Methylene cyclopropanes

The strength of the carbon-carbon bond adjacent to a difluoromethylene group IS hardly affected by the presence of fluorine on the cyclopropyl ring When heated, 2,2 difluoromethylenecyclopropane undergoes methylene-cyclopropane rearrangement [/2 ] Under kinetic control, 2,2 difluoro 1 methylenecyclopropane and (difluoromethylene)cyclopropane are formed in a 2 1 ratio, although the latter IS slightly more stable [129] (equation 27)... 

Brom-2-methyl-2-dthyl-I -methylen-cyclopropan 58% d.Th. Kp17 51-52°... 

Organometallic catalysis insertion and ring opening mechanism of methylene-cyclopropane over Cp2LaH and Cp2LuH catalysts... 

Recently, Y. Yamamoto reported a palladium-catalyzed hydroalkoxylation of methylene cyclopropanes (Scheme 6-25) [105]. Curiously, the catalysis proceeds under very specific conditions, i.e. only a 1 2 mixture of [Pd(PPh3)4] and P(o-tolyl)3 leads to an active system. Other combinations using Pd(0 or II) precursors with P(o-tolyl)3 or l,3-bis(diphenylphosphino)propane, the use of [Pd(PPh3)4] without P(o-tolyl)3 or with other phosphine ligands were all inefficient for the hydroalkoxylation. The authors assumed a mechanism in which oxidative addition of the alcohol to a Pd(0) center yields a hydrido(alkoxo) complex which is subsequently involved in hydropal-ladation of methylenecyclopropane. 

Fig. 16 The experimental geometries of methylenecyclopropane- HC1 and methylene-cyclopropane- -CIF, drawn to scale. The n-electron model for the Lewis base is also shown. The angles C- - H and C- Cl, respectively, where is the centre of the C - C double bond, are both close to 90°, as required by rule 2. The halogen bond again exhibits a smaller non-linearity 6 than the hydrogen bond. See Fig. 1 for key to the colour coding of atoms... 

A different result was obtained in the cycloaddition to methylenecyclo-propanes 216-218 tearing alkoxycarbonyl substituents on the cyclopropyl ring. In this instance, 1,2,3-triazoles 220 isomeric with the triazolines 219 were formed in the reaction [57]. The formation of triazoles 220 is rationalised by the intermediate formation of triazolines 219, which are unstable under the reaction conditions and undergo a rearrangement to the aromatic triazoles via a hydrogen transfer that probably occurs with the assistance of the proximal ester carbonyl (Scheme 35). The formation of triazoles 220 also confirms the regio-chemistry of the cycloaddition for the methylene unsubstituted methylene-cyclopropanes, still leaving some doubt for the substituted ones 156 and 157. 

Carbocyclic compounds containing an unsubstituted exocyclic methylene group give 1,2-diazetidines with PTAD. Methylene adamantane gives the adduct 47,8 5 and the methylene cyclopropane (48, R = H) gave the 1,2-diazetidine 49.86 The phenyl-substituted compound (48, R = Ph) behaved similarly to styrene and gave a 2 1 adduct with PTAD (see Section IV,D,1). 

Silaboration of bicyclopropylidene proceeds via proximal G-G bond cleavage at 130 °C in the presence of the palladium// r/-alkyl isocyanide catalyst (Equation (88)). In contrast to the aforementioned silaboration of methylene cyclopropanes, vinylsilane products are obtained with high regioselectivity.102... 

Cyelobutanone has been prepared by (1) reaction of diazomethane with ketene,4 (2) treatment of methylenecyclobutane with performic acid, followed by cleavage of the resulting glycol with lead tetraacetate,s (3) ozonolysis of methylenecyclobutane, (4) epoxidation of methylene-cyclopropane followed by acid-catalyzed ring expansion,7 and (5) oxidative cleavage of cyclobutane trimethylene thioketal, which in turn is prepared from 2-(co-chloropropyl)-l,3-dithiane.8... 

CavMon The yield isolated from this reaction depends on the efficiency of this condenser the epoxidation is exothermic and methylene-cyclopropane is volatile. 

At elevated temperatures, methylenecyclopropane and its derivatives undergo a rearrangement which maintains the methylenecyclopropane skeleton. Obviously, for the parent system this process is degenerate [216, 217-222]. The 2,2-dihalo-methylene-cyclopropanes behave analogously, providing either mixtures of the... 

Dolbier WR, Sellers SF (1982) J. Am. Chem. Soc. 104 2494. A review of the authors works on thermal rearrangements of gem-difluorocyclopropanes covering, inter alia, cyclopropane thermolysis, methylene cyclopropane and spiropentane rearrangements, vinyl-cyclopropane and cyclopropylcarbinyl isomerizations has been published Dolbier WR (1981) Acc. Chem. Res. 14 195... 

Monosubstituted methylenccyclopropanes 22 or unsymmetrically 1,1-disubstituted methylene-cyclopropane 24 give mixtures of cisftrans isomeric cyclobutanes 23 and 25, respectively.12-13 The [2 + 2]dimerization of 24 proceeded particularly cleanly under high pressure so that crystals of both diastereomers could be obtained and stereochemical assignment be made by X-ray crystal structure analysis.64... 

If small amounts of PhSSPh were deliberately added from the beginning, the more positive reduction potential could be applied from the start. Indirect electrochemical eliminations were also performed in the case of 1,2-dihalides Ringopening reactions by the redox catalytic method were possible in the case of methylene cyclopropanes and epoxides... 

The photolysis of the 4-alkylidene-l-pyrazoline (94) gives rise to two isomeric methylene cyclopropanes (95 and 96).76 The available evidence points to the intermediacy of a trimethylenemethyl species (97) in the triplet state which can cyclize in three ways. The same species is postulated in the photolysis of a series of 4-alkylidene-l-pyrazoline-3-carboxylates.77 This appears to be a general route to derivatives of trimethylenemethyl trimethylenemethyl itself has been generated from 4-methylene-1-pyrazoline and the triplet nature of the intermediate identified by electron spin resonance (ESR) spectroscopy.78... 

Difluoro(methylene)cyclopropanes have been obtained in modest yields from allene by difluorocarbene addition.61 The difluorocyclopropane moiety proved stable upon... 

Reactions of 2-phenyl- 1-methylenecyclopropane and of 2,2-diaryl-1-methylene-cyclopropane with hydrido complexes of Rh and Ir causes C-C bond cleavage of the three-membered ring (Scheme 87).130 The bond cleavage occurs via /3-alkyl... 

Pd-catalyzed C-H transformation at nitriles to methylenecydopropanes 8 affords the terminal 9 and internal alkenes 10, either exclusively or as a mixture of both (Scheme 2, Eq. 4) [9]. The selectivity of product formation depends on the mode of hydropalladation of the carbon-carbon double bond of the methylene-cyclopropane and ring opening. Ring opening normally occurs at the distal bond. Also, the mode of ring opening of methylenecydopropane depends on the chain length of the substituent at the exomethylene carbon. In the addition of malononi-trile lb to 8b, both monoalkylation and dialkylation products are obtained as a 1 1 mixture (Eq. 10). 

Methylene cyclopropanes are of interest as the substrate having high reactivity originated from the strained structure. The platinum-catalyzed reaction of diboron proceeded through the proximal bond cleavage of the cyclopropane ring (Equation (27)).223 Addition of diborons to enones provided 1,4-addition products in the presence... 

Silyl(pinacol)borane added to terminal alkenes in the presence of Pt(GH2=CH2)(PPh3)2 to give l-boryl-2-silylalk-anes 247 Platinum-catalyzed silylboration of methylene cyclopropanes provided alkenylboron derivatives via a proximal G-G bond cleavage (Equation (41))250 Nickel(0)-catalyzed silylboration of vinyl cyclopropanes and cyclobutanes provided allylsilane derivatives via analogous G-G bond cleavage (Equation (42)).251... 

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