between selenium and a cyclopropyl carbon atom. The few exceptions that exist involve polycyclic systems containing a cyclopropane moiety. When (l ,la 8,9b)8)-l-chloro-la-trimethylsilyl-1a,9b-dihydrocyclopropa[/lphenanthrene was stirred in a tetrahydrofuran solution of potassium /crr-butoxide and potassium benzeneselenolate a complex reaction mixture was obtained, from which la-phenylseleno-1a,9b-dihydrocyclopropa[/]phenanthrene (1) and (la,laa,9ba)-l-phenylseleno-1a,9b-dihydrocyclopropa[/]phenanthrene (2) were isolated in 10 and 15% yield, respectively, by preparative TLC. Both selenocyclopropanes arise from the same intermediate, la//-cyclopropa[/]phenanthrene, which means that the addition of benzeneselenolate to this alkene is nonregioselective. In contrast, the nucleophilic addition of meth-... 

In the presence of phosphane-free Ni(0) catalysts, substituted methylenecyclopropanes dimerize at low temperatures giving formal [2 + 2] and [3 -I- 2] cycloadducts 24 and 25, respectively. The chemoselectivity of [2 -t- 2] cycloaddition depends on the substitution pattern of the substrate and is restricted to systems without further substituents at the exocyclic double bond. [3 + 2] Cycloaddition and acyclic product formation depend on further substituents at the cyclopropane moiety. In general, the product is obtained as a mixture. The combined yield resulting from cycloaddition is higher in the presence of electron-deficient alkenes, such as dimethyl ( )-but-2-enedioate. ... 

The ketocyclopropane (40) photochemically rearranges into the cyclo-octadiene (41) which itself is photoconverted into the isomeric compound (42). The formation of the cyclo-octadiene involves the ring opening of the cyclohexenone and the cyclopropane moieties to afford the ketene (43). ... 

The reactions of Pd " with cyclopropanes lead to skeletal rearrangements and appear to be initiated by the heterolytic cleavage of the most substituted C—C cyclopropane moiety (equation 13). Two cleavage modes were observed which lead in the primary step to the formation of tertiary carbocations followed by successive deprotonation to the f/ -allyl complex 31 as the major pathway. 

However, the Ir(I)-catalysed rearrangement of exo- and ndo-tricyclo[3.2.1.0 ]-octane (36) preferentially leads to the conversion of the cyclopropane moiety into an exocyclic methylene group (equation 15). The different product distribution of exo- or endo-36 was rationalized in terms of the formation of iridacyclobutane- and iridacarbene-olefin intermediates. 

Vinyl cyclopropanes tethered to an aUcyne chain 127 were also subjected to the cycloisomerisation reaction in presence of the NHC-Ni catalyst system (Scheme 5.34) [39], The product formation depends on the substrate used and the NHC hgand. When SIPr carbene is used, three different products were obtained depending on the size of the R group attached to the alkyne moiety. If R is small (like a methyl) product 128 is obtained exclusively. If R is Et or Pr a mixture of 128 and 129 is obtained in 3 2 to 1 2 ratio, respectively. However, when R is large groups such as Bu or TMS only product 130 is obtained. When IfBu carbene 131 is used as the ligand, cycloisomerisation of 127 afforded product 128 exclusively, regardless of substituent size (Scheme 5.34) [39]. 

Fig. 30. Mechanism for C-C activation of propene. Decay of the allyl hydride complex may proceed via migration of the metal-bound H atom to the /3-carbon atom in the allyl moiety (i.e. reverse /3-H migration), leading to formation of the same metallacyclobutane complex implicated in the Y + cyclopropane reaction. The dynamically most favorable decay pathway is to YCH2 + C2H4.

It has been widely accepted that the carbene-transfer reaction using a diazo compound and a transition metal complex proceeds via the corresponding metal carbenoid species. Nishiyama et al. characterized spectroscopically the structure of the carbenoid intermediate that underwent the desired cyclopropanation with high enantio- and diastereoselectivity, derived from (91).254,255 They also isolated a stable dicarbonylcarbene complex and demonstrated by X-ray analysis that the carbene moiety of the complex was almost parallel in the Cl—Ru—Cl plane and perpendicular to the pybox plane (vide infra).255 These results suggest that the rate-determining step of metal-catalyzed cyclopropanation is not carbenoid formation, but the carbene-transfer reaction.254... 

The cyclopropane diester (800) bearing a vicinal acetylenic moiety, when treated with Co2(CO)s, affords the formation of the dicobalt hexacarbonyl complex (801). It undergoes a smooth cycloaddition with a,N -diphenylnitrone, in the presence of Sc(OTf)3, to form the corresponding dicobalt hexacarbonyl complex of tetraydro-l,2-oxazine (802). De-complexation of adduct (802) gives 6-ethynyl-tetrahydro-l,2-oxazine (803) (Scheme 2.332) (856). 

Numerous studies aimed at the understanding of the mechanism of these processes rapidly appeared. In this context, Murai examined the behavior of acyclic linear dienyne systems in order to trap any carbenoid intermediate by a pendant olefin (Scheme 82).302 A remarkable tetracyclic assembly took place and gave the unprecedented tetracyclo[6.4.0.0]-undecane derivatives as single diastereomer, such as 321 in Scheme 82. This transformation proved to be relatively general as shown by the variation of the starting materials. The reaction can be catalyzed by different organometallic complexes of the group 8-10 elements (ruthenium, rhodium, iridium, and platinum). Formally, this reaction involves two cyclopropanations as if both carbon atoms of the alkyne moiety have acted as carbenes, which results in the formation of four carbon-carbon bonds. 

The product possesses a homoallylic stannane moiety, which can be utilized as a useful synthon for cyclopropane formation (Scheme 68). Upon treatment of the homoallylstannane with HI, destannative cyclization takes place to give cyclopropylmethylsilane.271,272 A Lewis acid-catalyzed reaction with benzaldehyde dimethyl acetal affords vinylcyclopropane.273... 

Chrysanthemic acid (1) consists of ten carbons, suggesting that it is a monoterpene. The cyclopropane ring of the acid moiety is a feature of pyrethrins. Rivera et al. isolated chrysanthemyl pyrophosphate synthase (CPPase or alternatively referred to as chrysanthemyl diphosphate synthase) underlying the formation of chrysanthemyl pyrophosphate (16) containing a cyclopropane ring from two molecules of dimethylallyl pyrophosphate (15) (DMAPP) and the gene thereof [21]. They found that the reaction involves the cF-2-3 cyclopropanation of DMAPPs in a non-head-to-tail manner. 

The analogous process involving allylic epoxides is more complex, as issues such as the stereochemistry of substituents on the ring and on the alkene play major roles in determining the course of the reaction [38]. Addition of the Schwartz reagent to the alkene only occurs when an unsubstituted vinyl moiety is present and, in the absence of a Lewis acid, intramolecular attack in an anti fashion leads to cyclopropane formation as the major pathway (Scheme 4.10). cis-Epoxides 13 afford cis-cyclopropyl carbinols, while trans-oxiranes 14 give mixtures of anti-trans and anti-cis isomers. The product of (S-elimi-... 

Concerning the structure, the cyclopropane derivatives 524—526 deviate from the generally observed cycloadducts of cyclic allenes with monoalkenes (see Scheme 6.97 and many examples in Section 6.3). The difference is caused by the different properties of the diradical intermediates that are most likely to result in the first reaction step. In most cases, the allene subunit is converted in that step into an allyl radical moiety that can cyclize only to give a methylenecyclobutane derivative. However, 5 is converted to a tropenyl-radical entity, which can collapse with the radical center of the side-chain to give a methylenecyclobutane or a cyclopropane derivative. Of these alternatives, the formation of the three-membered ring is kinetically favored and hence 524—526 are the products. The structural relationship between both possible product types is made clear in Scheme 6.107 by the example of the reaction between 5 and styrene. 

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