The preparation of 3-substituted-1,5-dibromo-pentanes as precursors to heteracyclohexanes

Article abstract of DOI:10.3762/bjoc.7.49

The methodology to prepare 3-substituted 1,5-dibromopentanes I and their immediate precursors, which include 3-substituted 1,5-pentanediols VII or 4-substituted tetrahydropyrans VIII, is surveyed. Such dibromides I are important intermediates in the preparation of liquid crystalline derivatives containing 6-membered heterocyclic rings. Four dibromides 1a-1d containing simple alkyl and more complex fragments at the 3-position were prepared. 3-Propyl- and 3-pentyl-pentane-1,5-diol (2a,b) were prepared starting from either glutaconate or malonate diesters, while tetrahydropyrans 3c and 3d were obtained from tetrahydro-4H-pyran-4-one. The advantages and disadvantages of each route are discussed. Dibromides 1c and 1d were used to prepare sulfonium zwitterions 11c and 11d.

Full text of DOI:10.3762/bjoc.7.49

The preparation of 3-substituted-1,5-dibromo-
pentanes as precursors to heteracyclohexanes
Bryan Ringstrand1, Martin Oltmanns1,§, Jeffrey A. Batt1,§,
Aleksandra Jankowiak1, Richard P. Denicola1 and Piotr Kaszynski*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 386–393.
1Organic Materials Research Group, Department of Chemistry,
Vanderbilt University, Box 1822 Station B, Nashville, TN 37235, USA,
phone/fax: (615) 322-3458 and 2Faculty of Chemistry, University of
Łódź, Tamka 12, 91403 Łódź, Poland
doi:10.3762/bjoc.7.49
Received: 29 December 2010
Accepted: 15 March 2011
Published: 31 March 2011
Email:
Bryan Ringstrand - bryan.ringstrand@vanderbilt.edu;
Martin Oltmanns - martin-oltmanns@augustana.edu;
Jeffrey A. Batt - jeffrey-batt@augustana.edu;
Aleksandra Jankowiak - aleksandra.jankowiak@vanderbilt.edu;
Richard P. Denicola - richard.p.denicola@vanderbilt.edu;
Piotr Kaszynski* - piotr.kaszynski@vanderbilt.edu
Associate Editor: P. J. Skabara
© 2011 Ringstrand et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
§ Summer research student visiting from Augustana college, Rock
Island, IL
Keywords:
1,5-dibromopentanes; heterocycles; methodology; synthesis
Abstract
The methodology to prepare 3-substituted 1,5-dibromopentanes I and their immediate precursors, which include 3-substituted 1,5-
pentanediols VII or 4-substituted tetrahydropyrans VIII, is surveyed. Such dibromides I are important intermediates in the prepara-
tion of liquid crystalline derivatives containing 6-membered heterocyclic rings. Four dibromides 1a–1d containing simple alkyl and
more complex fragments at the 3-position were prepared. 3-Propyl- and 3-pentyl-pentane-1,5-diol (2a,b) were prepared starting
from either glutaconate or malonate diesters, while tetrahydropyrans 3c and 3d were obtained from tetrahydro-4H-pyran-4-one. The
advantages and disadvantages of each route are discussed. Dibromides 1c and 1d were used to prepare sulfonium zwitterions 11c
and 11d.
Introduction
3-Substituted 1,5-dibromopentanes I and disulfonates, typically III, thianes IV, silacyclohexanes V, and phosphorinanes VI
tosylates II, serve as useful intermediates in the preparation of (Figure 1). The piperidines III (Y = Ar) have been used as
six-membered heterocycles such as 4-substituted-piperidines structural elements of liquid crystals [1-6] as well as antithrom-
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Beilstein J. Org. Chem. 2011, 7, 386–393.
Figure 1: Methods for synthesis of dibromides I and their use for preparation of 6-membered heterocycles.
botic and neuroprotectant agents [7,8]. Some thianes IV are ition as intermediates for polar nematics. A number of such
found in petroleum distillates [9] and several have been used for dibromides I and ditosylates II have been reported in the litera-
the preparation of sulfones [7,10,11], sulfoxides [11,12], and ture and some are listed in Table 1. For completeness, thianes
sulfonium derivatives [13]. The latter, as well as silacyclo- IV, diols VII, and tetrahydropyrans VIII are also included.
hexanes V [14] and phosphorinanes VI [15,16], have been used
in conformational and stereochemical studies.
Diols VII represent a convenient intermediate for the prepara-
tion of dibromides I or ditosylates II, and four routes to VII are
The dibromides I or disulfonates II were prepared from presented in Scheme 1. The first route (Method 1A) involves
3-substituted 1,5-pentanediols VII. The former were obtained six steps starting from alkylation of malonate diester followed
using HBr and concd sulfuric acid [2], while the latter from by reduction of the esters, tosylation of the alcohols, carbon
p-toluenesulfonyl chloride in the presence of a base such as homologation with NaCN, hydrolysis of the cyano groups to
pyridine [13,17-20]. Dibromides I can also be obtained by the carboxylic acids, and lastly a second reduction to VII. Overall
ring-opening of tetrahydropyrans VIII under the same condi- yields of diols VII using Method 1A are around 10% [27,28].
tions employed for the diols VII [2,12,21,22], or via phase- The second route (Method 1B) consists of a Knoevenagel con-
transfer catalysis conditions with a phosphonium salt [23]. densation of malonate diester and an aldehyde followed by
Piperidines III (Y = H) can undergo degradation to form dibro- conjugate addition of a second equivalent of malonate diester.
mides I although this process is typically less efficient The tetraester is hydrolyzed, decarboxylated, esterified, and
[12,14,15]. The piperidine ring can be reconstituted by ring- then reduced to give the diol. Yields for tetraesters are in the
closure of I with an appropriate amine or aniline Y–NH2 [2,22]. range of 25–85%, whereas the decarboxylation step gives the
The thianes IV are typically obtained from I or from II by reac- corresponding glutaric acids in 60–80% yield [19,33]. Varia-
tion with Na2S [9,10,12,13], whereas the silacyclohexanes V tions of Method 1B involve the use of cyanoacetamide [34-36]
and phosphorinanes VI are prepared by reacting the Grignard or Meldrum’s acid [19,37] instead of malonate diester. For the
reagent derived from I with dichlorosilanes [14] or phospho- former, the condensation with aldehyde is efficient and the pro-
nous dichlorides [15], respectively (Figure 1).
duct is crystalline, which simplifies its purification.
Our research program focuses on the development of polar, Another route to VII involves a two-step sequence of conjugate
zwitterionic nematic liquid crystals having large longitudinal addition of an organocopper reagent to glutaconate diester [38]
dipole moments for formulation of nematic materials with posi- followed by reduction (Method 1C, Scheme 1). Yield of diols
tive dielectric anisotropy [24,25]. The zwitterions consist of six- from this sequence are typically around 70% [17,38]. The
membered sulfonium rings attached to a boron cluster, either downside to Method 1C is that an excess of Grignard reagent
[closo-1-CB9H10]− or [closo-1-CB11H12]−. Therefore, we have (3–4 equiv) is required; therefore, it is not economical with
interest in cycloalkylating reagents such as dibromides I and respect to the alkyl halide. Method 1D also uses glutaconate
disulfonates II containing alkyl or aryl substituents at the 3-pos- diester, which is reacted with an aryl iodide in the presence of
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Beilstein J. Org. Chem. 2011, 7, 386–393.
Table 1: Selected compounds reported in the literature.
R
Literature
Me, n-Pr, n-Bu, C5H11, C6H13, C7H15, C8H17, C9H19, C10H21, Ph,
[2,9,10,12,14,21,22,26]
4-C4H9OPh, 4-ClPh
Me, Et, n-Pr, Ph, 4-CH3OPh, 4-C5H11Ph
[13,17-20]
Me, Et, n-Pr, C6H13, Ph, 4-CH3OPh, 4-C4H9OPh, 4-ClPh, 4-C5H11Ph,
PhCH2CH2
[2,9,17-20,26-28]
[7,9-13,29,30]
Me, Et, n-Pr, n-Bu, C5H11, C6H13, Ph, 4-ClPh, 4-HOPh, 4-NH2Ph
Me, n-Pr, n-Bu, C5H11, C6H13, C7H15, C8H17, C9H19, C10H21, C11H23, Ph,
[2,7,12,21,22,31,32]
4-HOPh, 4-C4H9OPh, 4-CF3Ph, 4-NH2Ph
Scheme 1: General methods for preparation of diols VII.
Pd(0) under Heck conditions. The resulting unsaturated diester third route (Method 2C) begins from tetrahydropyran-4-
is reduced with excess LiAlH4 to give diol VII with an overall carboxylic acid: The acid chloride is reacted with a Grignard
yield of about 30% for the two-step process [19].
reagent, and the resulting ketone reduced under Wolff–Kischner
conditions. Typical yields for this sequence are around 40%
Tetrahydropyrans VIII represent a second general intermediate [21]. A recently described procedure [32] allows for efficient
for access to dibromides I, and four routes are presented in preparation of 4-aryltetrahydropyrans by the coupling of aryl-
Scheme 2. The first route involves the addition of a Grignard boronic acids and 4-chlorotetrahydropyran in the presence of Ni
reagent to tetrahydro-4H-pyran-4-one, elimination of water, and catalyst (Method 2D).
hydrogenation of the olefin (Method 2A). Typical yields for
Method 2A range from 20–30% [39,40]. The second route is the Here, we report the preparation of dibromides 1 having at the
Wittig olefination of tetrahydro-4H-pyran-4-one followed by 3-position propyl (1a), pentyl (1b), 2-(4-trans-pentylcyclo-
hydrogenation (Method 2B). Yields for the Wittig olefination of hexyl)ethyl (1c), or 4-propylphenethyl (1d) substituents
tetrahydro-4H-pyran-4-one range from 35–75% [41-43]. The (Figure 2). We investigate the preparation of appropriate diols 2
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Beilstein J. Org. Chem. 2011, 7, 386–393.
Scheme 2: General methods for preparation of tetrahydropyrans VIII.
starting from malonate (Method 1B, Scheme 1) and glutaconate
diesters (Method 1C) as well as appropriate tetrahydropyrans 3
via Wittig olefination (Method 2B, Scheme 2). These routes
promise the minimal number of steps and ease of chemical
transformations.
Scheme 3: Preparation of dibromides 1.
Figure 2: Structures of 1,5-dibromomopentanes 1a–1d.
Results
Preparation of dibromides
Dibromides 1a–1d were prepared in 67–91% yield in refluxing
48% HBr with tributylhexadecylphosphonium bromide [44] as
a phase-transfer catalyst either from diols 2a and 2b or tetrahy-
dropyrans 3a–3d (Scheme 3). When H2SO4 was used instead of
the phosphonium salt, the yield of 1b was lower (67%). The
dibromides were purified on silica gel and, in addition, 1a and
1b were distilled.
Scheme 4: Preparation of diol 2a.
Preparation of diols
Diols 2a and 2b, containing simple alkyl chains, were prepared
using both the malonate and glutaconate routes, Method 1B and Reaction of hexanal with excess dimethyl malonate under
Method 1C, respectively (Scheme 1). The reaction of a 4-fold Knoevenagel conditions gave dimethyl 3-pentylglutarate (5) in
excess of propylmagnesium chloride with diethyl glutaconate in 57% yield after decarboxylation and esterification (Scheme 5).
the presence of TMSCl with a catalytic amount of CuI gener- 3-Propylglutarate 4 was obtained in a similar way from butanal
ated diethyl 3-propylglutarate (4) in 79% yield. Reduction of 4 in 49% overall yield. Diester 5 was subsequently reduced to
gave diol 2a in nearly quantitative yield (Scheme 4).
give diol 2b in 87% yield.
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6a and 6b are volatile compounds so again care should be taken
during evaporation of solvent to maximize yields. Phosphor-
anes were generated using standard conditions by reacting phos-
phonium salts 7a–7c and 7e with either BuLi in anhydrous
THF, or NaHMDS in a 1:1 mixture of anhydrous Et2O and an-
hydrous CH2Cl2.
Scheme 5: Preparation of diol 2b.
Preparation of tetrahydropyrans
Tetrahydropyrans 3a–3c were prepared in 82%–96% yield by
hydrogenation of the corresponding 4-methylenetetrahydropy-
rans 6a–6c in THF (Scheme 6). Tetrahydropyrans 3a and 3b are
volatile so care should be exercised during evaporation of
solvent to ensure maximum yields.
Scheme 8: Preparation of methylenetetrahydropyrans 6.
Heating the corresponding alkyl halide (Br or I) with PPh3
under reflux in concentrated solutions of toluene, benzene, or
acetonitrile gave phosphonium salts 7a–7c and 7e. Starting
from 4-chlorophenylacetic acid, 4-chlorophenethyl bromide (8)
was prepared in 78% overall yield according to the literature
method [48] (Scheme 9). The phosphonium salt 7c, an inter-
mediate to dibromide 1c, was prepared starting from the known
2-(trans-4-pentylcyclohexyl)acetaldehyde (9) [49]. The alde-
Scheme 6: Preparation of tetrahydropyrans 3a–3c.
Negishi coupling of tetrahydropyran 3e with propylzinc chlo-
ride following a general literature method [45] using PEPPSI-IR
as the Pd(0) source gave tetrahydropyran 3d in 69% yield.
Tetrahydropyran 3e was prepared in 86% yield by hydrogena-
tion of the corresponding 4-methylenetetrahydropyran 6e in the
presence of ZnBr2 to prevent reductive dechlorination
(Scheme 7) [46]. Attempts to use diphenyl sulfide as a catalyst
poison [47] resulted only in recovery of the starting material.
Wittig olefination of tetrahydro-4H-pyran-4-one gave substi-
tuted 4-methylenetetrahydropyrans 6a–6c and 6e in yields
ranging from 20–40% (Scheme 8). Methylenetetrahydropyrans
Scheme 9: Preparation of bromides 8 and 10.
Scheme 7: Preparation of tetrahydropyran 3d.
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Beilstein J. Org. Chem. 2011, 7, 386–393.
hyde was reduced with NaBH4 and subsequently brominated Diols VII are obtained by reduction of a 3-substituted glutaric
with PBr3 to give 1-bromo-2-(trans-4-pentylcyclohexyl)ethane acid or ester. The latter can be prepared in a single step from
(10) in overall yield of 42% (Scheme 9). Bromide 10 has been glutaconate diester and a Grignard reagent. However, this
reported in the literature, but experimental details are lacking method is practically limited to simple 3-alkyl-1,5-pentanediols,
[50,51].
such as 1a and 1b, since large excess of Grignard reagents is
required. The second method, in which glutaric acids are
The utility of dibromides 1 was demonstrated by preparation of obtained from aldehydes and malonate ester, is more general
sulfonium derivatives 11. Thus, compound 11b was previously and suitable for large scale synthesis, but involves an additional
obtained in 39% yield (before recrystallization) by alkylative step.
cyclization of masked mercaptan [closo-1-CB9H8-1-
SCHNMe2-10-I] 12 with dibromide 1b in the presence of Tetrahydropyrans VIII can be prepared in two steps via Wittig
Me4N+OH−·5H2O in MeCN [25]. The same procedure gave 11c olefination. The efficiency of this method is limited by the
in 25% yield after recrystallization, when starting from dibro- initial olefination step of tetrahyro-4H-pyran-4-one, where the
mide 1c. Replacement of Me4N+OH−·5H2O with Cs2CO3 and a yields range from 20–40%. This step could potentially be im-
catalytic amount of Bu4N+Br− in analogous reactions with proved by using different reaction conditions or an excess of the
dibromides 1c and 1d led to the sulfonium derivatives 11c and ylide. The hydrogenation of the corresponding olefins is
11d in higher isolated yields of about 35% (Scheme 10). The straightforward and proceeds in high yield. In the case where a
higher yield for the cyclization in the presence of Cs2CO3 is Pd-sensitive group is present, such as a chlorine substituent in
attributed to less dehydrobromination of the electrophile than 6e, modification of the Pd catalyst with ZnBr2 permits efficient
observed with Me4N+OH−.
hydrogenation without loss of the halogen. The chlorophenyl
derivative 3e is a convenient precursor to a variety of
4-alkylphenyl and 4-arylphenyl derivatives that can be obtained
via Negishi or Suzuki coupling methods (i.e., 3d). Another such
potentially general intermediate is 4-(4-hydroxyphenyl)-
tetrahydro-4H-pyran [31], which can be O-alkylated to provide
4-alkoxyphenyl derivatives [2].
Overall, we have demonstrated that dibromides I containing
simple groups, such as propyl or pentyl, can be prepared in
yields of about 70% starting from glutaconate diester (three
steps, entry 2; Table 2), 36–38% from a malonate (four steps,
entries 1 and 5), and on average 25% starting from tetrahydro-
4H-pyran-4-one (three steps, entries 3 and 5). Dibromides I
containing larger fragments such as 3-propylphenethyl or 2-(4-
trans-pentylcyclohexyl)ethyl were prepared exclusively starting
from tetrahydro-4H-pyran-4-one in overall yields averaging
about 20% (three steps, entries 6 and 7). For comparison
Table 2 includes the preparation of dibromides I (R = aryl)
involving Heck-type arylation (entry 8) and dibromides I (R =
alkyl) starting from tetrahydropyran-4-carboxylic acid (entry 9).
Scheme 10: Preparation of sulfonium derivatives 11.
It appears that the most economical way to prepare simple
3-alkyl-1,5-dibromopentanes is by using the aldehyde/malonate
or aldehyde/cyanoacetamide method (Method 1B, Scheme 1),
Discussion
Evaluation of the syntheses described here reveals advantages while the glutaconate method (Method 1C) is the simplest and
and disadvantages of the methods used for preparation of diols most efficient. For the preparation of 3-aryl derivatives, the
VII and tetrahydropyrans VIII. Both VII and VIII can be effi- most direct methods are the Heck-type coupling with gluta-
ciently converted to dibromides I using HBr and tributylhexade- conate ester (Method 1D) and Suzuki-type coupling to
cylphosphonium bromide as the phase-transfer catalyst.
4-chlorotetrahydropyran (Method 2D).
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Beilstein J. Org. Chem. 2011, 7, 386–393.
Table 2: Summary of dibromide 1 syntheses.
Entry
Route (Method)
Number of steps
Overall yield (%)
1
2
3
4
5
6
7
8
9
R = C3H7 (1a)
Malonate (1B)
4
3
3
4
3
3
4
3
4
36
R = C3H7 (1a)
Glutaconate (1C)
Pyranone (2B)
Malonate (1B)
68
R = C3H7 (1a)
17
R = C5H11 (1b)
38
R = C5H11 (1b)
Pyranone (2B)
Pyranone (2B)
Pyranone (2B)
Glutaconate (1D)
4-COOH-pyran (2C)
33
R = C5H11C6H10CH2CH2 (1c)
R = C3H7C6H4CH2CH2 (1d)
R = C5H11C6H4
20
18a
26b
40c
R = C3H7–C10H21
a4th step includes replacement of Cl in Ar–Cl with propyl by Pd-catalyzed coupling reaction. Excluding this step the yield for the dibromide would have
been 17% over three steps. bIsolated as the ditosylate Ref. [19] cRef. [21].
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