Hydroalumination versus deprotonation of alkynes with sterically demanding substituents

Article abstract of DOI:10.5560/ZNB.2013-3070

Treatment of sterically highly shielded terminal alkynes, H-C≡C-aryl, with dialkylaluminium and dialkylgallium hydrides, R₂E-H, afforded by hydrogen release dimeric dialkylelement alkynides with a four-membered E ₂C₂ heterocycle independent of the bulk of the aryl groups. A rare example of a monomeric alkynylaluminium compound was only obtained with very bulky CH(SiMe₃)₂ groups attached to the metal atoms and by salt elimination reaction. The steric shielding by the bulky aryl groups did not prevent condensation reactions. Hydroalumination of 1-(trimethylsilyl)-2-(2,6-dimethylphenyl)ethyne using Me₂Al-H resulted in a divinyl compound by elimination of trimethyl-aluminium.

Full text of DOI:10.5560/ZNB.2013-3070

Hydroalumination versus Deprotonation of Alkynes with Sterically
Demanding Substituents
Werner Uhl, Marcus Layh, Ines Rhotert, Agnes Wollschla¨ger, and Alexander Hepp
Institut fu¨r Anorganische und Analytische Chemie der Universita¨t Mu¨nster, Corrensstraße 30,
48149 Mu¨nster, Germany
Reprint requests to Prof. Werner Uhl. E-mail: uhlw@uni-muenster.de
Z. Naturforsch. 2013, 68b, 503 – 517 / DOI: 10.5560/ZNB.2013-3070
Received February 27, 2013
Dedicated to Professor Heinrich No¨th on the occasion of his 85th birthday
Treatment of sterically highly shielded terminal alkynes, H–C≡C-aryl, with dialkylaluminium and
dialkylgallium hydrides, R2E–H, afforded by hydrogen release dimeric dialkylelement alkynides
with a four-membered E2C2 heterocycle independent of the bulk of the aryl groups. A rare exam-
ple of a monomeric alkynylaluminium compound was only obtained with very bulky CH(SiMe3)2
groups attached to the metal atoms and by salt elimination reaction. The steric shielding by the bulky
aryl groups did not prevent condensation reactions. Hydroalumination of 1-(trimethylsilyl)-2-(2,6-
dimethylphenyl)ethyne using Me2Al–H resulted in a divinyl compound by elimination of trimethyl-
aluminium.
Key words: Aluminium, Gallium, Hydroalumination, Hydrogallation, Alkynes
Introduction
droalumination and hydrogallation of phenyl-centred
tert-butylalkynes C6H6−n(C≡C–CMe3)n (n ≥ 2) with
Hydroalumination and to a smaller degree hydrogal- more than one alkyne group resulted in the case
lation reactions are important tools in organic synthe- of sterically shielded dialkylelement hydrides in the
sis and have often been applied for the regioselective elimination of ER3 and the formation of cyclophane-
reduction of unsaturated compounds with double or like condensation products while less shielded hy-
more importantly triple bonds by element hydrides drides gave one-dimensional coordination polymers
R2E–H (E = Al, Ga) [1 – 9]. The organic products are of the simple addition products [10]. The reactions
usually obtained after hydrolytic work-up and typ- of Ph–C≡C–CMe3 and phenyl-centred trimethylsily-
ically the organometallic intermediates are not iso- lalkynes C6H6−n(C≡C–SiMe3)n (n = 1 – 4) did not
lated. The intermediate hydrometalation products have result in condensation under mild conditions [10].
recently attracted considerable attention, and it was Hydrometalation of alkynes affords in the first step the
shown that the underlying chemistry is much more kinetically favoured cis-addition products which have
complex and interesting than postulated from the con- E and H atoms on the same side of the double bond
stitution of the isolated products after hydrolysis [10]. and rearrange in the absence of steric hindrance to the
It was shown that the reaction of sterically unhindered thermodynamically more stable trans-products (E and
alkylaluminium alkynides R2Al–C≡C-R0 with dialky- H trans to each other) [16].
laluminium hydrides R2Al–H led via dismutation and
Dialkylelement alkynides R2E–C≡C-R0 are accessi-
elimination of R3Al to carbaalanes which contain clus- ble on facile routes by the reaction of R0-C≡C–H with
ters of aluminium and carbon atoms with delocalised R2E–H or R3E by elimination of H2 or HR [17 – 19].
Al–C bonding interactions similar to the closely re- This method works well for relatively acidic alkynes
lated carbaboranes [11 – 14]. The related reactions of such as Ph–C≡C–H at low temperatures, while hy-
the corresponding gallium analogues were found to si- droalumination is observed as a competing reaction
milarly yield compounds with a Ga6C4 heteroadaman- with less acidic alkyl- or trimethylsilylethynes [20].
tane backbone and localised Ga–C bonds [15]. The hy- For these substrates, or for those with the sterically
© 2013 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
Scheme 1. Structural motifs of dimeric dialkylaluminium
alkynides.
demanding substituent R = CH(SiMe3)2, salt elimina-
tion by treatment of R0-C≡C–Li with Cl–ER2 is the
preferred method [17 – 19]. With very few exceptions
these alkynides are dimeric in the solid state with
two aluminium or gallium atoms bridged by the ter-
minal carbon atoms of two alkynido groups. Their
structures may be classified in two types A and B
(Scheme 1) [17 – 19]. Type A is found in the absence of
steric crowding for small substituents R, and type B for
bulky substituents R. An intermediate type of structure
that is somewhere between A and B was recently de-
scribed for alkynides with an 1,8-diethynylanthracene
backbone [21].
The present paper describes the reaction of steri-
cally encumbered aryl- and aryl(trimethylsilyl)ethynes
with element hydrides HER2. Within the scope of this (Me3C)3C6H2–C≡C–H independent of the stoichio-
study were the synthesis of sterically highly shielded metric ratio of the reactants only partial H2 elimina-
aluminium and gallium alkynides, the influence of the tion was observed resulting in adducts (2) of the ini-
substituents on the structures, in particular with respect tially formed alkynide and excess hydride [(ii), Eq. 1].
to the formation of monomeric species, and the modes Two metal atoms are bridged by one alkynide and
of reactivity with the possible generation of new types one hydride anion. For the relatively bulky dineopentyl
of carbaalane clusters or unsaturated heterocyclic com- derivatives (E = Al, Ga) 2a and 2b no further reac-
pounds by hydrometalation and condensation.
tion was observed even when the reaction time was
extended or the temperature raised. Prolonged heating
of 2b in the presence of an excess of (Me3CCH2)2Ga–
H to 70 ◦C resulted in the gradual formation of ele-
Results and Discussion
The dialkylelement alkynides [R2E–C≡C-R0]2 1a– mental gallium. In contrast, the adducts with the less
1f were easily accessible in moderate to high yields at bulky Et2Al or iBu2Al groups reacted further to yield
room temperature from the reaction of the correspond- over the course of several days the alkynides 1f and 1g
ing arylethynes R0-C≡C–H with hydrides R2E–H in [(iii), (i), Eq. 1]. The less reactive Et2GaH required re-
a stoichiometric 1 : 1 ratio [(i), Eq. 1]. As a result fluxing in benzene to give the alkynide 1h. The bulky
of the comparatively high acidity and the steric bulk (Me3C)2Al–H behaved again differently, and an inter-
of the acetylenes, low reaction temperatures were not mediate element hydride adduct analogous to 2 was not
necessary to prevent competing hydroalumination or observed. Instead prolonged stirring at room tempera-
-gallation reactions (c. f. refs. [17 – 19]). In case of ture resulted in the formation of compound 3 that fea-
the extremely shielded supermesitylacetylene 2,4,6- tures a bridging and a terminal alkynide group (Eq. 2),
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505
Table 1. Selected NMR (ppm) and
IR (cm−1) spectroscopic parameters
of the alkynyl compounds 1–4.
Compound
ECHx (1H) EH (1H) EC(sp3) (13C) EC≡C (13C) EC≡C (13C) C≡C (IR)
1a
1b
1c
1d
1e
1f
1g
1h
2a
2b
2c
2d
3a
0.06
–
–
−5.6
18.8
19.3
28.6
19.7
3.0
102.2
97.1
136.1
135.9
136.9
130.0
135.7
140.5
141.2
134.2
144.9
138.7
142.5
143.4
2062
2035
2050
2048
2019
2025
2025
2027
2029
2033
–
–
–
–
–
–
–
–
–
–
3.79
4.05
3.35
3.69
94.4
95.8
–
120.2
104.5
105.3
106.0
104.5
106.4
103.3
103.7
0.82
0.92
1.29
0.92
1.36
0.52
0.66/0.69
–
25.5
8.6
30.4
35.0
−1.0
22.0
2031
2010
2095
2108
3.60 17.9/18.6/19.7 101.8(brdg) 145.7(brdg)
115.6(term) 114.3(term)
4
−0.36
–
12.0
110.6
109.9
a
NMR at 230 K.
formed by hydrogen and concomitant isobutane elimi-
nation.
Previous investigations have shown the sterical- type A alkynides (Me2Al–C≡C–Ph 2089 cm−1 [11])
ly demanding dialkylaluminium hydride [(Me3Si)2- or terminal alkynes (Ph–C≡C–H 2119 cm−1, [23]).
CH]2Al–H to react even with relatively acidic alkynes The different structural motifs may be recognised in
such as Ph–C≡C–H exclusively by hydroalumina- the 13C NMR spectrum by a larger (except com-
tion [22]. The sterically encumbered alkynide 4 pound 1e) difference (≥ 30 ppm) between the chem-
was therefore synthesised using the salt elimination ical shifts of the carbon atoms of the C≡C triple
route (Eq. 3), which has previously been success- bond as compared to type A compounds (≤ 30 ppm)
ful in the preparation of alkynides that carry the with the signal of the metal bound carbon atom be-
Al[CH(SiMe3)2]2 substituent [17 – 19].
ing shifted significantly to higher field in compari-
The new dimeric alkynides [R2E–C≡C-R0]2 1 have son to that of the carbon atom bound to the sub-
type B structures in the solid state with symmetri- stituent R0. In terminal fragments, monomeric com-
cally bridging alkynide anions connecting two metal pounds (c. f. compound 4 and ref. [17 – 19]) or ter-
atoms via E–C–E 3c-2e bonds. In the related com- minal alkynes [24, 25] the difference is usually much
pounds 2 and 3 one of the alkynide groups is re- smaller. The characteristic shifts δ(1H) of the bridg-
placed by a bridging hydride anion. This is evident ing hydride anions in compounds 2 and 3 are found
from a modest lowering of the ν(C≡C) stretching between δ = 3 and 4. Further NMR parameters are
frequency in the IR spectrum (by 50 to 100 cm−1
)
summarised in Table 1. Compound 3 is fluxional in
of the bridging C≡C-R0 group as compared to ter- solution with the terminal and bridging alkyne frag-
minal alkynyl groups (Table 1, c. f. compound 3), ments and one tBu group on each Al atom rapidly ex-
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
Table 2. Selected structural parameters (pm, deg) of compounds 1–4.
Compound
E–C≡C
E–C0≡C0 E–C (av.)
E–H
C≡C
E–C–E0
E–H–E C–E–C0/H Puckering
ER2
angle
1a
1b
1c
209.8(2)
209.3(2)
205.9(2)
208.0(2)
208.5(2)
208.4(2)
206.6(2)
208.8(2)
209.4(2)
209.3(2)
212.6(2)
209.4(2)
208.6(2)
212.6(2)
213.9(2)
208.6(av)
192.1(3)
191.7(2)
203.7(2)
209.6(2)
212.6(2)
219.5(2)
208.5(2)
209.3(2)
208.7(2)
207.3(2)
207.5(2)
213.2(2)
209.6(2)
–
194.7
200.5
200.7
202.0
201.4
200.7
196.6
–
–
–
–
–
121.4(2)
121.8(2)
121.7(3)
121.6(2)
122.2(2)
122.2(2)
122.6(3)
122.0(3)
121.9(3)
122.5(2)
122.4(2)
122.4(3)
86.94(6)
88.28(6)
88.94(7)
89.22(6)
88.92(7)
87.89(6)
85.62(7)
85.45(7)
85.30(7)
85.50(6)
85.25(6)
81.27(7)
–
–
–
–
–
93.06(6)
91.72(6)
91.06(7)
90.78(6)
91.1(av)
92.09(6)
94.22(8)
93.99(8)
94.59(7)
94.33(7)
94.42(7)
87.7(8)
89.6(8)
90(1)
0
0
0
0
0
124.64(8)
118.53(8)
119.68(9)
121.82(7)
116.6(av)
117.69(9)
119.5(1)
118.0(1)
127.8(1)
124.0(1)
121.1(1)
128.7(av)
1d
1ea
1f
–
–
8.78
1g
1h
196.6
197.5
–
–
–
–
4.81
7.34
2a
2b
3b
197.2
198.0
179(2)
173(3)
169(3)
179(3)
169(3)
173(3)
–
101
104
105
–
(0.41)
(1.90)
(10.53)
–
–
–
–
122.1(3)
79.94(6)
81.4(1)
–
131.7(av)
86.5(9)
87(1)
85(1)
199.2
198.6
194.4
122.7(4)
121.3(4)
121.3(3)
122.5(2)
117.0(1)c
120.12(8)
4
–
a
Two independent molecules in the asymmetric unit; b second row values refer to the terminal alkinide; c this value refers to AlR(CCR0).
Fig. 1. Molecular structure and atomic numbering scheme of compound 1c. Displacement ellipsoids are drawn at the 40%
level. Hydrogen atoms have been omitted for clarity.
changing at room temperature. Surprisingly, the sec- as in the hydride adducts 2 and 3 (see representa-
ond CMe3 group in the Al(CMe3)2 fragment does not tive Figs. 1, 2 and 3). The alkynide/hydride anions are
participate in the exchange process. To slow down the bridging the metal atoms symmetrically with approx-
dynamic exchange and identify individual groups by imately equal bond lengths between the anions and
sharp resonances the NMR spectra were recorded at the metal atoms. The angles in the ring are close to
230 K.
90◦ with the CE2 angles being only slightly smaller
While compounds 1a–1e are all dimers with than the corresponding endocyclic EC2 angles at the
a symmetry-imposed planar four-membered Al2C2 metal atoms and significantly different from the larger
ring, a small puckering (5 – 9◦) is observed in (117 – 132◦) exocyclic ER2 angles. As expected, the
case of the extremely bulky supermesityl [2,4,6- exocyclic E–C bonds are much shorter than the endo-
(Me3C)3C6H2] derivatives 1f–1h (Table 2) as well cyclic 3c-2e E–C bonds. For steric reasons the planes
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
507
Fig. 2. Molecular structure and atomic numbering scheme of compound 2a. Displacement ellipsoids are drawn at the 40%
level. Hydrogen atoms (except H1; arbitrary radius) have been omitted for clarity.
Fig. 3. Molecular structure and atomic numbering scheme of compound 3. Displacement ellipsoids are drawn at the 40%
level. Hydrogen atoms (except H1; arbitrary radius) and methyl groups of the (tert-butyl) substituents on aluminium have
been omitted for clarity.
of the aromatic rings are approximately perpendicu-
Interestingly, the extreme steric shielding of the
lar (76 – 87◦ for 1, 65◦ for 2, 56◦ for 3) to the cen- ethynyl moieties by the bulky aryl groups does not in-
tral Al2C2 heterocycle. The C≡C bond lengths are fluence the structures of these alkynides, and the usual
unexceptional and only slightly longer than those re- dimeric formula units were observed in all cases. It
ported for Me–C≡C–Me (121.1 pm [26]) or H–C≡C– was only the very bulky CH(SiMe3)2 substituent at-
H (120.3 pm [27, 28]). The overall geometry and the tached to the aluminium atom of compound 4 (Fig. 4)
bond lengths are in good agreement with typical type which prevented dimerisation resulting in the forma-
B structures published previously [17 – 20, 29, 30].
tion of a rare example of a monomeric alkynide similar
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
Fig. 4. Molecular structure and atomic numbering scheme of compound 4. Displacement ellipsoids are drawn at the 40%
level. Hydrogen atoms have been omitted for clarity.
Fig. 5. Molecular structure and atomic numbering scheme of compound 5a. Displacement ellipsoids are drawn at the 40%
level. Hydrogen atoms (except H11, arbitrary radius) have been omitted for clarity.
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
509
Fig. 6. Molecular structure and atomic numbering scheme of compound 6. Displacement ellipsoids are drawn at the 40%
level. Hydrogen atoms (except H11, H21, arbitrary radius) have been omitted for clarity.
to related examples with the same substituent [17 – 19]. tion considerably and yielded the condensation product
The three-coordinate aluminium atom is in a planar 6 that is likely generated from the initial hydroalumi-
configuration (sum of angles 360◦) and has much nation product by elimination of AlMe3 (Eq. 4). The
shorter Al–C bonds (Table 2) than those found in the extended heating at higher temperature also led to an
dimeric species with four-coordinate aluminium atoms isomerisation of the cis-addition products to the ther-
discussed above. The Al–C distances are different modynamically more stable E-products in which the
with Al–C(sp) bonds (191.7(2) pm) expectedly being vinylic H and Al atoms are in trans positions to each
shorter than Al–C(sp3) bonds (194.4(2) pm).
other. Similar dismutation reactions between Me2E–H
The reaction of arylsilylethynes, R0-C≡C–SiMe3 (E = Al, Ga) and alkynes have been observed previ-
(R0 = aryl), with R2E–H has been investigated in some ously [16].
detail and was shown to lead in most cases to the re-
The configuration at the double bonds in solution
spective simple addition products [16 – 19, 31, 32]. To is evident from the characteristic 3JSi−H coupling con-
investigate the influence of steric bulk on the course stants [16] which are consistent with the formation of
of this reaction, 2-iPrC6H4-C≡C–SiMe3 was treated Z(5a)/E(5b)- (JSi−H ca. 20 Hz; Si trans to H) and E-
with (Me3C)2E–H (E = Al, Ga). While the reaction isomers (6, JSi−H ca. 12.5 Hz; Si cis to H). This was
proceeded smoothly at room temperature in case of confirmed by the solid-state structures (Figs. 5 and 6).
the aluminium hydride to give the corresponding ki- All compounds are monomeric with a planar three-
netically favoured cis-addition product (Eq. 4), the less coordinate metal atom (Σ of angles 360◦; Table 3) and
reactive gallium hydride required a double molar ex- nearly planar alkene fragments. Compounds 5 follow
cess of hydride and refluxing in hexane for three days the expected trend with E–C(sp2) bond lengths be-
to yield the respective gallium derivative (Eq. 4). Re- ing shorter than the corresponding E–C(sp3) distances.
ducing the steric bulk of the aluminium hydride using Surprisingly this trend is reversed in case of compound
Me2AlH and increasing the reaction temperature (three 6 probably caused by the increased steric bulk in these
days reflux in hexane) changed the course of the reac- compound (c. f. [31, 32]). Other bond lengths and an-
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
Table 3. Selected structural parameters (pm, deg) of compounds 5–6.
Compound
5a
E-alkyl
E–C=C
196.4(2)
C=C
134.5(3)
Σ angles (El) d (E to C3)a
E–C=C–C(aryl)
−178.7(2)
199.2(2)
199.4(2)
200.6(2)
201.0(2)
194.6(2)
359.9
360.0
360.0
3.1
1.8
0.2
5b
198.4(2)
134.3(3)
179.1(2)
6a
195.9(2)
196.1(2)
197.8(2)/198.7(2)
197.9(3)/198.0(3)
134.1(2)
133.9(2)
135.0(3)/134.0(3)
134.1(4)/133.4(4)
6.3(2)/4.5(2)
6bb
194.7(3)
193.9(3)
360.0
359.9
3.4
3.6
−7.8(3)/−7.0(3)
−4.6(3)/−3.8(4)
a
Deviation of E from the average plane of the connecting carbon atoms; b two independent molecules.
pend on steric shielding and result in the products of
the expected hydroalumination or the thermally in-
duced condensation.
Experimental Section
All procedures were carried out under an atmosphere
of purified argon in dried solvents (n-hexane, cyclopen-
tane and n-pentane with LiAlH4; benzene with Na, 1,2-
difluorobenzene and pentafluorobenzene with molecular
sieves). NMR spectra were recorded in C6D6 or C7D8
at ambient probe temperature (except compound 3) us-
ing the Bruker instruments Avance I (1H, 400.13; 13C,
100.62; 29Si, 79.49 MHz) or Avance III (1H, 400.03; 13C,
100.59; 29Si 79.47 MHz) and referenced internally to resid-
ual solvent resonances (chemical shift data in δ). 13C
NMR spectra were all proton decoupled. IR spectra were
recorded as Nujol mulls between CsI plates on a Shi-
madzu Prestige 21 spectrometer. ClAl[CH(SiMe3)2]2 [34],
Me2AlH [35], (Me3C)2AlH [36], (Me3CCH2)2AlH [37],
(Me3C)2GaH [36], 2-iPrC6H4-C≡C–H [38], 2,6-Me2C6H3-
C≡C–SiMe3 [39, 40], 2,4,6-iPr3C6H2-C≡C–H [41], 2-
iPrC6H4-C≡C–SiMe3 [38], 2,6-Me2C6H3-C≡C–H [40] and
2,4,6-iPr3C6H2-C≡C–Li [41] were obtained according to lit-
erature procedures. 2,4,6-(Me3C)3C6H2-C≡C–H [42] was
prepared from 2,4,6-(Me3C)3C6H2Br [42, 43] via 2,4,6-
(Me3C)3C6H2-C≡C–CO2–H [42] which was, in a variation
of the procedure described in ref. [42], decarboxylated at
260 ◦C and sublimed in vacuo at 200 ◦C. Et2GaH was syn-
thesised from Et2GaCl and Li[Et2GaH2] [44]. The starting
material Et2GaCl [45, 46] previously obtained from GaCl3
and GaEt3 [47] (synthesised from EtLi and GaCl3), was iso-
lated in high yield from the reaction of commercially avail-
able ZnEt2 (Sigma Aldrich) and GaCl3 (see below). Et2AlH
and iBu2Al–H were purchased from MOCHEM GmbH,
Marburg, Germany, and used without further purification.
The assignment of NMR spectra is based on HMBC, HSQC
gles are unexceptional and may be compared to those
of previously described related aluminium and gallium
derivatives [31 – 33].
We have synthesised a large number of different
dialkylaluminium and dialkylgallium alkynides with
bulky substituents bound to the ethynyl groups. Inde-
pendent of steric shielding, dimeric compounds were
formed in which two metal atoms are bridged by
two alkynido groups. A rare example of a monomeric
dialkylaluminium alkynide was only obtained with
two bulky CH(SiMe3)2 groups bound to aluminium.
This observation clearly underscores that the bulki-
ness of the substituents at aluminium is much more
important for the resulting structure than the sub-
stituents at the ethynyl group which obviously are
too far from the coordinatively unsaturated aluminium
atoms to prevent dimerisation via donor-acceptor in-
teractions. However, the secondary reactions with ex-
cess dialkylaluminium hydrides are influenced, and
in no case did we observe the addition of an Al–
H bond to the C≡C triple bonds of these alu-
minium alkynides. Also the reactions of trimethylsilyl-
arylethynes with dialkylaluminium hydrides do not de- and DEPT135 data.
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511
Improved synthesis of Et2GaCl
300 K): δ = 18.8 (br., CMe3), 22.1 (aryl-Me), 32.1 (CMe3),
97.1 (AlC≡C), 119.8 (ipso-C), 127.9 (m-C), 132.1 (p-C),
135.9 (AlC≡C), 145.5 (o-C). – MS (EI, 20 eV, 453 K): m/z
(%) = 270 (18) [1/2M]+, 213 (100) [1/2M–CMe3]+.
A solution of ZnEt2 (36 mL, 36 mmol, 1 M in hexane) was
added dropwise to a solution of GaCl3 (6.34 g, 36 mmol)
in hexane (20 mL) at room temperature. The reaction mix-
ture was heated and stirred under reflux conditions for 2 h.
The precipitated ZnCl2 was removed by filtration, the fil-
trate was collected and the solvent removed in vacuo to yield
Et2GaCl as a pale-yellow, analytically pure liquid. Yield:
5.88 g (100%). – 1H NMR: (400 MHz, C6D6): δ = 0.80 (q,
Synthesis of [(Me3C)2Al-µ-C≡C-2-iPrC6H4]2, 1c
A solution of (Me3C)2AlH (0.467 g, 3.29 mmol) in n-
hexane (15 mL) was added at room temperature to a solu-
tion of HC≡C-2-iPrC6H4 (0.448 g, 3.11 mmol) in n-hexane
(10 mL), and the mixture was stirred for 22 h. The solu-
tion was concentrated and cooled to −5 ◦C to yield colour-
less to pale-yellow crystals of compound 1c. Yield: 0.667 g
(76%). – M. p. (under argon; sealed capillary): 184 ◦C (dec.).
– IR (CsI, paraffin): ν = 2050 vs, 2023 sh ν(C≡C), 1929
vw, 1850 vw, 1817 vw, 1707 vw 1628 vw, 1593, 1580 s,
1566 m (phenyl); 1462 vs, 1375 vs (paraffin); 1362 s, 1306
vw, 1277vw δ(CH3); 1217 w, 1177 w, 1103 vw, 1080 s,
1030 w, 1003 w, 937 w, 891 m, 812 s, 756 s ν(CC); 720
(paraffin); 667 vw, 644 vw (phenyl); 588 s 536 vs, 478
vw, 442 w cm−1δ(CC), ν(AlC). – 1H NMR (400 MHz,
3JH−H = 8.0 Hz, CH2, 4 H), 1.18 (t, JH−H = 8.0 Hz, CH3,
3
6H) [46].
Synthesis of (Me2Al-µ-C≡C-2,6-Me2C6H3)2, 1a
A solution of HC≡C-2,6-Me2C6H3 (0.451 g, 3.47 mmol)
in n-hexane (10 mL) was added at room temperature to a so-
lution of Me2AlH (0.201 g, 3.47 mmol) in n-hexane (10 mL),
and the mixture was stirred for 22 h and then concentrated
and cooled to −5 ◦C to give yellow crystals of compound
1a. Yield: 0.396 g (61%). – M. p. (under argon; sealed capil-
lary): 215 ◦C (dec.). – IR (CsI, paraffin): ν = 2062 vs, 2018
m ν(C≡C), 1879 w, 1680 w, 1593 w, 1574 w (phenyl); 1466
vs, 1379 vs (paraffin); 1302 w, 1279 w, 1250 vw δ(CH3);
1182 s, 1165 s, 1088 w, 1030 m, 843 w, 810 w, 781 w, 770
w ν(CC); 721 vs (paraffin); 694 s (phenyl); 579 w, 557 m,
505 w cm−1 ν(AlC). – 1H NMR (400 MHz, C6D6, 300 K):
δ = 0.06 (s, 6H, AlCH3), 2.37 (s, 6H, aryl-CH3), 6.68 (d, 2H,
3JH−H = 7.6 Hz, m-H), 6.86 (t, 1H, 3JH−H = 7.6 Hz, p-H). –
13C NMR (100 MHz, C6D6, 300 K): δ = −5.6 (AlMe), 21.0
(aryl-CH3), 102.2 (AlC≡C), 120.4 (ipso-C), 127.4 (m-C),
3
C6D6, 300 K): δ = 1.21 (d, 6H, JH−H = 6.9 Hz, CHMe2),
3
1.46 (s, 18H, CMe3), 3.95 (sept, 1H, JH−H = 6.9 Hz,
CHMe2), 6.78 (m, 1H, m-H(5)), 6.99 (m, 1H, m-H(3)),
3
7.00 (m, 1H, p-H), 7.78 (d, 1H, JH−H = 7.7 Hz, o-H). –
13C NMR (100 MHz, C6D6, 300 K): δ = 19.3 (br., CMe3),
23.9 (CHMe2), 32.0 (CMe3), 32.1 (CHMe2), 94.4 (AlC≡C),
118.7 (ipso-C), 126.1 (m-C(3)), 126.5 (m-C(5)), 132.7 (p-
C), 136.2 (o-C(6)), 136.9 (AlC≡C), 154.8 (o-C(2)). – MS
(EI, 20 eV, 393 K): m/z(%) = 511 (5) [M–CMe3]+, 393 (3)
[M–butene–C6H4iPr]+, 284 (3) [1/2M]+, 227 (13) [1/2M–
CMe3]+, 144 (65) [alkyne]+.
131.3 (p-C), 136.1 (AlC≡C), 144.2(o-C). – MS (EI, 20 eV,
298 K): m/z (%) = 357 (13) [M–CH3]+, 186 (4) [1/2M]+,
130 (100) [alkyne]+.
Synthesis of [(Me3C)2Ga-µ-C≡C-2-iPrC6H4]2, 1d
A solution of (Me3C)2GaH (0.782 g, 4.23 mmol) in n-
hexane (10 mL) was added at room temperature to a solu-
Synthesis of [(Me3C)2Al-µ-C≡C-2,6-Me2C6H3]2, 1b
A solution of (Me3C)2AlH (0.685 g, 4.82 mmol) in n- tion of HC≡C-2-iPrC6H4 (0.589 g, 4.09 mmol) in n-hexane
hexane (20 mL) was added at room temperature to a solution (10 mL), and the mixture was stirred for 21 h and then con-
of HC≡C-2,6-Me2C6H3 (0.628 g, 4.83 mmol) in n-hexane centrated and cooled to 5 ◦C to yield colourless crystals
(5 mL), and the mixture was stirred for 26 h. A colour- of compound 1d. Yield: 0.773 g (58%). – M. p. (under ar-
less solid precipitated which was isolated and recrystallised gon; sealed capillary): 165 ◦C (dec.). – IR (CsI, paraffin):
from 1,2-difluorobenzene at 5 ◦C to give pale-yellow crys- ν = 2048 vs, 2018 s sh ν(C≡C); 1956 vw, 1925 vw, 1898
tals of compound 1b. Yield: 1.11 g (85%). – M. p. (under vw, 1846 vw, 1815 vw, 1744 w, 1709 w, 1680 w, 1653 w,
argon; sealed capillary): 156 ◦C (dec.). – IR (CsI, paraffin): 1626 w, 1593 s, 1576 m, 1532 vw, 1516 w (phenyl); 1462
ν = 2035 m, 2000 sh ν(C≡C); 1744 vw, 1694 vw, 1589 vs, 1379 vs (paraffin); 1362 vs, 1310 vw, 1273 w, 1248 vw
vs, 1580 vs, 1560 vs (phenyl); 1461 vs (paraffin); 1402 s δ(CH3); 1215 w, 1184 vs, 1165 vs, 1103 vw, 1080 s, 1011
δ(CH3); 1375 vs (paraffin), 1304 w, 1275 w δ(CH3); 1194 vs, 941 s, 893 w, 839 w, 810 vs, 756 vs ν(CC); 715 vs
m, 1167 s, 1117 w, 1090 s, 1045 m, 999 vw, 934 w, 891 vw, (paraffin); 654 vw (phenyl); 615 w, 590 w, 571 w, 532 vs,
814 s, 772 w ν(CC); 721 vs (paraffin); 654 m (phenyl); 590 509 s cm−1 δ(CC), ν(GaC). – 1H NMR (400 MHz, C6D6,
3
m, 563 w, 507 s, 444 s cm−1 δ(CC), ν(AlC). – 1H NMR 300 K): δ = 1.23 (d, 6H, JH−H = 7.0 Hz, CHMe2), 1.51
3
(400 MHz, C6D6, 300 K): δ = 1.41 (s, 18H, CMe3), 2.60 (s, (s, 18H, CMe3), 4.00 (sept, 1H, JH−H = 7.0 Hz, CHMe2),
3
6H, aryl-Me), 6.69 (d, 2H, JH−H = 7.6 Hz, m-H), 6.86 (t, 6.83 (m, 1H, m-H(5)), 7.02 (m, 1H, m-H(3)), 7.04 (m, 1H,
1H, JH−H = 7.6 Hz, p-H). – 13C NMR (100 MHz, C6D6, p-H), 7.77 (d, 1H, JH−H = 7.7 Hz, o-H). – 13C NMR
3
3
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
(100 MHz, C6D6, 300 K): δ = 24.0 (CHMe2), 28.6 (CMe3), (AlCH2), 9.7 (CH2CH3), 31.1 (p-CMe3), 31.6 (o-CMe3),
32.0 (CHMe2), 32.3 (CMe3), 95.8 (GaC≡C), 120.3 (ipso-C), 35.6 (p-CMe3), 37.4 (o-CMe3), 104.5 (AlC≡C), 115.2 (ipso-
125.9 (m-C(3)), 126.4 (m-C(5)), 130.0 (GaC≡C), 131.6 (p- C), 121.7 (m-C), 140.5 (AlC≡C), 154.2 (p-C), 158.5 (o-C).
C), 135.9 (o-C(6)), 154.1 (o-C(2)). – MS (EI, 20 eV, 353 K): – MS (EI, 20 eV, 413 K): m/z(%) = 354 (3) [1/2M]+, 270
m/z(%) = 327 (4) [1/2M]+, 269 (100), 271 (66) [M1/2
(tBu)]+, 183 (3), 185 (2) [Ga(tBu)2]+.
–
(88) [alkyne]+.
Synthesis of [iBu2Al-µ-C≡C-2,4,6-(Me3C)3C6H2]2, 1g
iBu2AlH (0.158 g, 1.11 mmol) was added at room
Synthesis of [(Me3C)2Al-µ-C≡C-2,4,6-iPr3C6H2]2, 1e
A solution of (Me3C)2AlH (0.464 g, 3.27 mmol) in n- temperature to a solution of HC≡C-2,4,6-(Me3C)3C6H2
hexane (20 mL) was added at room temperature to a solu- (0.300 g, 1.11 mmol) in n-pentane (10 mL), and the reaction
tion of HC≡C-2,4,6-iPr3C6H2 (0.720 g, 3.16 mmol) in n- mixture was stirred for 12 h. The solvent was removed in
hexane (15 mL), and the mixture was stirred for 22 h. The vacuo and recrystallised from pentafluorobenzene at −20 ◦C
solution was concentrated and cooled to −45 ◦C to yield to yield colourless crystals of 1g. Yield: 0.092 g (20%);
colourless crystals of compound 1e. Yield: 0.846 g (73%). pure samples can only be obtained with difficulties by re-
– M. p. (under argon; sealed capillary): 194 ◦C (dec.). – IR peated recrystallisation. – M. p. (under argon; sealed cap-
(CsI, paraffin): ν = 2019 vs ν(C≡C); 1775 vw, 1653 w, illary): 176 ◦C (dec.). – IR (CsI, paraffin): ν = 2025 w
1605 vs, 1578 vs, 1558 vs (phenyl); 1460 vs (paraffin); 1404 ν(C≡C); 1599 w (phenyl); 1458 vs, 1377 vs (paraffin); 1306
m δ(CH3); 1337 vs (paraffin); 1315 m, 1252 w δ(CH3); m, 1280 m δ(CH3); 1219 w, 1169 m, 1155 m, 1059 w,
1173 m, 1152 w, 1113 s, 1070 m, 1057 w, 1001 m, 941 1009 w, 964 m, 932 m, 891 w, 845 w ν(CC); 721 s paraf-
m, 880 s, 847 w, 812 vs, 774 w ν(CC); 723 vs (paraf- fin); 673 w (phenyl); 629 w, 594 w, 561 w, 515 w, 455 w
fin); 656 w (phenyl); 584 m, 561 s, 513 s, 461 m, 434 cm−1 δ(CC), ν(AlC). – 1H NMR (400 MHz, C6D6, 300 K):
3
m cm−1 δ(CC), ν(AlC). – 1H NMR (400 MHz, C6D6, δ = 0.92 (d, 4H, JH−H = 7.1 Hz, AlCH2), 1.16 (s, 9H, p-
300 K): δ = 1.09 (d, 6H, 3JH−H = 6.9 Hz, p-CHMe2), 1.34 CMe3), 1.19 (d, 12H, JH−H = 6.5 Hz, CHMe2), 1.76 (s, 18H,
(d, 12H, JH−H = 6.8 Hz, o-CHMe2), 1.45 (s, 18H, CMe3), o-CMe3), 2.29 (m, 2H, CHMe2), 7.50 (s, 2H, m-CH). – 13
C
3
3
2.65 (sept, 1H, JH−H = 6.9 Hz, p-CHMe2), 4.21 (sept, NMR (100 MHz, C6D6, 300 K): δ = 25.5 (AlCH2), 26.4
3
2 H, JH−H = 6.8 Hz, o-CHMe2), 7.06 (s, 2 H, m-H). – (CHMe2), 28.5 (CHMe2), 31.0 (p-CMe3), 32.0 (o-CMe3),
13C NMR (100 MHz, C6D6, 300 K): δ = 19.7 (br, CMe3), 35.5 (p-CMe3), 37.6 (o-CMe3), 105.3 (AlC≡C), 115.6 (ipso-
23.6 (p-CHMe2), 24.8 (o-CHMe2), 32.2 (CMe3), 32.2 (o- C), 121.7 (m-C), 141.2 (AlC≡C), 154.0 (p-C), 158.5 (o-C).
CHMe2), 35.0 (p-CHMe2), 115.2 (ipso-C), 120.2 (AlC≡C),
Synthesis of [Et2Ga-µ-C≡C-2,4,6-(Me3C)3C6H2]2, 1h
122.1 (m-C), 135.7 (AlC≡C), 154.2 (p-C), 156.6 (o-C). –
MS (EI, 20 eV, 393 K): m/z(%) = 368 (8) [1/2M]+, 311 (24)
A
solution of HC≡C-2,4,6-(Me3C)3C6H2 (0.322 g,
[1/2M–CMe3]+.
1.19 mmol) in benzene (5 mL) was added to a solution of
Et2GaH (0.307 g, 2.39 mmol), and the mixture was stirred
under reflux conditions for 3.5 h. The solvent was re-
Synthesis of [Et2Al-µ-C≡C-2,4,6-(Me3C)3C6H2]2, 1f
Et2AlH (0.128 g, 1.49 mmol) was added at room temper- moved in vacuo, and the residue was recrystallised from 1,2-
ature to a solution of HC≡C-2,4,6-(Me3C)3C6H2 (0.401 g, difluorobenzene at 5 ◦C to give colourless crystals of com-
1.49 mmol) in n-pentane (10 mL), and the reaction mixture pound 1h. Yield: 244 mg, (52%); samples of 1h may con-
was stirred for three days. The solvent was removed in vacuo, tain the terminal alkyne as an impurities. – M. p. (under ar-
and the residue was recrystallised from 1,2-difluorobenzene gon; sealed capillary): 147 – 150 ◦C (dec.). – IR (CsI, paraf-
at 2 ◦C to yield colourless crystals of compound 1f. Yield: fin): ν = 2027 s, 2000 sh ν(C≡C); 1769 w, 1599 s, 1535 w
0.451 g (86%); the product 1f may contain the alkyne as (phenyl); 1464 s, 1454 s (paraffin); 1418 w δ(CH3); 1375
an impurity. – M. p. (under argon; sealed capillary): 173 ◦C m (paraffin); 1362 m, 1308 m, 1292 m δ(CH3); 1234 vs,
(dec.). – IR (CsI, paraffin): ν = 2025 s ν(C≡C); 1769 w, br., 1153 vs, 1140 s, 1115 s, 1103 sh, 1026 w, 984 s, 928 w,
1599 s, 1508 s (phenyl); 1462 vs, 1377 vs (paraffin); 1364 880 m, 795 w, 750 w ν(CC); 721 w (paraffin); 692 vw, 640
s, 1269 m δ(CH3); 1219 m, 1101 w, 1053 w, 1024 vw, m (phenyl; 554 w, 509 s, 440 m cm−1 δ(CC), ν(GaC). –
974 w, 949 w, 928 w, 881 m, 795 w, 750 m ν(CC); 725 1H NMR (400 MHz, C6D6): δ = 1.25 (s, 9H, p-CMe3), 1.29
w (paraffin); 698 w, 658 m (phenyl); 629 m, 611 m, 594 m, (q, 4H, 3JH−H = 7.9 Hz, CH2), 1.48 (t, 6H, 3JH−H = 7.9 Hz,
567 m, 532 w, 513 w, 451 m, 434 w cm−1 δ(CC), ν(AlC). CH2Me), 1.72 (s, 18H, o-CMe3), 7.51 (s, 2H, m-H). – 13C
–
1H NMR (400 MHz, C6D6, 300 K): δ = 0.82 (q, 4 H, NMR (100 MHz, C6D6): δ = 8.6 (CH2), 11.1 (CH2Me),
3JH−H = 8.1 Hz, CH2), 1.21 (s, 9H, p-CMe3), 1.46 (t, 6H, 31.2 (p-CMe3), 31.4 (o-CMe3), 35.5 (p-CMe3), 37.4 (o-
3JH−H = 8.1 Hz, CH2CH3), 1.70 (s, 18H, o-CMe3), 7.50 (s, CMe3), 106.0 (br., GaC≡C), 116.7 (ipso-C), 121.5 (m-C),
2H, m-CH). – 13C NMR (100 MHz, C6D6, 300 K): δ = 3.0 134.2 (br., GaC≡C), 152.8 (p-C), 157.2 (o-C). ). – MS (EI,
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
513
20 eV, 303 K): m/z(%) = 396 (0.5) [1/2M]+, 367 (9), 369 1.19 (s, 9H, p-CMe3), 1.24 (s, 36H, CH2CMe3), 1,36 (s, br.,
(6) [1/2M–CH3]+, 270 (56) [alkyne]+.
8H, CH2), 1.70 (s, 18H, o-CMe3), 4.05 (s, br., 1H, GaHGa),
7.48 (s, 2H, m-H). – 13C NMR (100 MHz, C6D6, 275 K):
δ = 31.1 (p-CMe3), 31.4 (o-CMe3), 32.1 (br., CH2CMe3),
34.3 (br., CH2CMe3), 35.0 (CH2), 35.4 (p-CMe3), 37.4 (o-
CMe3), 106.4 (br., GaC≡C), 116.8 (ipso-C), 121.5 (m-C),
138.7 (br., GaC≡C), 152.7 (p-C), 156.6 (o-C). – MS (EI,
20 eV, 333 K): m/z (%) = 480 (1), 482 (1) [M–HGaNp2]+,
423 (1), 425 (1) [M–HGaNp2–CMe3]+, 409 (21), 411 (14)
[M–HGaNp2–CH2CMe3]+, 270 (86) [alkyne]+.
Synthesis of
[(Me3CCH2)2Al]2(µ-H)[µ-C≡C-2,4,6-(Me3C)3C6H2], 2a
A solution of (Me3CCH2)2AlH (0.342 g, 2.01 mmol) in
n-pentane (20 mL) was added at room temperature to a so-
lution of HC≡C-2,4,6-(Me3C)3C6H2 (0.272 g, 1.01 mmol)
in n-pentane (5 mL), and the mixture was stirred for two
days. The solvent was removed in vacuo, and the residue was
recrystallised from pentafluorobenzene at −30 ◦C to give
compound 2a. Yield: 0.475 g (78%). – M. p. (under argon;
sealed capillary): 94 ◦C (dec.). – IR (CsI, paraffin): ν = 2029
s, 2000 sh ν (C≡C); 1771 w ν (AlHAl); 1645 w, 1597
s, 1533 m (phenyl); 1460 vs, 1377 vs (paraffin); 1364 vs,
Synthesis of (Et2Al)2(µ-H)[µ-C≡C-2,4,6-(Me3C)3C6H2],
2c
Et2AlH (0.237 g, 2.76 mmol) was added at room temper-
1306 s, 1270 s δ (CH3); 1221 vs, 1125 m, 1099 m, 1070 ature to a solution of HC≡C-2,4,6-(Me3C)3C6H2 (0.285 g,
w, 999 s, 932 s, 883 s, 781 s, 745 s ν (CC); 723 s (paraf- 1.06 mmol) in n-hexane (10 mL), and the reaction mixture
fin); 677 s (phenyl); 633 s, 594 m, 554 w, 517 m, 463 m was stirred for 12 h. The solvent was removed in vacuo
cm−1 δ (CC), ν (AlC). – 1H NMR (400 MHz, C6D6, 300 K): to give a yellow oil which showed only the signals of 2c
1
δ = 0.92 (s, 8 H, CH2), 1.17 (s, 9H, p-CMe3), 1.27 (s, 36 and of the excess of Et2AlH. – H NMR (400 MHz, C6D6,
H, CH2CMe3), 1.64 (s, 18 H, o-CMe3), 3.79 (s, br., 1H, 300 K): δ = 0.52 (dq, 8H, 3JH−H = 3.4 Hz, 3JH−H = 8.0 Hz,
3
AlHAl), 7.45 (s, 2H, m-H). – 13C NMR (100 MHz, C6D6, CH2), 1.21 (s,9H, p-CMe3), 1.35 (t, 12H, JH−H = 8.0 Hz,
300 K): δ = 30.4 (s, br., CH2), 31.0 (p-CMe3), 31.7 (o- CH2CH3), 1.55 (s, 18H, o-CMe3), 3.35 (s, br., 1H, AlHAl),
CMe3), 31.8 (CH2CMe3) 35.2 (CH2CMe3), 35.6 (p-CMe3), 7.43 (s,2H, m-CH). – 13C NMR (100 MHz, C6D6, 300 K):
37.4 (o-CMe3), 104.5 (AlC≡C), 115.1 (ipso-C), 121.7 (m- δ = 3.0 (AlCH2), 9.7 (CH2Me), 31.1 (p-CMe3), 31.6 (o-
C), 144.9 (AlC≡C), 154.3 (p-C), 157.8 (o-C).
Synthesis of
[(Me3CCH2)2Ga]2(µ-H)[µ-C≡C-2,4,6-(Me3C)3C6H2], 2b
CMe3), 35.6 (p-CMe3), 37.4 (o-CMe3), 104.5 (AlC≡C),
115.2 (ipso-C), 121.7 (m-C), 140.5 (AlC≡C), 154.2 (p-C),
158.5 (o-C).
Synthesis of (iBu2Al)2(µ-H)[µ-C≡C-2,4,6-(Me3C)3C6H2],
A solution of (Me3CCH2)2GaH (0.342 g, 1.61 mmol) in
n-hexane (10 mL) was added at room temperature to a so-
lution of HC≡C-2,4,6-(Me3C)3C6H2 (0.163 g, 0.604 mmol)
in n-hexane (10 mL), and the mixture was stirred under re-
2d
iBu2AlH (0.292 g, 2.06 mmol) was added at room
flux conditions for one day and filtered to remove traces temperature to a solution of HC≡C-2,4,6-(Me3C)3C6H2
of elemental Ga. The solvent was removed in vacuo, and (0.278 g, 1.03 mmol) in difluorobenzene (2 mL), and the re-
the residue was recrystallised from 1,2-difluorobenzene at action mixture was stirred for 12 h. The solvent was removed
−20 ◦C to give colourless crystals of compound 2b. Yield: in vacuo to give 2d as an analytically pure colourless oil.
0.371 g (89%); the samples generally contain small quanti- – IR (CsI, paraffin): ν = 2031 vs ν(C≡C); 1769 w, 1695
ties of the free alkyne. – M. p. (under argon; sealed capillary): w, br. ν(AlHAl); 1599 s, 1537 w, 1508 w (phenyl); 1460
128 ◦C (dec.). – IR (CsI, paraffin): ν = 2033 w ν(C≡C), vs (paraffin); 1423 s, 1395 s δ(CH3); 1375 s (paraffin);
1769 vw, 1700 w, br., 1597 m, 1533 w (phenyl), ν(GaHGa); 1362 vs, 1319 s, 1267 m, 1248 m δ(CH3); 1219 s, 1200
1466 vs, 1377 vs (paraffin); 1306 s, 1270 s δ(CH3); 1233 m, 1180 s, 1159 m, 1115 w, 1065 s, 1018 vs, 945 m, 910 w,
m, 1167 vw, 1136 vw, 1099 w, 1003 w, 930 w, 883 w, 845 880 m, 820 s, 806 s, 775 s ν(CC); 731 m (paraffin); 665 s,
w, 770 s ν(CC); 723 vs (paraffin); 625 w, 594 w, 544 vw, br. (phenyl); 505 vw, 476 vw, 451 w, 426 w cm−1 δ(CC),
513 w, 455 m cm−1δ(CC), ν(GaC). – H NMR (400 MHz, ν(AlC). – 1H NMR (400 MHz, C6D6, 300 K): δ = 0.66
1
2
3
C6D6, 300 K): δ = 1.22 (s, 45 H, p-CMe3 and CH2CMe3), (dd, 4H, JH−H = 14.0 Hz, JH−H = 7.2 Hz, AlCH2), 0.69
2
3
1.32 (s, br., 8H, CH2), 1.72 (s, 18H, o-CMe3), 4.06 (s, br., (dd, 4H, JH−H = 14.0 Hz, JH−H = 6.8 Hz, AlCH2), 1.17
1H, GaHGa), 7.49 (s, 2H, m-H). – 13C NMR (100 MHz, (d, 12H, JH−H = 6.5 Hz, CHMe2), 1.19 (d, 12H, JH−H
=
C6D6, 300 K): δ = 31.2 (p-CMe3), 31.4 (o-CMe3), 32.1 (br., 6.5 Hz, CHMe2), 1.20 (s, 9H, p-CMe3), 1.60 (s, 18H, o-
CH2CMe3), 34.3 (br., CH2CMe3), 35.5 (p-CMe3), 35.6 (br., CMe3), 2.17 (m, 4H, CHMe2), 3.69 (s br, 1H, AlHAl),
CH2), 37.4 (o-CMe3), 117.2 (ipso-C), 121.4 (m-C), 152.3 (p- 7.44 (s, 2H, m-CH). – 13C NMR (100 MHz, C6D6, 300 K):
1
C), 156.2 (o-C). – H NMR (400 MHz, C6D6, 275 K): δ = δ = 22.0 (AlCH2), 26.8 (CHMe2), 28.2 and 28.3 (CHMe2),
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
31.1 (p-CMe3), 31.5 (o-CMe3), 35.6 (p-CMe3), 37.3 (o- (under argon; sealed capillary): 103 ◦C. – IR (CsI, paraffin):
CMe3), 103.7 AlC≡C), 114.8 (ipso-C), 121.7 (m-C), 143.4 ν = 2108 vs, 2074 sh ν(C≡C); 1927 w, 1883 vw, 1856
(AlC≡C), 154.6 (p-C), 158.5 (o-C).
w, 1765 w, 1730 vw, 1630 w, 1603 s, 1562 w, 1541 w
(phenyl); 1443 vs, 1379 vs (paraffin); 1361 vs, 1316 s,
1248 vs δ(CH3); 1209 m, 1175 s, 1107 s, 1072 w, 1055
w ν(CC); 1018 vs δ(CHSi2); 920 s, 843 vs, 774 s, 752 s
ρ(CH3(Si)); 723 s (paraffin); 671 s, 656 m, 637 m, 611
w ν(SiC), phenyl: 569 m, 550 w, 519 m, 503 m, 461 w
cm−1 δ(CC), ν(AlC). – 1H NMR (400 MHz, C6D6):
δ = −0.36 (s, 2H, CH), 0.34 (s, 36H, SiMe3), 1.21 (d, 6H,
3JH−H = 7.0 Hz, p-CHMe2), 1.36 (d, 12H, 3JH−H = 6.9 Hz,
Synthesis of (Me3C)2Al(µ-H)[µ-C≡C-2,4,6-
(Me3C)3C6H2]Al(CMe3)[C≡C-2,4,6-(Me3C)3C6H2], 3
A solution of (Me3C)2AlH (0.246 g, 1.73 mmol) in n-
pentane (10 mL) was added at room temperature to a so-
lution of HC≡C-2,4,6-(Me3C)3C6H2 (0.460 g, 1.70 mmol)
in n-pentane (10 mL), and the reaction mixture was stirred
for 3 d. The solvent was removed in vacuo and the residue
recrystallised from 1,2-difluorobenzene at −20 ◦C to give
colourless crystals of compound 3. Yield: 0.55 g (85%); im-
purity of the alkyne. – M. p. (under argon; sealed capillary):
78 ◦C (dec.). – IR (CsI, paraffin): ν = 2010 m ν(C≡C); 1771
w, 1601 m, 1537 vw ν(AlHAl), (phenyl); 1458 vs, 1375 vs
(paraffin); 1302 m, 1269 w, 1248 w δ(CH3); 1219 m, 1200
vw, 1169 w, 1153 w, 1115 w, 1032 vw, 1003 w, 932 m, 880
m, 847 vw, 810 m, 773 m ν(CC); 723 s (paraffin); 656 w
(phenyl); 631 w, 592 m, 554 m, 517 w, 476 vw, 453 w cm−1
δ(CC), ν(AlC). – 1H NMR (400 MHz, C7D8, 230 K): δ =
1.20 (s, 9H, p-CMe3, bridge), 1.35 (s, 9H, p-CMe3, termi-
nal), 1.45 (s, 9H, CMe3 (exchange), Al(CMe3)2), 1.47 (s, 9H,
3
o-CHMe2), 2.77 (sept, 1H, JH−H = 7.0 Hz, p-CHMe2,
3
1H), 3.98 (sept, 2H, JH−H = 6.9 Hz, o-CHMe2), 7.09 (s,
2H, m-H). – 13C NMR (100 MHz, C6D6): δ = 4.0 (SiMe3),
12.0 (AlCH), 23.9 (o-CHMe2), 24.1 (p-CHMe2), 32.2
(o-CHMe2), 35.0 (p-CHMe2), 109.9 (AlC≡C), 110.6 (br.,
AlC≡C), 119.4 (ipso-C), 120.8 (m-C), 149.8 (p-C), 151.9
(o-C). – 29Si NMR: (79 MHz, C6D6): δ = −3.1 (SiMe3). –
MS (EI, 20 eV, 313 K): m/z(%) = 572 (17) [M]+, 557 (82)
[M–Me]+, 413 (100) [M–CH(SiMe3)2]+.
Synthesis of Z-(Me3C)2Al-(Me3Si)C=C(H)-2-iPrC6H4, 5a
A
suspension of Me3Si–C≡C-2-iPrC6H4 (0.431 g,
CMe3 (no exchange), Al(CMe3)2), 1.50 (s, 9H, Al-CMe3), 1.99 mmol) in n-hexane (10 mL) was added to a solution of
1.74 (s, 18H, o-CMe3, bridge), 1.83 (s, 18H, o-CMe3, ter- (Me3C)2Al–H (0.283 g, 1.99 mmol), and the mixture was
2
3
minal), 3.60 (s, br., 1H, JH−C = 25.5 Hz, JH−C = 7.6 Hz, stirred under reflux conditions for 3 d. The reaction mixture
AlHAl), 7.49 (s, 2H, m-H, bridge), 7.56 (s, 2H, m-H, ter- was concentrated and kept at room temperature to yield
minal). – 13C NMR (100 MHz, C7D8, 230 K): δ = 17.9 colourless crystals of compound 5a. Yield: 0.299 g (42%). –
(AlCMe3), 18.6 (Al(CMe3)2, exchange), 19.7 (Al(CMe3)2, M. p. (under argon; sealed capillary): 58 ◦C (dec.). – IR (CsI,
no exchange), 30.8 (o-CMe3, terminal and p-CMe3, bridge), paraffin): ν = 1921 vw, 1695 vw, 1576 vs, 1560 vs, 1547
31.0 (AlCMe3), 31.2 (Al(CMe3)2, exchange, and o-CMe3, vs ν(C=C), phenyl; 1462 vs (paraffin); 1402 w δ(CH3);
bridge), 31.3 (Al(CMe3)2, no exchange, and p-CMe3, termi- 1377 s (paraffin); 1337 w, 1302 vw, 1244 w δ(CH3); 1213
nal), 35.2 (p-CMe3, terminal), 35.5 (p-CMe3, bridge), 37.1 vw, 1184 w, 1078 m, 1036 m, 1007 w ν(CC); 934 w, 814
(o-CMe3, bridge), 37.2 (o-CMe3, terminal), 101.8 (AlC≡C, w, 764 vw ρ(CH3(Si)); 721 s (paraffin); 592 w, 559 w,
bridge), 113.9 (ipso-C, bridge), 114.3 (AlC≡C, terminal), 509 vw, 469 w, 438 w cm−1 δ(CC), ν(AlC). – 1H NMR
115.6 (AlC≡C, terminal), 118.8 (ipso-C, terminal), 121.0 (400 MHz, C6D6, 300 K): δ = 0.05 (s, 9H, SiMe3), 1.19 (s,
3
(m-C, terminal), 145.7 (AlC≡C, bridge), 149.5 (p-C, termi- 18H, CMe3), 1.21 (d, 6H, JH−H = 6.9 Hz, CHMe2), 3.29
nal), 153.4 (o-C, terminal), 154.1 (p-C, bridge), 157.5 (o-C, (sept, 1H, 3JH−H = 6.9 Hz, CHMe2), 7.05 (m, 1H, m-H(5)),
bridge).
7.16 (m, 2H, m-H(3) and p-H), 7.23 (d, 1H, 3JH−H = 7.4 Hz,
3
o-H), 7.93 (s, 1H, JH−Si = 20.5 Hz, C=CH). – 13C NMR
Synthesis of [(Me3Si)2HC]2Al-C≡C-2,4,6-iPr3C6H2, 4
(100 MHz, C6D6, 300 K): δ = 1.7 (SiMe3), 23.3 (CHMe2),
29.8 (CMe3), 30.1 (CHMe2), 30.2 (CMe3), 124.8 (m-C(3)),
125.6 (m-C(5)), 128.3 (p-C), 129.1 (o-C(6)), 142.5 (ipso-C),
145.7 (o-C(2)), 154.9 (C=CH), 159.0 (br, AlC =CH). 29Si
NMR (75 MHz, C6D6, 300 K): δ = −12.5 (SiMe3).
A
suspension of LiC≡C-2,4,6-iPr3C6H2 (1.13 g,
4.83 mmol) in n-hexane (25 mL) was added during a period
of one hour to a solution of ClAl[CH(SiMe3)2]2 (1.84 g,
4.84 mmol) in n-hexane (50 mL) at −45 ◦C. The reaction
mixture was stirred for another 10 min, the cooling bath
was removed, and the mixture was allowed to warm to
room temperature and filtered. The LiCl residue was washed
Synthesis of E-(Me3C)2Ga-(Me3Si)C=C(H)-2-iPrC6H4, 5b
A
suspension of Me3Si–C≡C-2-iPrC6H4 (0.750 g,
with n-hexane (3 × 10 mL), the filtrates were collected 3.47 mmol) in n-hexane (10 mL) was added to a solution
and the solvent removed in vacuo. The residue was then of (Me3C)2Ga–H (0.320 g, 1.73 mmol), and the mixture
recrystallised from n-hexane at −15 ◦C to yield colourless was stirred under reflux conditions for 3 d. The reaction
crystals of compound 4. Yield: 0.55 g (20%). – M. p. mixture was concentrated and stored at −45 ◦C to yield
Unauthenticated
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W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
Table 4. Crystal data and structure refinement of compounds 1a–1h.
515
1a
1b·(C6H4F2)2/31c
1d
1e·(hexane)0.5 1f·(C6H4F2) 1g
1h·(C6H4F2)
Crystal data
Empirical formula
Mr
Crystal system
Space group
a, pm
C24H30Al2
372.44
monoclinic
P21/c
1043.8(2)
1285.7(3)
920.2(2)
90
111.20(3)
90
1.1514(4)
1.07
C40H54Al2F1.33 C38H58Al2
C38H58Ga2
654.28
C53H89Al2
780.20
monoclinic
P2/c
1819.57(2)
1460.09(2)
1955.96(3)
90
95.133(1)
90
5.1756(1)
1.00
C54H82Al2F2 C56H94Al2
823.16 821.27
orthorhombic monoclinic
C54H82F2Ga2
908.64
614.13
568.80
hexagonal
R3
triclinic
P1
triclinic
orthorhombic
P212121
990.53(2)
1962.87(4)
2631.09(5)
90
90
90
5.1156(2)
1.18
4
¯
¯
¯
P1
P212121
992.72(2)
1964.46(3)
2631.86(4)
90
C2/c
3282.72(2)
3282.72(2)
904.81(1)
90
90
120
878.26(2)
1016.98(3)
1123.42(3)
77.685(2)
73.875(2)
71.098(2)
0.90352(4)
1.05
878.11(6)
1017.64(7)
1123.50(7)
77.892(1)
74.149(1)
70.727(1)
0.9037(1)
1.20
2740.92(7)
993.20(3)
2122.13(5)
90
b, pm
c, pm
α, deg
β, deg
90
90
102.977(2)
90
γ, deg
V, × 10−30 m3
8.4441(1)
1.09
5.1326(2)
1.07
5.6295(3)
0.97
ρ
calcd., g cm−3
2
9
1
1
4
4
4
Z
F(000), e
400
2988
312
0.9 (CuKα )
348
1732
1800
0.8 (CuKα )
1824
0.7 (CuKα )
1944
1.1 (MoKα )
µ, mm−1
0.1 (MoKα ) 0.9 (CuKα )
1.5 (MoKα ) 0.7 (CuKα )
Data collection
T, K
Unique reflections
Reflections I > 2σ(I)
Refinement
153(2)
2738
1884
153(2)
3653
3080
153(2)
2789
2370
153(2)
4358
3997
153(2)
8893
7471
153(2)
9682
9242
153(2)
5214
3849
153(2)
14943
13515
Refined parameters
Final R1a [I > 2σ(I)]
Final wR2b (all data)
Flack (x)c
122
256
220
189
546
596
306
624
0.0337
0.0835
–
0.0532
0.1607
–
0.0442
0.1219
–
0.0255
0.0682
–
0.0496
0.1475
–
0.0476
0.1368
0.00(3)
0.0589
0.1687
–
0.0330
0.0847
0.006(6)
−3
˚
∆ρfin (max / min), e A
0.199 / −0.155 0.787 / −0.355 0.334 / −0.186 0.570 / −0.392 0.546 / −0.364 0.701 / −0.387 0.327 / −0.241 0.413 / −0.509
a
R1 = Σ||Fo|−|F ||/Σ|Fo|; b wR2 = [Σw(Fo2 −F2)2/Σw(Fo2)2]1/2, w = [σ2(Fo2)+(AP)2 +BP]−1, where P = (Max(Fo2,0)+2F2)/3; c absolute structure
c
c
c
parameter.
colourless crystals of compound 5b. Yield: 0.434 g (62%). under reflux conditions for 3 d. The reaction mixture was
– M. p. (under argon; sealed capillary): 72 ◦C (dec.). – IR concentrated and recrystallised from cyclopentane at −20 ◦C
(CsI, paraffin): ν = 1948 vw, 1921 vw, 1840 vw, 1697 to yield colourless crystals of compound 6. Yield: 0.814 g
vw, 1578 vs, 1555 vs ν(C=C); 1466 vs (paraffin); 1402 (42%). – IR (CsI, paraffin): ν = 1930 vw, 1859 vw, 1786
vs δ(CH3); 1377 s (paraffin), 1335 w, 1300 w, 1242 m vw, 1699 vw, 1580 s, 1557 s ν(C=C); 1462 vs (paraffin);
δ(CH3); 1215 w, 1173 m, 1109 w, 1078 s, 1034 m, 1009 1402 s δ(CH3); 1377 s (paraffin); 1341 m, 1304 m, 1244
w ν(CC); 972 vw, 949 w, 885 vw, 847 w, 814 m, 766 w m δ(CH3); 1188 m, 1167 m, 1155 m, 1090 m, 1045 m
ρ(CH3(Si)); 723 vs (paraffin); 683 m, 644 m ν(SiC); 594 ν(CC); 922 vw, 847 w, 827 w, 818 w, 766 w ρ(CH3(Si));
m, 556 m, 517 w, 498 w, 467 m, 428 m cm−1 δ(CC), 721 s (paraffin); 592 w, 561 w, 511 vw, 473 m, 449 w
ν(GaC). – 1H NMR (400 MHz, C6D6, 300 K): δ = 0.03 (s, cm−1 δ(CC), ν(AlC). – 1H NMR (400 MHz, C6D6, 300 K):
3
9H, SiMe3), 1.23 (d, 6H, JH−H = 6.9 Hz, CHMe2), 1.26 δ = −0.33 (s, 3H, AlMe), 0.03 (s, 18H, SiMe3), 2.17 (s,
3
3
(s, 18H, tBu), 3.32 (sept, 1H, JH−H = 6.9 Hz, CHMe2), 12H, ArMe), 6.92 (d, 4H, JH−H = 7.5 Hz, m-H), 6,99 (m,
7.06 (m, 1H, m-H(5)), 7.16 (s, 1H, p-H), 7.17 (s, 1H, 2H, p-H), 7.48 (s, 2H, JH−Si = 12.5 Hz, C=CH). – 13C
3
3
m-H(3)), 7.26 (d, 1H, JH−H = 7.4 Hz, o-H), 7.63 (s, 1H, NMR (100 MHz, C6D6, 300 K): δ = −3.8 (br, AlMe), 0.3
3JH−Si = 19.5 Hz, C=CH). – 13C NMR (100 MHz, C6D6, (SiMe3), 20.9 (ArMe), 127.4 (p-C), 128.5 (m-C), 135.0 (o-
300 K): δ = 1.6 (SiMe3), 23.3 (CHMe2), 29.6 (CMe3), 30.1 C), 144.0 (ipso-C), 155.6 (AlC= CH), 160.5 (br, AlC=CH).
(CMe3 and CHMe2), 124.7 (m-C(3)), 125.6 (m-C(5)), 128.2
–
29Si NMR (75 MHz, C6D6, 300 K): δ = −5.8 (SiMe3). –
(p-C), 129.4 (o-C(6)), 142.0 (ipso-C), 145.9 (o-C(2)), 151.1 MS (EI, 20 eV, 353 K): m/z(%) = 448 (2) [M]+, 433 (4) [M–
(AlC= CH), 162.9 (br, AlC =CH). – 29Si NMR (75 MHz, Me]+, 245 (100) [MeAlC(SiMe3)=CHAr]+.
C6D6, 300 K): δ = −12.2 (SiMe3). – MS (EI, 20 eV, 298 K):
m/z(%) = 344 (100), 346 (74) [M–butene]+.
X-Ray crystallography
Crystals suitable for X-ray crystallography were obtained
by recrystallisation from cyclopentane (6), n-hexane (1a, 1c,
Synthesis of MeAl[E-(Me3Si)C=C(H)-2,6-Me2C6H3]2, 6
A
solution of Me3Si–C≡C-2,6-Me2C6H3 (0.870 g, 1d, 1e, 2b, 4, 5a, 5b), 1,2-difluorobenzene (1b, 1f, 1h, 3)
4.30 mmol) in n-hexane (10 mL) was added to a solution of or pentafluorobenzene (1g, 2a). Intensity data was collected
Me2AlH (0.249 g, 4.29 mmol), and the mixture was stirred on a Bruker APEX II diffractometer or a Stoe IPDS II (6)
Unauthenticated
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516
W. Uhl et al. · Hydroalumination versus Deprotonation of Alkynes
Table 5. Crystal data and structure refinement of compounds 2a–6.
a
2a
2b
3·(C6H4F2)1/2 4
5a
5b
6
Crystal data
Empirical formula
Mr
Crystal system
Space group
a, pm
C40H74Al2
608.95
orthorhombic orthorhombic triclinic
P212121
1063.44(4)
1906.63(7)
2103.45(7)
90
90
90
4.2649(3)
0.95
C40H74Ga2
694.43
C55H88Al2F C31H61AlSi4 C22H39AlSi C22H39GaSi C27H41AlSi2 C29H45AlSi2
822.21
573.14
monoclinic
P21
358.60
401.34
448.76
476.81
triclinic
triclinic
triclinic
triclinic
¯
P1
¯
¯
¯
¯
P212121
1061.28(1)
1903.28(2)
2103.74(3)
90
P1
P1
P1
P1
1414.66(3)
1487.05(3)
1514.87(3)
61.742(1)
84.845(1)
74.557(1)
2.7032(1)
1.01
949.05(5)
1073.89(5)
1838.5(1)
90
92.835(4)
90
908.99(1)
960.31(2)
1562.67(2)
75.6000(8)
73.2219(8)
65.0612(9)
1.17150(3)
1.02
908.72(1)
961.08(1)
1567.74(2)
90.095(1)
106.796(1)
115.168(1)
1.17402(2)
1.14
875.99(4)
1020.04(5)
1705.52(8)
86.351(4)
75.213(4)
71.753(3)
1.3992(1)
1.07
1357.7(3)
1450.5(3)
1640.9(3)
102.06(3)
98.03(3)
95.41(3)
3.104(1)
1.02
b, pm
c, pm
α, deg
β, deg
90
90
γ, deg
V, × 10−30 m3
4.24937(9)
1.09
4
1.8715(2)
1.02
ρ
calcd., g cm−3
4
2
2
2
2
2
4
Z
F(000), e
1360
1504
906
632
396
432
488
1040
µ, mm−1
0.8 (CuKα ) 1.7 (CuKα ) 0.7 (CuKα ) 0.2 (MoKα ) 1.2 (CuKα ) 2.1 (CuKα ) 0.2 (MoKα ) 0.2 (MoKα )
Data collection
T, K
Unique reflections
Reflections I > 2 σ(I)
Refinement
153(2)
7776
6910
153(2)
7950
7688
153(2)
8570
6493
153(2)
8268
7981
153(2)
3716
3555
153(2)
3657
3465
153(2)
6703
4771
153(2)
14186
8187
Refined parameters
Final R1b [I > 2 σ(I)]
Final wR2c (all data)
Flack (x)d
404
404
656
343
254
259
282
594
0.0443
0.1190
−0.01(3)
0.0258
0.0671
−0.02(1)
0.0719
0.2134
–
0.0395
0.1117
0.01(8)
0.0517
0.1373
–
0.0405
0.1061
–
0.0387
0.0950
–
0.0559
0.1430
–
−3
˚
∆ρfin (max / min), e A
0.299 / −0.154 0.346 / −0.265 0.980 / −0.348 0.254 / −0.464 0.459 / −0.377 0.877 / −0.398 0.315 / −0.382 0.454 / −0.609
a
Only a few crystals of MeAl[E-(Me3Si)C=C(H)-2-iPrC6H4]2 were isolated from a reaction analogous to that which produced compound 6
after recrystallisation from n-hexane; R1 = Σ||Fo| − |F ||/Σ|Fo|; wR2 = [Σw(Fo2 − F2)2/Σw(Fo2)2]1/2, w = [σ2(Fo2) + (AP)2 + BP]−1, where
b
c
c
c
P = (Max(Fo2,0)+2F2)/3; d absolute structure parameter.
c
with monochromatised MoKα [1a, 1d, 1h·(C6H4F2), 4, 6] 1e (0.55 : 0.45), 1f (0.275 : 0.525 : 0.200), 1g (0.63 : 0.37),
or CuKα [1b·(C6H4F2)2/3, 1c, 1e, 1f·(C6H4F2), 1g, 2a, 2b, 1h (0.48 : 0.15 : 0.37), 3 (0.74 : 0.26, 0.46 : 0.54, 0.60 : 0.40),
3·(C6H4F2)1/2, 5a, 5b] radiation. The collection method in- 5a (0.62 : 0.38) and 5b (0.55 : 0.45). Further crystallographic
volved ω scans. Data reduction was carried out using the data is summarised in Tables 4 and 5.
program SAINT+ [48]. The crystal structures were solved
CCDC 926663 (1a), 926664 (1b(C6H4F2)2/3), 926665
by Direct Methods using SHELXTL [49, 50]. Non-hydrogen (1c), 926666 (1d), 926667 (1e(hexane)1/2), 926668
atoms were first refined isotropically followed by anisotropic (1f), 926669 (1g), 926670 (1h(C6H4F2), 926671 (2a),
refinement by full-matrix least-squares calculations based on 926672 (2b), 926673 (3(C6H4F2)1/2), 926674 (4), 926675
F2 using SHELXTL [49, 51]. Hydrogen atoms were posi- (5a), 926676 (5b), 926677 (6), and 926678 (MeAl[E-
tioned geometrically and allowed to ride on their respective (Me3Si)C=C(H)-2-iPrC6H4]2) contain the supplementary
parent atoms, except the bridging hydrogen atoms in com- crystallographic data for this paper. This data can be ob-
pounds 2 and 3 which were refined isotropically. The co- tained free of charge from The Cambridge Crystallographic
crystallised 1,2-difluorobenzene molecules were disordered Data Centre via www.ccdc.cam.ac.uk/data request/cif.
in all structures and refined in split positions (1b, 1f, 1h). In
the case of 3 the solvent molecule was only refined isotrop-
Acknowledgement
ically. tert-Butyl groups were disordered and refined in split
positions for compounds 1b (0.48 : 0.52), 1c (0.84 : 0.16),
We are grateful to the Deutsche Forschungsgemeinschaft
for generous financial support.
[1] G. Zweifel, J. A. Miller, Org. React. 1984, 32, 375.
[2] E. Winterfeld, Synthesis 1975, 10, 675.
[3] J. Marek, J.-F. Normani, Chem. Rev. 1996, 96, 3341.
[4] J. J. Eisch, in Comprehensive Organic Synthesis, Vol. 8,
(Eds.: B. Frost, I. Flemming), Pergamon, Oxford, 1991,
pp. 733.
[5] G. Zweifel, in Aspects of Mechanism and Organo-
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