Steric effects, benzene-alkene

Reaction 31 appears to be little affected by substituent electronic effects or by steric effects of either sulfonyl chloride or styrenes. Treatment of /5-chlorosulfones with triethylamine in benzene affords the corresponding a, /5-unsaturated sulfones in excellent yield. The copper-catalyzed addition of sulfonyl iodides to simple and cyclic alkenes has also been exploited76. 

Haloform reaction, 237, 296 Halogenation alkanes, 300, 323 alkenes, 179,186, 313 benzene, 138,316 ketones, 295 Hammett equation, 362 additional parameters, 374, 388, 395 derivation of, 362 deviations from, 375 empirical nature of, 395 implications of, 394 reaction pathway, and, 375 solvent effects and, 388 spectroscopic correlations, 392 standard reaction for, 362, 395 steric effects and, 361, 383 thermodynamic implications of, 394 Hammett plots, 359 change in rate-limiting step and, 383 change in reaction pathway and, 378... 

In contrast to proton shifts, carbon-13 shifts cannot be used as criteria for aromaticity (Section 3.1.3.10). No difference exists between aromatic (128.5 ppm for benzene) and comparable alkene carbon nuclei (127.5 ppm for cyclohexene). Aromatic ring carbon nuclei are practically not influenced by the ring current (Section 3.1.3.4), which makes up a deshielding of about 2 ppm and thus is small compared with other (e.g. steric) effects on carbon-13 shifts. 

The only example of an N-sulfonyl ketimine participating in an ene reaction involves the tosyl imine of trifluoroacetone 266 [90] [Eq.(62)]. When heated with a terminal olefin such as allyl benzene (265) in refluxing xylene, imine 266 leads to ene product 267 in moderate yields. However, internal alkenes gave significantly lower yields of ene products. The inefficiency of the ene process with more highly substituted olefins was ascribed to unfavorable steric effects due to the bulky trifluoromethyl groups. Interestingly, with /J-methyl styrene and allyl thiophenyl ether, [2+2]-cycloadducts were detected rather than ene products. 

Dilute solutions ( 10" moldm" ) of Wilkinson s complex in organic solvents such as ethanol/benzene actively catalyse the hydrogenation of alkynes and alkenes at ambient temperature and pressure (1 atm, H2). Traces of oxygen must be excluded. Hydrogenation rates are dependent on steric effects. The following general order has been established 1-alkenes > cis-2-alkenes > trans-2-alkenes > trans-3-alkenes. An exception is ethene itself, which forms a rather stable complex (Fig. 5.19) so that higher temperatures are required. Arenes, esters, ketones, carboxylic acids, amides and nitro compounds are unaffected, but aldehydes are slowly decarbonylated. 

The results confirm that It is the state of the arene which reacts with ground state alkene but do not allow any conclusion to be made about the intermediacy of an exciplex or any other species along the reaction path. Also in the same laboratory, the products of the meta photo-addition of cyclopentene with alkyl and alkoxy benzenes have been determined and the effect of increasing size of the alkyl or alkoxy group examined. The major product isolated is (80). although (81) and other isomers become more important for alkyl benzenes as the substituent s size is increased. The results are consistent with a polar mechanism involving the formation of (82) where the substituent stabilises the positive charge and steric interactions are avoided by the endo orientation of the cyclopentane. Also consistent with a polar intermediate are the meta adducts... 

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