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2
Tetrahedron
activity, condensed phase action, eventually combined with the
phosphorus acts as nucleophile which is added to an alkene or
alkyne. This reaction is an effective method for the formation of
phosphorus-carbon bonds with several electrophiles25 and is often
applied to synthesize phosphorus-containing flame retardant
additives.13,16,26,27
ACCEPTED MANUSCRIPT
formation of an intumescent layer.20 The impact of the oxidation
state of phosphorus in flame retardants for polyurethanes has
already been studied by Modesti.21
In this paper, the synthesis of compounds having a different
phosphorus-containing
ring
system,
dibenzo[d,f][1,3,2]
The reaction mechanism between a phosphinate and an
acrylate is known from literature.25 It is assumed that a similar
mechanism applies for the reaction with a H-phosphonate, as
shown in Scheme 3.28
dioxaphosphepine 6-oxide (BPPO, 1), is presented. BPPO differs
from DOPO by one additional oxygen atom between phosphorus
and the biphenyl moiety. BPPO is therefore a phosphonate
analogue to the phosphinate DOPO. BPPO has not been
described yet as a subunit for flame retardants.
Here, it is used as educt for phospha-Michael additions to a
series of unsaturated compounds. As the BPPO ring system
represents
a
phosphonate structure, while DOPO is
a
phosphinate, a significant change in flame retardant action
towards gas phase activity of the resulting compounds and
polymeric materials is expected. The present report is, however,
restricted to the synthesis of the respective BPPO-containing
compounds resulting in non-reactive (without functional groups)
as well as reactive flame retardant additives (with functional
hydroxyl and amide groups).
Scheme 3. Phospha-Michael addition of H-phosphonate to acrylates.
In the first step, the phosphonate is deprotonated by the base
(in this case triethyl amine) generating the phosphonate ion.
Then, a nucleophilic attack on the β-C atom takes place, followed
by delocalization of the anion. After protonation and
tautomerization the product is formed.25 If R1 and R2 are
connected as in the case of BPPO, P-containing heterocyclic
compounds result.
2. Results and discussion
2.1. Synthesis of BPPO
The synthesis of BPPO via three-component condensation of
2,2´-biphenol, phosphorus trichloride and water was described
for the first time by Natchev in 1988 (Scheme 2).22
BPPO was subsequently reacted with a number of unsaturated
molecules in order to prepare BPPO-containing compounds with
systematically changed chemical structure. These unsaturated
molecules were selected to obtain BPPO-derivatives with
reactive groups (esters, amides and alcohols). Like DOPO, these
BPPO-derivatives can be used as monomers for
polycondensation and polyaddition reactions. Additionally, the
ester group was varied (methyl, ethyl, tert.-butyl and phenyl)
giving non-reactive compounds.
Scheme 2. Synthesis of BPPO.
Another method describing the synthesis of different
heterocyclic hydrogen phosphine oxides by using P4O6 was
patented in 2016.23 P4O6 is not commercially available. Its
synthesis from white phosphorus requires very high reaction
temperatures and specific atmospheric conditions (high pressure)
and was not employed here.
The detailed procedures of the addition of BPPO to acrylates
(2-6), unsaturated diesters (7-9), and to p-benzoquinone (10, 11)
are described in the Experimental section. A summary of the
compounds obtained is given in Table 1.
All reactions except 10 were performed in presence of the
base triethyl amine. In most experiments, yields between 77 and
87% were reproducibly achieved. For compound 7 both educts,
dimethyl fumarate and dimethyl maleate, provided similar yields
of 77%. In case of the phenyl esters 5 and 9, the yields were
lower (between 57 and 71%) due to the lower reactivity of the
double bond. In case of p-benzoquinone as educt and in the
presence of triethyl amine Michael 1,4-addition did not take
place, but BPPO is oxidized to the corresponding aromatic
phosphate ester 11. The mechanism could be envisioned as a 1,2-
addition to the hydroquinone C=O bond. This oxidation has been
described for the addition of DOPO to p-benzoquinone under
comparable conditions, too.29
Therefore, we worked with the procedure described by
Natchev22 and optimized the reaction conditions. So, the
concentration of 2,2´-biphenol in the reaction mixture was raised
from 0.1 M to 2.7 M. Additionally, the workup was modified.
After evaporation of the solvent, Natchev treated the residue with
sodium hydroxide solution. During this step, we observed a
partial decomposition of BPPO and the expected product could
not be isolated. Therefore, the workup was simplified by letting
the residue crystallize and washing it with diethyl ether. Thus, the
yield was raised to 84% compared to the Natchev procedure with
a reported yield of 72% and comparable puirty.
2.2. The phospha-Michael addition of H-phosphonates to
unsaturated compounds
The chemical structure of compounds 1 – 11 was verified by
1H, 13C and 31P NMR spectroscopy (see Experimental section).
The mechanism of the reaction of BPPO with unsaturated
compounds follows a phospha-Michael addition. The Michael
addition is a base-catalyzed 1,4-addition of a C-H-acidic
compound to an α,β-unsaturated carbonyl compound.24 The
phospha-Michael addition proceeds similarly. Here, the
Lenz, Johannes
