Related Strained Hydrocarbons

On the basis of the above analysis a more reasonable estimate of the pK of cyclopropane would be about 51 + 5. Elevation of the pKa to this level places the acidity of cyclopropane in better perspective. Thus, although the accepted Walsh and valence bond (bent bond) bonding schemes assign nominal sp character to the exocyclic carbon bonds of cyclopropane and this is supported by the C-H coupling value (161 Hz) a cyclopropane C-H is several orders of magnitude less acidic than a vinyl C-H or benzene C-H. Nevertheless, cyclopropane is, relatively, more acidic than the other saturated cycloalkanes and within this narrow series Streitwieser and coworkers have de- 

Various bridged bicyclobutane analogs have been synthesized and the acidity of their bridgehead hydrogens exemplified by kinetic or synthetic means. A partial listing would include the following systems tricyclo[4.1.0.0 ] heptane (4)  

Although the ultimate bridged bicyclobutane system, tetrahedrane (8), has resisted synthetic efforts to date, its tetralithio derivative (9) is claimed to be a product of low temperature ( —45°C) photolysis of dilithioacetylene in liquid ammonia. Calculations suggest that the most stable structure of tetralithiotetrahedrane should have a lithium cation centered at each of the four faces of the tetraanion rather than at the corners as indicated by structure (9). 

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Sydnor, K.L., Butenandt, O., Brillantes, F.P., and Huggins, C. (1962). Race-strain factor related to hydrocarbon-induced mammary cancer in rats, J. Natl. Cancer Inst. 29, 805. 

It appears likely that transient metallacyclobutanes are involved in a variety of organic reactions which are catalyzed by transition metal complexes. Thus, cycloadditions of activated alkenes to strained hydrocarbons such as quadricyclane and bicyclo[2.1.0]pentane are catalyzed by complexes such as Ni(CH2=CHCN)2 and probably involve initial formation of a nickelacyclobutane (Scheme 2) (79MI12200). The nature of the organometallic intermediates in related metal-catalyzed rearrangements (72JA7757) and retro-cyclo-addition reactions (76JA6057) of cyclopropanoid hydrocarbons, e.g. bicyclo[n.l.O]alkanes, has been discussed. 

Centropolyindanes constitute a complete family of arylaliphatic polycyclic hydrocarbons containing several indane units. Mutual fusion of the five-membered rings leads to three-dimensional, carbon-rich molecular frameworks bearing a central carbon atom, such as benzo-annelated [3.3.3]propellanes, triquinacenes, and [5.5.5.6]- and [5.5.5.5]fenestranes. In this review, the structural concept of centropolyindanes is contrasted to other fused indane hydrocarbons. Besides the syntheses of the parent centropolyindanes and recently described related indane hydrocarbons, the preparation of a large variety of bridgehead and arene substituted centropolyindanes is presented including strained, heterocyclic, and centrohexacyclic derivatives. In appropriate cases, the particular reactivity and some structural features of these unusual, sterically rigid polycyclic compounds are pointed out. 

Commercial PCBs Toxic and Biochemical Effects. PCBs and related halogenated aromatic hydrocarbons ehcit a diverse spectmm of toxic and biochemical responses in laboratory animals dependent on a number of factors including age, sex, species, and strain of the test animal and the dosing regimen (single or multiple) (27—32). In Bobwhite and Japanese quad, the LC q dose for several different commercial PCB preparations ranged from 600 to 30,000 ppm in the diet the LC q values for mink that were fed Aroclors 1242 and 1254 were 8.6 and 6.7 ppm in the diet, respectively (8,28,33). The... 

The conversion of alcohols directly into the structurally related hydrocarbons by ionic hydrogenation can provide a means of synthesis for compounds that would be extremely difficult or impossible to obtain by other methods. A good example is the synthesis of 2-terr-butyladamantane (12, R = Me). This interesting, highly strained compound may be synthesized in moderate overall yield by a conventional multiple-step route.149 Alternatively, it is obtained in 90% isolated yield upon treatment of a dichloromethane solution of the readily available 2-/c/7-bulyI -2-adamantanoI (11, R = Me)150 and one equivalent of either tri-n-hexylsilane151152 or triethylsilane153 with trifluoroacetic acid at room temperature (Eq. 16). 

Metallocyclobutanes from cyclopropanes have been frequently invoked in transition metal-catalyzed rearrangements of strained ring hydrocarbons, and this body of chemistry is quite rich and diverse, as evidenced in the excellent review by Bishop (72). Because of this diversity, the significance of isolated observations should not be overstated nevertheless, certain reactions outlined by Bishop are closely related to the carbene retroadditions reported by Gassman and co-workers using metathesis catalysts. 

Benzoin derivatives are used as initiators for the photochemical curing of printing inks, lacquers and other surface coatings, since the intermediate radicals in a reaction such as (4.7) can be diverted to initiate the polymerization of vinyl monomers. The use of an unsvm-metrical ketone (4.8) also shows that discrete radicals are produced in the cleavage reaction, since the ratio of hydrocarbon products is close to that expected tor a random combination of separated radicals. Esters that give rise to similar stabilized radicals undergo loss of carbon dioxide (decarboxylation) by a closely related mechanism, and this has proved useful in making quite strained cyclic systems bv irradiation of readily prepared cyclic diesters (4.9). 

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