Synthesis and Characterization of Divalent Main Group Diamides and

Chapter 29

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CO2

1

1

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Yongjun Tang , Ana M . Felix , Virginia W. Manner , Lev N. Zakharov , Arnold L . Rheingold , Bahram Moasser , and Richard A. Kemp * 2

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3

1,4,

1Department

of Chemistry, University of Mexico, MSC03 2060, Albuquerque, NM 87131 Deparment of Chemistry and Biochemistry, University of California at San Diego, 9500 Gillman Drive, La Jolla, CA 92093 General Electric Global Research Center, Building CEB, Room 134, One Research Circle, Niskayuna, NY 12309 Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Boulevard, SE, Albuquerque, NM 87106

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It is known that bulky Sn and Ge N-silylamides insert CO2 to form either silyl isocyanides or silyl carbodiimides, albeit relatively slowly. As part of a research effort to eventually prepare C-labelled radiopharmaceuticals derived from 1 1 C O2, we have been interested in expanding the scope of this reaction by investigating other species that may react more rapidly. We have synthesized and structurally characterized a large variety of new diamides based on other metals such as Mg, Ca, Ba, and Zn, as well as new divalent Sn species with several new sterically-demanding, trimethylsilyl-containing ligands. Reactions of these species with C O are discussed. 11

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410

© 2006 American Chemical Society Lattman and Kemp; Modern Aspects of Main Group Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

411 The insertion of carbon dioxide into well-defined organometallic species in order to prepare more valuable organic molecules has been an active subject of study over the past several years (/). Goals of these previous investigations have ranged from detailed mechanistic studies of how CO2 interacts with metals to synthetic schemes that use CQ2 as reactant and/or solvent. While most of the previous work has focused on the interactions of CO2 with transition metals, more recently the insertion of CQ2 into main group element bonds has been of interest. More specifically insertion of CQ2 into main group element-nitrogen bonds to form carbamates has been under investigation, which has culminated in a recent review (2). Sita and coworkers have very recently studied the insertion of C 0 into divalent Sn and Ge bis(bistrimethylsilylamides) (Figure 1) (J). They discovered that under relatively mild conditions CO2 would insert into 2

SiMe

3

TMS-N=C=N-TMS

Figure 1. Overall reaction scheme to generate trimethylsilylisocyanate and bis(trimethylsilyl)carbodiimide from CO2 and Group 14 divalent amides}

these Sn or Ge amide bonds to form in situ carbamates, and these carbamates would subsequently extrude either trimethylsilylisocyanate or bis(trimethylsilyl)carbodiimide. We were intrigued with the possibility that by using these divalent metal amides we might be able to use radio-labeled C 0 2 as a reagent for preparing C-carbamate or urea-containing radiopharmaceuticals that could be useful in diversifying the options for positron emission tomography (PET) (4). However, the rates of CQ2 insertion observed by Sita, while impressive, were not nearly fast enough for our purposes. Since the halflife of C is 20.3 minutes (J) we are limited to 3-4 half lives (or approximately 60-80 minutes) to accomplish the chemistry we need to do in order to possibly make radiopharmaceuticals effectively. Thus, we were interested in reactions of CO2 with other main group amides that would mimic the reaction chemistry seen by Sita but at a significantly faster rate than that observed with the subN

u

n

Lattman and Kemp; Modern Aspects of Main Group Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

412 valent Sn or Ge amides. Additionally, we were interested in new ligands that with Sn or other Group 14 congeners might increase reaction rates beyond those seen earlier by Sita.

Syntheses of Divalent Metal Amides Our initial investigations have involved the synthesis and characterization of a variety of divalent metal amides with both known and new silyl-containing ligands. Towards this end we have prepared and structurally characterized new metal amides based on Mg, Ca, Ba, Zn, Hg, and Sn. Magnesium-Based Diamides Interactions of CO2 with magnesium compounds are well-known, ranging as far back as the preparation of organic acids from Grignard reagents (ό). More relevant recent research comes from the group of Chang ( 7) who examined the insertions of CO2 into Mg and mixed Mg-Al species to generate carbamato-type complexes of Mg. Of much interest to us was the observation that the insertions of C 0 were much more rapid than those seen by Sita earlier; however, the complexes formed in the Chang work did not lose trimethylsilylisocyanate spontaneously since his work was done with alkyl or aryl amides. We thus set forth to synthesize a range of Mg(NRR')2 complexes, where at least one of the R groups was a trimethylsilyl (TMS) group. 2

We prepared the magnesium diamides by either of two routes commonly used to prepare metal amides (8). The first method involved alkane elimination upon direct reaction of dibutylmagnesium with the desired silylamine. The second route was a metathesis reaction, whereby L i X (or KX) was eliminated from the interaction of lithium (or potassium) amide with MgBr2. In both of these routes solvents were found to play non-innocent roles, much as they do in the preparation of related metal alkoxides (9). Figure 2 shows the types of ligands used ( l a / l b , benzyl; 2, adamantyl; 3, i-propyl; 4a/4b, mesityl; 5, i-butyldimethylsilyl-; 6, i-butyldiphenylsilyl-; 7, cyclohexyl; 8, 4-adamantyl2,6-di-/-propylphenyl-) along with the range of magnesium coordination geometries seen in these preparations. While detailed discussions of the preparations and X-ray structures will be published elsewhere, several salient points can be made here. Generally, the [(TMS)(R)N]- ligands are insufficient in steric bulk to keep solvent molecules from entering the coordination sphere of the Mg atom. The majority of the structures are consistent with a Mg atom containing two solvent molecules in addition to the two Mg-N bonds, leading to a pseudo-tetrahedral geometry around Mg. In two cases we see a Mg atom with only one solvent ligand coordinated (the bulky R groups adamantyl 2 and /-butyldimethylsilyl 5) and in one case we see no solvent molecules attached to Mg (the extremely bulky R group /-butyldiphenylsilyl 6). The ligand used to prepare 8 deserves some comment as well. We were attempting to prepare the


413 bulky amine (adamantyl)(2,6-dw-propylphenyl)NH by reacting adamantyl bromide with 2,6-dw-propylphenylamine at 240 °C. Rather than forming the desired product we obtained instead the new compound formed by attack of the adamantyl bromide at the para-position of the aromatic ring to liberate HBr and couple the rings. Table 1. Magnesium Disilylamides [Mg(N(TMS)R) ] Prepared and Characterized 2

Solvent

R Group

/7—\ \ 7~CH = N

a) HMPA

ether

Mg(N(TMS)R) [Et 0] (2)

4-DMAP

Mg(N(TMS)R) [DMAP] (3)

a) THF

Pseudo-tetrahedral Mg

(t-Butyl)Ph Si-

2

2

Tri-coordinate Mg

2


Mg(N(TMS)R) [Solv] 2

2

(4)


3

(t-Butyt)Me Si2

2

b) pyr

CH

Ο

(1)

2

3

- Q -

2

2

b) 4-DMAP

,CH 3

Mg(N(TMS)R) [Solv]

2

: /

i-Propy!

H C

X-Ray Structure Comments

Resulting Formula

pyr

Mg(N(TMS)R) [pyr] (5)

ether

Mg(N(TMS)R) (6)

4-DMAP

Mg(N(TMS)R) [DMAP] (7)


THF

Mg(N(TMS)R) [THF] (8)


2

Monomelic, Di-coordinate Mg

2

2

2

2

2

Tri-coordinate Mg

Solvents: HMPA - hexamethylphosphoramide; 4-DMAP - 4-dimethylaminopyridine;pyr -pyridine; THF - tetrahydrofuran; ether - diethyl ether


414 AU of these magnesium species have been subjected to reaction with CCfe. In each case, simple bubbling of CQ2 through a pentane solution of the Mg diamide at room temperature led to an extremely rapid reaction (

Synthesis and Characterization of Divalent Main Group Diamides and

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