Reactions in the absence of Lewis acids

Although phosgene is commercially available in toluene solution, this is not an inert solvent under photochemical conditions. Thus, irradiation of a solution of COCl, in toluene with light from a mercury lamp produced PhCHjCI in 82.1% yield [1150]. 

Phosgene in toluene reacts with azulene at room temperature in the absence of a Friedel-Crafts catalyst according to [2052a]  

The azulene derivative, guaiazulene (1,4-dimethyl-7-isopropylazulene), also reacts with COClj to give the expected ketone, (10.3) [1698a]  

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As anticipated from the complexation experiments, reaction of 4.42 with cyclopentadiene in the presence of copper(II)nitrate or ytterbium triflate was extremely slow and comparable to the rate of the reaction in the absence of Lewis-acid catalyst. Apparently, Lewis-acid catalysis of Diels-Alder reactions of p-amino ketone dienophiles is not practicable. 

Oppolzer sultam-like chiral auxiliary (e.g., Xc in 304) has been studied in Diels-Alder cycloaddition reactions (Scheme 43) <2003JP0700>. The TiCU-promoted reaction of dienophile 304 and 1,3-cyclopentadiene 305 in DCM is complete within 18h and excellent diastereoselectivity of product 306 is observed. The same reaction in the absence of Lewis acid provides product 306 in very low yield. However, switching to trifluoroethanol as the solvent, the cycloaddition reaction proceeds to completion, albeit with slightly diminished levels of diastereoselectivity for Diels-Alder adduct 306. Surprisingly, the use of hexane as the solvent affords the opposite (23, J 31-diastereomer of 306 as the major product. 

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