Triplet Diphenylcarbenes Protected by Trifluoromethyl and Bromine Groups. A Triplet Carbene Surviving a Day in Solution at Room Temperature

• Published in: Journal of the American Chemical Society Vol. 128; no. 3; pp. 957 - 967
• Main Authors: Itoh, Tetsuji, Nakata, Yoshimaru, Hirai, Katsuyuki, Tomioka, Hideo
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 25-01-2006
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Reaction intermediate; Azo compound; Kinetic control; Electronic structure; Near ultraviolet radiation; Laser beam; Organic free biradical; Electronically excited state; EPR spectrometry; Experimental study; Flash photolysis; Ultraviolet visible spectrometry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja056575j

Two types of diphenyldiazomethanes having two trifluoromethyl and two bromine groups at the ortho positions, either in unsymmetrical or in symmetrical fashion, that is, (2,6-dibromo-4-phenylphenyl)[4-phenyl-2,6-bis(trifluoromethyl)phenyl]diazomethane (U-1 −N2) and bis(2-bromo-4-phenyl-6-trifluoromethylphenyl)diazomethane (S-1 −N2), are prepared. Triplet diphenylcarbenes (U-3 1 or S-3 1) are generated from those precursors and are characterized by ESR, UV/vis spectroscopy at low temperature, as well as time-resolved UV/vis spectroscopy at room temperature. Those carbenes are shown to be at least 2 orders of magnitude less reactive than the most stable triplet diphenylcarbene thus far known, that is, bis(2,6-dibromo-4-phenylphenyl)carbene. It has been also shown that S-3 1 is significantly more stable than U-3 1 even though both have the same two kinds of substituents. It is suspected that the perpendicular alignment of the two most bulky groups is a more effective way to shield the carbenic center than the planar one. By this way, triplet substituted diphenylcarbene surviving nearly a day in solution at room temperature is realized for the first time.