Stereochemistry of the methoxide induced rearrangement of an α-bromophosphonamidate: cleavage of the P–N and P–C bonds in the azaphosphiridine oxide intermediate 1
作者:Martin J. P. Harger、Ramesh Sreedharan-Menon
DOI:10.1039/a604093d
日期:——
Menthyl
P-(bromomethyl)-N-tert-butylphosphonamidate
5 has been prepared from
(1R,2S,5R)-(-)-menthol and the
S
P
diastereoisomer has been isolated. This
rearranges with methoxide [Me
3
(PhCH
2
)N
+
MeO
-
in THFâMeOH] to give only the
S
P
diastereoisomer of menthyl methyl
(tert-butylamino)methylphosphonate 6 and very largely (95%) the
S
P
diastereoisomer of menthyl methyl
N-tert-butyl-N-methylphosphoramidate 7. The
configurations of these products show that when the (postulated)
azaphosphiridine oxide intermediate 16 suffers ring opening by
methoxide, the PâN bond is cleaved (to give 6) with inversion of
configuration but the PâC bond is cleaved (to give 7) with
predominant retention. These contrasting stereochemistries suggest that
nucleophilic attack on the PO group of the azaphosphiridine
oxide generates a five-coordinate intermediate (not merely a transition
state) that exists long enough to undergo
pseudorotation.
薄荷醇 P-(溴甲基)-N-叔丁基膦酰胺盐 5 是由 (1R,2S,5R)-(-)-薄荷醇合成的,且 S P 对映体已被分离。该化合物在甲醇氧化物 [Me3 (PhCH2)N+ MeO- 在四氢呋喃–甲醇中] 的作用下重排,仅生成薄荷基甲基(叔丁基氨基)甲基膦酸酯 6 的 S P 对映体,以及很大比例(95%)的薄荷基甲基 N-叔丁基-N-甲基膦氨基酸酯 7 的 S P 对映体。这些产物的构型表明,当(假设的)氮磷环氧中间体 16 被甲醇氧化物开环时,P–N 键的断裂(生成 6)伴随构型反转,而 P–C 键的断裂(生成 7)则主要保持构型。这些对比的立体化学特征表明,对氮磷环氧中 PO 基团的亲核攻击生成了一个五配位中间体(不仅仅是过渡态),该中间体存在的时间足够长以进行伪旋转。