The yield of triptane from the reaction of methanol with zinc iodide is dramatically increased by addition of phosphorous or hypophosphorous acid, via transfer of hydride from a P–H bond to carbocationic intermediates

Bercaw, John E.

Grubbs, Robert H.

Hazari, Nilay

Labinger, Jay A.

Li, Xingwei

English

Caltech Authors - Main

Supplementary Material (ESI) for Chemical Communications 
This journal is (c) The Royal Society of Chemistry 2007 
Supplementary Information 
Enhanced Selectivity in the Conversion of Methanol to 2,2,3-
Trimethylbutane (Triptane) over Zinc Iodide by Added 
Phosphorous or Hypophosphorous Acid 
John E. Bercaw, Robert H. Grubbs, Nilay Hazari, Jay A. Labinger, Xingwei Li 
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute 
of Technology, Pasadena, California, USA 91125 
jal@its.caltech.edu 
Experimental protocol 
Zinc iodide, methanol, isopropanol (used as initiator) and phosphorus compounds were 
reagent-grade commercial samples, used without further purification.  GC analyses were 
performed on an HP model 6890N chromatograph equipped with a 10 m x 0.10 mm x 
0.40 μm DB-1 column. 31P and 13C NMR spectra were obtained on a Varian 300 MHz 
instrument. Reactions were carried out in thick-walled pressure tubes equipped with 
Teflon stopcocks (Ace Glassware), as previously described.i  In a typical experiment, 
zinc iodide (2.444 g, 7.65 mmol), methanol (1.0 ml, 791 mg, 24.7 mmol), isopropanol 
(50μL, 39.2 mg, 0.65 mmol), and P(OCH3)3 (200 μL, 210 mg, 1.69 mmol) were added to 
the pressure tube under argon.  The mixture was stirred until dissolution was complete, 
and the tube placed into a preheated oil bath at 200 oC for 3 hours, after which time it was 
cooled to room temperature to give two layers.  A solution of 83.4 mg cyclohexane (as 
reference standard) in 1 mL chloroform followed by 1 mL water was added; the organic 
layer was extracted, diluted with acetone, and analyzed by gas chromatography (GC).  
Reactions with different additives and quantities were carried out analogously.  Since 
hypophosphorous acid is available commercially only as a 50% aqueous solution, and 
since water has an inhibitory effect,1 a small modification was introduced for its reaction: 
- S2 - 
the tube was first charged with an amount of the solution corresponding to the desired 
molar quantity of H3PO2 and evacuated for 12-36 h to remove most of the water.  The 
remaining components were then added and the reaction carried out as above.  If water 
removal is omitted, substantially longer reaction times are required for complete 
conversion. 
 
Selected results are shown in Table 1 of the main text, and a more complete listing is 
given in Table S1 below. 
 
Table S1.  Yields for reactions with varying additives 
Additive, mol % 
(rel. to MeOH) Reaction time, h Temp., °C 
Triptyl yield, mg Triptyl yield, % 
based on MeOH 
Triptyl yield, % 
based on total C 
none 2 200 43 12 11 
“ 3 200 66 19 18 
P(OMe)3, 6.8 % 2 200 113 32 24 
“ 3 200 108 31 23 
“ 24 175 121 34 26 
P(OMe)3, 1.7%b 2 200 65 18 17 
P(OMe)3, 3.4%b 2 200 100 28 23 
P(OMe)3, 10.2%b 2 200 102 29 21 
P(OEt)3, 6.8 % 2 200 67 19 13 
H3PO3, 6.8 % 2 200 86 24 22 
“ 3 200 89 25 23 
H3PO2, 7.4 % 3 200 122 35 32 
“ 5 200 127 36 33 
“ 12a 200 130 37 34 
“ 24a,b 170 129 36 36 
“ 42a,b 170 132 37 37 
H3PO2, 3.7 % 19.5a 180 92 26 26 
H3PO2, 5.5 % 5a 200 110 31 29 
“ 30.5a 160 130 37 34 
“ 66a 150 138 39 36 
H3PO2, 8.8 % 8a 200 130 37 34 
H3PO2, 11.1 % 8a 200 133 38 35 
a Water not removed before reaction.  b No isopropanol (promoter) added 
 
PIANO analyses 
Analysis using a standard refinery “PIANO” method (for 
paraffins/isoparaffins/aromatics/naphthenes/olefins) was carried out at the BP Carson 
refinery by Eugene Zaluzec, on the organic products resulting from both a control 
reaction (no additive) and one containing H3PO2.  The results for a selected group of 
individual compounds and for the aggregated classes of compounds are shown in Table 
- S3 - 
S2.  It should be noted that the latter will not be completely accurate, as some of the peak 
assignments, especially minor peaks, may be open to question (we have verified the 
major peak assignments by comparison to our GC studies with authentic standards), but it 
should be a good approximation.  It is clear that the addition of the phosphorus compound 
substantially increases (branched) alkanes relative to olefins, both individually and 
collectively, and decreases arenes as well. 
 
Table S2.  PIANO analytical results for organic liquid fractions 
Compound or class Wt. % in control Wt. % with H3PO2 
2-methylbutane 2.9 5.8 
2-methylbut-2-ene 1.5 0.6 
2,3-dimethylbutane 1.8 3.0 
2,3-dimethylbut-2-ene 0.9 0.3 
triptane 24.9 40.0 
triptene 5.6 1.5 
hexamethylbenzene 3.4 1.3 
methyl iodide 1.4 13.7 
   
n-alkanes 1.3 0.7 
branched alkanes 45.0 63.0 
olefins 14.2 4.9 
arenes 10.7 4.9 
naphthenes 5.2 2.0 
 
Reactions of olefins with H3PO2 
A solution of equimolar triptene and H3PO2 in methanol was heated at 160° for 12 h.  13C 
NMR spectroscopy showed a number of new peaks, but no triptane.  The 31P spectrum 
(Figure S1) shows, inter alia, a signal centered at δ 37.8 consisting of a doublet (1JPH = 
537 Hz) of complex multiplets (2JPH ≈ 15 Hz).  This is consistent with the structure that 
would be obtained by adding the P-H bond to triptene, HP(O)(OH)(CH2CH(Me)But); the 
chiral center renders the methylene protons non-equivalent.  Similar parameters have 
- S4 - 
been observed for a variety of HP(O)(OX)(CH2R) derivatives.ii  While addition of 
hypophosphite derivatives to olefins normally require a radical initiator (such as a 
peroxide, or Et3B/O2) and temperatures from 25-80°C, it seems reasonable that 
adventitious initiators could suffice for the small degree of reaction observed here at a 
much higher temperature. 
 
 
Figure S1.  Top: 31P spectrum of reaction mixture after heating triptene and 
hypophosphorous acid in methanol.  Bottom: Expansion of downfield signal. 
- S5 - 
In contrast, when a solution of equimolar triptene and H3PO2 containing 0.5 equivalents 
of p-toluenesulfonic acid in tetraglyme was heated at 160° for 3 h, GC analysis showed 
significant hydrogenation of olefin — the ratio of triptane to triptene was about 1:4 (other 
species were also present) — and the 31P spectrum showed the presence of H3PO3.  2,3-
dimethylbut-2-ene behaved quite similarly, but hex-1-ene gave no hexane at all, only 
isomerization to hex-2-ene and hex-3-ene.  We have not attempted to optimize these 
reactions in any way. 
 
H3PO2 plus a catalytic amount of I2 has been found to reduce diaryl alkenes such as 
stilbene in refluxing acetic acid; the proposed mechanism involves addition of HI 
(obtained by reduction of I2 by H3PO2) to the double bond followed by C-I bond 
cleavage, so that H3PO2 serves only to continually regenerate HI.iii  One might conceive 
of a related process operating in our system — some HI should be present in the acidic 
methanolic ZnI2 solution.  However, the reductions described immediately above involve 
H3PO2 but no HI, and substitution of HI for toluenesulfonic acid does not give any 
increase in hydrogenation rate, so it does not appear that HI does participate here.  
Conversely, the idea that the stilbene reduction might involve direct reaction of a 
carbocationic intermediate with a P-H bond, as in our proposed mechanism, appears to be 
excluded by the fact that HI (or I2) is a required co-reagent.3  Most likely the two 
reactions involve completely separate mechanisms; the radical path implicated for the 
earlier work would be expected to be much more favorable for the aryl alkenes studied 
there than for the aliphatic alkenes intermediate in triptane formation. 
- S6 - 
 
References 
 
i. J. E. Bercaw, P. Diaconescu, R. H. Grubbs, R. D. Kay, S. Kitching, J. A. Labinger, X. 
Li, P. Mehrkhodavandi, G. E. Morris, G. J. Sunley and P. Vagner, J. Org. Chem., 
2006, 71, 8907-8917. 
ii. S. Deprèle and J.-L. Montchamp, J. Org. Chem., 2001, 66, 6745-6755 
iii. A. J. Fry, M. Allukian and A. D. Williams, Tetrahedron, 2002, 58, 4411-4415. 


Enhanced selectivity in the conversion of methanol to 2,2,3-trimethylbutane (triptane) over zinc iodide by added phosphorous or hypophosphorous acid

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Enhanced selectivity in the conversion of methanol to 2,2,3-trimethylbutane (triptane) over zinc iodide by added phosphorous or hypophosphorous acid

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