Cyclopropene Diels-Alder adducts from

Thus photolysis of the tosylhydrazone sodium salt of 5//-dibenzo[a,c]cyclohepten-5-one (14) at — 60 °C in the presence of cyclopentadiene or furan with tetrahydrofuran as cosolvent gave the cyclopropene Diels-Alder adducts 17 and 18 in 73 and 47% yields, respectively. If the photolysis was stopped shortly after all the tosylhydrazone salt had decomposed, adduct 17 was the only isomer found in the cyclopentadiene reaction. In the formation of the furan adduct 18, the reaction was not so clean and a number of unidentified products were also formed. Unfortunately, adduct 18 is thermally unstable at the temperature necessary for thermal formation of carbene 15. No trace of adduct 18 was detected when the arylcarbene 19 was generated directly from its tosylhydrazone salt in the presence of furan. ... 

Fluoride-ion promoted elimination of /J-halosilylcyclopropanes has also been utilized to prepare Diels-Alder adducts from less strained cyclopropene intermediates. Thus, when 1,1 -dibromo-2-trimethylsilylcyclopropane was reacted with potassium fluoride in diglyme containing 1,3-diphenylisobenzofuran, ( nc o-2-bromo-l,5-diphenyl-8-oxabenzo[/]bicyclo[3.2.1.0 ]octane (5) was obtained in 77% yield.Other examples can be found in refs 92,254, 347, 722, 728-737, 942. 

Liquid-phase photolysis of furan atroom temperature occurred in very low yields (1 % conversion), giving a mixture of Diels-Alder adducts deriving from the reaction of cyclopropene-3-carbaldehyde and formylallene with furan (85JOC3034). 

The cyclopropenes were distilled directly from the reaction mixture and were characterized by trapping as Diels-Alder adducts with 1,3-diphenylisobenzofuran (X = Br, 77%, mp 107-109"C X = Cl, 54%, mp 104-106 C). The same method was used to generate the unstable 7-chlorobicyclo[4.1.0]hept-l(7)-ene which was trapped in situ by 1,3-diphenylisobenzofuran. Reaction of the silanes 2 with tetrabutylammonium fluoride in tetrahydrofuran gave 1,2-dibromocyclopropene (3a) or 1-bromo-2-chlorocyclopropene (3b) ... 

A variety of diverse synthetic methods have been empioyed for the preparation of cyclopropane (1 j. Schlatter and Demjanov and Dojarenko pyrolyzed cyclopropyltrimethylammonium hydroxide at 320°C using platinized asbestos as the catalyst. About equal amounts of cyclopropene (1) and cyclopropyidimethylamine are formed, contaminated with some dimethyl ether and ethylene. Treatment with dilute hydrochloric acid removed the amine from the gas stream and 1 was separated from the other products by gas chromatography. Alder-Rickert cleavage of the Diels-Alder adduct formed from cycloheptatriene and dimethyl acetylenedicarboxylate resulted only in the formation of a polymer and trace amounts of 1. A simple approach by Closs and Krantz based on the synthesis of 1-methylcyclopropene involved the addition of allyl chioride to a suspension of sodium amide in mineral oil at 80°C. Under the conditions employed, 1 could readily escape from the reaction mixture. Though a number of variations were tried, the yield of 1 never exceeded 10%. 

Cycloheptatrienes were isolated from reactions of 1,2,3-triphenylcyclopropane with cyclopentadienes. Thermolysis of the Diels-Alder adduct of cyclopropene... 

The addition of dihalocarbenes to alkynes is again a rather inefficient process and usually leads, to the isolation of the cyclopropenone rather than the 3,3-dichlorocyclo-propene. In a rather unusual example, however, 2-butyne is reported to be converted to (67). This product is apparently derived by addition of dichlorocarbene to the corresponding methylenecyclopropene, derived in turn by elimination of HC1 from the primary adduct (68). The cyclopropene (67) does not appear to ring open to a vinylcarbene, but can be trapped in Diels-Alder reactions with cyclopentadiene 60). A related addition of dichlorocarbene to ethyl 2-butynoate also leads to a low yield of the 3,3-dichlorocyclopropene, which may be hydrolysed to the cyclopropenone 6l). 

The pressure-promoted [4 + 2]-cycloaddition of 2-pyrone 318 with cyclo-propenone ketal 377 (25 C, 6.2 kbar) affords a mixture of reaction products exo-adduct 378a, cycloheptatrienone ketal 379, and cycloheptatrienone 380 (resulting from Si02 hydrolysis of 379), each representing the product derived from the Diels-Alder reaction of 2-pyrone 318 with cyclopropen-one ketal 377 (86JA6695). The mdo-adduct 378b loses carbon dioxide upon depressurization, while the exo-adduct 378a is thermally stable. 3-... 

Wiberg reported the Diels-Alder reaction of butadiene and cyclopropene [53] and Baldwin estimated from the reaction between cyclopropene and 1-deuteriobutadiene at 0°C that 99.4% of the formed cycloadduct was the endo isomer [54], There are many suggestions which attempt to explain endo selectivity in Diels-Alder reactions (Alder s rule [55]), but none are firmly established. According to Woodward and Hoffmann [56], the preference is the result of favorable Secondary Orbital Interactions (SOI) or secondary orbital overlap [57-59] between the diene and dienophile in the corresponding transition state structure. One can also find an explanation for the reaction preference in the difference between primary overlap [60], volumes of activation [61], and the polarity of the transition states [62]. Secondary orbital overlap between the diene and the dienophile does not lead to bonds in the adduct, but primary orbital overlaps do. 

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