Styrenes trifluoromethyl

Bis(trifluoromethyl)-l,l-dicyanoethylene is a very reactive dienophile. It undergoes facile and high-yield [2+4] cycloadditions with 1,3-dienes, cyclopen-tadiene, and anthracene [707] (equation 86). It is reactive enough in a Diels-Alder reaction with styrene [702] (equation 86). 

When strong electron-withdrawing substituents were introduced at the a-or )S-carbon of the vinyl group, the styrenes acted as dienophiles. Thus cycloaddition of a-trifluoromethyl styrene (58) with Danishefsky s diene 59 afforded regioselectively a 1 1 mixture of cycloadducts which were then converted (Equation 2.20) into 4-phenyl-4-trifluoromethyl-2-cyclohexen-l-one [54]. 

More recently, Carreira reported a [Fe(TPP)Cl]-catalyzed diastereoselective synthesis of trifluoromethyl-substituted cyclopropane in aqueous media [56]. The carbene precursor trifluoromethyl diazomethane is difficult to be handled, generated in situ from trifluoroethyl amine hydrochloride, and reacts with styrene in the presence of [Fe(TPP)Cl] to give the corresponding cyclopropanes in high yields and with excellent diastereoselectivities (Scheme 12). 

However, the discovery with the most far-reaching consequences arose from the close scrutiny of the results of Kunitake and Takarabe on the polymerisation of styrene by trifluoromethyl sulphonic acid. It became evident to the reviewer that in these reactions an important contribution to monomer consumption must have been made by the ester, and he was able to extract from the results the corresponding propagation rate-constants, kpu, for that cationoid insertion polymerisation. This became one of the most convincing supports for the author s views on cationoid insertions. 

An asymmetric intermolecular carbonyl-ene reaction catalyzed by 1 mol% of chiral A-triflyl phosphoramide (/ )-4t (1 mol%, R = 4-MeO-CgH ) was developed by Rueping and coworkers (Scheme 69) [88], Various a-methyl styrene derivatives 163 underwent the desired reaction with ethyl a,a,a-trifluoropyruvate 164 to afford the corresponding a-hydroxy-a-trifluoromethyl esters 165 in good yields along with high enantioselectivities (55-96%, 92-97% ee). The presence of the trifluoromethyl group was crucial and the use of methyl pyruvate or glyoxylate instead of 164 resulted in lower reactivities or selectivities. 

Table 1 Polyfunctional products obtained by the Ni(ll)-cat zedcross<50UFling of Arylzinc bromides and alkyl iodides in the presence of 4-(trifluoromethyl)styrene. 

Morken and Burton extended this methodology to the synthesis of /3,/3-difluoro-a-(trifluoromethyl)styrenes, as illustrated in equation 5334. The steric hindrance presented by... 

Styrene and 1,1-diphenylethene are frequently used as target molecules in investigations of the role of reagent structure on its reactivity toward organic compounds. Low-temperature fluorination with trifluoromethyl hypofluorite gives up to six products.32-33 The Hammett correlation value q was established to be — 2.48 and a mechanism suggested in which spin-paired free-radical pairs arc formed, which are then converted by electron transfer into an ion pair.33... 

The quinone methide (284) is stabilized by the trifluoromethyl groups but was not isolated. Trapping with styrene gave the fluorinated chroman (Scheme 76) (68JOC3297). 

Trifluoromethyl ethylenic substrates have been used as dipolarophiles. The remarkable activation effect provided by the trifluoromethyl group is illustrated by the large increase in yield from the reaction of styrene (20%)416 to that of a-trifluoromethylstyrene (80%)417,418... 

The stabilizing effect of the secondary orbital interactions of the trifluoromethyl substituent with dienes is exemplified in the cydoaddition reaction of a-(trifluorornethyI)styrene (7) with Danishefsky s diene 4. A phenyl substituent is known to stabilize the c t/o-phenyl transition state. Furthermore, the steric hindrance of a trifluoromethyl group should favor its exo approach. However, cycloadducts 8 are obtained in a 1 1 mixture of tran.sicis stereoisomers which are converted into the cyclohexenone 9. Under the same thermal conditions, st-mcthyl-styrene is unreactive. ... 

With 1,1-disubstituted trifiuoromelhylated alkenes, such as -(trifluoromethyl)styrene, cy-doaddilion with nitrone 1 is regioselective, and a 50 50 mixture of the a. s/trani -isomers is obtained, as observed in Dicls-Alder cycloaddition reactions (see Section 2.1.1.6.2.1.1.). When the reaction is performed solvent-free under microwave irradiation, instead of under thermal conditions (boiling toluene), the yield is improved from 65% to 98% and the reaction time decreases from 48 hours to 4 minutes, however, stereoselectivity is not improved. ... 

Unlike alkenes directly substituted with fluorine atoms (see Section 2.1.1.6.1.2.), fluoroalkyl-substituted alkenes are not prone to undergo [2 + 2] cycloadditions. An early report concerns the peculiar case of 2-[bis(trifluoromethyl)methylene]propanedinitrile(l) which reacts remarkably easily with electron-rich alkenes, such as methyl vinyl ether and styrene derivatives. ... 

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