Hydrocarbation chemistry proceeding from nickel carbenes

Article abstract of DOI:10.1021/ja030108p

Nickel carbene complex 2 [Ni(triphos)C(H)N(H)xylyl]²⁺(BF₄^-)₂ reacts with alkenes quantitatively and regiospecifically to give the anti-Markovnikov hydrocarbation products. X-ray crystallography shows significant iminium alkyl character of the hydrocarbation products, similar to that observed in parent carbene 2. Mechanistic studies suggest the importance of a hydride pathway over alkene (metallocycle formation or carbocation) pathways. Copyright

Full text of DOI:10.1021/ja030108p

Published on Web 07/22/2003
Hydrocarbation Chemistry Proceeding from Nickel Carbenes
Hongyi Hou, Peter K. Gantzel,§ and Clifford P. Kubiak*
Department of Chemistry and Biochemistry, UniVersity of California, San Diego, La Jolla, California 92093-0358
Received February 13, 2003; E-mail: ckubiak@ucsd.edu
Transition metal-mediated carbon-carbon bond formations have
provided important new synthetic routes to many organic com-
pounds.1,2 In many cases, mechanistic studies demonstrate that metal
carbene complexes are intermediates.3 We recently reported an
efficient route to N-aryl nickel (II) carbenes by protonation of the
corresponding nickel (0) isocyanide complexes, eq 1.4
Figure 1. ORTEP diagram of 3. Phenyl rings on the phosphorus atoms
are not included. Selected bond distances (Å) and angles (deg) are as
follows: Ni(1)-C(5), 1.901(2); N(1)-C(5), 1.302(3); N(1)-C(1F), 1.452-
(3); C(5)-C(8), 1.512(4); C(5)-N(1)-C(1F), 128.4(2); N(1)-C(5)-Ni(1),
120.70(19); C(8)-C(5)-Ni(1),120.22(18); N(1)-C(5)-C(8), 119.0(2).
Nearly 20 years ago, Casey and co-workers reported a new class
of carbon-carbon bond-forming reactions “hydrocarbations” be-
cause they involved the direct C-H bond addition across the C-C
double bond of alkenes.5 They showed that a cationic bridging iron-
methylidyne complex undergoes this type of reaction with alkenes
with anti-Markovnikov regioselectivity.6 To our knowledge, no
additional examples of hydrocarbations have been subsequently
reported. In principle, hydrocarbation, together with hydroboration7
and hydrozirconation,8 represents a potentially general reaction type
for adding carbon-to-hydrogen bonds to alkenes. Here we report
our ongoing studies of the cationic nickel aminocarbene complex
2,4 its hydrocarbation chemistry with alkenes, and mechanistic
studies.
Scheme 1
Scheme 2. Hydrocarbation of Alkenes with Complex 2
Refluxing a solution of 2 in methylene chloride at 40 °C under
an ethylene atmosphere for 8 h results in complete conversion to
the ethylcarbene complex 3 [Ni(triphos)C(CH2CH3)N(H)xylyl]2+
-
(BF4-)2, (triphos ) Bis(2-diphenylphosphinoethyl)phenylphosphine)
eq 2. Recrystallization of the product from methylene chloride/
ether gave single crystals of 3 suitable for X-ray crystallography,
Figure 1.
The coordination geometry about the nickel center is ap-
proximately square planar, as expected for a 16e- Ni (II) complex.
The C(5)-N(1) bond distance (1.302 (3) Å) is shorter than a typical
C-N single bond (1.472 (5) Å) but longer than the C-N triple
bond of isocyanides (1.157 (5) Å). The three substituents on the
carbene carbon C(5) are coplanar, consistent with sp2 character of
the carbon atom. The nickel-carbon bond distance of 3, d (Ni1-
C5) )1.901 (2) Å is 0.114 Å longer than the Ni-C distance of
isocyanide complex 1, and 0.041 Å longer than the Ni-C distance
of 2. The metrical parameters therefore show significant iminium
alkyl character (c), similar to that observed in parent carbene, 2
(Scheme 1).4
Complex 2 reacts with cyclohexene, propene, 1-butene, and 3,3-
dimethyl 1-butene quantitatively and regiospecifically to give the
anti-Markovnikov hydrocarbation products (Scheme 2). Thus, the
regiochemistry is the same as that of hydroboration reactions and
the hydrocarbation reactions reported by Casey.6 The structures of
propyl carbene complex [Ni(triphos)C(CH2CH2CH3)N(H)xylyl]2+
-
-
(BF4
) (4), isoheptyl carbene [Ni(triphos)C(CH2CH2C(CH3)3)-
2
-
N(H)xylyl]2+(BF4
) (5) and cyclohexyl carbene [Ni(triphos)-
2
C(C6H11)N(H)xylyl]2+(BF4-)2 (7) are further established by X-ray
crystallography, Figures 2, 3.9
Mechanistic studies suggest the importance of a “hydride”
pathway over “alkene” (metallocycle formation or carbocation)
§ Address correspondence pertaining to crystallographic studies to this author.
E-mail: pgantzel@ucsd.edu.
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J. AM. CHEM. SOC. 2003, 125, 9564-9565
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C O M M U N I C A T I O N S
nickel (II) hydride complex (f). Subsequent olefin insertion and
isocyanide insertion gives hydrocarbation product 3.
The hydride mechanism is supported by the following observa-
tions: Isotope labeling experiments by using d4-ethylene or
[Ni(triphos)C(D)N(D)xylyl]2+(CF3SO3-)2 showed deuterium at both
the methylene group and the methyl group of the R-ethyl carbene
3.10 The alkene pathways (either form metallocycle or carbocation)
would not give a mixed result.
When ethylene was bubbled into the hydrocarbation product of
propene [Ni(triphos)C(CH2CH2CH3)N(H)xylyl]2+(BF4-)2 (4), there
was no alkyl exchange after refluxing for several days. The
corresponding ethyl complex reaction with propene showed no alkyl
exchange either. This result rules out a reversible alkene pathway
as the source of H/D scrambling.
Figure 2. ORTEP diagram of 4. Phenyl rings on the phosphorus atoms
are not included. Selected bond distances (Å) and angles (deg) are as
follows: Ni(1)-C(5), 1.914(4); N(1)-C(5), 1.305(5); N(1)-C(1F), 1.458-
(5); C(5)-C(6), 1.500(5); C(5)-N(1)-C(1F), 130.2(3); N(1)-C(5)-Ni-
(1), 120.3(3); C(6)-C(5)-Ni(1),120.6(3); N(1)-C(5)-C(6), 119.1(3).
Upon addition of weak, noncoordinating bases such as 2,6-
lutidine to methylene chloride solutions of 2, the formation of nickel
hydride complex f was detected by 1H NMR showing the hydride
peak (dt) centered at -11.5 ppm, while IR showed ν(CN) for the
isocyanide ligand at 2129 cm-1. Deliberate formation of hydride
intermediate f greatly accelerates the rate of formation of hydro-
carbation product.
These results demonstrate a surprisingly ‘classical’ organome-
tallic mechanism for a decidedly “nonclassical” class of carbon-
carbon bond-forming “hydrocarbation” reactions.
Figure 3. ORTEP diagram of 5. Phenyl rings on the phosphorus atoms
are not included. Selected bond distances (Å) and angles (deg) are as
follows: Ni(1)-C(5), 1.908(6); N(1)-C(5), 1.318(8); N(1)-C(1F), 1.461-
(8); C(5)-C(6), 1.503(8); C(5)-N(1)-C(1F), 132.3(5); N(1)-C(5)-Ni-
(1), 120.0(4); C(6)-C(5)-Ni(1),120.7(4); N(1)-C(5)-C(6), 119.2(5).
Acknowledgment. We gratefully acknowledge the DOE
(DE-FG03-99ER14992) for support.
Supporting Information Available: Tables of crystallographic data
collection and refinement parameters, position and thermal parameters,
bond distances and angles for 3, 4, 5, and 7 (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Scheme 3. Hydride Pathway of Hydrocarbation
References
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pathways. This mechanism is briefly summarized here (Scheme
3). From the preparation of 2, one would expect very acidic protons
on both the carbene carbon and the nitrogen. The crystal structure
of 2 further indicates significant iminium formyl ground-state
character of 2.4 R-Hydrogen elimination is the microscopic reverse
of hydride insertion/imino formyl formation and would give the
(9) For crystal structure of complex 7, see Supporting Information.
(10) The reaction of 2 with d4-ethylene gives 60% [Ni(triphos)C(CD2CD2H)-
N(H)xylyl]2+(BF4
) -
and 40% [Ni(triphos)C(CDHCD3)N(H)xylyl]2+
-
(BF4-)2, while reacti2on of [Ni(triphos)C(D)N(D)xylyl]2+(CF3SO3-)2 with
ethylene gives 74% [Ni(triphos)(CH2CH2D)N(D)xylyl]2+(CF3SO3-)2 and
26% [Ni(triphos)(CHDCH3)N(D)xylyl]2+(CF3SO3-)2.
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