Cyclopropanes 1,3-Dienes

The dienyne 394 undergoes facile polycyclization. Since the neopentylpalla-dium 395 is formed which has no hydrogen /J to the Pd after the insertion of the disubstituted terminal alkene, the cyclopropanation takes place to form the tt-allylpalladium intermediate 396, which is terminated by elimination to form the diene 397(275]. The dienyne 398 undergoes remarkable tandem 6-e. o-dig. 5-cxo-trig. and -exo-trig cyclizations to give the tetracycle 399 exclu-sively(277]. 

Cyclopropanes can also be obtained by the reaction of vinyltrialkylborates with aldehydes followed by treatment with phosphoms pentachloride and base (300), and by the rearrangement of 5-substituted alkynyltrialkylborates (308). It is also possible to utilize this approach for the synthesis of five- and six-membered rings (3). Trans-1,4-elimination ia cycHc systems leads to the formation of stereodefined acycHc 1,5-dienes or medium-ring dienes, depending on the starting compound (309). 

The photochemical isomerization of 1,4-dienes 1, bearing substituents at C-3, leads to vinyl-cyclopropanes 2, and is called the di-n-methane rearrangement This reaction produces possible substrates for the vinylcyclopropane rearrangement. 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

Small amounts of cyclopentene derivatives are detected in cyclopropanation reactions of electron-deficient dienes, but they may result from thermal rearrangement of the corresponding vinyl cyclopropanes and not from a direct [4+1] cycloaddition... 

The /lomo-Diels-Alder reaction is a [2 + 2 + 2] cycloaddition of a 1,4-diene with a dienophile which produces two new bonds and a cyclopropane ring. This reaction is an example of a multi-ring-forming reaction that to date has found few applications in synthesis, since the use of 1,4-dienes has been limited mainly to bridged cyclohexa-1,4-dienes and almost exclusively to norbornadiene. Lewis-acid catalysts accelerate /lowo-Diels-Alder reactions and increase the selectivity for the [2 + 2 + 2] vs. [2 + 2] cycloaddition. 

When free radicals are added to 1,5- or 1,6-dienes, the initially formed radical (9) can add intramolecularly to the other bond, leading to a cyclic product (10). When the radical is generated from an precursor that gives vinyl radical 11, however, cyclization leads to 12, which is in equilibrium with cyclopropylcarbinyl radical 13 via a 5-exo-trig reaction. A 6-endo-trig reaction leads to 14, but unless there are perturbing substituent effects, however, cyclopropanation should be the major process. 

Alkenes of all types can be converted to cyclopropane derivatives by this reaction (though difficulty may be encountered with sterically hindered ones). Even tetracyanoethylene, which responds very poorly to electrophilic attack, gives cyclopropane derivatives with carbenes.Conjugated dienes give 1,2 addition ... 

Two competing reactions are the homodienyl [1,5] shift (if a suitable H is available, see 18-29), and simple cleavage of the cyclopropane ring, leading in this case to a diene (see 18-3). 

Dihydro-2iy-thiopyrans, derived from dimethylbuta-1,3-dienes, Na2S203-5H20 and various activated alkyl h des, ring contract on treatment with a strong base leading to vinyl cyclopropanes and cyclopentenes <96JOC4725>. 

The work of Sharp and his many co-workers at Edinburgh cannot be underestimated. In a more recent communication they have extended the scope of his cyclisation of diene-conjugated nitrile ylides to triene homologues <96CC2739>. Thus cyclisation of the triene 21 afforded the cyclopropa[c]isoquinoline 22, which on heating gave a mixture of 23 and 24. The isomeric triene 25 also gave 24 as the sole product (Scheme 4). In this instance the intermediate cyclopropane could not be isolated. 

Cyclopropanation of the 4,6-diene function proceeds selectively at the 5,6-double bond. Thus, reaction of 185 with the ylide from trimethyl sulfonium iodide and sodium hydride, in DMSO, affords predominantly the a-cyclopropyl compound (187) accompanied... 

Some years ago we began a program to explore the scope of the palladium-catalyzed annulation of alkenes, dienes and alkynes by functionally-substituted aryl and vinylic halides or triflates as a convenient approach to a wide variety of heterocycles and carbocycles. We subsequently reported annulations involving 1,2-, 1,3- and 1,4-dienes unsaturated cyclopropanes and cyclobutanes cyclic and bicyclic alkenes and alkynes, much of which was reviewed in 1999 (Scheme l).1 In recent days our work has concentrated on the annulation of alkynes. Recent developments in this area will be reviewed and some novel palladium migration processes that have been discovered during the course of this work will be discussed. 

The simple hydrocarbon substrates included ethene, 1,2-propa-diene, propene and cyclopropane (22). Their reactivity with Sm, Yb and Er was surveyed. In contrast to the reactions discussed above, lanthanide metal vapor reactions with these smaller hydrocarbons did not provide soluble products (with the exception of the erbium propene product, Er(C H ) ). Information on reaction pathways had to be obtained primarily by analyzing the products of hydrolysis of the metal vapor reaction product. 

When the cyclopropane homolog 24, with the same 1,1,2,2-tetrabromide array, was treated with zinc, the analogous fragmentation reaction occurred, leading in this case to an 84% yield of 2,4-dimethyl-penta-1,4-diene, 25 4g). This is in contradistinction to the reaction of the corresponding tetramesylate 16 with sodio-diethyl malonate 4f>, (vide supra). 

The limitation to electron-rich alkenes in Rh(II)-catalyzed cyclopropanation with phenyldiazomethane leaves untouched the great versatility of zinc halides for this purpose with this, catalyst, efficient and very mild cyclopropanation of 1,3-dienes and unactivated alkenes has been reported 46). 

Apart from these findings, the limited application of ZnCl2 (cyclopropanation of some cyclic 1,3-dienes, isoprene and ethyl vinyl ether 4S-49)) and copper(II) acetyl-acetonate (cyclopropanation of enamines 50)) still stand alone. 

Unless indicated otherwise, reactions were performed at 25 °C b For reactions with dienes, yields are presented as (% yield of cyclopropane isomers/(total % yield of cyclopropane products) Precision 5% of reported value d Reaction performed at 0 °C e Reactions performed at 60 °C. 

Pd(OAc)2 works well with strained double bonds as well as with styrene and its ring-substituted derivatives. Basic substituents cannot be tolerated, however, as the failures with 4-(dimethylamino)styrene, 4-vinylpyridine and 1 -vinylimidazole show. In contrast to Rh2(OAc)4, Pd(OAe)2 causes preferential cyclopropanation of the terminal or less hindered double bond in intermolecular competition experiments. These facts are in agreement with a mechanism in which olefin coordination to the metal is a determining factor but the reluctance or complete failure of Pd(II)-diene complexes to react with diazoesters sheds some doubt on the hypothesis of Pd-olefin-carbene complexes (see Sect. 11). 

The catalytic cyclopropanation of 1,3-dienes leads exclusively or nearly so to mono-cyclopropanation products, as long as no excess of diazocarbonyl compound is applied. The regioselectivity has been tested for representative rhodium, copper and palladium catalysts 59 7 ,72), and the results are displayed in Table 9. 

Table 9. Regioselectivities in the cyclopropanation of 1,3-dienes with EDA in the presence of various catalysts". The yields [%] of cyclopropanation at each of the double bonds are given"... 

Only a few results are available concerning competitive cyclopropanation of non-conjugated dienes. The case of 1,4-hexadiene72 (mixture of Z and E isomers) illustrates the reactivity difference between a monosubstituted and a 1,2-disubstituted double bond, whereas in limonene (24)47, a 1,1-disubstituted and a trisubstituted double bond compete for the carbenoid derived from ethyl diazoacetate. In both cases, the less substituted double bond reacts preferentially (Scheme 8). 

A similar, although less marked difference characterizes the cyclopropanation of olefins 41 and 42. In the presence of either copper or copper complexes whose chelating ligands contain an azomethine moiety derived from an a-amino acid, no stereoselectivity was observed with diene 41, whereas the cyclopropanes derived from 42 occur with cisjtrans ratios of 57 43 to 69 31, depending on the catalyst93). 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

Furans and some of its derivatives have been cyclopropanated with the ketocarbenoids derived from ethyl diazoacetate and copper catalysts. The 2-oxabicyclo[3.1.0]hex-3-enes thus formed are easily ring-opened to 1,4-diacylbutadienes thermally, thermo-catalytically or by proton catalysis 14,136). The method has been put to good use by Rh2(OAc)4-catalyzed cyclopropanation of furan with diazoketones 275 to bicyclic products 276. Even at room temperature, they undergo electrocyclic ring-opening and cis, trans-dienes 277a are obtained with fair selectivity 257,258). These compounds served as starting materials in the total syntheses 257 259) of some HETE s (mono-... 

When the cis/trans stereoselectivity of cyclopropanation with ethyl diazoacetate in the presence of CuCl P(0-z-Pr)3, Rh6(CO)16 or PdCl2 2 PhCN was plotted against that obtained with Rh2(OAc)4, a linear correlation was observed in every case, with slopes of 1.74,1.04 and 0.59, respectively (based on 22 olefins, T = 298 K) S9). These relationships as well as the results of regioselectivity studies carried out with 1,3-dienes point to the similar nature of the intermediates involved in Cu-, Rh-and Pd-catalyzed cyclopropanation. Furthermore, obvious parallels in reactivity in the transformations of Scheme 45 for a variety of catalysts based on Cu, Rh, Fe, Ru, Re and Mo suggest the conclusion that electrophilic metal carbenes are not only involved in cyclopropanation but also in ylide-forming reactions66. ... 

Strong evidence exists for the intermediacy of a tungsten ethoxycarbonyl carbene 425 in cyclopropanation of various enol ethers, 1,3-dienes and cyclohexene with ethyl diazoacetate in the presence of catalytic amounts of (CO)5W = C(OMe)Ph 413). The following equations could account for the obtained products ... 

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