My PhD dissertation - Institut Fresnel

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Primary alkyl substituents were then compared to secondary ones by means of
alkaline decomposition of two mixed phosphonium halides. Butyltricyclohexylphosphonium
bromide (28f) was first decomposed under the usual butanol/water alkaline medium for 72 h
at 110 °C to give a mixture of butyldicyclohexylphosphine oxide (27g) and
tricyclohexylphosphine oxide in a 76 : 24 ratio.
P
3
28f
H 9C 4OH / H 2O (8 : 2)
Br
KOH, 110 °C
P
2
O
27g
+
76 : 24
+ +
Statistical corrections finally gave a relative rate constant of k1/2 = 1.02 (±0.06)
which lead to a difference in activation free enthalpy of activation between cyclohexyl and
butyl expulsion of ∆∆G ≠ ~ 0.0 kcal·mol -1 . Triisopropylmethylphoshonium iodide (28g) was
then also submitted to the usual alkaline treatment but, in this case, triisopropylphosphine
oxide was the sole product. This result, once again attested to the relatively large difference in
activation free energy between methyl and other alkyl expulsion since it can be assumed that
∆∆G ≠ > 4.60 kcal·mol -1 .
To assess the relative ease of alkyl cleavage between primary and tertiary groups,
dibutyldi-tert-butylphosphonium iodide (28c) and di-tert-butyldimethylphosphonium iodide
(28b) [164] were used as starting substrates. Phosphonium iodide 28c after alkaline
decomposition gave a mixture of butyldi-tert-butylphosphine oxide (27a) and dibutyl-tert-
butylphosphine oxide (27d) in a 97 : 3 ratio as revealed by 31 P NMR analysis.
P I P O
H9C4OH / H2O (8 : 2)
28c
KOH, 110 °C
O
P
27a 27d
97 : 3
+ +
The relative rate constant k1/2 was in this of 30.8 (±3.90) and indicated a marked
difference in promptness to form tert-butane and butane [∆∆G ≠ = 2.30 (±0.10) kcal·mol -1 ]
even if alkaline decomposition gave rise to the formation of both possible phosphine oxides.
Finally the alkaline decomposition of di-tert-butyldimethylphosphonium iodide (28b) [164] only
+
H
P
3
O