Categories: Synthesis of O-Heterocycles, C-O Bond Formation >
Synthesis of lactones and related compounds
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 Butenolides |
 5,6-Dihydropyran-2-ones |
 3,4-Dihydropyran-2-ones |
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 Benzofuranones |
 Isochromanones |
 Coumarins |
 Isocoumarins |
Recent Literature
Cu/nitroxyl catalysts promote a highly efficient and selective aerobic oxidative
lactonization of diols under mild reaction conditions using ambient air as the
oxidant. A Cu/ABNO catalyst system shows excellent reactivity with symmetrical
diols and hindered unsymmetrical diols, whereas a Cu/TEMPO catalyst system
displays excellent chemo- and regioselectivity for the oxidation of less
hindered unsymmetrical diols.
X. Xie, S. S. Stahl, J. Am. Chem. Soc., 2015,
137, 3767-3770.
Air-stable iron carbonyl compounds bearing cyclopentadienone ligands with TMS
groups in the 2- and 5-positions catalyze dehydrogenative diol lactonization
reactions using acetone as both the solvent and hydrogen acceptor. Lactones
containing five-, six-, and seven-membered rings were successfully synthesized,
and no over-oxidations to carboxylic acids were detected.
Y. Tang, R. I. L. Meador, C. T. Malinchak, E. E. Harrison, K. A. McCaskey, M.
C. Hempel, T. W. Funk, J. Org. Chem., 2020, 85,
1823-1834.
Cyclic ketones were quickly and quantitatively converted to 5-, 6-, and
7-membered lactones by treatment with Oxone, a cheap, stable, and nonpollutant
oxidizing reagent in 1 M NaH2PO4/Na2HPO4
water solution (pH 7). These simple and green conditions avoid the formation of
hydroxyacid. With some changes, the method can also be applied to
water-insoluble ketones.
V. Bertolini, R. Appiani, M. Pallavicini, C. Bolchi, J. Org. Chem., 2021, 86,
15712-15716.
A new copper-catalyzed oxidative [3 + 2] cycloaddition of alkenes with
anhydrides using oxygen as the sole oxidant affords γ-lactones in good to
excellent yield. This catalyzed cyclization process has a broad substrate scope.
L. Huang, H. Jiang, C. Qi, X. Liu, J. Am. Chem. Soc., 2010,
132, 17652-17654.
The use of Ru(bpy)3Cl2 as photocatalyst enables a mild
and reproducible synthesis of γ-lactones. This photocatalytic reaction
has been monitored using high-resolution mass spectrometry.
I. Triandafillidi, M. G. Kokotou, C. G. Kokotos, Org. Lett.,
2018, 20, 36-39.
Various γ-lactones can be accessed readily by utilizing a Au-catalyzed tandem
cycloisomerization/oxidation of homopropargyl alcohols. Notably, the mechanism
of this strategy is distinctively different from the related Ru-catalyzed
reactions where a ruthenium vinylidene intermediate occurs.
C. Shu, M.-Q. Liu, Y-Z. Sun, L.-W. Ye, Org. Lett., 2012,
14, 4958-4961.
An alkyne deprotonation/boronation/oxidation sequence enables a one-pot
γ-lactonization of homopropargyl alcohols. Oxidation of the generated alkynyl
boronate affords the corresponding ketene intermediate, which is trapped by the
adjacent hydroxy group to furnish the γ-lactone.
D. Yamane, H. Tanaka, A. Hirata, Y. Tamura, D. Takahashi, Y. Takahashi, T.
Nagamitsu, M. Ohtawa, Org. Lett., 2021, 23,
2831-2835.
The use of Re2O7 in hexafluoroisopropanol (HFIP) enables an activation of
inert as well as electronically deactivated olefins to facilitate a challenging
intramolecular hydroacyloxylation reaction to provide various γ- and δ-lactones
in good yields.
Y. Liu, L. Hu, Y. Zheng, X. Fang, Y. Xie, Org. Lett., 2023, 25,
64-69.
Palladium catalysis enables an unprecedented, highly effective synthesis of
γ-lactones from homoallylic alcohols in one step. The protocol affords aryl,
alkyl, and spiro γ-lactones directly from readily available homoallylic alcohols
in good yields with excellent functional group tolerance and high
chemoselectivity under mild conditions.
M. Zheng, P. Chen, L. Huang, W. Wu, H. Jiang, Org. Lett.,
2017, 19, 5756-5759.
[4-Iodo-3-(isopropylcarbamoyl)phenoxy]acetic acid is a highly reactive and
easily separable catalyst for the oxidative cleavage of
tetrahydrofuran-2-methanols to γ-lactones in the presence of Oxone as the
terminal oxidant.The catalyst and product were easily separated by only
liquid-liquid separation without chromatography.
T. Yakura, T. Fujiwara, H. Nishi, Y. Nishimura, H. Nambu, Synlett, 2018, 29,
2316-2320.
An intramolecular coupling of epoxides with alcohols provides α- and
β-substituted γ-butyrolactones depending on the transition metal catalyst used.
This methods offers an efficient and practicable way to afford γ-butyrolactones
with good efficiency, excellent regioselectivity, and broad substrate scope.
F. Liu, Z. Chen, X. Fang, X. Wang, L. Zhao, Z.-Q. Rong, Org. Lett., 2023, 25,
3618-3622.
An intramolecular coupling of epoxides with alcohols provides α- and
β-substituted γ-butyrolactones depending on the transition metal catalyst used.
This methods offers an efficient and practicable way to afford γ-butyrolactones
with good efficiency, excellent regioselectivity, and broad substrate scope.
F. Liu, Z. Chen, X. Fang, X. Wang, L. Zhao, Z.-Q. Rong, Org. Lett., 2023, 25,
3618-3622.
An abundant and low toxicity iron catalyst enables a regioselective annulation
of alkenes with α-halocarboxylic acids and their derivatives in the absence of
ligands, bases, and additives to afford various γ-lactones in good yields.
M. Iwasaki, N. Miki, Y. Ikemoto, Y. Ura, Y. Nishihara, Org. Lett.,
2018, 20, 3848-3852.
A combined palladium-catalyzed arylation/hydrogenation of
α-methylene-γ-butyrolactones provides functionalized α-benzyl-γ-butyrolactones
in good yields.
A. Arcadi, M. Chiarini, F. Marinelli, Z. Berente, L. Kollŕr,
Org. Lett., 2000, 2, 69-72.
(HMe2SiCH2)2 is a useful reagent for a
reductive, B(C6F5)3-catalyzed lactonization of
keto acids to provide γ- and δ-lactones. The process enables the synthesis of
(-)-cis-whisky and (-)-cis-cognac lactones in good overall yields.
H. Xie, J. Lu, Y. Gui, L. Gao, Z. Song,
Synlett, 2017, 28, 2453-2459.
A mild, general, and functional group tolerant intramolecular
hydroalkoxylation and hydroacyloxylation of unactivated olefins using a Co(salen)
complex, an N-fluoropyridinium salt, and a disiloxane reagent at room temperature
provides five- and six-membered cyclic ethers and lactones. The powerful Co catalyst system
also enables the deprotective hydroalkoxylation of O-protected alkenyl alcohol
and hydroacyloxylation of alkenyl esters.
H. Shigehisa, M. Hayashi, H. Ohkawa, T. Suzuki, H. Okayasu, M. Mukai, A.
Yamazaki, R. Kawai, H. Kikuchi, Y. Satoh, A. Fukuyama, K. Hiroya, J. Am. Chem. Soc., 2016,
138, 10084-10087.
AuCl3-catalyzed electrophilic cyclization of 4-bromo-3-yn-1-ols via
hydroxyl-assisted regioselective hydration in wet toluene enables the synthesis
of γ-butyrolactones in good yields. Various secondary and tertiary
alcohols including benzylic systems were found to be equally reactive.
M. S. Reddy, Y. K. Kumar, N. Thirupathi, Org. Lett., 2012,
14, 824-827.
The use of 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) as catalyst allows a
regiocontrolled construction of cyclic ethers and lactones via
visible-light-induced intramolecular C-O bond formation. This approach provides
valuable five- and six-membered cyclic ethers and lactones with a unified
protocol.
H. Im, D. Kang, S. Choi, S. Shin, S. Hong, Org. Lett.,
2018, 20, 7437-7441.
A new and reliable method for the direct construction of biologically important
aryl lactones and phthalides from carboxylic and benzoic acids is based on
selective benzylic C-H abstraction in the presence of hypervalent iodine(III)
reagents and KBr.
T. Dohi, N. Takenaga, A. Goto, A. Maruyama, Y. Kita, Org. Lett., 2007,
9, 3129-3132.
An efficient, chemo- and enantioselective ketone hydroacylation enables the
direct preparation of lactones from keto alcohols in the presence of Noyori's
asymmetric transfer hydrogenation catalyst. The alcohol is oxidized in situ to
an aldehyde, obviating the need to prepare sensitive keto aldehyde substrates.
S. K. Murphy, V. M. Dong, J. Am. Chem. Soc., 2013,
135, 5553-5556.
Tetrabutylammonioum fluoride is a catalyst for the nucleophilic addition of
silyl ketene acetals to epoxides providing γ-lactones with high
regioselectivities and yields. This metal-free catalytic approach works in a
very efficient manner under solvent-free conditions.
S. Bonollo, A. Z. Ahmady, C. Petrucci, A. Marocchi, F. Pizzo, L. Vaccaro, Org. Lett.,
2014,
16, 5721-5723.
Silver(I) triflate catalyzes intramolecular additions of hydroxyl or carboxyl groups to olefins
in good to excellent yields for a range of substrates under relatively mild conditions. This
reaction is one of the simplest methods to construct cyclic ethers or
lactones.
C.-G. Yang, N. W. Reich, Z. Shi, C. He, Org. Lett.,
2005, 7, 4553-4556.
Stereoselective Synthesis of δ-Lactones from 5-Oxoalkanals via One-Pot
Sequential Acetalization, Tishchenko Reaction, and Lactonization by Cooperative
Catalysis of Samarium Ion and Mercaptan
J.-L. Hsu, J.-M. Fang, J. Org. Chem., 2001,
66, 8573-8584.
A highly enantioselective chromium-catalyzed carbonyl
2-(alkoxycarbonyl)allylation followed by lactonization enables the synthesis of
enantioenriched α-exo-methylene γ-butyrolactones. Various functional
groups are compatible under the mild reaction conditions.
W. Chen, Q. Yang, T. Zhou, Q. Tian, G. Zhang, Org. Lett.,
2015,
17, 5236-5239.
The room-temperature nucleophilic addition of vinyl azides to propargylic
alcohols in the presence of a catalytic amount of BF3·Et2O
provides 4-ynamides. The procedure is operationally convenient, shows broad
substrate scope, and tolerates many functional groups. Further, a Vilsmeier
intramolecular cyclization of 4-ynamides gives dihydrofuran-2(3H)-ones
with the alkyne group as the nucleophile.
J. Zheng, J.-H. Lin, L.-Y. Yu, Y. Wei, X. Zheng, J.-C. Xiao, Org. Lett.,
2015,
17, 6126-6129.
A desymmetrization of prochiral diesters with a chiral phosphoric acid catalyst
produces highly enantioenriched lactones in excellent yield. Various
substitution patterns are tolerated, many of which result in the generation of
an enantioenriched all-carbon quaternary center. Manipulation of the lactone
products to useful small building blocks is also described.
J. Wilent, K. S. Petersen, J. Org. Chem., 2014,
79, 2303-2307.
A cyanide-catalyzed ring-expansion of cyclic α-hydroxy-β-oxoesters provides
δ-valerolactone derivatives in up to quantitative yields. Several
alkyl-substituted as well as benzo- and heteroarene-annulated starting materials
are converted without problems. As an additional benefit, the substrates are
straightforwardly accessed by cerium-catalyzed aerobic α-hydroxylation of
readily available β-oxoesters.
D. Kieslich, J. Christoffers, Org. Lett., 2021, 23,
953-957.
The hypervalent iodine reagent PIFA promotes the efficient intramolecular
electrophilic cyclization of easily accessible alkynylamides and alkynyl
carboxylic acids, leading to pyrrolidinone and lactone skeletons, respectively.
A synthetic study and a mechanistic proposal for these transformations are
presented.
I. Tellitu, S. Serna, M. T. Herrero, I. Moreno, E. Domínguez, R. SanMartin, J. Org. Chem., 2007,
72, 1526-1529.
A metal-free photoredox system, consisting of an acridinium photocatalyst, an
organic base, and molecular sieve (MS) 4 Ĺ, promotes chemoselective
photooxidation of aryl alkenes in the presence of oxygen. This oxo-acyloxylation
of aryl alkenes provides a green, practical, and metal-free protocol for a wide
range of α-acyloxy ketones.
Q.-B. Zhang, Y.-L. Ban, D.-G. Zhou, P.-P. Zhou, L.-Z. Wu, Q. Liu, Org. Lett.,
2016, 18, 5256-5259.
A multifunctional modular organocatalysis enables a simple and efficient
approach to enantioenriched α,β-disubstituted γ-butyrolactones via a one-pot
sequential Michael-hemiacetalization-oxidation reaction. The catalytic process
offers good substrate compatibility, and the products can be transformed to
synthetically useful molecules.
P. Mahto, N. K. Rana, K. Shukla, B. G. Das, H. Joshi, V. K. Singh,
Org. Lett., 2019, 21, 5962-5966.
Homogeneous carboamination, carboalkoxylation and carbolactonization of terminal
alkenes are realized via oxidative gold catalysis, providing expedient access to
various substituted N- or O-heterocycles. Deuterium-labeling studies established
the nature of the alkene functionalization and the indispensible role of Au(I)/Au(III)
catalysis.
G. Zhang, L. Cui, Y. Wang, L. Zhang, J. Am. Chem. Soc., 2010,
132, 1474-1475.
Several Pd-catalyzed oxidative cyclizations proceed in excellent yield under
simple aerobic conditions. Importantly, this system provided entry into
enatioselective catalysis with a readily available Pd-sparteine complex.
R. M. Trend, Y. K. Ramtohul, E. M. Ferreira, B. Stoltz,
Angew. Chem. Int. Ed., 2003, 42, 2892-2895.
Heterogeneous gold catalysts were easily assembled from readily available silica
materials and gold complexes. Dramatic enhancement in regio- and
enantioselectivity was observed when compared to the homogeneous unsupported
gold catalysts in various reactions where protodeauration is the rate-limiting
step. The catalysts can be recovered and recycled up to 11 times without loss of
enantioselectivity.
X.-Z. Shu, S. C. Nguyen, Y. He, F. Oba, Q. Zhang, C. Canlas, G. A. Somorjai, A.
P. Alivisatos, F. D. Toste, J. Am. Chem. Soc., 2015,
137, 7083-7086.
In the presence of CuI/trans-N,N′-dimethylcyclohexane-1,2-diamine as
catalyst, a number of carboxylic acids underwent efficient intramolecular O-vinylation
with vinyl bromides leading to the corresponding five- and six-membered enol
lactones. The same catalytic system also enabled an efficient cycloisomerization
of alkynoic acids.
C. Sun, Y. Fang, S. Li, Y. Zhang, Q. Zhao, S. Zhu, C. Li, Org. Lett., 2009,
11, 4084-4087.
C. Sun, Y. Fang, S. Li, Y. Zhang, Q. Zhao, S. Zhu, C. Li, Org. Lett., 2009,
11, 4084-4087.
In the presence of a cyclometalated iridium catalyst modified by (−)-TMBTP,
catalytic C-C coupling of acrylic ester with alcohols provides enantiomerically
enriched 5-substituted α-exo-methylene γ-butyrolactones. Bromination of the
methylene butyrolactone products followed by zinc-mediated reductive aldehyde
addition provides the disubstituted α-exo-methylene γ-butyrolactones with very
good levels of diastereoselectivity.
T. P. Montgomery, A. Hassan, B. Y. Park, M. J. Krische, J. Am. Chem. Soc., 2012,
134, 11100-11103.
A general, efficient, and convenient cyclization of alkynes bearing carboxylic
acids to the corresponding γ-alkylidene-γ-butyrolactones in the presence of
commercially available Au2O3 shows a high degree of
chemo-, regio-, and stereoselectivity. The 5-exo mode of cyclization and anti
auration are a general trend for the Au2O3 catalyst.
P. Y. Toullec, E. Genin, S. Antoniotti, J.-P. Genęt, V. Michelet, Synlett, 2008,
707-711.
A highly efficient gold-catalyzed cyclization reaction of various
functionalized acetylenic acids leads to γ-lactones in good to excellent yields.
The reaction conditions are compatible with several functional groups, such
as ester, alkene, alkyne, chloro, and free or protected alcohol.
E. Genin, P. Y. Toullec, S. Antioniotti, C. Brancour, J.-P. Genęt, V.
Michelet, J. Am. Chem. Soc.,
2006, 128, 3112-3113.
Imidazolinium-derived carbenes catalyze an efficient ring-expansion
lactonization of oxacycloalkane-2-carboxaldehydes to give various functionalized
five-, six-, and seven-membered lactones under mild reaction conditions. The
electronic nature of the carbene catalyst plays a crucial role for the success
of this method.
L. Wang. K. Thai, M. Gravel, Org. Lett., 2009,
11, 891-893.
A mechanistic investigation on the effect of substrate on stereoselectivity in the triflic acid-catalyzed allylboration
reaction between 2-alkoxycarbonyl allylboronates and aldehydes confirms the
involvement of a carbocation intermediate as the source of stereochemical
inversion. This methodology allows a facile access to β,γ-disubstituted
five-membered ring lactones.
T. G. Elford, Y. Arimura, S. H. Yu, D. G. Hall, J. Org. Chem., 2007,
72, 1276-1284.
T. G. Elford, Y. Arimura, S. H. Yu, D. G. Hall, J. Org. Chem., 2007,
72, 1276-1284.
Depending on the strength of a Lewis or Brřnsted acid catalyst, borate intermediates resulting from the crotylboration of aliphatic aldehydes
with ester-containing crotylboronates form either γ-substituted-α-alkylidene-γ-butyrolactones
via oxonia cope
rearrangement-lactonization or
β,γ-disubstituted-α-methylene-γ-butyrolactones via lactonization.
P. V. Ramachandran, D. Pratihar, Org. Lett., 2007,
9, 2087-2090.
P. V. Ramachandran, D. Pratihar, Org. Lett., 2007,
9, 2087-2090.
The dynamic kinetic resolution of β-aryl α-keto esters using a newly designed (arene)RuCl(monosulfonamide)
transfer hydrogenation catalyst generates three contiguous stereocenters with
remarkable diastereoselectivity through a reduction/lactonization sequence. The
resulting enantioenriched, densely functionalized γ-butyrolactones are of high
synthetic utility.
K. M. Steward, E. C. Gentry, J. S. Johnson, J. Am. Chem. Soc., 2012,
134, 7329-7332.
The Reformatsky reaction of α-hydroxy ketones with indium enolates furnished
highly diastereoselective lactones, while α-alkoxy ketones gave acyclic
esters in moderate selectivities. A boat-type of chelated bicyclic
transition state involving highly diastereoselective construction of three
contiguous stereogenic centers is proposed.
S. A. Babu, M. Yasuda, Y. Okabe, I. Shibata, A. Baba, Org. Lett.,
2006, 8, 3029-3032.
Treatment of 3-[(alkoxycarbonyl)alkyl]-substituted conjugated cycloalkenones
with diisobutylaluminum hydride at -78 °C followed by acid quenching
furnishes spiro ethers, whereas the corresponding 3-(carboxyalkyl)-substituted
cycloalkenones generate spiro lactones upon reaction with sodium borohydride
at 30 °C followed by acid quenching.
M.-C. P. Yeh, Y.-C. Lee, T.-C. Young, Synthesis, 2006, 3621-3624.
The Brřnsted acid catalyzed formal insertion of an isocyanide into a C-O bond
of various acyclic and cyclic acetals can be applied to form α-alkoxy imidates.
Functional groups, such as nitro, cyano, halogen, ester, and alkoxy groups, are
tolerant to the reaction conditions employed. The course of the reaction is
highly dependent on the structure of the isocyanide.
M. Tobisu, A. Kitajima, S. Yoshioka, I. Hyodo, M. Oshita, N. Chatani, J. Am. Chem. Soc.,
2007,
129, 11431-11437.
The synthesis of a planar-chiral bisflavin catalyst (1) and its use
in asymmetric Bayer-Villiger-Oxidations is described.
S. Murahashi, S. Ono, Y. Imada, Angew. Chem. Int. Ed., 2002, 41,
2366-2368.
d2 Re(V) alkylidyne complexes bearing two decorated
phosphino-phenolates (POs) and a labile pyridine ligand can efficiently promote
homometathesis as well as ring-closing alkyne metathesis (RCAM) of methyl-capped
diynes. A wide range of functional groups including ethers,
esters, ketones, aldehydes, alcohols, phenols, amines, amides, and heterocycles
are tolerated.
M. Cui, H. H. Y. Sung, I. D. Williams, G. Jia, J. Am. Chem. Soc.,
2022, 144, 6349-6360.
An effective Pd-catalyzed hydroesterification of alkenylphenols with phenyl
formate as CO surrogate enables the synthesis of various lactones in generally
high yields with high regioselectivities. In one case, 76% ee is obtained with a
chiral ligand.
H. Wang, B. Dong, Y. Wang, J. Li, Y. Shi, Org. Lett., 2014,
16, 186-189.
Pd(II)-catalyzed C-H activation of phenylacetic acids followed by an
intramolecular C-O bond formation afforded benzofuranones. A modified reaction
provides the first example of enantioselective C-H functionalizations through
Pd(II)/Pd(IV) redox catalysis.
X.-F. Cheng, Y. Li, Y.-M. Su, F. Yin, J.-Y. Wang, J. Sheng, H. U. Vora, X.-S.
Wang, J.-Q. Yu, J. Am. Chem. Soc., 2013,
135, 1236-1239.
X.-F. Cheng, Y. Li, Y.-M. Su, F. Yin, J.-Y. Wang, J. Sheng, H. U. Vora, X.-S.
Wang, J.-Q. Yu, J. Am. Chem. Soc., 2013,
135, 1236-1239.
A metal-free tandem Friedel-Crafts/lactonization reaction to 3,3-diaryl or
3-alkyl-3-aryl benzofuranones is catalyzed by HClO4. The reaction of
various tertiary α-hydroxy acid esters with substituted phenols affords the
desired products in rich diversity. 1H NMR studies supports that this
tandem reaction proceeds via tandem Friedel-Crafts/lactonization sequence.
L. Chen, F. Zhou, T.-D. Shi, J. Zhou, J. Org. Chem., 2012,
77, 4354-4362.
A convenient radical oxidative cyclization mediated by N-iodosuccinimide
(NIS) enables the synthesis of a series of dibenzopyranones from a wide scope of
2-arylbenzoic acids. The methodology offers good functional group tolerance and
mild reaction conditions without the use of transition metals.
P. Gao, Y. Wei, Synthesis, 2014, 46,
343-347.
A quinine tethered Co(III)-salen complex promotes as a Lewis acid-Lewis base
(LA*-LB*) bifunctional catalyst a rapid asymmetric [2+2] cycloaddition reaction
between ketene and aldehydes to produce C4-substituted β-lactones in uniformly
>99% ee and high isolated yields.
S. Chidara, Y.-M. Lin, Synlett, 2009,
1675-1679.
Chiral N-hetereocyclic carbenes are efficient catalysts for the formal [2 + 2]
cycloaddition reactions of alkyl(aryl)ketenes with 2-oxoaldehydes to afford
highly substituted β-lactones in high yields with good diastereoselectivities
and excellent enantioselectivities. Both alkyl(aryl)ketenes and diarylketene
worked well in this reaction.
L. He, H. Lv, Y.-R. Zhang, S. Ye, J. Org. Chem., 2008,
73, 8101-8103.
The regioselective opening of Bn2N-α-methylserine-β-lactone
with organocuprates gave enantiopure α-methyl amino acids in excellent yields.
N. D. Smith, A. M. Wohlrab, M. Goodman, Org. Lett., 2005, 7, 255-258.
The merger of photoactivation and
asymmetric Pd catalysis enables an enantioselective [5+2] cycloaddition of vinylethylene carbonates and
α-diazoketones to provide 7-membered lactones bearing challenging chiral quaternary
stereocenters.
Y. Wei, S. Liu, M.-M. Li, Y. Li, Y. Lan, L.-Q. Lu, W.-J. Xiao, J. Am. Chem. Soc.,
2019,
141, 133-137.
A new Pd-catalyzed oxidation reaction for the stereospecific conversion of
enynes into cyclopropyl ketones proceeds with net inversion of geometry with
respect to the starting olefin. This result is consistent with a mechanism in
which the key cyclopropane-forming step involves nucleophilic attack of a
tethered olefin onto the PdIV-C bond.
L. L. Welbes, T. W. Lyons, K. A. Cychosz, M. S. Sanford, J. Am. Chem. Soc., 2007,
129, 5836-5837.