α,β-unsaturated α′-halomethylsulfones: Prepackaged Ramberg-Baecklund reagents for tandem synthetic processes

Article abstract of DOI:10.1080/10426500590910783

The synthesis and reactions of several α,β-unsaturated chloromethyl sulfones are presented, for example [(chloromethyl) sulfonyl]-1,3-propadiene, [(chloromethyl) sulfonyl]ethene, [(dichloromethyl) sulfonyl]ethene and (E,Z)-1,2-bis[(chloromethyl)-sulfonyl]ethene. These compounds serve as prepackaged Ramberg-Backlund reagents, which, following an appropriate first step, such as Diels-Alder addition, react with a base, giving Ramberg-Backlund products. Copyright Taylor & Francis Inc.

Full text of DOI:10.1080/10426500590910783

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α,β-Unsaturated α′-
Halomethylsulfones:
Prepackaged
Ramberg–Bäcklund Reagents
for Tandem Synthetic Processes
Eric Block a , Hak Rim Jeon a , David Putman a &
Shao-Zhong Zhang a
a Department of Chemistry, University at Albany,
State University of New York, Albany, USA
Version of record first published: 16 Aug 2006.
To cite this article: Eric Block, Hak Rim Jeon, David Putman & Shao-Zhong Zhang
(2005): α,β-Unsaturated α′-Halomethylsulfones: Prepackaged Ramberg–Bäcklund
Reagents for Tandem Synthetic Processes, Phosphorus, Sulfur, and Silicon and the
Related Elements, 180:5-6, 1197-1202
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Phosphorus, Sulfur, and Silicon, 180:1197–1202, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500590910783
α,β-Unsaturated αꢀ-Halomethylsulfones:
Prepackaged Ramberg–Ba¨cklund Reagents
for Tandem Synthetic Processes
Eric Block
Hak Rim Jeon
David Putman
Shao-Zhong Zhang
Department of Chemistry, University at Albany, State University
of New York, Albany, USA
The synthesis and reactions of several α,β-unsaturated chloromethyl sulfones are
presented, for example [(chloromethyl) sulfonyl]-1,3-propadiene, [(chloromethyl)
sulfonyl]ethene, [(dichloromethyl)sulfonyl]ethene and (E,Z)-1,2-bis[(chloromethyl)-
sulfonyl]ethene. These compounds serve as “prepackaged” Ramberg–Ba¨cklund
reagents, which, following an appropriate first step, such as Diels–Alder addition,
react with a base, giving Ramberg–Ba¨cklund products.
Keywords α-Halosulfones; alkene synthesis; Ramberg–Ba¨cklund reaction
INTRODUCTION
The Ramberg–Ba¨cklund (RB) reaction is a synthetically useful pro-
cess in which α-haloalkyl sulfones afford olefins upon treatment with
a base. As shown in Scheme 1, the requisite α-haloalkyl sulfones are
typically prepared in multistep processes.1,2 We sought “prepackaged”
RB reagents that incorporate all of the components required for the RB
reaction into one reactive unit, requiring only an olefinic substrate and
base to give olefinic end products with an increase in total number of
carbon atoms. In 1983 we described the use of bromomethanesulfonyl
bromide (BrCH2SO2Br, 1) as a one-carbon example of such a prepack-
aged reagent (Scheme 2).3 Reagent 1 readily undergoes light-induced
free radical addition to alkenes. Dehydrobromination of the addition
Received July 9, 2004; accepted October 5, 2004.
This work was supported by the National Science Foundation (CHE-9906566) and the
Petroleum Research Fund, administered by the American Chemical Society.
Address correspondence to Eric Block, Department of Chemistry, University at Albany,
State University of New York, Albany, NY 12222, USA. E-mail: eb801@albany.edu
1197

1198
E. Block et al.
SCHEME 1
SCHEME 2
products gives α,β-unsaturated αꢀ-bromoalkyl sulfones 2, which un-
dergo vinylogous RB reaction, giving 1,3-dienes as E,Z-mixtures. Rep-
etition of the process gives isomeric 1,3,5-trienes. Reaction of 1 with
1,3-butadiene followed by dehydrobromination affords 1,3-butadienyl
bromomethylsulfone (CH2 CHCH CH2SO2CH2Br, 3). The latter com-
pound can be considered as a five-carbon prepackaged RB reagent be-
cause it undergoes Michael-induced RB reactions, for example with
sodium isopropoxide, giving i-PrO CH2CH CHCH CH2.3
In
a
1972 communication, 1-bromo-1-(methylsulfonyl)ethene
(CH2 C(Br)SO2CH3), which can be considered to be a three-carbon
prepackaged RB reagent, was reported to undergo Diels–Alder addition
followed by RB reaction, although only a single reaction was described.4
We sought to further develope α,β-unsaturated αꢀ-halomethylsulfones
akin to 2 and 3 as prepackaged RB reagents, focusing in particular
on tandem processes, for example where the RB reagent undergoes a
pericyclic reaction followed by a RB reaction upon addition of a base.
RESULTS
We find that the four-carbon prepackaged RB reagent [(chloromethyl)-
sulfonyl]-1,2-propadiene (CH2 C CHSO2CH2Cl, 4; Scheme 3), upon
tandem Diels–Alder RB reaction, gives the formal Diels–Alder adducts
of buta-1,2,3-triene.5 Reagent 4 has been employed in synthe-
ses of [n]beltenes.6 Use of [(trichloromethyl)sulfonyl]-1,2-propadiene
(CH2 C CHSO2CCl3) as a 1,1-dichloro-1,2,3-butatriene synthon has
also been described.7

α,β-Unsaturated αꢀ-Halomethylsulfones
1199
SCHEME 3
We recently described the syntheses and applications of the
three- and four-carbon pre-packaged RB reagents [(chloromethyl)-
sulfonyl]ethene (CH2 CHSO2CH2Cl, 5), [(dichloromethyl)sulfonyl]-
ethene (CH2 CHSO2CHCl2, 6), (E,Z)-1,2-bis[(chloromethyl)sulfonyl]-
ethene (ClCH2SO2CH CHSO2CH2Cl, 7), and [(chloromethyl)sulfo-
nyl]ethyne (8).5,8 To prepare 5, chloromethanesulfenyl chloride
(ClCH2SCl) was added to ethylene, giving the known9 1-chloro-2-
[(chloromethyl)thio]ethane (90%), which was oxidized with two equiv-
alents of mCPBA to 1-chloro-2-[(chloromethyl)sulfonyl]ethane. The
latter is directly stirred with aqueous NaHCO3, giving 5 (99%)
as a colorless oil. A similar sequence of steps was used to con-
vert Cl2CHSCl (from chlorination of ClCH2SCl)10 into 6 (78%)
by way of 1-chloro-2-[(dichloromethyl)-thio]ethane and 1-chloro-2-
[(dichloromethyl)sulfonyl]ethane, neither of which is isolated. Both 5
and 6 readily add to a variety of 1,3-dienes, giving high yields of the
corresponding Diels–Alder adducts, for example 9 and 11 as shown in
Scheme 4. As anticipated, treatment of the Diels–Alder adducts of 5
with KOt-Bu/THF give good yields of the diene-allene adducts, for ex-
ample 10, via Ramberg–Ba¨cklund reaction.
SCHEME 4
Base treatment of adducts of 6 gave mixtures of the isomeric adducts
of chloropropa-1,2-diene together with lesser amounts of the adducts
of allene and 1,1-dichloropropa-1,2-diene (13, 10, 12, respectively,
Scheme 5). The latter two compounds are presumably formed by nu-
cleophilic attack of the α-sulfonyl-α-chlorocarbanion 11a of the Diels–
Alder adduct 11 of 6 on the chlorine of a second adduct molecule, giv-
ing [(trichloromethyl)sulfonyl]ethene adduct 14, which is not isolated.

1200
E. Block et al.
SCHEME 5
We reasoned that if base treatment of Diels–Alder adducts of 6
were conducted in the presence of an excess of a suitably reactive
source of chlorine, chlorine transfer might be possible prior to the
Ramberg–Ba¨cklund reaction, leading to exclusive formation of Diels–
Alder adducts of 1,1-dichloropropa-1,2-diene. To test this hypothesis,
[(trichloromethyl)sulfonyl]methane, MeSO2CCl3, was used as a novel
chlorine source. This little studied compound11 was conveniently pre-
pared in 91% yield by bubbling Cl2 into a refluxing, UV-irradiated solu-
tion of dimethyl sulfone in SO2Cl2. We were pleased to find that when
Diels–Alder adducts of 6 were treated at 0◦C with KOt-Bu-THF in the
presence of two equivalents of MeSO2CCl3, the corresponding Diels–
Alder adducts of 1,1-dichloropropa-1,2-diene were formed in good yield
(e.g., 12, Scheme 5).
Because compound 7 showed good reactivity as a dienophile (see the
following), we examined its reactivity in the ene reaction. When (E)-
7 was heated with two equivalents of β-pinene in toluene at 135◦C
for 1.5 h, the ene product 2-{[(1ꢀ,2ꢀ-bis(chloromethyl)sulfonyl]propyl}-
6,6-dimethylbicyclo[3.1.1]hept-2-ene (15) was formed in 72% yield
as a crystalline solid. Base treatment (t−BuOK) of 15 in refluxing
THF afforded (E)-2-(buta-1,3-dienyl)-6,6-dimethylbicyclo[3.3.1] hept-
2-ene (17) in low yield. If treatment with base was conducted
at 0◦C, low yields of 2-[(3-chloromethyl)sulfonyl]allyl-6,6-dimethyl-
bicyclo[3.3.1]hept-2-ene (16) could be isolated, leading to the over-
all proposed mechanism shown in Scheme 6. Although not of
SCHEME 6

α,β-Unsaturated αꢀ-Halomethylsulfones
1201
synthetic utility because of the low yield, the reaction in Scheme 6 rep-
resents the first example of a tandem reaction sequence incorporating
an ene reaction with a Ramberg–Ba¨cklund reaction.
Both (E)- and (Z)-7 readily form Diels–Alder adducts (18 and 19, re-
spectively) with 1,3-cyclopentadiene (Scheme 7). Base treatment of the
SCHEME 7
adducts gave unstable 2-[(chloromethyl) sulfonyl]bicyclo[2.2.1]hepta-
2,5-diene (20) rather than 5,6-bis(methylene)bicyclo[2.2.1]hept-2-ene
(21). Thus, 1,2-elimination from the activated α-sulfonyl carbanions
is favored over 1,3-elimination (RB reaction), even with the adduct of
(Z)-7, where a coplanar 1,2-elimination transition state is impossible.
Compound 20 could be directly prepared in high yield from Diels–Alder
addition of [(chloromethyl)sulfonyl]ethyne (8) to cyclopentadiene.
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