40
L.-Y. Jiao et al. / Journal of Catalysis 379 (2019) 39–45
and economics, the diphenylphosphoryl azide (1a) was subjected
to methanol (2a) to identify the catalytic system under air. As
the results depicted in Table 1, methanol (2a) was in excess and
served both as reactant and solvent. However, no conversion was
observed initially (entry 1). Satisfyingly, the addition of 5 mol%
CuCl led to the formation of desired product 3aa in almost full con-
version and 87% isolated yield over a prolonged reaction time
(entry 2). It is especially noteworthy that copper was chosen as
the metal because of its square planar geometry reducing the
impact of steric effects [16]. Interestingly, unlike classical transfor-
mations [15], the unexpected PAN bond rather than the N ANb
a
bond was activated in this transformation. Doubling the loading
of copper salts led to exclusive increasing the isolated yield, yet
at the same time, shorten reaction time significantly (entry 3).
The effect of copper source on the reaction system was further
investigated. As shown in Table 1, Cu(I) and Cu(II) were efficient
to furnish the product with good yield in range of 78–95%, wheres
a reduced yield was observed in case of Cu(0) (entries 4–12). It was
found that the counterion has an influence on reactivity as well,
replacement of CuCl with other copper source all reduced the yield
to varying degree. We are pleased to find the reaction is not sensi-
tive to the water, thus the reaction could be performed under air
and in the presence of moisture, which exhibited the convenience
of our protocol. Additional efforts was dedicated to promote the
yields by screening the temperature and additives. A correlation
between yields and temperature revealed the best result was
obtained in 40 °C (entries 3, 13, 14). With this result in hand, we
next explored the additives. The presence of base improved the
yield, which might due to the generation of more nucleophilic
alkoxyl aion under base condition (entries 15, 16). Considering
the high cost and viscosity of aliphatic alcohols, we turned our
attention to employ other reaction media. After a solvent screening
(entries 17–22), the best result was outlined in entry 21, 96% yield
was achieved in cyclohexane at 40 °C for 24 h, which represents a
highly efficient and convenient approach for this transformation.
Inspired by this result, a variety of aliphatic alcohols were
explored under optimized reaction conditions (Table 2). Alcohols
bearing alkyl, aryl, and alkyene functionalities all undertaken the
reaction smoothly, leading to the corresponding phosphates in
excellent yields. The electron withdrawing substituted alkyl alco-
hol was inferior to electron donating substituents, affording the
products in slightly lower yield (2b ? 3ab vs 2c ? 3ac). The reac-
tivity pattern may indicate the increased nucleophicility of alcohol
would facilitate the reaction. It appeared that the steric effect has
an important influence on the reactivity. In general, the primary
alcohols were superior to the secondary ones (2d ? 3ad, 2f ?
3af vs 2e ? 3ae, 2g ? 3ag), however, almost no conversion was
observed when tertiary alcohol with a tert-butyl substituent was
employed (2h ? 3ah). In addition, the diminished yields were
obtained in case of cyclopentanol (2m) and cyclohexanol (2n),
respectively. These results suggested the reaction might involve-
Scheme 1. Representative examples of aryl/alkyl mixed phosphates.
reagents are corrosive, unstable, difficult to prepare and expensive.
Apart from this, these reactions always suffer from drawbacks in
terms of the requirements of excess additives, harsh reaction
conditions, narrow substrate scope and low reactivity. From the
viewpoint of synthesis application, it is highly desirable but
challenging to develop a new synthetic method from a low cost
and readily available phosphoric reagent.
Being recognized as an ‘‘energetic reagent”, diphenylphosphoryl
azide (DPPA) was widely used in many kinds of transformations
due to its various reactivities under distinguished reaction condi-
tions [12,13]. Generally, it could serve as an azido source through
the cleavage of the PAN bond under base conditions [14]
(Scheme 3a). On the other hand, an amino moiety could be deliv-
ered deriving from DPPA via the cleavage of N ANb bond in the
a
presence of transition metals, such as Cu, Co, Ru, and Ir, followed
by CAH bond insertion to afford aminadion products [15] (Sche-
me 3b). Enlightened by this, we envisioned that the DPPA, which
are readily accessible and inexpensive, might act as potential phos-
phorylation reagents through the cleavage of PAN bonds through a
different reaction setup.
Herein, we developed a novel and practical method to access a
wide range of mixed phosphates from DPPA under mild reaction
conditions, which serve as newly phosphorylatipon reagents
through the unusual copper catalyzed cleavage of PAN bond
(Scheme 3c).
2. Results and discussion
Then we concentrated on the establishment of reaction condi-
tion in a systematic manner. From the standpoint of simplicity
Scheme 2. Well developed methods to prepare aryl/alkyl mixed phosphates.