1.3- dimethylcyclopropene

Strained bicyclic compounds can be obtained e.g. when cyclopropenes are used as dipolarophiles. Reaction of 3,3-dimethylcyclopropene 7 with diazomethane 4 gives the heterobicyclic cycloaddition product 8 in 85% yield ... 

Cycloaddition of cyclopropenes is catalyzed by transition metal complexes. 1-Methylcyclopropene 118 undergoes a facile PdCl2(PhCN)2-catalyzed cyclodimerization to dimethyltricyclo[3.1.0.02,4]hexanes 119. In contrast, cyclo-trimerization of 3,3-dimethylcyclopropene 120 occurs in the presence of a catalytic amount of Pd(PPh3)4 to give hexamethyl-frans-ff-trishomobenzene 121... 

The reactions between cyclopropenes and carbon monoxide in the presence of transition metals have been of some use in synthesis,93 and in 1978 Binger initiated a study of the reactions between metal carbonyls and cyclopropenes in order to elucidate the generality of these reactions.75 It was found that dicarbonyl 775-cyclopentadienyl(tetrahydrofuran)manganese(I) reacted with 3,3-dime thy Icy clopropene at 0°C to produce -(vinylketene) complex 81 in fair yield. The only other transition metal in Binger s study that was found to react with 3,3-dimethylcyclopropene in this manner was iron (see Section VI,B). 

An exceptionally interesting example of an intramolecular [3 + 2] cycloaddition, in which the diazo dipole and the olefinic C=C bond are separated by only one carbon atom, is outlined in Scheme 8.65. The thermal decomposition of the allenic tosylhydrazone sodium salt 267 produced 1,4-dihydropyridazine 269 (57). It is assumed that diazabicyclohexene 268 is a short lived reaction intermediate. This suggestion is supported by the observation that the generation of the diazocumulene l-diazo-2-methyl-l-propene in the presence of 3,3-dimethylcyclopropene also leads to 269. 

As for additions of allylic Grignard reagents, the relative reactivity order of the olefins appears to be 1-alkenes < styrene < 1,3-butadiene < ethylene and a,oi-or a,/3-disubstituted alkenes do not react94. However, strained alkenes such as cyclopropenes constitute an exception. Indeed, dicrotylzinc smoothly reacted with 3,3-dimethylcyclopropene and afforded the dicyclopropylzinc reagent 130 resulting from a syn addition process (equation 62)93. 

Dimethylcyclopropene undergoes efficient cyclodimerization giving 3 (R1 = R2 = Me) in the presence of palladium(O), nickel(O) complexes16,17 or Lewis acids such as boron trifluoride and aluminum trichloride-diethyl ether complexes.16... 

In cycloadditions of enones to alkenes novel strategies have been adopted for ring expansion of the cycloadducts, either by the choice of appropriate alkenes, e.g. 2-(trimethylsiloxy)buta-1,3-diene,70 vmv-2-trimethylsiloxybuten-2-oales71 or 3,3-dimethylcyclopropene,72 or by using 3-oxo-l-cyeloalkene-l-carboxylates as enones.73 Asymmetric [2 + 2] photocycloaddition of cyclopent-2-enone to a (+ )-dihydrofuran acetonide constitutes the cornerstone of the synthetic strategy in the first total synthesis of the novel antitumor metabolite ( )-echinosporin.74 The cycloaddition product 25 from treatment of 2-(2-carbomethoxyethyl)-2-cyclopentenone (24) with ethene has been used as a precursor for the preparation of tricyclo[4.2.0.01,4]octane.75... 

Similarly, the complex 3 was formed from 3,3-dimethylcyclopropene and 2,2 -bipyridyl( 4-cy-cloocta-l,5-diene)nickel, which on treatment with maleic anhydride at 25 °C gave anti-3,3,6,6-tetramethyltricyclo[3.1.0.02,4]hexane (4) in >90% yield. Displacement of the hydrocarbon ligand from 3 with 3,3-dimethylcyclopropene proceeds at > 90"C. Since the complex 3 is regenerated in this step, 3 is the catalyst in the cyclodimerization of the cyclopropene.120... 

The stability of the metallocene complexes is strongly dependent on the nature of the cyclopropene substituents, and the reaction conditions. Thus, when equimolar amounts of 3,3-dimethylcyclopropene and Cp2Ti(PMe3)2 react at 0 °C, a 2 1 mixture of alkylidene and cyclopropene complexes is formed. However, when excess of cyclopropene is used, a dicyclopropyl titanacycle is exclusively formed by oxidative coupling reaction of the intermediate cyclopropene complex (equation 215)77. The analogous zirconium oxidative-coupling product is obtained upon reaction of 3,3-dimethylcyclopropene with Cp2(PMe3)Zr( j2-CH2=CHEt) (Section IV.B.2). 

Suppose one added hydrogen as H2 to the cyclopropene double bond of 1-methoxy-2-phenyl-3,3-dimethylcyclopropene. Explain how the proton nmr of the product can be used to infer whether the hydrogen added to the double bond in the suprafacial or antarafacial manner. (Review Section 9-10G and 9-1 OH.)... 

Cycloaddition reactions of phenyl and tosyl azides to the strained double bond in cyclopropenes have been investigated.170il The reaction products from 3,3-dimethylcyclopropene indicate that the initially formed intermediate is a normal 1,3-dipolar adduct. Tetrachlorocyclopropene yields the primary adducts with several aryl azides. 70b However, cyclopropenedicarboxyl ester gives only unstable triazolines with phenyl and methyl azides.170 ... 

Fused-ring triazolines derived from 3,3-dimethylcyclopropene dicarboxyl-ate,442 Dewar thiophene (as was shown in Scheme 20)161 and cyclobutadiene undergo thermal isomerization to diazoimino compounds in a retro-1,3-cycloaddition (Scheme 146). 

Spontaneous isomerization of triazolines to diazo compounds can lead to addition of the latter to a second molecule of olefin, especially in the case of acrylic derivatives, resulting in a A pyrazoline, which by proto tropic rearrangement, gives the A2-compound (Scheme 149). Pyrazolines have been observed in the reactions of alkyl,67 aryl,32,282 heterocyclic,283,453 and gly-cosyl288 azides. A A pyrazoline is reported from the addition of phenyl and tosyl azides to 3,3-dimethylcyclopropene in this case the diazoimine formed by a retro-1,3-addition of the primary cyclopropanotriazoline adduct reacts with another olefin molecule.82... 

In an interesting and unusual reaction, Paquette and Maynard have shown that peracid oxidation of l,2-disiIyl-3,3-dimethylcyclopropene gives rise to an o -silyl-aq/i-unsaturated acyl silane, through rearrangement of the intermediate epoxycyclopropane. Further treatment with peracid gave the a-epoxy acyl silane (Scheme 54)145. 

The most strained cycloolefins, which are substituted cyclopropenes, e.g. 3,3-dimethylcyclopropene or 3-methyl-3-ethylcyclopropene, appeared to be polymerised readily to respective substituted poly(l,2-cyclopropene)s in the presence of Pd-based catalysts containing very bulky non-labile ligands. Such catalysts are characterised by reduced activity in order to prevent ring opening of the cyclopropene monomer [23],... 

The ring-strain in 3,3-dimethylcyclopropene causes titanacyclobutane 10 to open exclusively in a productive fashion at temperatures above 23 °C. The absence of an equilibrium as shown in Eq. (17 a) and the lower reaction temperature for ring-opening provide a polymerization reaction with initiation faster than chain propagation, Eq. (19). 

Problems with subsequent reaction of a 1-halocyclopropene may be avoided by reduction of the dihalocyclopropane to a monohalocyclopropane prior to dehydrohalogenation. Thus 3,3-dimethylcyclopropene may be obtained in multi-gram quantities by treatment of l-bromo-3,3-dimethylcyclopropane with potassium t-butoxide in DMSO at —78 °C19). Dehydrohalogenation of a series of related 3,3-dialkyl substituted monochloro- or monobromo-cyclopropanes leads to moderate yields of cyclopropenes (11, R = alkyl, alkenyl, aryl, CN, Cl, Rl = H, Ph, t-Bu, R2 = H)20) Indeed, dehydrobromination of (12) leads to either mono- or di-cyclopropenes 2l>. Reaction of a dihalocyclopropane with an alkyl lithium at low temperature followed by carboxylation of the derived 1-lithio-l-halocyclopropane provides a convenient source of 1-halocyclopropane carboxylates dehydrohalogenation leads to cyclo-... 

As might be expected from their inherent strain, many cyclopropenes undergo rearrangement, dimerisation or even polymerisation under relatively mild conditions. The conditions required for reaction are, however, very variable and some cyclo-propenes, such as 3,3-dimethylcyclopropene, are stable at relatively high temperature (150 °C in this case). Three main reactions are described below — the ene-reaction, [2+ 2]-dimerisation, and rearrangement to vinylcarbenes. 

The ring opening of cyclopropenes may also be induced by metal salts or complxes. 3,3-Dimethylcyclopropene is converted to adducts (192) by treatment with Ni(COD)2 in the presence of electron poor alkenes with diethylmaleate the reaction proceeds with predominant retention of stereochemistry4,135. Other 3,3-disubstituted compounds are converted to adducts in good yield by reaction with (EtO)3P CuCl in the presence of alkenes at —40 to 20 °C. In the absence of a trap, a triene (193) is isolated103 ... 

Addition of halogens often occurs without ring opening. Early reports described a ciy-addition 2) though the stereochemistry of addition of chlorine to (229) is cis-160), addition of bromine is cis- in non-polar solvents in the presence of sunlight but trans-in relatively polar ones 161), and addition of bromine to l,2-dichloro-3,3-dimethylcyclopropene leads to a mixture of (E)- and (Z)-dibromodichlorides83). 

Addition of dimethylvinylidene or a related carbenoid, generated from the reaction of propanone with a diazophosphonate and base, to alkylcyclopropenes leads largely to trienes, eg. (283) from 3,3-dimethylcyclopropene. Apparently the same products are obtained when the carbenoid is generated from 1,1 -dibromo-2-methylpropene and base, but it is not clear whether the reactions involve formation and rearrangement of a bicyclobutane or rather collapse of a polar intermediate such as (284)2U). 

Addition of diazomethane to 3,3-dimethylcyclopropene leads to the pyrazolines (303, R = H) but methyl diazoacetate leads to (304) and (305) the former is apparently derived by base induced reaction of the pyrazoline, while the latter may be explained in terms of rearrangement to the diazocompound (306) followed by dipolar addition to the cyclopropene 242). 

The reaction of 3,3-dimethylcyclopropene with phenyl- or p-toluenesulphonylazides leads to 2 1 adducts (316). These are apparently derived by rearrangement of an... 

The readily available methyl 3,3-dimethylcyclopropene-l-carboxylate (III/54) undergoes [2+2] cycloaddition with enamines e.g. the morpholine derivative III/55 to give 2-aminobicyclo[2.1.0]pentane derivatives, e.g. III/56. These compounds are transformed into cyclopentane derivatives, e.g. methyl 4-hydroxy-2,5,5-trimethyl-1-cyclopentenecarboxylate (III/57) by treatment with dilute mineral acids. 

Details of the preparation of methyl (-)-cis-chrysanthemate from (+)-car-3-ene have appeared (Vol. 5, p. 15 is misleading).159 Both ( )-cis- and (+)-trans-chrysanthemic acids are again reported from (+)-car-3-ene via ozonolysis 160 this work is very similar to that reported (Vol. 5, p. 15) by Sukh Dev and illustrates the lamentable delay from receipt to publication in some journals. The decomposition of ethyl diazoacetate in 2,5-dimethylhexa-2,4-diene in the presence of the chiral copper complex (72) leads to cis- and frvms-chrysanthemic acids in 60—70% optical yield the degree of asymmetric induction is dependent upon the steric bulk of R1 and R2 in (72).161 cis-Chrysanthemic acid has also been prepared from 3,3-dimethylcyclopropene, isoprene, and 2-methylpropenylmagnesium bromide followed by treatment with carbon dioxide.162... 

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