Although the pyramidal inversion barriers in diphosphines (R(2)P-PR(2)) are similar to those in phosphines (PR(3)), P-stereogenic chiral diphosphines have rarely been exploited as building blocks in asymmetric synthesis. The synthesis, reactivity, and resolution of the benzodiphosphetane trans-1,2-(P(t-Bu))(2)C(6)H(4) are reported. Alkylation with MeOTf followed by addition of a nucleophile gave the useful C(2)-symmetric P-stereogenic ligand BenzP* and novel analogues.
The selectivity of
catalytic asymmetric transformations of bifunctional
symmetrical substrates often depends on the linker between the two
reactive sites. If the catalyst controls the selectivity of reactions
at both sites, the rac product will be formed in
high enantiomeric ratio (er) via asymmetric amplification. Substrate
control may augment this selectivity (positive cooperativity) or detract
from it (negative cooperativity). Here, we investigated the effect
of linker length on the selectivity of catalytic asymmetric alkylation
of the bis(secondary phosphines) PhHP(CH2)
n
PHPh (n = 2–6; 1a–e) with benzyl bromide using the base NaOSiMe3 and the catalyst precursor Pt((R,R)-Me-DuPhos)(Ph)(Cl).
These reactions yielded the diastereomerically and enantiomerically
enriched bis(tertiary phosphines) Ph(PhCH2)P(CH2)
n
P(CH2Ph)Ph (n = 2–6, 2a–e). Pt-catalyzed benzylation of the
model phosphines PH(Ph)((CH2)
n
H) (n = 2–6; 4a–e) gave enantiomerically enriched P(CH2Ph)(Ph)((CH2)
n
H) (n = 2–6; 5a–e). The partially alkylated bis(phosphines)
PhHP(CH2)
n
P(CH2Ph)Ph
(n = 2, 3, 5; 3a,b,d) were prepared with different degrees of enantiomeric enrichment
at the tertiary phosphine centers and then catalytically alkylated
to give 2a,b,d. From the diastereoselectivity
and enantioselectivity of the transformations 1 → 2 and 3 → 2, the selectivity
of each individual alkylation step (1 → 3R or 3S; 3R → 2RR or 2RS; 3S → 2SS or 2SR) could be determined as a function of linker length and
compared to the selectivity of alkylation of the model monophosphines 4. The two alkylations of bis(secondary phosphines) 1b–e with longer linker lengths (n = 3–6) showed identical selectivity, within experimental
error. This catalyst control resulted in asymmetric amplification
of rac-2. In contrast, the selectivity
of the first alkylation of ethano-bridged 1a was lower
than that in 1b–e (negative cooperativity),
but the selectivity of the second alkylation (of intermediate 3a) increased due to positive cooperativity. Possible mechanistic
explanations for the observed dependence of selectivity on linker
length are discussed.
A Grignard reagent derived from (−)-menthyl chloride has been reported to be a 1:1 mixture of menthyl magnesium chloride and neomenthyl magnesium chloride, which do not interconvert. Addition of an excess of this reagent to Au(PPh3)(Cl) or Pt(dppe)Cl2 gave Au(PPh3)(Men) (1) and Pt(dppe)(Men)(Br) (2), respectively. Crystallographic studies of these first well-characterized transition metal menthyl complexes showed that the menthyl group adopts a conformation with all three substituents in equatorial positions. NMR spectroscopic data for 2 showed that menthyl has a large trans influence, comparable to other alkyl groups. Decomposition of 1 in CDCl3 gave Au(PPh3)(Cl) and a mixture of menthyl chloride and neomenthyl chloride, while 2 formed the halide complexes Pt(dppe)Cl2, Pt(dppe)Br2, and Pt(dppe)(Br)(Cl) and a mixture of 2-menthene and 3-menthene.
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