Cyclopropanes cycloaddition with

The highly strained nature of methylene- and alkylidenecyclopropanes has been evidenced by spectroscopic measurements and X-ray analysis. The presence of the exocyclic double bond imposes a lengthening of the C(2)-C(3) bond as a result of an increase of the C(2)-C(l)-C(3) angle (compared to cyclopropane). This structural feature is reflected in a typical reactivity of these compounds which is a thermal or transition metal catalysed [3 + 2] cycloaddition with alkenes. This chemistry, usually referred to as TMM chemistry , has been the object of many studies and thoroughly reviewed by Binger and Buch [2] and Trost [8]. 

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). 

W(CO)6 in toluene at room temperature gave the tricyclic adduct 190a in 94% yield after acidic workup. This reaction forms the tungsten-containing azomethine ylide 191, which undergoes the [3 + 2]-cycloaddition with 189a. The rhodium(n)-catalyzed cyclization of the ene-yne-aldimine 192 with alkene 193 into the cyclopropane 194 was reported by Uemura and Ohe (Scheme 32).42c... 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

Section 14.2 describes the highly stereoselective cyclopropanation chemistry of the donor/acceptor-carbenoids (Fig. 14.1a) [16]. This section introduces the range of vinyl, aryl, alkynyl, and heteroaryl functionalities that have been used as donor groups in this chemistry. Also, chiral auxiliaries and chiral catalysts that achieve high asymmetric induction in this chemistry are described [25]. The next two sections cover chemistry that is unique to the vinylcarbenoid system, namely [3-t4] cycloaddition with dienes (Fig. 14.1b see also Section 14.3) [13] and [3-1-2] cycloaddition with vinyl... 

The reaction of vinylcarbenoids with vinyl ethers can lead to other types of [3 + 2] cycloadditions. The symmetric synthesis of 2,3-dihydrofurans is readily achieved by reaction of rhodium-stabilized vinylcarbenoids with vinyl ethers (Scheme 14.17) [107]. In this case, (J )-pantolactone is used as a chiral auxihary. The initial cyclopropanation proceeds with high asymmetric induction upon deprotection of the silyl enol ether 146, ring expansion occurs to furnish the dihydrofuran 147, with no significant epi-merization during the ring-expansion process. 

Cyclohexadiene derivatives are less reactive than butadiene derivatives, thus only a few examples of cycloadditions with these compoimds are known (Figure 4.3) [37 0]. The cyclohexadiene bicychc derivative 32 was synthesized by rhodium-catalyzed reaction of toluene with tert-butyldiazoacetate and cycloadds in about 40% yield to Cjq [39]. The product has anti-cyclopropane orientation relative to the entering dienophile Cjq. Valence isomerization of 33 (Scheme 4.4) leads to the cyclobutene-fused cyclohexene 35 that adds in good yields (50%) at moderate temperatures (110 °C) to Cjq [40]. The reaction of with the electron-deficient cyclohexene 34 is also possible in moderate yields [38]. 

The (ri" -diene tricarbonyliron)-substituted diazocarbonyl compounds 25 have been found to undergo 1,3-dipolar cycloaddition with methyl acrylate in high yield, but with little or no diastereoselectivity (56). Nevertheless, the facile chromatographic separation of the diastereomeric products 26a,b and 27a,b (Scheme 8.8), permits the synthesis of pure enantiomers when optically active diazo compounds (25) [enantiomeric excess (ee) >96%] are employed. When the reaction of 25 (R = C02Et) with methyl acrylate was carried out at 70 °C, cyclopropanes instead of A -pyrazolines were formed. The enantiomerically pure... 

With respect to the large number of unsaturated diazo and diazocarbonyl compounds that have recently been used for intramolecular transition metal catalyzed cyclopropanation reactions (6-8), it is remarkable that 1,3-dipolar cycloadditions with retention of the azo moiety have only been occasionally observed. This finding is probably due to the fact that these [3+2]-cycloaddition reactions require thermal activation while the catalytic reactions are carried out at ambient temperature. A7-AUyl carboxamides appear to be rather amenable to intramolecular cycloaddition. Compounds 254—256 (Scheme 8.61) cyclize intra-molecularly even at room temperature. The faster reaction of 254c (310) and diethoxyphosphoryl-substituted diazoamides 255 (311) as compared with diazoacetamides 254a (312) (xy2 25 h at 22 °C) and 254b (310), points to a LUMO (dipole) — HOMO(dipolarophile) controlled process. The A -pyrazolines expected... 

In a mechanism for the [67i+27t]/[67i+27i]/[2o +27t] process, the first two steps involved in this overall transformation were presumed to parallel the pathway observed for the above described [67t+27t], [67t+27t] process. A third cycloaddition of the cyclopropane unit with an additional alkyne component occurred via a [2[Pg.113]

In addition to the representative [3 + 2] cycloaddition reactions shown in Table I, the delocalized singlet vinyl carbenes have been shown to participate as it2a components of non-Hnear cheletropic [tt 2s + tt 2a] cycloadditions to provided cyclopropanes with an observable endo effect, and as7r2s components of [tt4s + tt 2S] cycloadditions with selected dienes to provide cyclo-... 

The cycloaddition of carbenes to glycals is an effective method for the formation of cyclopropanated carbohydrates with high stereoselectivity,60... 

Various cyclic compounds are prepared by the reaction of these carbene complexes with various unsaturated compounds [78-80]. The metallacyclobutane 246 is generated by [2+2] cycloaddition with electron-rich alkenes, and its reductive elimination affords the cyclopropanes 247. 

Radical iodine atom transfer [3 + 2]-cycloaddition with alkene (118) using dimethyl 2-(iodomethyl)cyclopropane-l,l-dicarboxylate (117) forms cyclopentane derivative (119), through the formation of an electron-deficient homoallyl radical, followed by the addition to alkene, and cyclization via 5-exo-trig manner as shown in eq. 4.41. 

Allylsilanes act as good acceptors of nitrones and oxyallyl cations. The 1,3-dipole species arising from electronically activated cyclopropanes can be trapped by allylsilanes.203 204 2043 Epoxides as well as aziridines act as 1,3-dipole precursors for inter- and intramolecular [3 + 2]-cycloadditions with allylsilanes.205 2053 206 2063... 

However, carbenes react with dienes to give vinylcyclopropanes 6.302, avoiding the [2 + 4] cycloaddition with a linear approach giving cyclopentenes 6.299. We have seen that the cyclopropane-forming reaction is allowed when it uses a nonlinear approach 6.130, but we need to consider why the nonlinear approach is preferred when the linear approach giving a cyclopentene could profit from overlap to the atomic orbitals with the two large coefficients at the ends of the diene. 

Double bonds adjacent to complexed dienes can be cyclopropanated using diazomethane, methyl diazoacetate, or sulfur-based ylids. Cycloheptatriene iron tricarbonyl undergo a [2 -F 2] cycloaddition with chloroketene derived from trichloroacetyl chloride (Scheme 161). 

Dialkyl diazenedicarboxylates do not undergo efficient cycloaddition with vinyl ethers. For example the [2 -I- 2] adduct was obtained in low yield from the cyclopropane-fused dihydropy-ran 11 even at high temperature28. From the same substrate the [2 + 2] cycloadduct with 4-phenyl-3//-1,2,4-triazole-3,5(4//)-dione was obtained quantitatively even at low temperature through a polar mechanism involving an aziridinium imide as the intermediate28. 

In a novel combination of Pauson-Khand cycloaddition with vinylcyclopropane chemistry, de Meijere has described an entry to linearly fused triquinanes beginning with cyclopropylalkynes. Cyclopentenone formation has been carried out with a variety of substitution patterns on the cyclopropane, and moderate yields achieved with both norbomene and cyclopentene as substrates. Thermal vinylcyclopropane-cy-clopentene rearrangement of the cycloaddition products leads to the final tricyclic system (Scheme... 

R and R may be H, methyl, cyclopropyl, cyano, or ester groups. The phenylcarbene formed on irradiation of trans-l,2-diphenyloxirane has been trapped and identified in the form of a cyclopropane derivative in methanol in the presence of benzyl methyl ether and alkenes. Photolysis in the presence of 2,3-dimethyl-2-butene proceeds by cycloaddition with the formation of cyclopropane-carboxylic acid and oxetane derivatives (Eq. 368). ... 

There are many cases where the [3 + 2]-cycloaddition leads to cycloadducts with high enantioselectivity. Cycloaddition of diazo esters with a cobalt catalyst having a chiral ligand leads to cyclopropane derivatives with good enantioselectivity. ... 

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