The nature of the ring-opening process in a substituted cyclopropylmethyliron complex

Article abstract of DOI:10.1021/om970906m

Irradiation of CpFe(CO)₂(2,2-dimethylcyclopropylmethyl) (4) in the absence and presence of triphenylphosphine leads to the ring-opened products CpFe(CO)CH₂CMe₂CH=CH₂ (6) and CpFe(CO)(PPh₃)CH₂CMe₂CH=CH₂) (7). These are interconverted on further irradiation and on heating are more slowly converted to the π-allylic complex CpFe(CO)(η³CH₂-CHCHCHMe₂) (8). Ring-opening rearrangement in the sense to produce mainly 6 or 7 (with a primary σ-bonded alkyl group) rather than products derived from a tertiary σ-bonded alkyl group is most consistent with a concerted four-center organometallic rearrangement process. A radical route, previously proposed for the unsubstituted cyclopropylmethyl complex, is unlikely. Complex 4 also undergoes normal migratory insertion to an acyl complex, without opening of the cyclopropylmethyl group.

Full text of DOI:10.1021/om970906m

2778
Organometallics 1998, 17, 2778-2783
Th e Na tu r e of th e Rin g-Op en in g P r ocess in a Su bstitu ted
Cyclop r op ylm eth ylir on Com p lex
E. Alexander Hill* and Baoju Li
The Department of Chemistry, University of Wisconsin-Milwaukee, P.O. Box 413,
Milwaukee, Wisconsin 53201
Received October 16, 1997
Irradiation of CpFe(CO)2(2,2-dimethylcyclopropylmethyl) (4) in the absence and presence
of triphenylphosphine leads to the ring-opened products CpFe(CO)CH2CMe2CHdCH2 (6)
and CpFe(CO)(PPh3)CH2CMe2CHdCH2) (7). These are interconverted on further irradiation
and on heating are more slowly converted to the π-allylic complex CpFe(CO)(η3CH2-
CHCHCHMe2) (8). Ring-opening rearrangement in the sense to produce mainly 6 or 7 (with
a primary σ-bonded alkyl group) rather than products derived from a tertiary σ-bonded alkyl
group is most consistent with a concerted four-center organometallic rearrangement process.
A radical route, previously proposed for the unsubstituted cyclopropylmethyl complex, is
unlikely. Complex 4 also undergoes normal migratory insertion to an acyl complex, without
opening of the cyclopropylmethyl group.
In tr od u ction
products were formed in its absence, accompanied by
much decomposition. Irradiation of 1a or 1b or their
migratory insertion products led to more rapid forma-
tion of the crotyl π-complexes. The crotyl compounds
were considered to be the result of a cyclopropylmethyl/
3-butenyl rearrangement, followed by shift of a hydro-
gen (eq 2). The presumed intermediates were not
Rearrangements interconverting cycloalkylmethyl and
unsaturated organometallic compounds have long been
known2,3 (e.g., the conversion of cyclopropylmethyl to
3-butenyl Grignard reagents, eq 1).4 In 1987, Pannell
and co-workers5 published a study of transition-metal
cyclopropylmethyl derivatives 1a -c. When treated
with triphenylphosphine in THF, all three underwent
facile migratory insertion without rearrangement of the
cyclopropylmethyl group to yield the corresponding
metal acyls, CpM(CO)n-1(PPh3)COCH2C3H5. However,
in refluxing hexane, 1a and 1b were converted into
phosphine-free η3-crotyl complexes 3a and 3b. Reaction
was faster in the presence of PPh3, but the same
detected, but irradiation of 1c in hexane with Ph3P
did produce a rearanged migraory insertion product,
(1) (a) Taken in part from the Ph.D. Thesis of B. Li, University of
Wisconsin-Milwaukee, 1995. (b) Presented in preliminary form at the
204th American Chemical Society National Meeting, Chicago, IL,
August 1993, Abstract O-280 and at the IUPAC 12th Conference on
Physical Organic Chemistry, Padova, Italy, 1994.
(2) (a) Hill, E. A. J . Organomet. Chem. 1975, 91, 123. (b) Hill, E. A.
In Advances in Organometallic Chemistry; Stone, F. G. A., West, R.,
Eds.; Academic: New York, 1977; Vol. 16, p 131. (c) Stirling, C. M. J .
Chem. Rev. 1978, 78, 517.
(3) For example, see: (a) Hill, E. A.; Li, B. J . Organomet. Chem.
1993, 448, 9. (b) Hill, E. A.; Park, Y.-W. J . Organomet. Chem. 1988,
356, 1. (c) Bailey, W. F.; Punzalan, E. R.; Della, E. W.; Taylor, D. K. J .
Org. Chem. 1995, 60, 297. (d) Chum, P. W.; Wilson, S. E. Tetrahedron
Lett. 1976, 1257. (e) Flood, T. C.; Statler, J . A. Organometallics 1984,
3, 1795. (f) Marks, T. J .; Wachter, W. A. J . Am. Chem. Soc. 1976, 98,
703. (g) Miginiac, P.; Daviaud, G. J . Organomet. Chem. 1976, 104, 139.
(h) Whitesides, G. M.; Bergbreiter, D. E.; Kendall, P. E. J . Am. Chem.
Soc. 1974, 96, 2806. (i) St. Denis, J .; Dolzine, T.; Oliver, J . P. J . Am.
Chem. Soc. 1972, 94, 8260.
CpW(CO)2(COCH2CH2CHdCH2).
The authors assumed that rearrangement of the
cyclopropylmethyl group had occurred by a radical
mechanism. Loss of a CO ligand from the metal would
leave a 16-electron intermediate 2, and subsequent
homolytic cleavage of the metal-C bond would form a
15-electron species and a cyclopropylmethyl radical. The
known, very rapid rearrangement of the latter6 to a
3-butenyl radical, followed by radical recombination,
would complete conversion to the 3-butenylmetal com-
pound.
(4) Silver, M. S.; Shafer, P. R.; Nordlander, J . E.; Ruchardt, C.;
Roberts, J . D. J . Am. Chem. Soc. 1960, 82, 2646.
(5) Pannell, K. H.; Kapoor, R. N.; Wells, M.; Giasolli, T.; Parkanyi,
L. Organometallics 1987, 6, 663.
(6) (a) Roberts, J . D.; Mazur, R. H. J . Am. Chem. Soc. 1951, 73, 2509.
(b) Beckwith, A. L. J .; Bowry, V. W. J . Org. Chem. 1989, 54, 2681. (c)
Newcomb, M.; Glenn, A. G. J . Am. Chem. Soc. 1989, 111, 275. (d)
Bowry, V. W.; Ingold, K. U. J . Am. Chem. Soc. 1992, 114, 4992.
S0276-7333(97)00906-0 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 06/05/1998

Ring-Opening in a Cyclopropylmethyl Fe Complex
Organometallics, Vol. 17, No. 13, 1998 2779
Sch em e 1
The proposal of only the radical mechanism for
rearrangement of the cyclopropylmethyl group was
unexpected for two reasons. First, a number of other
rearrangements of cycloalkylmethylmetal compounds
are likely to occur via concerted mechanisms involving
neither radical nor carbanionic intermediates.2,3 It
would seem reasonable to at least consider a similar
mechanism in the current case. Second, the mechanism
proposed involves cleavage of the metal-carbon σ bond
in the short-lived, reactive, coordinatively unsaturated
intermediate 2, forming a 15-electron species which is
also expected to be unstable. To be significant, this
cleavage would have to be very fast. The case has been
made for homolytic cleavage of this sort in similar 16-
electron intermediates.5,7 However, it also appears that
alternative pathways for reaction of the 16-electron
intermediate are preferred if they are available,8 and
the mechanism involving a 15-electron intermediate has
been questioned.8e For this reason, we have investi-
gated an analogous cyclopropylmethyliron compound,
substituted on the cyclopropane ring in a manner which
distinguishes between a radical mechanism and a
nonradical “organometallic” mechanism. Our conclu-
sion, as detailed below, is that the reaction does not
involve rearrangement of a cyclopropylmethyl radical,
but is instead a concerted rearrangement of the 16-
electron intermediate.
Resu lts
The 2,2-dimethylcyclopropylmethyliron complex 4
[CpFe(CO)2DMCPM] was prepared by reaction of
-
Na+CpFe(CO)2 with 2,2-dimethylcyclopropylmethyl
bromide or chloride. Its NMR and IR spectra are
consistent with published data for other Fp-alkyl
complexes;5,9-11 in common with other 2,2-dimethylcy-
clopropylmethyl compounds prepared, the diastereotopic
methylene protons are magnetically nonequivalent. The
reactions of 4, described below, are summarized in
Scheme 1.
(7) (a) Severson, R. G.; Wojcicki, A. J . J . Organomet. Chem. 1978,
157, 173. (b) Mahmoud, K. A.; Narayanaswamy, R.; Rest, A. J . J . Chem.
Soc., Dalton Trans. 1981, 2199. (c) Gismondi, T. E.; Rausch, M. D. J .
Organomet. Chem. 1985, 284, 59. (d) Pourreau, D. B.; Geoffroy, G. L.
In Advances in Organometallic Chemistry; Stone, F. G. A., West R.,
Eds.; Academic: New York, 1985; Vol. 24, p 249.
(8) (a) Young, K. M.; Wrighton, M. J . Am. Chem. Soc. 1990, 112,
157. (b) Crowther, D. J .; Tivakornpannarai, S.; J ones, W. M. Organo-
metallics 1990, 9, 739. (c) Virrels, I. A.; George, M. W.; J ohnson, F. A.
P.; Turner, J . J .; Westwell, J . R. Organometallics 1995, 14, 5203. (d)
Yang, G. K.; Peters, K. S.; Vaida, V. J . Am. Chem. Soc. 1986, 108,
2511. (e) Hooker, R. H.; Mahmoud, K. A.; Rest, A. J .; Alt, H. G. J .
Organomet. Chem. 1991, 419, 101.
(9) (a) Cameron, A. D.; Laycock, D. E.; Smith, V. H., J r.; Baird, M.
C. J . Chem. Soc., Dalton Trans. 1987, 2857. (b) San Filippo, J ., J r.;
Silbermann, J .; Fagen, P. J . J . Am. Chem. Soc. 1978, 100, 4834. (c)
Bly, R. S.; Bly, R. K.; Hossain, M. M.; Silverman, G. S.; Wallace, E.
Tetrahedron 1986, 42, 1093. (d) Giese, B.; Thoma, G. Helv. Chim. Acta
1991, 74, 1143. (e) Li, H.; Turnbull, M. M. J . Organomet. Chem. 1991,
419, 245.
(10) (a) Scharrer, E.; Brookhart, M. J . Organomet. Chem. 1995, 497,
61. (b) Amiens, C.; Balavoine, G.; Guibe´, F. J . Organomet. Chem. 1993,
443, 207. (c) Su, S. R.; Wojcicki, A. J . Organomet. Chem. 1971, 27,
231. (d) van Doorn, J . A.; Masters, C.; Volger, H. C. J . Organomet.
Chem. 1976, 105, 245.
(11) Green, M. L. H.; Smith, M. J . J . Chem. Soc. A 1971, 3220.

2780 Organometallics, Vol. 17, No. 13, 1998
Hill and Li
Refluxing 4 with excess Ph3P in THF or hexane led
to the corresponding migratory insertion product, 5,
with no apparent side reactions. Spectroscopic data
were again consistent with published data for similar
compounds.5,10 The presence of two diastereomers of 5
in approximately equal amounts was evident from the
As in the reported5 study of compounds 1a -c, we
presume that the photochemical reactions leading to
ring-opened products are initiated by the dissociation
of a CO ligand from the metal, forming the 16-electron
intermediate 9. Homolytic cleavage of the Fe-C bond
at this stage, as proposed5 for compounds 1a and 1b
and detailed in Scheme 2, would generate a 2,2-
dimethylcyclopropylmethyl radical, 11, paired with the
15-electron iron species 10. Radical 11 has been studied
previously by Newcomb, Glenn, and Williams18 and by
Beckwith and Bowry,6b using scavenging methods, and
in our laboratory19 via perester photolysis. It undergoes
ring cleavage with an overall rate constant (at 25 °C)
about 6-9 times greater than that of the unsubstituted
cyclopropylmethyl radical, and the tertiary radical 12
is kinetically favored by a ratio of about 7:1 over the
primary radical 13 in this rearrangement. Therefore,
the major intermediate formed by radical recombination
after ring cleavage should be the tertiary iron alkyl 14.
Subsequent hydrogen migration (as in the reactions of
1a and b) would produce the η3-allylic complex 8 as the
principal kinetically controlled product. Although 8 is
formed on heating or further photolysis of 6 or 7, it is
not a significant initial reaction product. Alternative
disproportionation of the radical pair [10• •12] would
produce CpFe(CO)H and dienes. The former is likely
to decompose, and the latter might polymerize under
reaction conditions. The formation of products 6 and 7
in moderately good yields, the absence of 8 as an
important initial product, and the lack of major decom-
position during the photolysis combine to make it
unlikely that the radical-pair mechanism contributes
significantly in the photochemical reaction of 4 to 6 and
7.
An alternative to the radical-pair mechanism is a
concerted molecular mechanism of rearrangement
(Scheme 3). On the basis of several kinds of mechanistic
evidence, this type of mechanism has been favored over
alternative radical-pair or ion-pair routes for the ring-
opening rearrangements of cyclopropylmethyl and cy-
clobutylmethyl organomagnesium compounds.2a For
steric and, probably, electronic reasons, these rear-
rangements favor cleavage of the less-substituted ring
bond, leading to the preferential formation of 1° > 2° >
3° organometallic product.2,3b,20 This is the opposite of
the radical’s behavior but in line with observations for
4.
Seen in the reverse direction, the mechanism of
Scheme 3 is analogous to the addition step in transition-
metal-catalyzed coordination polymerization reactions,
in which a coordinated alkene molecule is believed to
insert into an alkyl-metal bond.21 This mechanism
implies that a vacant coordination site on the metal is
important, perhaps essential, for the ring cleavage of
the cyclopropylmethyl organometallic compound to oc-
cur. In this context, it might be noted that analogous
rearrangements of cyclopropylmethyl- and cyclobutyl-
methylboranes are strongly retarded by the addition of
pyridine, which coordinates to the vacant orbital on the
1
13C and H NMR spectra, and the diastereotopic R-me-
thylene protons of both were distinguishable. In the
absence of Ph3P, no sign of reaction was noted in the
IR spectrum after 54 hours of refluxing in hexane. In
refluxing heptane, decomposition occurred without for-
mation of identifiable organometallic products.
Irradiation of 4 in THF or hexane, in the absence of
Ph3P, led to the loss of CO and formation of the ring-
opened complex 6, in which the newly generated double
bond is coordinated to the iron. This compound has
previously been prepared in another fashion by Green
and Smith.11 The relatively high-field shift of the
“olefinic” carbons and protons is characteristic of the
iron-coordinated double bond.11,12 The same photo-
chemical reactions in the presence of excess Ph3P
yielded 7, in which the phosphine replaces the double
bond in its coordination to the metal. The olefinic
carbon and proton resonances of 7 are within normal
ranges,13 and other NMR parameters are consistent
with those of analogous complexes.16 In the NMR
spectra of both 6 and 7, the methylene hydrogens H1
and H1′ and the methyl groups are nonequivalent
because of the stereogenic iron center. In the presence
of a 2-fold excess of Ph3P, irradiation led to an apparent
photostationary mixture of 6 and 7 with approximate
composition 40:60.
When either 6 or 7 was heated at reflux in hexane,
or when 4, 6, or 7 was subjected to an extended period
of irradiation, the allylic π-complex 8 was formed. Its
proton and carbon NMR parameters are consistent with
the stereochemical features shownsexo orientation of
the allyl fragment and syn configuration of the isopropyl
groupswhich are favored in other allylic complexes.17
The structure is chiral, so the methyl groups are
nonequivalent.
Discu ssion
The migratory insertion reaction of 4 to form 5
appears unexceptional and warrants no discussion
beyond noting that the absence of cyclopropylmethyl
ring opening confirms the lack of involvement of radicals
in that process.
(12) Mahmoud, K. A.; Rest, A. J .; Alt, H. G. J . Chem. Soc., Dalton
Trans. 1985, 1365.
(13) The olefinic carbon shifts of 106.01 and 153.37 ppm are at
relatively high and low fields, respectively, in comparison with the
hydrocarbon 3,3-dimethyl-1-butene at 109.0 and 149.8 ppm.14 These
shifts are consistent with a “linear electric field” effect, with the metal
as a moderately electropositive group.15
(14) Sadtler Standard Carbon-13 NMR Spectra, Sadtler Research
Laboratories, Philadelphia, #282.
(15) Hill, E. A.; Guenther, H. R. Org. Magn. Reson. 1981, 16, 177.
(16) (a) Davies, S. G.; Dordor-Hedgecock, I. M.; Sutton, K. H.;
Whittaker, M. J . Am. Chem. Soc. 1987, 109, 5711. (b) Idmoumaz, H.;
Lin, C. H.; Hersh, W. H. Organometallics 1995, 14, 4051.
(17) (a) Mercour, J .-Y.; Charrier, C.; Roustan, J .-L.; Benaim, J . C.
R. Seances Acad. Sci., Ser. C 1971, 273, 285. (b) Fish, R. W.; Giering,
W. P.; Marten, D.; Rosenblum, M. J . Organomet. Chem. 1976, 105,
101. (c) Faller, J . W.; J ohnson, B. V.; Dryja, T. P. J . Organomet. Chem.
1974, 65, 395. (d) Faller, J . W.; Chen, C.-C.; Mattina, M. J .; J akubows-
ki, A. J . Organomet. Chem. 1973, 52, 361. (e) Conti, N. J .; Crowther,
D. J .; Tivakornpannarai, S.; J ones, W. M. Organometallics 1990, 9,
175.
(18) Newcomb, M.; Glenn, A. G.; Williams, W. G. J . Org. Chem.
1989, 54, 2675.
(19) Li, B.; Hill, E. A. Unpublished results.
(20) Hill, E. A.; Chen, A. T.; Doughty, A. J . Am. Chem. Soc. 1976,
98, 167.
(21) Lukehart, C. M. Fundamental Transition Metal Organometallic
Chemistry; Brooks-Cole: Monterey, CA, 1985; pp 256-9.

Ring-Opening in a Cyclopropylmethyl Fe Complex
Organometallics, Vol. 17, No. 13, 1998 2781
Sch em e 2
Sch em e 3
boron.22 In the present reaction, rearrangement of the
cyclopropylmethyl group appears to require initial loss
of a carbonyl ligand in order to free a coordination site
on the iron, after which rearrangement occurs very
rapidly.
In the absence of Ph3P, the product isolated (6) retains
coordination of the double bond with the iron. With the
phosphine present, the major product is 7, in which the
double bond is replaced in the coordination sphere of
the metal. Since photochemical equilibration between
6 and 7 occurs faster than the photolysis of 4, it is
reasonable that 7 resulted from subsequent displace-
ment by the added phosphine.
(22) (a) Koster, R.; Arora, S.; Binger, P. Angew. Chem., Int. Ed. 1969,
8, 205. (b) Hill, E. A.; Nylen, P. A.; Fellinger, J . H. J . Organomet. Chem.
1982, 239, 279.

2782 Organometallics, Vol. 17, No. 13, 1998
Hill and Li
hexane solution in a 0.1-mm sodium chloride cell on a Nicolet
5DXC FT-IR spectrometer. NMR spectra were recorded in
CDCl3 solution on a Bruker WM 250 spectrometer (1H, 250.1
MHz; 13C, 62.9 MHz). Chemical shifts are vs internal TMS,
either directly or via solvent at 7.24 or 77.0 ppm, respectively.
The numbering of the protons and carbons is as shown in
structures 4, 7, and 8. Resonances were assigned with the
assistance of DEPT, single-frequency off resonance, and homo-
nuclear H-H decoupling experiments and comparison of
chemical shifts and coupling constants with those of analogous
compounds. Mass spectra were run on a Hewlett-Packard
5985 mass spectrometer using electron impact ionization (solid
inlet, 70 eV). Abbreviations Cp and DMCPM represent η5-
cyclopentadienyl and 2,2-dimethylcyclopropylmethyl, respec-
tively.
Heating or further irradiation of either 6 or 7 leads
to a slower formation of the η3-allylic complex 8, the
apparent thermodynamic sink in the system. This final
conversion might be formulated to involve reversible
recyclization of 6 to the 16-electron intermediate 9,
followed by the alternative ring cleavage of the dimeth-
ylcyclopropane ring to produce the tertiary organoiron
complex 14. The latter ring opening might occur by the
same type of molecular mechanism as proposed above
for interconversion of 4 and 6 but in the kinetically less-
favored sense. Alternatively, it might, in fact, occur by
a radical pathway. In any event, the gem-dimethyl
substitution of the alkenyl ligand blocks direct conver-
sion of 6 or 7 to an η3-allylic complex, in addition to
directing the regiochemistry of the ring opening.
2,2-Dim eth ylcyclop r op ylm eth yl Ch lor id e. 2,2-Dimeth-
ylcyclopropanecarboxylic acid was prepared via the corre-
sponding nitrile;23 bp 195 °C (lit.23 bp 198 °C). The acid was
then reduced to 2,2-dimethylcyclopropylmethanol with lithium
aluminum hydride in ether; bp 135 °C (lit.24 bp 93-94 °C (118
Torr)).
Con clu sion
The present study appears to definitively rule out a
radical mechanism in the rearrangement of 4 to 6 and
7 and to cast doubt upon its occurrence in the rear-
rangements of 1a and 1b.5
Tri-n-butylphosphine (90 mL, 0.36 mol) was added dropwise
to a vigorously stirred solution of the alcohol (21 g, 0.21 mol)
in 350 mL of CCl4, chilled to 0 °C. The solution became
viscous, and after 72 h at 0 °C, the 1H NMR indicated that
reaction was complete. Pentane (100 mL) was added, the
upper (pentane) layer was separated, and the lower layer was
extracted three more times with pentane. Solvent was stripped
(10 Torr), and the product was transferred under high vacuum
to a cold trap and redistilled; 17.9 g, 72%; bp 120 °C (760 Torr).
1H NMR: δ 0.19 (H3t, t, J 3c,3t ≈ J 1,3t ) 4.8 Hz), 0.59 (H3c, d/d,
J 3c,3t ) 4.8, J 1,3c ) 8.3 Hz), 0.96 (H1, m), 1.07 and 1.11 (3H
each, H4 and H5, s), 3.41 (HR, d/d, J R,R′ ) 11.2, J R,1 ) 9.3 Hz),
3.71 (HR′, d/d, J R,R′ ) 11.2, J R′,1 ) 6.8 Hz). 13C NMR: δ 18.13
(C2), 19.40 (C5), 20.82 (C3), 26.55 (C1), 26.95 (C4), 47.43 (CR).
2,2-Dim et h ylcyclop r op ylm et h yl Br om id e. Tri-n-bu-
tylphosphine (17 mL, 0.068 mol) was added dropwise to a
vigorously stirred solution of 2,2-dimethylcyclopropylmethanol
(5 g, 0.05 mol) and carbon tetrabromide (20 g, 0.06 mol) in 75
mL of pentane, cooled on a -78 °C bath. The reaction flask
was wrapped with a black cloth, warmed to room temperature,
and stirred for 60 h. A viscous lower layer separated, and the
absence of alcohol peaks in the 1H NMR spectrum indicated
that reaction was complete. The lower layer was extracted 3
times with pentane, and solvent was stripped (10 Torr) from
the combined pentane layers. The bromide (6 g, 73%) was
transferred under high vacuum to a -78 °C trap, stored in
the dark at -78 °C, and used within 8 h. The 1H NMR
spectrum indicated less than 3% of rearrangement to open-
chain bromide. 1H NMR: δ 0.20 (H3t, t, J 3c,3t ≈ J 1,3t ) 4.9 Hz),
0.64 (H3c, d/d, J 3c,3t ) 4.9, J 1,3c ) 8.5 Hz), 1.07 and 1.11 (3H
each, H4 and H5, s), 1.1 (H1, m), 3.28 (HR, d/d, J R,R′ ) 10.2, J R,1
) 9.3 Hz), 3.59 (HR′, d/d, J R,R′ ) 10.2, J R′,1 ) 7.1 Hz). 13C
NMR: δ 18.97 (C5), 19.46 (C2), 22.31 (C3), 26.80 (C4), 26.89
(C1), 36.19 (CR).
However, the behavior of 4 does not completely
parallel that of 1a and 1b, and so it might be argued
that our results do not preclude the previously sug-
gested homolytic pathway for rearrangement of the
latter. Most notably, we found only the migratory
insertion product 5 on heating 4 with PPh3 in either
hexane or THF. We observed the ring-opened rear-
ranged products from 4 only on irradiation. In contrast,
the cyclopropylmethyl groups of 1a and 1b did rear-
range on heating in hexane solution to form the π-crotyl
complexes 3a and 3b (both in the presence of PPh3 or
more slowly in its absence). Also, 1a appears to have
reacted more rapidly than 4, particularly in the migra-
tory insertion, though not all of the experiments were
reported in detail.
The methyl substituents in 4 clearly influence the
quantitative balance among competing pathways of
decomposition, migratory insertion, and rearrangement,
relative to compound 1a . We do not have a simple
rationalization for differences in reactivity between 1a
and 4. It remains possible that the methyl groups have
in fact produced a qualitative change in the mechanism
of the rearrangement of 4, either preventing the ho-
molytic dissociation of the Fe-C bond in the 16-electron
intermediate 9 or accelerating its molecular rearrange-
ment. However, we prefer the alternative that a single
mechanism, the molecular mechanism consistent with
the results in the present study of 4, is responsible for
the cyclopropylmethyl ring openings in all cases.
Cp F e(CO)2(DMCP M) (4). A solution of Na+[CpFe(CO)2]-
was prepared8a,25 by addition of [CpFe(CO)2]2 (1.167 g, 3.30
mmol) in 20 mL of THF to sodium amalgam (from 0.76 g, 33
mmol of Na, and 5.8 mL of Hg) and 10 mL of THF. The
supernatant was transferred to centrifuge tubes via cannula,
centrifuged until clear, transferred to a reaction flask, and
cooled to 0 °C. 2,2-Dimethylcyclopropylmethyl bromide (1.0
g, 6.2 mmol) was added by syringe, and the reaction mixture
was stirred at 0 °C for 3 h. The solvent was distilled to a cold
trap under vacuum, and the residue was dissolved in pentane
and transferred by cannula to a column of alumina (Brockman
Exp er im en ta l Section
Reactions involving organometallic compounds were carried
out under dry nitrogen that had additionally passed through
a tube of Drierite. Photochemical reactions were run under
nitrogen in a quartz tube in a Rayonet photochemical reactor
(Srinivasan-Griffin, with 16 low-pressure mercury lamps).
During photolysis, the temperature held at 31 ( 1 °C. Column
chromatography was generally performed under nitrogen on
a 2 × 50-cm column of Brockman grade I acidic alumina;
variations from this are noted. THF and ether were distilled
under nitrogen from sodium/benzophenone. Pentane, hexane,
and heptane were distilled from sodium metal. Melting and
boiling points are uncorrected. IR spectra were taken in
(23) Nelson, E. R.; Marienthal, M.; Lane, L. A.; Benderly, A. A. J .
Am. Chem. Soc. 1957, 79, 3467.
(24) Bly, R. S.; Swindell, R. T. J . Org. Chem. 1965, 30, 10.
(25) King, R. B. Organometallic Synthesis; Academic: New York,
1965; Vol. 1.

Ring-Opening in a Cyclopropylmethyl Fe Complex
Organometallics, Vol. 17, No. 13, 1998 2783
grade I acidic, deactivated with 10% of water). A yellow band
eluted with pentane was concentrated to an orange oil (0.77
g, 48%). Similar preparation from the chloride yielded 4 of
Cp F e(CO)(P P h 3)(σ-2,2-d im eth ylbu t-3-en yl) (7); Ir r a -
d ia tion of 4 w ith P P h 3. A solution of 4 (2.5 g, 9.6 mmol)
and PPh3 (3.85 g, 14.68 mmol) in 100 mL of hexane was
irradiated for 62 h. The solution was chromatographed.
Unreacted starting materials were eluted with hexane, and
an orange product band followed with hexane/ether (1/1).
After removal of solvent under vacuum, 3.32 g (70%) of 7 was
obtained as an orange oil. IR: 1912 cm-1. MS (15 eV): 466
(M - CO, 0.2), 464 (0.3), 383 (2), 277 (11), 262 (100), 232 (5),
204 (31), 200 (29), 183 (59), 176 (7), 162 (14), 160 (9), 148 (21)
134 (19), 121 (21), 108 (64). 1H NMR: δ 0.83 (H1, partially
obscured, but consistent with d/d, J 1,1′ ) 9.8, J 1,P ) 15.0 Hz),
0.85 and 0.86 (3H each, H5 and H6, s), 1.96 (H1′, d, J 1,1′ ) 9.8,
J 1′,P ≈ 0 Hz), 4.27 (5H, Cp, d, J P,H ) 1.0 Hz), 4.65 (H4c, d/d,
J 4c,4t ) 1.85, J 3,4c ) 10.5 Hz), 4.73 (H4t, d/d, J 4c,4t ) 1.82, J 3,4t
) 17.65 Hz), 5.80 (H3, d/d, J 3,4c ) 10.7, J 3,4t ) 17.5 Hz), 7.28-
7.45 (Ph, m). 13C NMR: δ 17.26 (C1, d, J P,C ) 18 Hz), 29.74
and 31.05 (C5 and C6), 41.73 (C2), 84.62 (Cp), 106.01 (C4),
127.87 (Ph Cm, d, J P,C ) 8.8 Hz), 129.31 (Ph Cp), 133.25 (Ph
Co, d, J P,C ) 9.4 Hz), 136.80 (Ph Cipso, d, J P,C ) 40 Hz), 153.37
(C3), 224.5 (CO, d, J P,C ) 32 Hz).
similar purity in 61% yield. IR: 1955 and 2010 cm-1
. MS
260 (M+, 2), 232 (3), 204 (16), 200 (21), 178 (31), 177 (10), 162
(13), 160 (9), 149 (32), 134 (18), 121 (100). 1H NMR: δ -0.10
(H3t, t, J 3c,3t ≈ J 3t,1 ) 3.9 Hz), ca. 0.55 (2H, H3c and H1, m),
0.97 and 1.03 (3H each, H4 and H5, s), 1.39 (HR, t, J R,R′ ≈ J R,1
) 8.6 Hz), 1.70 (HR′, d/d, J R,R′ ) 8.6, J R′,1 ) 5.1 Hz), 4.73 (5H,
Cp, s). 13C NMR: δ 4.16 (CR), 19.56 (C5), 20.53 (C2), 24.95 (C3),
27.66 (C4), 33.48 (C1), 85.25 (Cp), 217.67 (CO). Anal. Calcd
for C13H16FeO2: C, 60.03; H, 6.20. Found: C, 60.45; H, 6.40.
Cp F e(CO)(P P h 3)(CO-DMCP M) (5); Rea ction of 4 w ith
P P h 3. A solution of 4 (1.1 g, 4.2 mmol) and PPh3 (2.22 g, 8.46
mmol) in 75 mL of hexane was refluxed under nitrogen. After
48 h, the color had changed from yellow to pink and changes
in the IR carbonyl absorption indicated that the reaction was
nearly complete. The reaction mixture was passed through a
column of silica gel. Unreacted 4 and PPh3 were eluted with
hexane, followed by an orange fraction with ether-hexane (1:
4). Removal of the solvent under vacuum yielded 5 as a yellow
powder (1.57 g, 71%); mp 112-117 °C. IR: 1920 and 1619
cm-1
. MS 439 (M - C6H11, 37), 411 (2), 383 (99), 262 (54),
Similar irradiation of 4 (0.132 g, 0.51 mmol) and PPh3 (0.35
g, 1.3 mmol) in 10 mL of THF for 16 h yielded 0.166 g (66%)
of 7.
204 (9), 200, (11), 183 (100), 152 (15), 149 (17), 121 (57), 108
(38). The NMR spectra indicated an inseparable mixture of
diastereomers in a ratio of about 1:1; spectra for the two are
summarized together, because resonances are not clearly
assignable to the two isomers. 1H NMR: δ -0.47 and -0.18
Equ ilibr a tion of 6 a n d 7. A solution of 6 (0.4 g, 1.72
mmol) and PPh3 (0.81 g, 3.08 mmol) in 10 mL of hexane was
irradiated for 22 h. Monitoring by IR showed that the reaction
had reached a stationary state with 6 and 7 in a ratio of 40/
60. The mixture was separated by chromatography; unreacted
PPh3 and 6 were eluted with pentane, followed by 7 (0.29 g,
34%) with 1:1 pentane-ether. Repetition in THF led to similar
results; 7 was isolated in 29% yield.
(H3t, t, J 3t,3c ≈ J 1,3t ) 4.5 Hz), 0.17 and 0.33 (H3c, d/d, J 3t,3c
)
4.2, J 1,3c ) 8.5 Hz), 0.58 (H1, m), 0.79, 0.90, 0.91, and 0.94 (each
3H, H4 and H5, s), 2.36 and 2.50 (HR, d/d, J R,R′ ) 16.8 and 16.1,
J R,1 ) 8.0 and 6.1 Hz), 3.01 and 2.81 (HR′, d/d, J R,R′ ) 16.9 and
16.0, J R′,1 ) 6.0 and 7.7 Hz), 4.38 and 4.40 (Cp, d, J PH ) 1.15
and 1.12 Hz), 7.35 and 7.45-7.53 (Ph). 13C NMR: δ 14.33 (C2),
19.17 and 19.38 (C3), 19.81 and 19.90 (C5), 20.87 (C1), 26.79
and 26.94 (C4), 65.98 and 66.98 (CR), 84.85 (Cp), 127.64 (Ph
Cm, d, J PC ) 9 Hz), 129.27 (Ph Cp), 133.01 (Ph Co, d, J PC ) 11
A solution of 6 (0.6 g, 2.59 mmol) and PPh3 (1.00 g, 3.8
mmol) in 20 mL of hexane was refluxed for 3.5 h. The IR
carbonyl spectrum indicated the presence of 6 and 7 in a ratio
of 55/45. Compound 7 was isolated by column chromatography
in 30% yield.
Cp F e(CO)[η3-(1-isop r op yla llyl)] (8); Th er m a l R ea r -
r a n gem en t of 6 or 7. A solution of 6 (0.6 g, 2.59 mmol) in
100 mL of hexane was refluxed for 4 h and chromatographed.
A yellow band eluted with hexane yielded 8 as an orange oil
after removal of solvent (0.45 g, 75%). IR: 1950 cm-1. MS:
232 (M+, 11), 204 (38), 200 (51), 186 (44), 176 (9), 148 (12),
134 (30), 121 (100). 1H NMR: δ 0.32 (H1a, d/d, J 1a,2 ) 10.6,
J 1a,1s ) 1.4 Hz), ca. 1.15 (H3, m, obscured), 1.13 and 1.17 (H5
and H6, 3H each, d, J 4,5 ) J 4,6 ) 6.55 Hz), 1.82 (H4, d/sept, J 3,4
Hz), 136.25 (Ph Cipso, d, J PC ) 42 Hz), 220.29 (CO, d, J PC
)
31.5 Hz). Anal. Calcd for C31H31FeO2P: C, 71.27; H, 5.98.
Found: C, 71.62; H, 5.93.
A similar reaction of 4 (1.1 g, 4.2 mmol) with PPh3 (3.0 g,
11.4 mmol) in THF (60 mL) yielded 1.35 g (61%) of 5 (77 h
reflux).
A solution of 4 (0.538 g, 2.07 mmol) in 60 mL of hexane was
heated at reflux for 54 h without PPh3. No reaction was
indicated in the IR or 1H NMR spectra.
Cp F e(CO)(σ-2,2-d im eth ylbu t-3-π-en yl) (6); Ir r a d ia tion
of 4. A solution of 4 (1.62 g, 6.23 mmol) in 75 mL of hexane
was irradiated for 28 h; the reaction was judged by IR to be
complete. On chromatography, a yellow band eluted with
ether/pentane (1/1) yielded, after vacuum removal of solvent,
1.26 g (87%) of 6 as an orange oil.11 IR: 1947 cm-1. MS: 232
(M+, 9), 204 (44), 200 (56), 186 (11), 176 (10), 167 (10), 162
(25), 160 (18), 149 (28), 148 (26), 134 (36), 121 (100). 1H
NMR: δ -1.13 (H1, d, J 1,1′ ) 5.7 Hz), 0.20 (3H, H5 or H6, s),
1.04 (H1′, d, J 1,1′ ) 5.4 Hz), 1.12 (3H, H5 or H6, s), 1.83 (H4t, d,
) 9.5, J 4,5 ) J 4,6 ) 6.5 Hz), 2.55 (H1s, d/d, J 1s,2 ) 6.9, J 1a,1s
)
1.4 Hz), 4.32 (H2, d/t, J 1s,2 ) 7.1, J 1a,2 ) J 2,3 ) 10.5 Hz), 4.46
(Cp, 5H, s). 13C NMR: δ 24.13 and 26.15 (C5 and C6), 28.30
(C1), 34.70 (C4), 70.12 (C3), 71.48 (C2), 78.85 (Cp), 221.8 (CO).
Similar reaction of 7 (0.5 g, 1.01 mmol) in hexane (50 mL)
also produced 8 (0.17 g, 71%).
J 3,4t ) 12.5 Hz), 2.37 (H4c, d, J 3,4c ) 8.0 Hz), 3.03 (H3, d/d, J 3,4t
)
Ack n ow led gm en t. We are grateful to Dr. Eugene
DeRose for advice and assistance with NMR measure-
ments, Frank Laib for mass spectra, and Keith Krum-
now for elemental analyses.
12.4, J 3,4c) 8.05 Hz), 4.51 (5H, Cp, s). 13C NMR: δ -22.37
(CH2), 23.97 and 33.14 (CH3), 34.30 (C), 35.61 (CH2), 45.03
(CH), 83.31 (Cp), 227 (CO).
Similar irradiation of 4 (0.81 g, 3.11 mmol) in THF (37 mL)
for 35 h yielded 0.47 g (65%) of 6.
OM970906M