Phosphonate-Directed Catalytic Asymmetric Hydroboration: Delivery of Boron to the More Substituted Carbon, Leading to Chiral Tertiary Benzylic Boronic Esters

Article abstract of DOI:10.1021/acscatal.8b03591

Phosphonate-directed catalytic asymmetric hydroboration (CAHB) of β-aryl/heteroaryl methylidenes and trisubstituted alkenes by pinacolborane enables facile access to functionalized, chiral tertiary benzylic boronic esters. Hydroboration is catalyzed by a chiral rhodium catalyst prepared in situ from a Rh(I) precursor in combination with a simple TADDOL-derived chiral cyclic monophosphite in a 1:1 ratio. The regio- and stereochemistry arise from the combined effects of the relative disposition of the directing group to the alkene, the alkene substitution pattern, and the necessity of an aryl substituent attached to the alkene. A range of aryl and heteroaryl substituents can be accommodated, and for several chiral substrates, the reactions are efficiently catalyst-controlled, enabling the choice of diastereomeric products as desired. Stereospecific transformations of the chiral boronic ester afford chiral phosphonates bearing a quaternary carbon stereocenter. The synthetic utility of the products is further demonstrated by α-oxidation of the phosphonate, leading to hydroxy- and oxophosphonates; the latter readily undergo elimination/substitution reactions to unmask the phosphonate functionality with the formation of aldehydes, alcohols, esters, amides, acids, and ketones.

Full text of DOI:10.1021/acscatal.8b03591

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Article
Phosphonate-Directed Catalytic Asymmetric Hydroboration:
Delivery of Boron to the More Substituted Carbon
Leading to Chiral Tertiary Benzylic Boronic Esters
Suman Chakrabarty, and James M. Takacs
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03591 • Publication Date (Web): 03 Oct 2018
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Phosphonate-Directed Catalytic Asymmetric Hydroboration: Delivery of Boron to
the More Substituted Carbon Leading to Chiral Tertiary Benzylic Boronic Esters
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Suman Chakrabarty and James M. Takacs*
Department of Chemistry and Center for Integrated Biomolecular Communication, University of Nebraska–Lincoln, Lin-
coln, Nebraska 68588–0304, United States.
Supporting information placeholder
ABSTRACT: Phosphonate-directed catalytic asymmetric hydroboration (CAHB) of β-aryl/heteroaryl methylidenes and tri-
substituted alkenes by pinacolborane enables facile access to functionalized, chiral tertiary benzylic boronic esters. Hydrob-
oration is catalyzed by a chiral rhodium catalyst prepared in situ from a Rh(I)-precursor in combination with a simple
TADDOL-derived chiral cyclic monophosphite in a 1:1 ratio. The regio- and stereochemistry arises from the combined effects
of the relative disposition of the directing group to the alkene, the alkene substitution pattern, and the necessity of an aryl
substituent attached to the alkene. A range of aryl and heteroaryl substituents can be accommodated, and for several chiral
substrates, the reactions are efficiently catalyst-controlled enabling the choice of diastereomeric products as desired. Stereo-
specific transformations of the chiral boronic ester afford chiral phosphonates bearing a quaternary carbon stereocenter. The
synthetic utility of the products is further demonstrated by α-oxidation of the phosphonate leading to hydroxy- and oxo-
phosphonates; the latter readily undergo elimination/substitution reactions to unmask the phosphonate functionality with
the formation of aldehydes, alcohols, esters, amides, acids and ketones.
KEYWORDS: chiral tertiary boronic esters, rhodium catalysis, asymmetric hydroboration, trisubstituted alkene hydrobora-
tion, oxophosphonates
INTRODUCTION
substrates 1 and 3 result in boration at the less substituted
terminus of the alkene affording γ-borylated products (S)-2
(71%, 96.5:3.5 er) and (R)-4 (70%, 95:5 er), respectively
(Figure 1).4d Others have found that methylidene substrates
bearing at least one aryl substituent undergo iridium-,3g co-
balt-,3h-i or copper-catalyzed3j-k CAHB by pinacolborane
(pinBH) or pinacol diborane (B2pin2) with boration occur-
ring at the less substituted carbon.
Chiral boronic esters are useful synthons for diversity-
oriented synthesis1 due to the relative ease with which
these compounds can be refunctionalized via stereospecific
transformations of the C–B bond.2 While an increasing vari-
ety of modern methods are available for the enantioselec-
tive preparation of primary and secondary organoboron de-
rivatives,3,4 fewer methods efficiently access chiral tertiary
boronic esters.5 The latter are important since they can
serve as precursors to chiral tertiary alcohols and amines,
and to the construction of all-carbon quaternary stereocen-
ters via stereospecific C–B to C–C bond substitution.1,2 The
latter structural motif is common in natural products and
compounds of biological interest and often presents a chal-
lenge for their synthesis.6
We have been exploring rhodium-catalyzed, directed-cat-
alytic asymmetric hydroborations (CAHB) of functionalized
alkene substrates with the goal of obtaining multifunctional,
chiral boronic ester synthons.4,5a-b We find that several fac-
tors influence the regio- and stereochemical outcomes of
these reactions; the nature of the directing group, the alkene
substitution pattern, the nature of the alkene substituent
and the nature of the chiral ligand each play pivotal roles.
Using the TADDOL-derived phosphite T1 in combination
with a common Rh(I) catalyst-precursor, the amide- and ox-
ime ether-directed CAHB of β-aryl substituted methylidene
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Figure 1. Directed-CAHB of methylidene vinyl arenes: compar-
ison of amide, oxime and phosphonate directing groups.
CAHB conditions: 0.5 mol% [Rh(cod)Cl]2, 1.0 mol% AgBF4, 1.0
mol% (R,R)-T2, 1 eq. pinBH. rt. 3 h; yields are for the isolated
major regioisomer.
We now report that incorporating phosphonate function-
ality in otherwise similar β-aryl or β-heteroaryl substituted
methylidene (and trisubstituted alkene) substrates leads to
delivery of boron predominantly to the more substituted β-
position. For example, allyl phosphonate 5a (R = H) under-
goes efficient CAHB to afford the chiral tertiary benzylic bo-
ronic ester (R)-6a (81%, 97:3 er);7 less than 15% of the re-
gioisomeric γ-borylated product is formed along with traces
of the alkene reduction product.8,9 While the predominant
regioisomer contrasts that obtained from the amide and ox-
ime ether substrates, 6a arises via the same sense of facial
induction, that is, B-H adds to the si-face of the alkene.10 Sim-
ilar results are obtained for related allylic phosphonates
bearing trisubstituted alkenes (vide infra).
Figure 2. Substrate scope of phosphonate-directed CAHB of β-
aryl methylidenes.
Halogenated aromatics can be versatile intermediates for
subsequent cross-coupling chemistry, and substrates bear-
ing 3- or 4-halogenated phenyl substituents react smoothly
under standard conditions. β-Borylated products 6h–6k are
obtained with high enantioselectivity (94:6 to 97:3 er) and
in moderate-to-very good yields (62–82%). Substrates
bearing the thienyl ring system also undergo highly regiose-
lective β-boration in good yield. However, the 2-thienyl de-
rivative exhibits higher enantioselectivity than the corre-
sponding 3-thienyl derivative; 6l (85%, 97:3 er) and 6m
(80%, 88:12 er) are obtained.
RESULTS AND DISCUSSIONS
Given the contrasting results obtained with phosphonate
compared to amide or oxime ether directing groups, the
question naturally arises as to whether the phosphonate di-
recting group is solely responsible for the observed β-bora-
tion regiochemistry. In fact, this appears not to be the case
(Figure 3). While aryl substrates bearing ring substituents
in the meta- and para-positions undergo efficient β-bora-
tion, substrates having ortho-substituted phenyl groups
(e.g., 5n and 5o) completely change the regiochemistry; γ-
borylated products 6n (82%, 78:22 er) and 6o (80%, 74:26
er) are obtained with high regioselectivity but poor levels of
asymmetric induction. Related phosphonates also exhibit
differences in regioselectivity. Phosphonate 7 is similar to 5
except the methylidene bears a simple alkyl rather than an
aryl substituent (i.e., benzyl vs. phenyl). CAHB of 7 under the
standard conditions proceeds via predominant si-face-addi-
tion to afford γ-borylated product 8 (71%, 88:12 er) as the
major product. The β-borylated isomer 9 is a minor product
Phosphonate–Directed CAHB of β-Aryl Methylidene Sub-
strates. Figure 2 summarizes the isolated yield of chiral ter-
tiary boronic ester obtained from a series of methylidenes
differing in the nature of the aromatic substituent appended
to the alkene. In all cases, the major byproduct is the regioi-
someric terminal boronic ester. The 3-methoxyphenyl (5b)
and 4-methoxyphenyl (5c) substrates undergo efficient β-
boration to yield the chiral tertiary benzylic boronic esters
6b (76%, 97:3 er) and 6c (70%, 94:6 er), respectively. Sim-
ilarly, the 3,4-dimethoxyphenyl derivative 5d and 3,5-dime-
thylphenyl analog 5e undergo CAHB in good yield and with
high enantioinduction to afford 6d (77%, 94:6 er) and 6e
(76%, 96:4 er), respectively. Changing the electronic char-
acter of the aryl group does not significantly affect the out-
come of CAHB. For example, substrates bearing 4-
methylphenyl (5f) and 4-trifluoromethylphenyl (5g)
groups undergo CAHB with similarly high efficiency yield-
ing 6f (81%, 96:4 er) and 6g (77%, 96:4 er), respectively.
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along with traces of alkene reduction product. To further
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probe the role the phosphonate group in directing the
course of reaction, we prepared 10, the one-carbon homo-
logue of 5a, and subjected it to CAHB conditions. Unlike 5a,
the γ,δ-unsaturated phosphonate 10 affords a 1:1 mixture
of regioisomers with neither regioisomer exhibiting high
levels of enantioselectivity. The results in Figure 3 indicate
that the phosphonate group, as well as its disposition rela-
tive to the alkene, and the nature of the alkene substituent
combine to control the regio- and enantioselectivity ob-
served in the phosphonate-directed CAHB.
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Figure 4. Aryl-substituents lead to the same regioselectivity
but opposite π-facial selectivity as substrates bearing all alkyl-
substituents in phosphonate-directed CAHB of trisubstituted
alkenes.
Using the same configuration of ligand T2, 13 and 15a
both result in -boration, however, the addition of pinBH
occurs from opposite faces of the two π-systems. As illus-
trated in Figure 4, pinBH is added to 13 from the “bottom-
face” in the perspective shown, while pinBH adds to 15a
from the “top-face”. This disparity in the π-facial selectivity
may stem from π-stacking interactions between substrate
and ligand or via involvement of the aromatic ring stabiliz-
ing a Rh–C intermediate. The latter argument has been long
used to explain the regioselective hydroboration of simple
vinylarenes.11 The sense of π-facial selectivity observed for
15a (and related substrates 15b–j, vide infra) is consistent
with the si-face addition of pinBH found for β-aryl methyli-
dene substrates 5. As a practical matter, the re/si-sense of
π-facial selectivity is of no consequence; either configura-
tion of ligand T2 is equally accessible and hence both dia-
stereomeric products can be prepared with comparable fa-
cility.
Figure 3. CAHB regioselectivity differs for some methylidene
substrates.
Phosphonate-Directed CAHB of β-Aryl Trisubstituted Al-
kene Substrates. Few catalyst systems have been demon-
strated to be effective for the CAHB of trisubstituted al-
kenes.3j,5a-b We recently reported that a variety of prochiral
and chiral β,γ-unsaturated phosphonates possessing a tri-
substituted alkene, one that bears only alkyl substituents,
are excellent substrates for CAHB (Figure 4).5a For example,
13 undergoes efficient catalyst-controlled diastereoselec-
tive CAHB. The use of (R,R)-T2 leads to the formation of
(2R,5S)-14 (82%, 97:3 dr); (S,S)-T2 affords (2S,5S)-14
(81%, 98:2 dr). We now report that the corresponding β-
aryl trisubstituted alkene substrate (Z)-15a also undergoes
facile CAHB to afford highly hindered, chiral tertiary ben-
zylic boronic esters. The use of (R,R)-T2 leads to the for-
mation of chiral tertiary benzylic boronic ester (2S,5S)-16a
(82%, 95:5 dr), while (S,S)-T2 affords the diastereomeric
product (2R,5S)-16a (81%, 94:6 dr). The synthetically more
challenging (E)-15a substrate behaves similarly; (R,R)-T2
affords (2R,5S)-16a (81%, 95:5 dr) and (S,S)-T2 affords
(2S,5S)-16a (79%, 95:5 dr).
Substrates bearing a 2-thienyl moiety in the -position as
in 15a, but with a simple alkyl substituent in the -position
(e.g., 15b), also undergo -boration yielding the corre-
sponding tertiary benzylic boronic ester product 16b (85%,
93:7 er) (Figure 5). The chiral substrate derived from cit-
ronellal (i.e., 15c) bears a pendant alkene as well as a re-
mote stereocenter. It undergoes catalyst-controlled -bora-
tion leaving the more distant alkene intact; (R,R)-T2 affords
(2S)-16c (55%, 92:8 dr) and (S,S)-T2 affords (2R)-16c
(57%, 92:8 dr).12 The benzothiophene derivative 15d gives
(2S)-16d (68%, 92:8 dr) using (R,R)-T2 and (2R)-16d
(69%, 92:8 dr) with (S,S)-T2. The benzofuran derivative
15e gives comparable results using (R,R)-T2, affording
(2R)-16e (76%, 93:7 dr), but exhibits a mismatched effect
with (S,S)-T2 to give (2S)-16e (73%, 80:20 dr) with a re-
duced level of diastereoselectivity.
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Substrate (E)-15j bears phenyl groups in both β- and γ-
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positions presenting a potential competition for regiocon-
trol. Which of the two phenyl groups will determine the re-
giochemistry? In the event, CAHB with (R,R)-T2 yields the
-borylated product (R)-16j (60%, 97:3 er) with good en-
antioinduction. However, unlike the improved enantiose-
lectivity seen for the (Z)-isomer of 15f, CAHB of the dia-
stereomeric (Z)-15j affords (R)-16j (71%, 70:30 er) with a
much lower degree of stereocontrol.
Synthetic Versatility of Chiral Borylated Phosphonate
Products. The CAHB of 5a proceeds smoothly on gram scale
using a reduced catalyst loading (0.5 mol% Rh) and one
equivalent of pinBH to give 6a (80% yield, 97:3 er). Several
common transformations of boronic esters were carried out
using 6a to demonstrate the potential synthetic utility of
phosphonate functionalized, chiral tertiary benzylic bo-
ronic esters (Figure 6). The chiral tertiary boronic ester is
easily converted to the corresponding trifluoroborate salt
17 (87%),13 the latter are generally more stable than pina-
col boronates and are of interest in metal-catalyzed cross-
coupling chemistry.14,15 Oxidation of 6a by NaBO3.4H2O af-
fords the chiral tertiary alcohol 18a (90%, 97:3 er). Proto-
deboronation16 of 6a using TBAF/H2O yields the known chi-
ral phosphonate 19 (85%, 96:4 er).17 Cross-coupling of 6a
with 2-lithiofuran is very efficient under the conditions re-
ported by Aggarwal1e to produce the furan derivative 20
(91%, 97:3 er); the latter contains a quaternary stereocen-
ter bearing two aryl groups, a structural motif attracting re-
cent interest in medicinal chemistry.18 Cross-coupling with
vinylmagnesium bromide under similar conditions affords
the vinyl derivative 21 (70%, 97:3 er).19
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Standard CAHB conditions: 0.5 mol% [Rh(cod)Cl]2, 1.0 mol%
AgBF4, 1.0 mol% (R,R)/(S,S)-T2, 1 eq. pinBH. rt. 3 h.
Figure 5. Substrate scope and catalyst control with trisubsti-
tuted alkenes. Note: Data highlighted in yellow are from reac-
tions carried out using (R,R)-T2; non-highlighted data are from
reactions carried out using (S,S)-T2.
Chiral allylic phosphonates bearing β-phenyl substitu-
ents, for example (E)-15f–i, tend to undergo highly regiose-
lective -boration albeit with somewhat lower stereoselec-
tivity. For example, CAHB of 15f using (R,R)-T2 affords
(2R)-16f (80%, 91:9 dr) while the reaction with (S,S)-T2 ex-
hibits a modest mismatched effect in giving predominantly
(2S)-16f (78%, 85:15 dr). Improved diastereoselection is
observed for the isomeric substrate (Z)-15f; (R,R)-T2 af-
fords (2R)-16f (83%, 96:4 dr) and (S,S)-T2 gives (2S)-16f
(84%, 97:3 dr). A significant electronic effect is observed.
The 4-methoxyphenyl substrate (E)-15g behaves like (E)-
15f; (2R)- and (2S)-16g are formed with catalyst control in
75% yield (90:10 dr). However, the presence of an electron
withdrawing substituent lowers the yield and diastereose-
lectivity. Substrates 15h (4-chlorophenyl derivative) and
15i (4-trifluoromethylphenyl derivative) afford 56-63%
yields of 16h (83:17 dr) and 16i (ca. 85:15 dr).
Reagents and conditions: (a) KHF2, MeOH; (b) NaBO3.4H2O; (c)
o
TBAF.H2O; (d) (i) nBuLi, furan, –78 C, THF; (ii) NBS; (iii) aq.
Na2S2O3; (e) (i) CH2=CHMgBr; (ii) I2 (iii) MeONa, MeOH.
Figure 6: Synthetic utility of phosphonate functionalized, chi-
ral tertiary benzylic boronic esters is illustrated by selected
transformations of 6a.
Phosphonates and phosphonic acids are components of
important biologically active compounds20 principally due
to their capacity to act as stable phosphate surrogates by re-
sisting hydrolysis. In our prior work on phosphonate-di-
rected CAHB, we demonstrated the versatility of the phos-
phonate functionality through its use in thiophosphonate
olefination to complete the synthesis of all-carbon
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quaternary allylic stereocenter of bakuchiol.5a Here, we fur-
α-hydroxyphosphonate as a mixture of diastereomers.
Treatment of the latter with aqueous base liberates the
known chiral phenyl ketone30 (S)-30 (78% overall yield).31
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ther highlight the utility of the phosphonate moiety by fo-
cusing on the chemistry of α-oxophosphonates (i.e.,
acylphosphonates) (Figure 7). The preparation of
acylphosphonates typically proceeds via the reaction of tri-
alkylphosphite with an acid chloride,21 a modification of the
Michaelis-Arbuzov reaction, or via addition of a dial-
kylphosphite anion to an aldehyde followed by oxidation.22
The direct α-oxidation of phosphonates is not common,
however, Wiemer reported the oxidation of benzylic phos-
phonates to chiral α-hydroxyphosphonates by treatment of
the phosphonate α-carbanion with camphorsulfonyl oxa-
ziridine.23 We used a similar strategy to oxidize our phos-
phonate intermediates using 2-(phenylsulfonyl)-3-phenyl-
oxaziridine (Davis' oxaziridine).24
CONCLUSIONS
Phosphonate-directed CAHB of β-aryl methylidenes and
trisubstituted alkenes provides facile access to functional-
ized, chiral tertiary benzylic boronic esters for which few di-
rect methods of synthesis are available. A simple and read-
ily accessible TADDOL-derived chiral monophosphite lig-
and in 1:1 combination with a commercially available Rh(I)-
precatalyst is used to generate an active catalyst in situ. Re-
actions can be carried out under conditions of ambience and
are scalable without loss of yield or enantiopurity; catalyst
loading as low as 0.5% has been demonstrated. A range of
aryl and heteroaryl substituents can be accommodated, and
in cases of chiral substrates, the reactions are often effi-
ciently catalyst-controlled enabling the synthesis of dia-
stereomeric products as desired. Stereospecific transfor-
mations of the chiral boronic ester afford chiral phospho-
nates bearing a quaternary carbon stereocenter. The syn-
thetic utility of the products obtained via CAHB is further
demonstrated via α-oxidation of the phosphonate leading to
α-hydroxy- and α-oxophosphonates; the latter readily un-
dergo elimination or substitution reactions to unmask the
phosphonate functionality with the formation of aldehydes,
alcohols, esters, amides, acids and ketones.
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The regioselective CAHB of simple vinyl arenes is well
precedented in the literature. Depending on the nature of
chiral catalyst used benzylic selectivity is frequently ob-
served, presumably due to formation of a π-benzyl com-
plex.11 Although the regiochemistry differs between phos-
phonate-directed CAHB of β-aryl trisubstituted allylic phos-
phonates and methylidene derivatives and the correspond-
ing amide- and oxime-directed β-aryl methylidene sub-
strates, the sense of -facial discrimination remains the
same. This contrasts with the π-facial selectivity observed
from phosphonate-directed CAHB of all-alkyl trisubstituted
alkenes. The change based on the alkyl/aryl-nature of an al-
kene is surprising given that our previous studies suggested
changes in regioselectivity often coincide with changes in
alkene substitution pattern or the directing group. The re-
gio- and stereochemistry for the phosphonate-directed bo-
ration likely arises from the combined effects of the relative
disposition of the directing group to the alkene, the alkene
substitution pattern, and the necessity for a conjugated al-
kene. Further studies of CAHB are in progress.
Reagents and conditions: (a) nBuLi, –78 oC, THF; Davis' oxaziri-
dine; (b) aq. NaHCO3 reflux; (c) Dess-Martin Periodinane,
CH2Cl2; (d) LiAlH4, THF; (e) EtOH, DBU; (f) aq. NH3, THF; cat.
(nBu)4NBr; (g) aq. NaOH; (h) (i) PhMgBr, –78 C, THF; (ii) aq.
NH4Cl; (iii) aq. NaOH.
o
Figure 7: Oxidations leading to α-hydroxy and oxophospho-
nates and their synthetic utility.
Our initial attempts to α-oxidize the boronic ester func-
tionalized phosphonate 6a were unsuccessful. However,
catalytic hydrogenation of 21 gives 22 which when treated
with nBuLi and Davis' oxaziridine yields α-hydroxyphos-
phonate 23 (83%, 2:1 dr). α-Hydroxyphosphonates such as
23 are aldehyde equivalents and readily undergo elimina-
tion.25 Thus, treating 23 with aqueous NaHCO3 affords the
chiral aldehyde (S)-24 (88%, 95:5 er). Alternatively, oxida-
tion of α-hydroxyphosphonate 23 with Dess-Martin perio-
dinane affords acylphosphonate 25 (92%).20 Reduction of
25 with LiAlH4 affords the known chiral alcohol (S)-26
(89%, 95:5 er)19,26, confirming the absolute configuration
assigned to 6a. Acylphosphonates have recently attracted
interest as surrogates for esters and amides in asymmetric
organocatalysis by serving the role of an active ester.21,22
Thus, treatment of 25 with: (i) ethanol/DBU converts 25 to
the chiral ester 27 (95%, 96:4 er), (ii) aqueous ammonia af-
fords the chiral amide (S)-28 (85%, 97:3 er),27 or (iii) aque-
ous sodium hydroxide forms the known chiral carboxylic
acid28 (S)-29 (91%, 96:4 er).29 Treatment of 25 with phenyl-
magnesium bromide gives after aqueous workup the stable
AUTHOR INFORMATION
Corresponding Author Email
* jtakacs1@unl.edu
ORCID
Suman Chakrabarty: 0000-0002-6611-3839
James M. Takacs: 0000-0002-1903-6535
Notes:
The authors declare no competing financial interest.
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3067–3070. (k) Corberan, R.; Mszar, N. W.; Hoveyda, A. H. NHC‐Cu‐
Experimental procedures, characterization data (SI-1; PDF)
and NMR Data (SI-2; PDF).
Catalyzed Enantioselective Hydroboration of Acyclic and Exocyclic
1,1‐Disubstituted Aryl Alkenes. Angew. Chem. Int. Ed. 2011, 50,
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ACKNOWLEDGMENT
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4. (a) Hoang, G. L.; Zhang, S.; Takacs, J. M. Rhodium-catalyzed
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191 and references cited therein.
Funding from the NIH National Institutes of General Medical
Sciences (R01 GM100101)) is gratefully acknowledged. We
thank M. Morton for helpful discussions and for assistance with
NMR experiments and A. J. Bochat and V. M. Shoba for prelimi-
nary data on the CAHB of 3 in advance of publication.
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a comparatively lower level of enantioinduction (95:5 er). See the
SI for characterization data and assignment of the absolute config-
uration of the minor regioisomer.
8. Unless otherwise noted, the reported yields and enantiomer
and diastereomer ratios are of isolated products and the average
of at least 2 runs. Enantiomer ratios (er) are determined by chiral
HPLC analysis after oxidation to the corresponding alcohols; dia-
stereomer ratios (dr) are determined by 31P NMR analysis of the
inseparable mixture of diastereomers. The absolute configuration
of 6a is established via protodeboration to a known phosphonate;
other products are assigned in analogy. For the assignments of ab-
solute configurations of the products obtained from trisubstituted
alkenes, see the SI.
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