Bicyclic species

The Boger pyridine synthesis involves the reaction of triazine 1 with activated alkene 2 in a hetero-Diels-Alder fashion. The intermediate bicyclic species 3 is unstable and a facile cycloreversion takes place due to the loss of nitrogen gas to afford the appropriately substituted pyridine derivative 4. 

Another related bicyclic species (Figure 2.12) has been produced in 20% yield by the reaction of red phosphorus with 1,2-dichlorobenzene on heating in the presence of a Lewis acid catalyst.41 This reaction presumably occurs by an addition-elimination route. 

The spirocyclic pentacoordinate germanium zwitterion 74 was prepared from 2,3-naphthalenediol in three steps starting with a trichlorogermane (Equation (132)).172 The bicyclic species 75 can be prepared in modest yield... 

The chloride analog of 157 can be employed in similar processes to generate digermacyclobutanes 158 and 159 (Scheme 58), and 158 can be converted into a bicyclic species (Equation (293)).348 The germylene 50 reacts with ethylene to yield a related germacyclobutane 160 via a germirane intermediate (Equation (294)).349... 

Cyclic periodinanes also tend to be more stable than their acyclic analogues. The acyclic periodinane C6H5IF4 is moisture sensitive and decomposes upon storage (68ZOB2813), while the cyclic species (45), (49) and (51) are stable at room temperature and not markedly moisture sensitive. The spiro bicyclic or fused-ring bicyclic species are more stable than... 

Bicycio[m. .0]alkenes (25 in. n) with the double bond linking the bridgehead carbons are another class of relevant compounds. Enthalpy-of-formation data are available for the m = n = 1 case through analysis of gas-phase acidity and electron-affinity measurements76, and for m = 2, n = 2, 3 and 4 from hydrogenation data54 from reaction in a thermochemically innocuous hydrocarbon solvent. Parallelling the earlier transalkylation reactions (equation 14), perhaps the simplest comparison we can make is to use the enthalpy of the reaction of the bicyclic species with ethylene to form two mono-cyclic ones. 

Indeed, this abundance of data is the reason why we have chosen to discuss these bicyclic species rather than the simpler, monocyclic five-membered ring compounds recognized as C2H2XYZ (XVI) for which the desired thermochemical data is very often lacking. For these monocyclic compounds we can write the related reaction ... 

Most assuredly, there is a difference in strain energy of the mono and bicyclic species, as well as in phenylene (-C6H4-) and vinylene (-CH=CH-) bonding. However, the difference is small. Taking C2H2(CH3)2 as the (Z)-isomer of 2-butene (choosing the ( )-isomer introduces just an additive constant), we find the reaction ... 

Deprotonation by alkali or amines to azaazulenes is commonly observed with jV-protonated pyrrolotropylium salts, often reversing their formation. Examples are those of bicyclic species like 603 (to give 604, Scheme 162 ... 

The internal cyclization-isomerization route (ICI), which involves the formation of a bicyclic species featuring a single bond between the ring-atoms in the 2- and 5-positions of the original heterocycle. Sigmatropic shifts then occur, leading to different bicyclic intermediates from which the final products will arise (see e.g., Scheme 12.2). 

The bicyclic species (111) are unknown but some reactions of the monobenzo (112) and dibenzo (113) species have been described and they closely resemble those of the isomeric betaines (107) and (108) (Section 4.37.4.4). Electrophilic substitution occurs on the ben-zenoid rings of compound (113) and treatment with methyl iodide slowly gives an adduct. The monobenzo derivative is more reactive, readily giving a methiodide and rapidly undergoing electrophilic substitution reactions. Reaction of compound (112 R = H) with TCNE results in elimination of hydrogen cyanide and formation of the tricyanovinyl derivative (112 R = C(CN)=C(CN)2) (67JA2633). 

Linear species Cyclic species Bicyclic species... 

The Diels-Alder addition of the 1,2-dihydrophosphete complexes 26 and 74 to /V-phenylmaleimide proceeds smoothly, yielding the bicyclic species 87a and 87b. The improved reactivity of 74 over 26 is reflected by the considerably milder reaction conditions (Scheme 22).44a,b... 

Tattenden, G. Roberts, L. Blake, A.J. J. Chem. Soc., Perkin Trans. 11998, 863 Batsanov, A. Chen, L. Gill, G.B. Pattenden, G. J. Chem. Soc., Perkin Trans. 11996, 45. Also see, Pattenden, G. Smithies, A.J. Tapolczay, D. Walter, D.S. ]. Chem. Soc., Perkin Trans. 1 1996, 1 for a related reaction that generates a bicyclic species from an initially generated alkyl radical. 

Cross-conjugated Dienones.- The photochemical rearrangement of a series of cyclohexadienones (244 - 247) has been studied by Schultz and his coworkers. The reactions encountered are typical of such compounds and involve rearrangement to a bicyclic species. In the first example, the irradiation of (244), the bicyclic species could not be isolated or detected since rapid ring opening was presumed to take place affording... 

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