Published on Web 10/09/2004
Phosphaalkynes from Acid Chlorides via P for O(Cl) Metathesis:
A Recyclable Niobium Phosphide (P3-) Reagent that Effects C-P Triple-Bond
Formation
Joshua S. Figueroa and Christopher C. Cummins*
Room 2-227, Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139-4307
Received June 29, 2004; E-mail: ccummins@mit.edu
With the advent of niobaziridine-hydride Nb(H)(t-Bu(H)Cd
NAr)(N[Np]Ar)2 (1, Np ) neopentyl, Ar ) 3,5-Me2C6H3)1 has
emerged new potential for P-atom transfer stemming from the
ability of 1 to serve as a synthon for the reactive Nb(N[Np]Ar)3
fragment.2 As shown in Figure 1, white phosphorus (P4) reacts with
1 to provide the µ-P2 diniobium complex 2. The latter is cleaved
reductively by sodium amalgam in THF to provide the [Na(THF)x]+
salt of the terminal niobium phosphide anion, [PNb(N[Np]Ar)3]-
(3). The latter sequence (reactions i and ii in the synthetic cycle at
left in Figure 1) was reported recently.1,2 Featured in the present
communication is the fact that phosphide anion 3 serves to transform
acid chlorides into corresponding phosphaalkynes via four-
membered metallacyclic intermediates, examples of which have
been isolated and fully characterized. The transformation represents
a new, mild method for access to phosphaalkynes, the high inherent
reactivity of which has led to their implementation in organic,3,4
polymer,5 and coordination chemistry.6
Treatment of [Na(THF)x]3 with either pivaloyl chloride (t-BuC-
(O)Cl) or 1-adamantoyl chloride (1-AdC(O)Cl) in cold THF solution
elicits a color change from dark yellow to red over a period of 1.0
h. A standard workup followed by crystallization from Et2O
produced the cherry-red metallacyles, (t-BuC(O)P)Nb(N[Np]Ar)3
(4-t-Bu) and (1-AdC(O)P)Nb(N[Np]Ar)3 (4-1-Ad), in yields of 70
and 80%, respectively (reaction iii, Figure 1). Crystallographic
structure determination of both derivatives 47 revealed the presence
of four-membered, Nb-P-C-O rings containing both Nb-P and
Nb-O linkages (an ORTEP rendering of 4-1-Ad is included in
Figure 1, top right). Whereas metallacycles 4 might be viewed as
acylphosphinidenes8 (NbdPC(dO)R) with bending at phosphorus9
and carbonyl oxygen coordination, we suggest that they are best
formulated as heteroatom-substituted, niobacyclobutene (Nb-Pd
C(-O)R) complexes. Significant electronic rearrangement is thus
brought about by acylation of the phosphide (M-P triple bond10)
moiety.2 Structural parameters in support of this formulation for
4-1-Ad include a C-O distance of 1.314(4) Å and a (short) P-C
distance of 1.757(7) Å. The latter parameter is in the range typical
for phosphaalkene P-C double bonds,11 while the former parameter
exceeds the maximum value expected for a carbonyl group.
Furthermore, the Nb-P distance of 2.491(2) Å is substantially
longer than expected for a doubly bound terminal phosphinidene,9
more closely approximating a Nb-P single bond.12
Hz (4-1-Ad), are likewise strongly suggestive of the presence of
P-C multiple bonding in these niobacycles. In addition, the increase
in P-C bond order at the expense of carbonyl CdO bonding is
reflected in the infrared spectrum of complexes 4, where carbonyl
stretching vibrations in the usual energy range are not observed.14
Niobacycles 4 are valence-isoelectronic to the metallacyclic in-
termediates postulated by Chisholm in explanation of NW(O-t-Bu)3-
catalyzed nitrogen atom exchange between PhCN and MeC15N.15
Accordingly, metallacycles 4 have been found to readily undergo
retro [2 + 2]-fragmentation in solution, leading smoothly to known
phosphaalkynes3 5 (t-BuCP or 1-AdCP) along with oxoniobium-
(V) complex1 6 (reaction iv in Figure 1). Cherry red C6D6 solutions
of either 4-t-Bu or 4-1-Ad left standing at room temperature are
observed to pale in color, becoming yellow over several hours.
Intermittent assay of these solutions by either 1H or 31P{1H} NMR
spectroscopy revealed the disappearance of 4 concomitant with
formation of oxo 6 and either t-BuCP (5-t-Bu) or 1-AdCP (5-1-
Ad). Phosphaalkynes 5 are readily identified by their characteristic
resonances3,16 in both 1H and 31P{1H} NMR spectra of the reaction
mixtures, and the reaction can be conveniently monitored as a
function of time.
Figure 1 includes (bottom, right) a stacked plot of the 31P{1H}
NMR spectrum as a function of time corresponding to the 4-t-Bu
f 5-t-Bu + 6 conversion at 45 °C in C6D6 solution. Clearly evident
is the smooth decay of the resonance at 261 ppm (4-t-Bu)
simultaneous with the appearance of a resonance located at -64
ppm (4-t-Bu, lit. (20 °C) ) -69 ppm). At 45 °C, phosphaalkyne
formation is complete in ca. 4 h, and it is noteworthy that no
additional signals are observed in the 31P{1H} NMR spectrum over
the range +700 to -300 ppm during the reaction. Upon complete
thermolysis, solutions containing equimolar amounts of 5-t-Bu and
6 stored at room temperature do not revert back to 4-t-Bu when
monitored for several weeks. Thus, the retro [2 + 2]-fragmentation
reaction is irreversible under the conditions probed. Preliminary
kinetic studies for the 4-t-Bu f 5-t-Bu + 6 reaction over the
temperature range 308-338 K in C6D6 solution are consistent with
a first-order fragmentation process. Activation parameters ∆Hq )
24.9 ( 1.4 kcal/mol and ∆Sq ) 2.4 ( 4.3 cal/mol K were obtained
over this range, with the small entropic barrier adding weight to
the notion of niobacycles 4 as intermediates on the phosphalkyne
formation pathway, requiring little reorganization to reach the
transition state for fragmentation.
The foregoing description of metallacycles 4 finds further support
by way of spectroscopic signatures. In C6D6 solution, the 31P{1H}
NMR resonances for 4-t-Bu (261 ppm) and 4-1-Ad (258 ppm) are
upfield of the chemical shift range characteristic of early transition
metal, terminal phosphinidene complexes,9 while lying in a range
typical of phosphaalkenes.11 The 13C{1H} NMR resonances of 260
ppm (4-t-Bu and 4-1-Ad) for the metallacyclic carbon,13 with asso-
For synthetic purposes, thermolysis of solutions containing 4-t-
Bu or 4-1-Ad at 85 °C provides 6 and the corresponding phos-
phaalkyne within 30 min. After thermolysis of a solution containing
4-t-Bu, colorless solutions of the pure phosphaalkyne, 5-t-Bu, are
obtained by simple vacuum transfer. The yield of 5-t-Bu obtained
in this fashion is greater than 90%, on scales from 20 to 200 mg of
1
ciated large JPC coupling constants of 113 Hz (4-t-Bu) and 107
9
13916
J. AM. CHEM. SOC. 2004, 126, 13916-13917
10.1021/ja0461522 CCC: $27.50 © 2004 American Chemical Society
Figueroa, Joshua S.
