Experimental determination of the nN --> sigma*P-O interaction energy of O-equatorial C-apical phosphoranes bearing a primary amino group.
The reaction of a chlorophosphorane (9-Cl) with primary amines produced anti-apicophilic spirophosphoranes (5, O-equatorial phosphoranes), which violate the apicophilicity concept, having an apical carbon-equatorial oxygen configuration, along with the ordinarily expected O-apical stereoisomers (6) with the apical oxygen-equatorial carbon configuration. Although the amino group is electronegative in nature, the O-equatorial phosphoranes were found to be stable at room temperature and could still be converted to their more stable O-apical pseudorotamers (6) when they were heated in solution. X-ray analysis implied that this remarkable stability of the O-equatorial isomers could be attributed to the orbital interaction between the lone-pair electrons of the nitrogen atom (n(N)) and the antibonding sigma(P-O) orbital in the equatorial plane. A kinetic study of the isomerization of 5 to 6 and that between diastereomeric O-apical phosphoranes 13b-exo and 13b-endo revealed that 5b bearing an n-propylamino substituent at the central phosphorus atom was found to be less stable than the corresponding isomeric 6b by ca. 7.5 kcal mol(-1). This value was smaller than the difference in energy (11.9 kcal mol(-1)) between the O-equatorial (1b) and the O-apical n-butylphosphorane (2b) by 4.4 kcal mol(-1). This value of 4.4 kcal mol(-1) can be regarded as the stabilization energy induced by the n(N) --> sigma(P-O) interaction. The experimentally determined value was in excellent agreement with that derived from density functional theory (DFT) calculations at the B3PW91 level (4.0 kcal mol(-1)) between the nonsubstituted aminophosphoranes (5g is less stable than 6g by 10.1 kcal mol(-1)) and their P-methyl-substituted counterparts (1a is less stable than 2a by 14.1 kcal mol(-1)).
Adachi
Takahiro
T
Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan.
Matsukawa
Shiro
S
Nakamoto
Masaaki
M
Kajiyama
Kazumasa
K
Kojima
Satoshi
S
Yamamoto
Yohsuke
Y
Akiba
Kin-ya
KY
Re
Suyong
S
Nagase
Shigeru
S
D000588:Amines / Q000737:chemistry*; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D010753:Phosphoranes / Q000138:chemical synthesis / Q000737:chemistry*; D012995:Solubility; D013237:Stereoisomerism; D013816:Thermodynamics
D000588:Amines; D010753:Phosphoranes