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This document, authored by francisco zaera from the university of california, riverside, explores the organic chemistry of alkanes and alkenes on metal solid surfaces. Topics include alkane-alkene conversion, h-d exchange, and stereoselectivity in hydrogenation. The document also discusses the role of cinchona derivatives in enhancing enantioselectivity in hydrogenation reactions.
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Francisco ZaeraDepartment of Chemistry University of California, Riverside
by Francisco Zaera Department of Chemistry University of California Riverside, CA 92521, USA Phone: 1 (951) 827-5498 Fax: 1 (951) 827-3962 Email: zaera@ucr.edu http://www.zaera.chem.ucr.edu
General References:
Z. Ma and F. Zaera,
Surf. Sci. Rep.
,^ 61(5) , 229-282 (2006).
F. Zaera,
Catal. Lett.
,^ 91(1-2)
, 1-10 (2003).
F. Zaera,
Chem. Rev.
,^^95 , 2651-2693 (1995).
F. Zaera,
Appl. Catal.
,^^229 , 75 (2002).Francisco ZaeraDepartment of Chemistry University of California, Riverside A selective process:• Consumes less reactants• Avoids separation problems
n-Hexane
AromatizationCyclizationIsomerizationHydrogenolysis
BenzeneMethylcyclopentane2-MethylpentanePropane
(ON ~ 40)
(ON ~ 95)(ON ~ 90)(ON ~ 80)(gas)
Dehydrogenation
2-Hexene
Desirable Undesirable
Introduction
Francisco ZaeraDepartment of Chemistry University of California, Riverside
Outline
CH^3
γ-H
Francisco ZaeraDepartment of Chemistry University of California, Riverside
Outline
CH^3
γ-H elimination.
β -hydride elimination
F. Zaera,
J. Am. Chem. Soc.
,^^111 , 8744 (1989).
A. J. Gellman,
Acc. Chem. Res.
,^^33 , 19 (2000).
B. E. Bent,
Chem. Rev.
^96 , 1361 (1996).
Francisco ZaeraDepartment of Chemistry University of California, Riverside
Alkane-alkene equilibria via
β -H
F. Zaera,
Molec. Phys.
,^^100 , 3065-3073 (2002). F. Zaera,
Catal. Lett.
,^ 91(1-2)
, 1-10 (2003).
F. Zaera,
Top. Catal.
,^ 34 (1-4)
, 129-141 (2005).
-^ β-hydride elimination accounts for fast alkane-alkene equilibria - Reforming requires
α- and
γ-hydride elimination
Francisco ZaeraDepartment of Chemistry University of California, Riverside
α -H vs.
β -H elimination Ethylidyne formation
F. Zaera, S. Tjandra and T. V. W. Janssens,
Langmuir
,^^14 , 1320-1327 (1998).
Francisco ZaeraDepartment of Chemistry R(β)/R( University of California, Riverside
α) ~ 10
but decreases with T D H C–C^ H ethylene D
CH^3 D C (^2) ethyl
CH DC ethylidene 3 CH^3 C ethylidyne
ethylidyne- C d^1 CDH^2
α-D
β-H
δ[CDH
δ[CH^3 α-D ]^ β-H
Wavenumber / cm
-
bancer Abso
CH^3
CD^2
I/Pt(111)
α -H vs.
β -H elimination
A. Loaiza, M. Xu and F. Zaera,
J. Catal.
,^^159 , 127 (1996).
A. Loaiza, D. Borchardt and F. Zaera,
Spectrochim. Acta A
,^^53 , 2481 (1997).
A. Loaiza and F. Zaera
J. Am. Soc. Mass Spectrom.
,^^15 , 1366 (2004).
Francisco ZaeraDepartment of Chemistry University of California, Riverside
amu b. unitsra / er essur tial Pr Pa
x C^ D^2 MS/CID-MS 2nd Stage, 34 amuP= 50 TorrCD^2 6 P= 500 TorrH^2 T = 675 K55% Conversion
+ H 6
/Pt 2
P= 50 TorrCH^26 P= 500 TorrD^2 T = 625 K20% Conversion^ δ^ / ppm C^2 13 C NMR of Mixture H^ + D 6
/Pt 2
5.^
H^ ethylene
H C–C H H
CH HC ethylidene α-H^3 β-H^
H^ C–CH^3 ethaneCH^3 H^ C^2 ethyl
3 ethane-
d^2 DH^ C–CH^2
D 2
ethane-
d^2 D^ HC–CH^2
3
R(β) ~ 5·R(
α) @ 625K
Hydrocarbon reforming^ Isomerization vs. hydrogenolysis
T. V. W. Janssens, G. Jin and F. Zaera,
J. Am. Chem. Soc.
,^^119 , 1169 (1997).
F. Zaera and S. Tjandra,
J. Am. Chem. Soc.
,^^118 , 12738 (1996).
Francisco ZaeraDepartment of Chemistry University of California, Riverside Ni^ Ni
PtNi Pt CH^2 C–CH
3 H HC H
CH^3 CH^3 C H H^3 C^ CH^2 H HC
CH^2 C–CH
3 H HC
CH^3
CH^2 C–CH
3 H HC
CH^3 CH^3 H 3C C CH H H 2C
Metallacyclobutane
CH^3 H 3C C CH H H 2C
CH^3 CH 3 –C=CH^2 C
H^2
CH^ 3–C–CH^2 CH
H CH^3 Neopentylidene
CH^3 CH 3 –C–CH^2 H CH H Neopentyl
α-H elimination(Hydrogenolysis)^ γ-H elimination(Isomerization)
F. Zaera,
Chem. Rev.
,^^95 , 2651 (1995). F. Zaera,
Appl. Catal.
,^^229 , 75-91 (2002).Francisco ZaeraDepartment of Chemistry University of California, Riverside
-^ β-H eliminationis much fasterthan^ α
-H or^ γ
elimination• Nevertheless,
α-H and^ γ-H eliminationsare required forreforming• Selectivity is definedearly by initialdehydrogenation steps
Hydrocarbon reforming
F. Zaera,
Catal. Lett.
,^^91 , 1 (2003),
F. Zaera,
Molec. Phys.
,^^100 , 3065 (2002).
Francisco ZaeraDepartment of Chemistry University of California, Riverside
β -H elimination in Horiuti-Polanyi mechanism
H^ CH^ alkene
3 C–C^ CH
3 H
H^ CH H C 2 alkyl
(^3) C CH 3
H^ CH H C 3 alkane
(^3) C CH 3
alkenes
H^ HC alkylidene β-H CH^3 C CH^3
smaller alkanes
α-H^
Hydrogenolysis
H H C (^2) metallacycle CH^3 C CH^2
branched alkanes
γ-H
Isomerization
Explains:
C=C double bond migration^ R. Morales and F. Zaera,
J. Phys. Chem. B
,^ 110(19)
, 9650-9659 (2006).Francisco ZaeraDepartment of Chemistry University of California, Riverside
CH^3 1-methyl cyclopentene (1MCp=)
CH methylenecyclopentane (MeCp) 2
1-methyl 1-cyclopentyl (1MCpyl)
CH^3
CH^3 1-methylcyclopentene (ads)
H H methylenecyclopentane (ads)
Isomerization depends onregioselectivity of
β-H
elimination step from thealkyl intermediate
Temperature / K
arb. units / Partial Pressure
200
300
x0.5 x
amu b. Molecular Ion
82 83 84 86
100
200
(^67686971300)
x0.1 x0.5 x
amu
D
DD
D D
DD
Francisco ZaeraDepartment of Chemistry University of California, Riverside
H-D exchange in methylene cyclopentane
R. Morales and F. Zaera,
J. Phys. Chem. B
,^ 110(19)
, 9650-9659 (2006).
C=C double bond migration^ Regioselectivity energetics
Francisco ZaeraDepartment of Chemistry University of California, Riverside
R. Morales and F. Zaera,
J. Phys. Chem. B
,^ 110(19) H H^ MeCp , 9650-9659 (2006). CH^3 1MCpyl CH^3 1MCp=
β-H from methyl
β-H from cycle
∆H° / kcal·mol
CH^3 1MCp=
CH^2 MeCp
~3 kcal/mol
