✍ Scribed by Nobuhiro Takeda; Prof. Dr. Norihiro Tokitoh; Prof. Dr. Renji Okazaki
No coin nor oath required. For personal study only.
In recent decades much attention has been paid to the chemistry of thiocarbonyl['] and selenocarbony1121 compounds. Selenoaldehydes, however, have been very little explored because they are highly reactive and difficult to prepare. Some selenoaldehydes stabilized by mesomeric effects due to heteroatoms such as nitrogen and sulfur[31 or by coordination to transition m e t a l ~[ ~% ~'
51 have been isolated. The only known stable, electronically unperturbed selenoaldehyde, 2,4,6-tri-tert-butylselenobenzaldehyde, is stabilized kinetically by a bulky substituent.I6] In addition, some transient selenoaldehydes have been reported. ['] Recently, we succeeded in synthesizing rotational isomers of 2,4,6-tris[bis(trimethylsilyl)methyl]thiobenzaldehydes as stable compounds[*] by the desulfurization of the corresponding cyclic polysulfides TbtCHS,, (n = 5, 8). ['] This synthesis takes advantage of an efficient steric protecting group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, which we will refer to Tbt. We now report here on 3) the synthesis of stable selenoaldehydes 1 a and 1 b (the rotational isomer of 1 a), 2) an equilibrium among 1,3diselenetane 2 and the seleoaldehydes 1 a and 1 b, and 3) the synthesis of q'-selenoaldehyde complexes 3 a and 3 b and the structure of 3 a, the first q'-selenoaldehyde complex characterized by X-ray crystallography.
Deselenation of cyclic polyselenide TbtCHSe, 4'' O1 with excess triphenylphosphane at room temperature resulted in the formation of the corresponding selenoaldehyde 1 a (greenish yellow solution), which was identified based on its spectroscopic data (G,(CHSe) = 16.06, GJCHSe) = 237.6, 'J,,(CHSe) = 161.2 Hz, G,, = 2075, A,,, =792 nm; Scheme 1, Table ). This deselenation of cyclic polyselenides with a trivalent phosphorous reagent is a novel synthetic approach to selenocarbonyl compounds. The isolation of 1 a was unsuccessful ; concentration of the reaction solution resulted in the complete conversion of l a into the head-to-tail dimer 2 (79%). Like reactive selenoaldehydes generated in situ,['] 1 a reacted with 2,3-dimethyl-1,3-butadiene and mesitonitrile oxide to afford the corresponding cycloadducts 5 (53%) and 7[12] (60%), respectively (Scheme 1). The formation of 5 and 7 indicates that 1 a still has a high reactivity toward these reagents in spite of its severe steric congestion, which prohibits its dimerization in a dilute solution for several hours. This is in sharp contrast to the fact that 2,4,6tri-tert-butylselenobenzaldehyde is too crowded to produce the corresponding cycloadducts. [' 31 When a solution of 2 was heated at 45 "C, the formation of 1 a and its isomer 1 b (rotation of the bis(trimethylsi1yl)methyl (disyl) group at the 2-p0sition['~I) was evident in the 'H NMR and UV/Vis spectra (2: 1 a: 1 b = 1 : 8: 3). In another experiment 1 a
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A series of η 1 -borazine complexes were obtained by the reac-ruled out from the spectroscopic and structural results. All new borazine complexes were isolated as orange crystalline tion of trichloroborazine with Na solids and characterized by multinuclear NMR methods and IR spectroscopy. The struc