Butene cyclo

Conrotatory and disrotatory eoncerted reaetions ean often be distinguished by chemical means. For example, using the results of the previous calculation, predict whether the cyclizations of hexa-2,4-diene will lead to cii or traa.i dimethyl cyclo-butene... 

Cyclopropenone formation should involve the bisketene 46 and its decarbonylation to the monoketene 47 (a valence tautomer of cyclopropenone 44), since photolysis in protic media like ethanol produces diethoxy diethyl tartrate 45) (meso and d,l). This method was also successful in the case of l,2-diphenyl-3,3-dichloro-cyclo-butene dione (48) giving rise to diphenyl cyclopropenone49 (but as for 44 only a moderate yield was produced) ... 

Allerdings ist die Abnahme der Quantenausbeute mit groBerer Cyclo-buten-Konzentration mit diesem Schema nicht in Ubereinstimmung. 

Cycloaddition of norbornadiene with allene takes place to yield the cyclo-butene derivative 10[5], Cyclodimerization of 1,2-cyclononadiene (11) affords a mixture of stereoisomers of the cyclobutane derivatives 12[6,7]. 

Fig. 13.10. The trench region of an Al/BCB (aluminium/benzo-cyclo-butene) damascene test structure (a-c) AFM (d-f) UFM different scan sizes. The contrast inverts between topography and elasticity scans. There is interfacial variation of the elasticity between the A1 and BCB regions. The interface between hardened and unmodified BCB is denoted by the white arrow (i) (Geer et al. 2002).

Examples of this are the photocycloaddition reactions of 3-methylcyclohex-2-enone with ethene96 and isophoronc with l-propen-2-yl acetate.97 The cycloaddition of isophorone to methyl cyclobutene-1-carboxylate98 or of ( —)-piperitone (28) to 1,2-bis(trimethylsiloxy)cyclo-butene (29)99 affords tricyelo[4.4.0.02 5]deean-7-ones, used as starting compounds in fragmentation reactions. 

BIS(SALICYCL1DENE)ETHYLENE-DIIM1NO COBALT(Il), 57, 78 BlS[2,2,2-TRIFLUORO-l-PHENYL-l-(TRIFLUOROMETHYL)ETIIOXY ] -DIPHENYL SULFURANE, 57, 22 l,2-BlS(TRIMETHYLSlLYLOXY)CYCLO BUTENE, 57, 1... 

CYCLOADDITION Alumina chloride. l,2-Bis(trimethylsilyloxy)cyclo-butene. 2,2-Dimcthyl-3(2//)-furanone. Lthylaluminum dichloridc. Kctcnc diethyl acetal.. Methyl cyclobutene-carboxylate. Trimethyl-1,3-dioxolen-one. Zinc chloride. 

It is also possible that the unimolecular reaction takes place with the molecule in the electronic ground state, but it requires very intense fields to generate so-called multiphoton or direct overtone transitions that is, transitions from the vibrational ground state of the type 0 —> n, where n > 1. The opening of the cyclo-butene ring to form butadiene is an example of a unimolecular reaction induced by direct overtone excitation ... 

In a normal-mode picture, the C-H stretching of cyclo-butene is, essentially, a pure normal mode. There are two types of CH bonds, and the normal modes involve either olefinic CH stretch or methylenic CH stretch. Experimentally, it has been demonstrated that direct overtone excitation of these modes, e.g., to the fourth excited C-H stretch mode, provides enough energy for the ring-opening to 1,3-butadiene. 

Unimolecular reactions induced by electromagnetic radiation are often further divided into direct and indirect reactions. In a direct reaction, the unimolecular reaction is over within the order of a vibrational period after the initial excitation. In an indirect reaction, the unimolecular reaction starts long after the initial excitation, since a long-lived intermediate complex is formed. Evidently, the cyclo-butene reaction is not... 

The (high-pressure) pre-exponential factor for the ring-opening of cyclo-butene into butadiene is 1013 4 s 1, and the activation energy is 137.6 kJ/mol. Using the RRK theory calculate the unimolecular rate constant for the reaction at an excitation energy of 200 kJ/mol. 

It is, however, also possible to find elementary reactions with very similar rates in gas and solution phases. Thus, the thermally-activated unimolecular isomerization of some substituted cyclo-butenes in a solution (of dimethyl phtalate at temperatures around 275°C) has been found to proceed at, essentially, the same rate as in the gas phase. 

Direkte UV-Bestrahlung von 2,3-Dimethyl-butadien-(l,3) fuhrt erwar-tungsgemaB zum l,2-Dimethyl-cyclobuten-(l) (Formel 65) (92), wah-rend mit Benzophenon als Sensibilisator keine Isomerisierung zum Cyclo-buten stattfindet. In einer langsamen, aber einheitlichen Reaktion ent-steht l,l-Diphenyl-2,3-dimethyl-butadien-(l,3) (Formel 66) (229). 

Scheme 5.11 Synthesis of tricyclic cyclo butene derivatives with intramolecular [2 + 2] photocycloadditions of aromatic compounds. The addition is followed by rearrangements leading to the final products.

Cyclobutan 3-Carboxy-l-oxo- E17e, 368 [l,l-(COOR)2-3,3-(OR)2 - l-H/COOH-3-oxo-] Cyclopropan 1 -Carboxy- 1-hydroxy-E17b, 1040 [1,2-(OTos)2 — cyclo-buten/Br2 OH ]... 

Carboxy-2,2-dimethyl-1 -hydroxy-El7b, 1040 [1,2-(OTos)2—cyclo-buten/Br2 NaOH]... 

OR—3,4-dioxo —cyclo-buten/hv) E17b, 998 (l-OR-2-R — 3,4-dioxo — cyclobuten/hv) E17d, 2997 (subst. Dioxocyclobu-ten/hv)... 

Figure 4.12. Natural orbital correlation for the disrotatory ring opening of cyclo-buten e to butadiene (by permission from Bigot, 1980).

Figure 29.16. Photochemical cyclization of a 1,3-butadiene to a cyclo-butene. Oisrotatory motion leads to bonding. Conrotatory motion leads to antibonding.

The reaction of enamines with acetylenic compounds has been the subject of intensive studies. In general, alkyl acetylenedicarboxylates and propiolates undergo initial cycloaddition (Chapter 18) to form cyclo butene derivatives. These cyclobutenes rearrange, in most cases spontaneously, to form products whose structures depend on the starting enamine and on the reaction conditions(Scheme 45). 

Keywords bisallene, cyclization, stereospecific, thermolysis, bismethylene-cyclo-butene... 

An analogous vinylketene intermediate (127, see Schemes 57 and 59) as proposed for the Dotz reaction has been assumed in the so-called cyclobutenedione methodology [161]. The key intermediate is a 4-aryl or 4-alkenyl substituted 2-cyclobutenone (128) that can be obtained e.g. by the reaction of the 3-cyclo-butene-1,2-dione (129) with the appropriate lithium reagent or Stille coupling with 4-chloro-3-cyclobutenone. Thermal cyclobutenone ring opening to the vinylketene 130 followed by electrocyclization furnishes the highly substituted aromatic compound 131 (see Scheme 59). 

Chlorobenzylidene chloride is hydrolysed by concentrated sulfuric acid in 5 hours at room temperature,531 and 4-chloro-3,3,4-trifluoro-l-phenyl-cyclo-butene gives 3-phenyl-3-cyclobutene-l,2-dione (75% yield) when treated with... 

As an example of mixed cycloaddition (the two reactants being different) the reaction of 2-butyne with maleic anhydride to form 3,4-dimethyl-3-cyclo-butene-l,2-dicarboxylic anhydride62 may be described ... 

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