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How is Phosphorus Bound in 2-Phospha- and 2-УPhosphoniaallylФ Cations.

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by irradiation of 2b in acetone. Transient 2b is thus firmly
established as an intermediate in the transformation of l b
into 3b.
The light-induced step 2b-3b is ca. 1.7 times faster in
acetone than in [D,]acetone. In both experiments, C,H,
(ca. 80%) and CH,, but neither CH,OH nor C,H,, were
found in the gas phase by GC-MS. The H / D isotope effect
shows that hydrogen abstraction is rate determining. The
formation of ethane indicates that the 8-methoxy group is
homolytically cleaved; moreover, the absence of methanol
and ethylene rules out heterolytic cleavage and carbene
elimination. Consequently, there are two possibilities for
the primary step in the transformation of 2 into 3 : either
hydrogen abstraction by the keto group (2- 11) or the regioselective[' ' ] p-cleavage of the cyclopropyl ketone following n+n* excitation (2-111). For 11, ring opening would
be expected to dominate over hydrogen abstraction from
isopropyl alcohol, as has been found even for less strained
a-hydroxycyclopropylcarbinyl radicals.i141
The existence of intermediate I11 is strongly supported
by an additional observation: On irradiation of 2d
(A= 300 nm, benzene) in the presence of 2-trimethylsiloxybutadiene (spectral cut-off at ca. 290 nm), three of the four
possible products with constitutions 5 and 6''.'s1 are
formed in a ratio of 1 :2 :4. Since direct excitation was
used in this experiment, the diene most likely quenched all
triplet-excited ketone by intersystem crossing. The conclusion drawn may therefore be valid only for the reaction of
the singlet excited state of 2b. That the addition occurs
with poor regioselectivity is in better accord with the trapping of a biradical than a zwitterion. Furthermore, the
mechanism of the transformations 2- 3 is very likely independent of the substitution patterns a-e.
How is Phosphorus Bound in 2-Phospha- and
2-"Phosphoniaallyl" Cations?**
By Roberta 0. Day. Angela Willhalm, Joan M . Holmes,
Robert R . Holmes,* and AIfred Schmidpeter*
Dedicated to Professor Karl Dimroth on his 75th birthday
The 2-phosphaallyl cationsi'.21are of practical interest as
building blocks in the synthesis of phosphaheterocyclesi2]
as well as of theoretical interest as models with phosphorus in the center of a potentially conjugated chain.I3' Their
bonding system may in principle be described by one of
the three possibilities A , B, and C (E=P), which should
result in different structures.
For E = N,IYstructurally investigated examples cover the
range from A (2-azaallenium ions) to B (2-azaallyl cations) depending on the nature of the substituent at C. For
E = P, the linear arrangement A was much less feasible
from the outset because two-coordinate phosphorus compounds have been found to be bent (by an angle of around
IOO", usually less than the tetrahedral angle) in every case
up to now. Whereas description C would account for such
a bond angle, it is consistent with neither the chemical behavior of the 2-phosphaallyl cations, nor with the strong
variation of S(31P)with the C-substituents."] The latter is
not understandable in the absence of rr-interactions between the phosphide center and the carbenio ligands.
Received: April 9. 1985;
revised: June 10, 1985 [ Z 1252 IE]
German version: Angew. Chem. 97 (1985) 777
[I] a) S . D. Parker, N. A. J. Rogers, Tetrahedron Lett. 1976, 4389; b) T. J.
Eckersley, S. D. Parker, N. A. J. Rogers, Tetrahedron 40 (1984) 3749.
121 M. Demuth, K. Schaffner, Angew. Chem. 94 (1982) 809; Angew. Chem.
fnt. Ed. Engl. 21 (1982) 820.
[3] L. A. Paquette, Top. Curr. Chem. 119 (1984) 1 .
141 M. Demuth, Chimia 38 (1984) 257.
[5] B. Wietfeld, Dissertation. Max-Planck-Institut fur Strahlenchemie, Mulheim a. d. Ruhr and Universitat Bochum 1984.
[6] The analytical data (IR, 400-MHz 'H-NMR, MS) for all new compounds are satisfactory.
[7] Actinometry: W. Amrein, J. Gloor, K. Schaffner, Chimia 28 (1974) 185:
extrapolation to zero conversion.
(81 The oxa-di-n-methane rearrangement and the 1.3-acyl shift are characteristic photoreactions of P,y-unsaturated ketones [9]. As for bicyclooctenones lacking methoxy substituents [lo], the 1,3 acyl shift (+4b,c) under our conditions probably results from residual direct absorption by
lb, c.
[Y] D. 1. Schuster in P. d e Mayo (Ed.): Rearrangements in Ground and Excited S/ates. Vof. 3, Academic Press, New York 1980, p. 167.
[lo] M. Demuth, P. R. Raghavan, C. Carter, K. Nakano, K. Schaffner, Helu.
Chrm. Acta 63 (1980) 2434.
[ I I ] L. D. Hess, J. L. Jacobson, K. Schaffner, J. N. Pitts, Jr., J. Am. Chem.
Soc. 89 (1967) 3684.
[I21 W. G. Dauben, G. W. Shaffer, Tetrahedron Lett. 1967, 4415.
[ 131 W. Amrein, K. Schaffner, unpublished results.
[I41 D. C. Neckers, A. P. Schaap, J . Hardy, J . Am. Chem. SOC.88 (1966)
1265; D. G. Marsh, J. N. Pitts, Jr., K. Schaffner, A. Tuinman, ibid. 93
(1971) 333.
(151 The major reaction path of Zd with trimethylsiloxybutadiene is oxetane
formation (>90?/0 at room temperature). The amount of 5 and 6 increased to about 10% at 5°C (the reaction at lower temperatures has not
yet been investigated). Recently, Isobe et al. [I61 photochemically generated and successfully trapped a similar but more delocalized biradical.
(161 N. Isobe, M. Matsuo, J . Miyamoto, Tetrahedron Letf. 25 (1984) 861.
0 VCH Verlagsgereilschaji mbH. 0-6940 Wernherrn. 1985
c c
/ \ / \
4 \Q/ \
The C-substituents also -govern the electrouhilicitv of
phosphorus. Thus, only cation 1 with four amino groups
can be methylated by methyl iodide to give the dication 2.
If derived from form B , the center in 2 should be planar,
as in the homologous biguanide d i c a t i o n ~ ' ~and
in the
isoelectronic tetraaminoallyl cations.['] On the other hand,
[*] Prof. Dr. A. Schmidpeter, Dipl.-Chem. A. Willhalm
lnstitut fur Anorganische Chemie der Universitat
Meiserstrasse 1 , D-8000 Miinchen 2 (FRG)
Prof. Dr. R. R. Holmes, Dr. R. 0. Day, J. M. Holmes
Department of Chemistry, University of Massachusetts
Amherst, MA 01003 (USA)
T h i s work was supported by the Deutsche Forschungsgemeinschaft and
Fonds der Chemischen Industrie / A . S . ) and by the NSF (CHE
820541 1) and NIH (GM 21 466, R. R . H . ) .
0570-0833/85/0909-0764 $ 03.50/0
Angew. Chem. Int. Ed Engl. 24 (1985) No 9
it should be pyramidal if derived from form C . With X-ray
structural investigations on crystals of the perchlorate of
and the diiodide of 2,18]it is now possible to compare 1
and 2 and their analogues.
The molecular structure of 1 (Fig. 1, top) shows the
usual bent geometry at phosphorus, as well as PC bond
lengths that are in accord with a bond order of 1.5, corresponding to B. Nevertheless, the coplanarity that would be
optimal for B is not quite attained. As in the ally1 cation, 1
(CH instead of P),Iblthe dimethylamino groups are twisted
somewhat out of the central plane as a result of their steric
interaction, and the interior angles at carbon are considerably larger than the exterior angles. The C N bond lengths
also agree with those found for the ally1 cation (average,
135 pm).
Fig. I . ORTEP plot of the structures of I (top) and 2 (bottom) in the crystals
The thermal ellipsoids are shown at the 50% probability
of 1 C104 and 2 I>,
level. Hydrogen atoms have been omitted for clarity. Bond lengths [pm] and
bond angles ["I (mean values).
The C2 symmetry that is approximately present in 1 is
fully lost as a consequence of methylation to give 2 (Fig. 1,
bottom). With a somewhat wider CPC angle than in 1, the
phosphorus coordination is now essentially trigonal pyramidal, and the planes of the two C(NMe2)2 groups are
rotated by an angle of 69" with respect to one another. The
PC bonds attain the single bond distance, while the C N
bonds are clearly shorter than in l.19]
Accordingly, the methylation of the phosphaallyl cation
is well described by 1B-2C. In 1 , where the phosphorus
is bicoordinated, there is obviously a normal conjugated
allylic system present. In this case, the phosphorus can
best be viewed as using its p orbitals for both TI- and 0bonding, the latter of which is consistent with the small
bond angle. The lone (s-)electron pair is accordingly not
available to a n electrophile without considerable promotion energy. It therefore remains in 2 and becomes stereochemically active. Instead, the n-electron pair is involved
(at least formally)"01 in the binding of the methyl group.
The dication 2, therefore, is no longer a conjugated allylic
system but merely a phosphane substituted with two independent amidinium groups. The conjugated form 2B
would require a nearly planar coordination of the phosphorus, the necessary energy for which is not sufficiently
provided by the resonance stabilization. In contrast, in the
isoelectronic tetraamino cations and dications with C R
Anyen,. Chem. I n r . Ed. Engl. 24 1198s) No. 9
and NR, respectively, instead of PMe, the allylic bonds exhibit the highest barriers to rotation.I5. ' I ]
Received: April 22, 1985;
revised: June 7, 1985 [Z 1274 IE]
German version: Anyew. Chem. 97 (1985) '775
A. Schmidpeter, S. Lochschmidt, A. Willhalm, Anyew. Chem. 9s (1983)
561; Angew. Chem. Inr. Ed. Engl. 22 (1983) 545: Anqew. Chem. Suppl.
1983, 710.
A. Schmidpeter, A. Willhalm, Angew. Chem. 96 (1984) 901 ; Anyew.
Chem. Int. Ed. Engl. 23 (1984) 903.
Heterocyclic phosphaallyl cations were among the first examples of
two-coordinate phosphorus: K. Dimroth, Top. Curr. Chem. 38 (1973)
E.-U. Wiirthwein, R. Kupfer, P. H. M. Budzelaar, C. Strobel, H. P. Beck,
Anyew. Chem. 97 (1985) 327; Angew. Chem. Int. Ed. Enql. 24 (1985) 340
and references cited therein.
V. J. Bauer, W. Fulmore, G. 0. Morton, S. R. Safir, J . Am. Cheni. SUC.90
(1968) 6845.
E. Oeser, Chem. Ber. 107 (1974) 627; H.-U. Wagner, ibid. I07 (1974)
Upon combination of concentrated ethanol solutions of 1 CI [ I ] and
N e C I O ? , 1 CIO, crystallizes at first as light yellow needles, then as
platelets; m.p. = 168-169°C. P2,/n (alterFate setting of P2,/c),
a=6.806(1) A, b= 12.887(3)A, c = 19.256(3)A, 8=97.99(2)", 2 = 4 . Enraf-Nonius CAD4 diffractometer; graphite-monochromated MoK.. rddiation for 2 " 5 2 8 5 5 0 " ; 2045 independent reflections with I 2 2 n ( l ) :
R=0.061, R,=0.075.
From a roughly 0 . 2 ~acetonitrile solution of 1 CI [ I ] and two molar
equivalents of methyl iodide, a light yellow precipitate of 2 I-.
within several days; almost colorless crystals after recrystallization from
acetonitrile, 80% yield, m.p.=207-209"C, G("P)jCHICN)= - 17.4.
PZ,/n (alternate setting of PZ,/c), a=8.012(2) A, h= 12.164(3) A,
c = 19.157(3) A,P=92.31(1)", Z = 4 . Enraf-Nonius CAD4 diffractometer,
graphite-monochromated MoK,<radiation for 2 ' 5 28143': 1'7x4 independent reflections with 12 2 0 ( l ) ; R =0.035, R, =0.056. Further details
of the crystal structure investigation are available on request from the
Fachinformationszentrum Energie Physik Mathematik, D-75 14 Eggenstein-Leopoldshafen 2, on quoting the depository number CSD 5 I 5 19,
the names of the authors. and full citation of the journal.
[9] A parallel yet less pronounced change of bond lengths is observed for
the transition from the monoprotonated to the diprotonated biguanide:
A. A. Pinkerton, D. Schwarzenbach, J . Chem. SOL Dullon Trans. IY78.
[lo] According to calculations (J. Kroner, A. Schmidpeter, unpublished),
however, this very likely describes the reaction pathway, too.
1111 R. Gompper, C . S. Schneider, unpublished results; C . S. Schneider, Dissertafion, Universitat Munchen 1977.
Allylation of Aldehydes with Etherification by
an Asymmetric Variant of the Sakurai Reaction**
By Rene Imwinkelried and Dieter Seebach*
Dedicated to Professor Hans Musso on the occasion of
his 60th birthday
The preparation of homoallylic alcohols by TiCl,-mediated allylation of aldehydes and ketones with allyltrimethylsilane was first described by Sakurai et al. in 1976."l
We have now found that dialkoxydichlorotitanium reagents behave quite differently from TiCI, under the same
conditions. Treatment of aldehydes with 1.1 equivalents of
dialkoxydichlorotitanium at - 75 oC,121
and subsequent addition of 1.2 equivalents of allyltrimethylsilane led to the
isolation in good yields of homoallylic ethers 1 (Table I).
The reagents are easily generated in situ either by mixing
equimolar amounts of TiCI, and commercial Ti(OR')4
[*] Prof. Dr. D. Seebach, DipLChem. R. lmwinkelried
Laboratorium fur Organische Chemie
der Eideenossischen Technischen Hochschule
ETH-Zentrum, Universitatstrasse 16, CH-8092 Zurich (Switzerland)
Part of the projected Ph. D. thesis of R . 1.. ETH Zurich.
0 VCH Verluqsgesellschaji mbH. 0-6940 Weinheim, 1985
OS70-0833/8S/0909-076S $ 02.50/0
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